Experimental simulation of impact cratering on icy satellites

NASA Technical Reports Server (NTRS)

Greeley, R.; Fink, J. H.; Gault, D. E.; Guest, J. E.

1982-01-01

Cratering processes on icy satellites were simulated in a series of 102 laboratory impact experiments involving a wide range of target materials. For impacts into homogeneous clay slurries with impact energies ranging from five million to ten billion ergs, target yield strengths ranged from 100 to 38 Pa, and apparent viscosities ranged from 8 to 200 Pa s. Bowl-shaped craters, flat-floored craters, central peak craters with high or little relief, and craters with no relief were observed. Crater diameters increased steadily as energies were raised. A similar sequence was seen for experiment in which impact energy was held constant but target viscosity and strength progressively decreases. The experiments suggest that the physical properties of the target media relative to the gravitationally induced stresses determined the final crater morphology. Crater palimpsests could form by prompt collapse of large central peak craters formed in low target strength materials. Ages estimated from crater size-frequency distributions that include these large craters may give values that are too high.

Increased depth-diameter ratios in the Medusae Fossae Formation deposits of Mars

NASA Technical Reports Server (NTRS)

Barlow, N. G.

1993-01-01

Depth to diameter ratios for fresh impact craters on Mars are commonly cited as approximately 0.2 for simple craters and 0.1 for complex craters. Recent computation of depth-diameter ratios in the Amazonis-Memnonia region of Mars indicates that craters within the Medusae Fossae Formation deposits found in this region display greater depth-diameter ratios than expected for both simple and complex craters. Photoclinometric and shadow length techniques have been used to obtain depths of craters within the Amazonis-Memnonia region. The 37 craters in the 2 to 29 km diameter range and displaying fresh impact morphologies were identified in the area of study. This region includes the Amazonian aged upper and middle members of the Medusae Fossae Formation and Noachian aged cratered and hilly units. The Medusae Fossae Formation is characterized by extensive, flat to gently undulating deposits of controversial origin. These deposits appear to vary from friable to indurated. Early analysis of crater degradation in the Medusae Fossae region suggested that simple craters excavated to greater depths than expected based on the general depth-diameter relationships derived for Mars. However, too few craters were available in the initial analysis to estimate the actual depth-diameter ratios within this region. Although the analysis is continuing, we are now beginning to see a convergence towards specific values for the depth-diameter ratio depending on geologic unit.

Lost Impacts (Invited)

NASA Astrophysics Data System (ADS)

Schultz, P. H.; Stickle, A. M.

2009-12-01

The absence of a clearly identified crater (or craters) for the proposed YDB impact has raised questions concerning the reality of such an event. Geologic studies have identified impact deposits well before recognizing a causative crater (e.g., Chicxulub and Chesapeake Bay); some have yet to be discovered (e.g., Australasian tektite strewnfields). The absence of a crater, therefore, cannot be used as an argument against the reality of the YDB impact (and its possible consequences). The study here addresses how a large on-land impact during the late Pleistocene or early Holocene could avoid easy detection today. It does not argue the case for a YDB impact, since such evidence must come from the rock record. During the late Pleistocene, the receding Laurentide ice sheet still covered a significant portion of Canada. While a large (1km) body impacting vertically (90°) would penetrate such a low-impedance ice layer and excavate the substrate, an oblique impact couples more of its energy into the surface layer, thereby partially shielding the substrate. Three approaches address the effectiveness of this flak-jacket effect. First, hypervelocity impact experiments at the NASA Ames Vertical Gun Range investigated the effectiveness of low-impedance layers of different thicknesses for mitigating substrate damage. Second, selected experiments were compared with hydrocode models (see Stickle and Schultz, this volume) and extended to large scales. Third, comparisons were made with relict craters found in eroding sediment and ice covers on Mars. Oblique impacts (30 degrees) into soft particulates (no. 24 sand) covering a solid substrate (aluminum) have no effect on the final crater diameter for layer thicknesses exceeding a projectile diameter and result in only plastic deformation in the substrate. In contrast, a vertical impact requires a surface layer at least 3 times the projectile diameter to achieve the same diameter (with significant substrate damage). Oblique impacts into ice and plasticene layers over clear acrylic blocks allow assessing internal damage. These experiments reveal that low-impedance surface layers approaching 1 to 2 projectile diameters effectively shield the substrate from shock damage for impact angles less than 30 degrees. Missing craters (and relict crater roots) within ice-rich deposits on Mars illustrate the rapid erasure the impact record. Numerous small pedestal craters (crater diameter < 5km) occur at high latitudes and reflect the cyclic expansion and disappearance of polar ice/dust deposits up to 0.5 km thick. Much larger examples (> 50km), however, occur at low latitudes but are localized in certain regions where even thicker deposits (locally >2km) have been removed, uncovering a preserved Noachian landscape. Crater statistics further document this missing cratering record. Thick Pleistocene ice sheets on Earth would have played a similar role for the removal of terrestrial cratering record. We calculate that a crater as large as 15km in diameter formed by an oblique impact could have been effectively erased, except for dispersed ejecta containing shocked impactor relicts and a disturbed substrate. While plausible, evidence for specific missing events (e.g., the proposed YB impact) must be found in still-preserved ice layers and sediments.

Penetration scaling in atomistic simulations of hypervelocity impact

NASA Astrophysics Data System (ADS)

Ruestes, C. J.; Bringa, E. M.; Fioretti, F.; Higginbotham, A.; Taylor, E. A.; Graham, G.

2011-06-01

We present atomistic molecular dynamics simulations of the impact of copper nano particles at 5 km/s on copper films ranging in thickness from 0.5 to 4 times the projectile diameter. We access both penetration and cratering regimes with final cratering morphologies showing considerable similarity to experimental impacts on both micron and millimeter scales. Both craters and holes are formed from a molten region, with relatively low defect densities remaining after cooling and recrystallisation. Crater diameter and penetration limits are compared to analytical scaling models: in agreement with some models we find the onset of penetration occurs for 1.0 < f/d < 1.5, where f is the film thickness and d is the projectile diameter. However, our results for the hole size agree well with scaling laws based on macroscopic experiments providing enhanced strength of a nano-film that melts completely at the impact region is taken into account. Penetration in films with pre-existing nanocracks is qualitatively similar to penetration in perfect films, including the lack of back-spall. Simulations using ``peridynamics'' are also described and compared to the atomistic simulations. Work supported by PICT2007-PRH, PICT-2008 1325, and SeCTyP.

Occurrence and mechanisms of impact melt emplacement at small lunar craters

NASA Astrophysics Data System (ADS)

Stopar, Julie D.; Hawke, B. Ray; Robinson, Mark S.; Denevi, Brett W.; Giguere, Thomas A.; Koeber, Steven D.

2014-11-01

Using observations from the Lunar Reconnaissance Orbiter Camera (LROC), we assess the frequency and occurrence of impact melt at simple craters less than 5 km in diameter. Nine-hundred-and-fifty fresh, randomly distributed impact craters were identified for study based on their maturity, albedo, and preservation state. The occurrence, frequency, and distribution of impact melt deposits associated with these craters, particularly ponded melt and lobate flows, are diagnostic of melt emplacement mechanisms. Like larger craters, those smaller than a few kilometers in diameter often exhibit ponded melt on the crater floor as well as lobate flows near the crater rim crest. The morphologies of these deposits suggest gravity-driven flow while the melt was molten. Impact melt deposits emplaced as veneers and ;sprays;, thin layers of ejecta that drape other crater materials, indicate deposition late in the cratering process; the deposits of fine sprays are particularly sensitive to degradation. Exterior melt deposits found near the rims of a few dozen craters are distributed asymmetrically around the crater and are rare at craters less than 2 km in diameter. Pre-existing topography plays a role in the occurrence and distribution of these melt deposits, particularly for craters smaller than 1 km in diameter, but does not account for all observed asymmetries in impact melt distribution. The observed relative abundance and frequency of ponded melt and flows in and around simple lunar craters increases with crater diameter, as was previously predicted from models. However, impact melt deposits are found more commonly at simple lunar craters (i.e., those less than a few kilometers in diameter) than previously expected. Ponded melt deposits are observed in roughly 15% of fresh craters smaller than 300 m in diameter and 80% of fresh craters between 600 m and 5 km in diameter. Furthermore, melt deposits are observed at roughly twice as many non-mare craters than at mare craters. We infer that the distributions and occurrences of impact melt are strongly influenced by impact velocity and angle, target porosity, pre-existing topography, and degradation. Additionally, areally small and volumetrically thin melt deposits are sensitive to mixing with solid debris and/or burial during the modification stage of impact cratering as well as post-cratering degradation. Thus, the production of melt at craters less than ∼800 m in diameter is likely greater than inferred from the present occurrence of melt deposits, which is rapidly affected by ongoing degradation processes.

Lunar crater volumes - Interpretation by models of impact cratering and upper crustal structure

NASA Technical Reports Server (NTRS)

Croft, S. K.

1978-01-01

Lunar crater volumes can be divided by size into two general classes with distinctly different functional dependence on diameter. Craters smaller than approximately 12 km in diameter are morphologically simple and increase in volume as the cube of the diameter, while craters larger than about 20 km are complex and increase in volume at a significantly lower rate implying shallowing. Ejecta and interior volumes are not identical and their ratio, Schroeters Ratio (SR), increases from about 0.5 for simple craters to about 1.5 for complex craters. The excess of ejecta volume causing the increase, can be accounted for by a discontinuity in lunar crust porosity at 1.5-2 km depth. The diameter range of significant increase in SR corresponds with the diameter range of transition from simple to complex crater morphology. This observation, combined with theoretical rebound calculation, indicates control of the transition diameter by the porosity structure of the upper crust.

Landform degradation on Mercury, the moon, and Mars - Evidence from crater depth/diameter relationships

NASA Technical Reports Server (NTRS)

Malin, M. C.; Dzurisin, D.

1977-01-01

Craters on Mercury, the moon, and Mars were classified into two groups, namely, fresh and degraded craters, on the basis of qualitative visual degradation as revealed by degree of rim crispness, terraced interior walls, slumping from crater walls, etc., and the depth/diameter relationship of craters was studied. Lunar and Mercurian crater populations indicate the existence of terrain-correlated degradational phenomena. The depth/diameter relations for Mercury and the moon display remarkably similar forms, suggesting similar degrees of landform degradation. Depth/diameter curves display a break in slope, dividing two distinct crater populations. Mars craters show few of the trends of those of Mercury and the moon. The depth/diameter curve has no definite break in slope, though there is considerable depth variation. The role of nonballistic degradation in connection with the early formation of large expanses of intercrater plains is discussed.

Crater dimensions from apollo data and supplemental sources

USGS Publications Warehouse

Pike, R.J.

1976-01-01

A catalog of crater dimensions that were compiled mostly from the new Apollo-based Lunar Topographic Orthophotomaps is presented in its entirety. Values of crater diameter, depth, rim height, flank width, circularity, and floor diameter (where applicable) are tabulated for a sample of 484 craters on the Moon and 22 craters on Earth. Systematic techniques of mensuration are detailed. The lunar craters range in size from 400 m to 300 km across and include primary impact craters of the main sequence, secondary impact craters, craterlets atop domes and cones, and dark-halo craters. The terrestrial craters are between 10 m and 22.5 km in diameter and were formed by meteorite impact. ?? 1976 D. Reidel Publishing Company.

Crater studies: Part A: lunar crater morphometry

USGS Publications Warehouse

Pike, Richard J.

1973-01-01

Morphometry, the quantitative study of shape, complements the visual observation and photointerpretation in analyzing the most outstanding landforms of the Moon, its craters (refs. 32-1 and 32-2). All three of these interpretative tools, which were developed throughout the long history of telescopic lunar study preceding the Apollo Program, will continue to be applicable to crater analysis until detailed field work becomes possible. Although no large (>17.5 km diameter) craters were examined in situ on any of the Apollo landings, the photographs acquired from the command modules will markedly strengthen results of less direct investigations of the craters. For morphometry, the most useful materials are the orbital metric and panoramic photographs from the final three Apollo missions. These photographs permit preparation of contour maps, topographic profiles, and other numerical data that accurately portray for the first time the surface geometry of lunar craters of all sizes. Interpretations of craters no longer need be compromised by inadequate topographic data. In the pre-Apollo era, hypotheses for the genesis of lunar craters usually were constructed without any numerical descriptive data. Such speculations will have little credibility unless supported by accurate, quantitative data, especially those generated from Apollo orbital photographs. This paper presents a general study of the surface geometry of 25 far-side craters and a more detailed study of rim-crest evenness for 15 near-side and far-side craters. Analysis of this preliminary sample of Apollo 15 and 17 data, which includes craters between 1.5 and 275 km in diameter, suggests that most genetic interpretations of craters made from pre-Apollo topographic measurements may require no drastic revision. All measurements were made from topographic profiles generated on a stereoplotter at the Photogrammetric Unit of the U.S. Geological Survey, Center of Astrogeology, Flagstaff, Arizona.

Relative depths of simple craters and the nature of the lunar regolith

NASA Astrophysics Data System (ADS)

Stopar, Julie D.; Robinson, Mark S.; Barnouin, Olivier S.; McEwen, Alfred S.; Speyerer, Emerson J.; Henriksen, Megan R.; Sutton, Sarah S.

2017-12-01

We assessed the morphologies of more than 930 simple impact craters (diameters 40 m-10 km) on the Moon using digital terrain models (DTMs) of a variety of terrains in order to characterize the variability of fresh crater morphology as a function of crater diameter. From Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) DTMs, we determined depth-to-diameter (d/D) ratios for an extremely fresh set of these craters with diameters less than 400 m and found that their d/D ratios range from 0.11 to 0.17. Using both NAC and Kaguya Terrain Camera DTMs, we also determined the d/D ratios for the set of fresh simple craters larger than 400 m in diameter. The d/D ratios of these larger craters are typically near 0.21, as expected of gravity-dominated crater excavation. Fresh craters less than ∼400 m in diameter, on the other hand, exhibit significantly lower d/D ratios. Various possible factors affect the morphologies and relative depths (d/D ratios) of small strength-dominated craters, including impactor and target properties (e.g., effective strength, strength contrasts, porosity, pre-existing weaknesses), impact angle and velocity, and degradation state. While impact conditions resulting from secondary impacts can also affect crater morphologies, we found that d/D ratio alone was not a unique discriminator of small secondary craters. To investigate the relative influences of degradation and target properties on the d/D ratios of small strength-dominated craters, we examined a subset of fresh craters located on the geologically young rim deposits of Tycho crater. These craters are deeper and steeper than other craters of similar diameter and degradation state, consistent with their relative freshness and formation in the relatively coherent, melt-rich deposits in this region. The d/D ratios of globally distributed small craters of similar degradation state and size range, on the other hand, are relatively shallow with lower average wall slopes, consistent with crater excavation in a weak or poorly cohesive layer. The widespread predominance of these small, shallow craters is consistent with the pervasive, poorly cohesive upper regolith.

Investigation of the relationship of crater depths and diameters in selected regions of Mars

NASA Astrophysics Data System (ADS)

Hsu, Hsin-Jen

2013-03-01

Impact craters are common geomorphological features on Mars. The density of craters is different among various regions. Higher crater density means older terrain. Craters can be divided into two types by the interior morphology: simple and complex. The cavity of Simple craters is bowl-shape, and complex craters display various interior features, such as central peaks. The depth/diameter ratio (d/D) of simple craters is larger than that of complex craters. The transition diameter from simple to complex morphologies ranges between 5 and 10 km, and is commonly cited to be about 7 km in the equatorial regions and 6 km near the poles, but the exact value also could vary with terrain type. In this research, seven regions, Amazonis Planitia, Arabia Terra, Chryse Planitia, Hesperia Planum, Isidis Planitia, Solis/Syria/Sinai Planum, and Terra Sirenum, were selected to investigate the onset diameter of complex craters and the relationship of crater diameter and depth in these regions on Mars in order to understand how the geology affects crater d/D. The analysis revealed that the slopes of the d/D relations are different, and these are linked to the surface material in different regions. The onset diameters in young volcanic regions with stronger material are slightly higher than older volcanic regions, and much higher than that of volatile regions. The research proves the different geological units can affect the morphology and morphometry of craters.

Global detection of large lunar craters based on the CE-1 digital elevation model

NASA Astrophysics Data System (ADS)

Luo, Lei; Mu, Lingli; Wang, Xinyuan; Li, Chao; Ji, Wei; Zhao, Jinjin; Cai, Heng

2013-12-01

Craters, one of the most significant features of the lunar surface, have been widely researched because they offer us the relative age of the surface unit as well as crucial geological information. Research on crater detection algorithms (CDAs) of the Moon and other planetary bodies has concentrated on detecting them from imagery data, but the computational cost of detecting large craters using images makes these CDAs impractical. This paper presents a new approach to crater detection that utilizes a digital elevation model instead of images; this enables fully automatic global detection of large craters. Craters were delineated by terrain attributes, and then thresholding maps of terrain attributes were used to transform topographic data into a binary image, finally craters were detected by using the Hough Transform from the binary image. By using the proposed algorithm, we produced a catalog of all craters ⩾10 km in diameter on the lunar surface and analyzed their distribution and population characteristics.

Phobos - Surface density of impact craters

NASA Technical Reports Server (NTRS)

Thomas, P.; Veverka, J.

1977-01-01

Revised crater counts for Phobos are presented which are based on uniform Mariner 9 imagery and Duxbury's (1974) map of the satellite. The contiguous portion of the satellite's surface on which all craters down to the limiting resolution of 0.2 to 0.3 km in diameter would be expected to be identified is delineated and found to contain 87 identifiable craters larger than 0.2 km in diameter. Analysis of the crater size distribution shows that the surface appears to be saturated for craters exceeding 1 km in diameter but the crater counts definitely fall below the saturation curve for smaller craters. Reasons for this fall-off are considered, and it is noted that too few craters are visible in Mariner 9 images of Deimos to permit meaningful crater counts on that satellite's surface. It is concluded that, contrary to a previous assertion, the surfaces of Phobos and Deimos are not known to be saturated with craters larger than 0.2 km in diameter.

Evolution of Lunar Crater Ejecta Through Time: Influence of Crater Size on the Record of Dynamic Processes

NASA Astrophysics Data System (ADS)

Ghent, R. R.; Tai Udovicic, C.; Mazrouei, S.; Bottke, W. F., Jr.

2017-12-01

The bombardment history of the Moon holds the key to understanding important aspects of the evolution of the Solar System at 1AU. It informs our thinking about the rates and chronology of events on other planetary bodies and the evolution of the asteroid belt. In previous work, we established a quantitative relationship between the ages of lunar craters and the rockiness of their ejecta. That result was based on the idea that crater-forming impacts eject rocks from beneath the regolith, instantaneously emplacing a deposit with characteristic initial physical properties, such as rock abundance. The ejecta rocks are then gradually removed and / or covered by a combination of mechanical breakdown via micrometeorite bombardment, emplacement of regolith fines due to nearby impacts, and possibly rupture due to thermal stresses. We found that ejecta rocks, as detected by the Lunar Reconnaissance Orbiter Diviner thermal radiometer disappear on a timescale of 1 Gyr, eventually becoming undetectable by the Diviner instrument against the ambient background rock abundance of the regolith.The "index" craters we used to establish the rock abundance—age relationship are all larger than 15 km (our smallest index crater is Byrgius A, at 18.7 km), and therefore above the transition diameter between simple and complex craters (15-20 km). Here, we extend our analysis to include craters smaller than the transition diameter. It is not obvious a priori that the initial ejecta properties of simple and complex craters should be identical, and therefore, that the same metrics of crater age can be applied to both populations. We explore this issue using LRO Diviner rock abundance and a high-resolution optical maturity dataset derived from Kaguya multiband imager VIS/NIR data to identify young craters to 5 km diameter. We examine the statistical properties of this population relative to that of the NEO population, and interpret the results in the context of our recently documented evidence for changes in the flux of impactors that create larger craters. Finally, we detail implications of our result for understanding the dynamic history of the lunar surface and the evolution of the asteroid belt.

Rock spatial densities on the rims of the Tycho secondary craters in Mare Nectaris

NASA Astrophysics Data System (ADS)

Basilevsky, A. T.; Michael, G. G.; Kozlova, N. A.

2018-04-01

The aim of this work is to check whether the technique of estimation of age of small lunar craters based on spatial density of rock boulders on their rims described in Basilevsky et al. (2013, 2015b) and Li et al. (2017) for the craters < 1 km in diameter is applicable to the larger craters. The work presents the rock counts on the rims of four craters having diameters 1000, 1100, 1240 and 1400 m located in Mare Nectaris. These craters are secondaries of the primary crater Tycho, whose age was found to be 109 ± 4 Ma (Stoffler and Ryder, 2001) so this may be taken as the age of the four craters, too. Using the dependence of the rock spatial densities at the crater rims on the crater age for the case of mare craters (Li et al., 2017) our measured rock densities correspond to ages from ∼100 to 130 Ma. These estimates are reasonably close to the given age of the primary crater Tycho. This, in turn, suggests that this technique of crater age estimation is applicable to craters up to ∼1.5 km in diameter. For the four considered craters we also measured their depth/diameter ratios and the maximum angles of the crater inner slopes. For the considered craters it was found that with increasing crater diameter, the depth/diameter ratios and maximum angles of internal slopes increase, but the values of these parameters for specific craters may deviate significantly from the general trends. The deviations probably result from some dissimilarities in the primary crater geometries, that may be due to crater to crater differences in characteristics of impactors (e.g., in their bulk densities) and/or differences in the mechanical properties of the target. It may be possible to find secondaries of crater Tycho in the South pole area and, if so, they may be studied to check the specifics and rates of the rock boulder degradation in the lunar polar environment.

Interior and Ejecta Morphologies of Impact Craters on Ganymede

NASA Astrophysics Data System (ADS)

Barlow, Nadine G.; Klaybor, K.; Katz-Wigmore, J.

2006-09-01

We are utilizing Galileo SSI imagery of Ganymede to classify impact crater interior and ejecta morphologies. Although we are in the early stages of compiling our Catalog of Impact Craters on Ganymede, some interesting trends are beginning to emerge. Few craters display obvious ejecta morphologies, but 68 craters are classified as single layer ejecta and 3 as double layer ejecta. We see no obvious correlation of layered ejecta morphologies with terrain or latitude. All layered ejecta craters have diameters between 10 and 40 km. Sinuosity ("lobateness") and ejecta extent ("ejecta mobility ratio") of Ganymede layered ejecta craters are lower than for martian layered ejecta craters. This suggests less mobility of ejecta materials on Ganymede, perhaps due to the colder temperatures. Interior structures being investigated include central domes, peaks, and pits. 57 dome craters, 212 central peak craters, and 313 central pit craters have been identified. Central domes occur in 50-100 km diameter craters while peaks are found in craters between 20 and 50 km and central pit craters range between 29 and 74 km in diameter. The Galileo Regio region displays higher concentrations of central dome and central pit craters than other regions we have investigated. 67% of central pit craters studied to date are small pits, where the ratio of pit diameter to crater diameter is <0.2. Craters containing the three interior structures preferentially occur on darker terrain units, suggesting that an ice-silicate composition is more conducive to interior feature formation than pure ice alone. Results of this study have important implications not only for the formation of specific interior and ejecta morphologies on Ganymede but also for analogous features associated with Martian impact craters. This research is funded through NASA Outer Planets Research Program Award #NNG05G116G to N. G. Barlow.

Crater density differences: Exploring regional resurfacing, secondary crater populations, and crater saturation equilibrium on the moon

USGS Publications Warehouse

Povilaitis, R Z; Robinson, M S; van der Bogert, C H; Hiesinger, Harald; Meyer, H M; Ostrach, Lillian

2017-01-01

The global population of lunar craters >20 km in diameter was analyzed by Head et al., (2010) to correlate crater distribution with resurfacing events and multiple impactor populations. The work presented here extends the global crater distribution analysis to smaller craters (5–20 km diameters, n = 22,746). Smaller craters form at a higher rate than larger craters and thus add granularity to age estimates of larger units and can reveal smaller and younger areas of resurfacing. An areal density difference map generated by comparing the new dataset with that of Head et al., (2010) shows local deficiencies of 5–20 km diameter craters, which we interpret to be caused by a combination of resurfacing by the Orientale basin, infilling of intercrater plains within the nearside highlands, and partial mare flooding of the Australe region. Chains of 5–30 km diameter secondaries northwest of Orientale and possible 8–22 km diameter basin secondaries within the farside highlands are also distinguishable. Analysis of the new database indicates that craters 57–160 km in diameter across much of the lunar highlands are at or exceed relative crater densities of R = 0.3 or 10% geometric saturation, but nonetheless appear to fit the lunar production function. Combined with the observation that small craters on old surfaces can reach saturation equilibrium at 1% geometric saturation (Xiao and Werner, 2015), this suggests that saturation equilibrium is a size-dependent process, where large craters persist because of their resistance to destruction, degradation, and resurfacing.

Bright crater outflows: Possible emplacement mechanisms

NASA Technical Reports Server (NTRS)

Chadwick, D. John; Schaber, Gerald G.; Strom, Robert G.; Duval, Darla M.

1992-01-01

Lobate features with a strong backscatter are associated with 43 percent of the impact craters cataloged in Magellan's cycle 1. Their apparent thinness and great lengths are consistent with a low-viscosity material. The longest outflow yet identified is about 600 km in length and flows from the 90-km-diameter crater Addams. There is strong evidence that the outflows are largely composed of impact melt, although the mechanisms of their emplacement are not clearly understood. High temperatures and pressures of target rocks on Venus allow for more melt to be produced than on other terrestrial planets because lower shock pressures are required for melting. The percentage of impact craters with outflows increases with increasing crater diameter. The mean diameter of craters without outflows is 14.4 km, compared with 27.8 km for craters with outflows. No craters smaller than 3 km, 43 percent of craters in the 10- to 30-km-diameter range, and 90 percent in the 80- to 100-km-diameter range have associated bright outflows. More melt is produced in the more energetic impact events that produce larger craters. However, three of the four largest craters have no outflows. We present four possible mechanisms for the emplacement of bright outflows. We believe this 'shotgun' approach is justified because all four mechanisms may indeed have operated to some degree.

Testing models for the formation of the equatorial ridge on Iapetus via crater counting

NASA Astrophysics Data System (ADS)

Damptz, Amanda L.; Dombard, Andrew J.; Kirchoff, Michelle R.

2018-03-01

Iapetus's equatorial ridge, visible in global views of the moon, is unique in the Solar System. The formation of this feature is likely attributed to a key event in the evolution of Iapetus, and various models have been proposed as the source of the ridge. By surveying imagery from the Cassini and Voyager missions, this study aims to compile a database of the impact crater population on and around Iapetus's equatorial ridge, assess the relative age of the ridge from differences in cratering between on ridge and off ridge, and test the various models of ridge formation. This work presents a database that contains 7748 craters ranging from 0.83 km to 591 km in diameter. The database includes the study area in which the crater is located, the latitude and longitude of the crater, the major and minor axis lengths, and the azimuthal angle of orientation of the major axis. Analysis of crater orientation over the entire study area reveals that there is no preference for long-axis orientation, particularly in the area with the highest resolution. Comparison of the crater size-frequency distributions show that the crater distribution on the ridge appears to be depleted in craters larger than 16 km with an abruptly enhanced crater population less than 16 km in diameter up to saturation. One possible interpretation is that the ridge is a relatively younger surface with an enhanced small impactor population. Finally, the compiled results are used to examine each ridge formation hypothesis. Based on these results, a model of ridge formation via a tidally disrupted sub-satellite appears most consistent with our interpretation of a younger ridge with an enhanced small impactor population.

Shatter cones: (Mis)understood?

PubMed

Osinski, Gordon R; Ferrière, Ludovic

2016-08-01

Meteorite impact craters are one of the most common geological features in the solar system. An impact event is a near-instantaneous process that releases a huge amount of energy over a very small region on a planetary surface. This results in characteristic changes in the target rocks, from vaporization and melting to solid-state effects, such as fracturing and shock metamorphism. Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events. They are one of the most used and trusted shock-metamorphic effects for the recognition of meteorite impact structures. Despite this, there is still considerable debate regarding their formation. We show that shatter cones are present in several stratigraphic settings within and around impact structures. Together with the occurrence of complete and "double" cones, our observations are most consistent with shatter cone formation due to tensional stresses generated by scattering of the shock wave due to heterogeneities in the rock. On the basis of field mapping, we derive the relationship D sc = 0.4 D a, where D sc is the maximum spatial extent of in situ shatter cones, and D a is the apparent crater diameter. This provides an important, new, more accurate method to estimate the apparent diameter of eroded complex craters on Earth. We have reestimated the diameter of eight well-known impact craters as part of this study. Finally, we suggest that shatter cones may reduce the strength of the target, thus aiding crater collapse, and that their distribution in central uplifts also records the obliquity of impact.

Shatter cones: (Mis)understood?

PubMed Central

Osinski, Gordon R.; Ferrière, Ludovic

2016-01-01

Meteorite impact craters are one of the most common geological features in the solar system. An impact event is a near-instantaneous process that releases a huge amount of energy over a very small region on a planetary surface. This results in characteristic changes in the target rocks, from vaporization and melting to solid-state effects, such as fracturing and shock metamorphism. Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events. They are one of the most used and trusted shock-metamorphic effects for the recognition of meteorite impact structures. Despite this, there is still considerable debate regarding their formation. We show that shatter cones are present in several stratigraphic settings within and around impact structures. Together with the occurrence of complete and “double” cones, our observations are most consistent with shatter cone formation due to tensional stresses generated by scattering of the shock wave due to heterogeneities in the rock. On the basis of field mapping, we derive the relationship Dsc = 0.4 Da, where Dsc is the maximum spatial extent of in situ shatter cones, and Da is the apparent crater diameter. This provides an important, new, more accurate method to estimate the apparent diameter of eroded complex craters on Earth. We have reestimated the diameter of eight well-known impact craters as part of this study. Finally, we suggest that shatter cones may reduce the strength of the target, thus aiding crater collapse, and that their distribution in central uplifts also records the obliquity of impact. PMID:27532050

Cratering Equations for Zinc Orthotitanate Coated Aluminum

NASA Technical Reports Server (NTRS)

Hyde, James; Christiansen, Eric; Liou, Jer-Chyi; Ryan, Shannon

2009-01-01

The final STS-125 servicing mission (SM4) to the Hubble Space Telescope (HST) in May of 2009 saw the return of the 2nd Wide Field Planetary Camera (WFPC2) aboard the shuttle Discovery. This hardware had been in service on HST since it was installed during the SM1 mission in December of 1993 yielding one of the longest low Earth orbit exposure times (15.4 years) of any returned space hardware. The WFPC2 is equipped with a 0.8 x 2.2 m radiator for thermal control of the camera electronics (Figure 1). The space facing surface of the 4.1 mm thick aluminum radiator is coated with Z93 zinc orthotitanate thermal control paint with a nominal thickness of 0.1 0.2 mm. Post flight inspections of the radiator panel revealed hundreds of micrometeoroid/orbital debris (MMOD) impact craters ranging in size from less than 300 to nearly 1000 microns in diameter. The Z93 paint exhibited large spall areas around the larger impact sites (Figure 2) and the craters observed in the 6061-T651 aluminum had a different shape than those observed in uncoated aluminum. Typical hypervelocity impact craters in aluminum have raised lips around the impact site. The craters in the HST radiator panel had suppressed crater lips, and in some cases multiple craters were present instead of a single individual crater. Humes and Kinard observed similar behavior after the WFPC1 post flight inspection and assumed the Z93 coating was acting like a bumper in a Whipple shield. Similar paint behavior (spall) was also observed by Bland2 during post flight inspection of the International Space Station (ISS) S-Band Antenna Structural Assembly (SASA) in 2008. The SASA, with similar Z93 coated aluminum, was inspected after nearly 4 years of exposure on the ISS. The multi-crater phenomena could be a function of the density, composition, or impact obliquity angle of the impacting particle. For instance, a micrometeoroid particle consisting of loosely bound grains of material could be responsible for creating the multiple craters. Samples were obtained from the HST largest craters for examination by electron microscope equipped with x-ray spectrometers to determine impactor source (micrometeoroid or orbital debris). In an attempt to estimate the MMOD particle diameters that produced these craters, this paper will present equations for spall diameter, crater depth and crater diameter in Z93 coated aluminum. The equations will be based on hypervelocity impact tests of Z93 painted aluminum at the NASA White Sands Test Facility. Equations inputs for velocities beyond the testable regime are expected from hydrocode simulations of Z93 coated aluminum using CTH and ANSYS AUTODYN.

Degradation studies of Martian impact craters

NASA Technical Reports Server (NTRS)

Barlow, N. G.

1991-01-01

The amount of obliteration suffered by Martian impact craters is quantified by comparing measurable attributes of the current crater shape to those values expected for a fresh crater of identical size. Crater diameters are measured from profiles obtained using photoclinometry across the structure. The relationship between the diameter of a fresh crater and a crater depth, floor width, rim height, central peak height, etc. was determined by empirical studies performed on fresh Martian impact craters. We utilized the changes in crater depth and rim height to judge the degree of obliteration suffered by Martian impact craters.

Moon-Mercury: Large impact structures, isostasy and average crustal viscosity

USGS Publications Warehouse

Schaber, G.G.; Boyce, J.M.; Trask, N.J.

1977-01-01

Thirty-five craters and basins larger than 200 km in diameter are recognized on the imaged portion (45%) of Mercury. If the unimaged portion of the planet is similarly cratered, a total of 78 such impact features may be present. Sixty-two craters and basins 200 km in diameter are recognized on the moon, a body with only half the cross-sectional area of Mercury. If surface areas are considered, however, Mercury is cratered only 70% as densely as the moon. The density of impact craters with diameters greater than 400 km on Mercury is only 30% of that on the moon, and for craters with diameters between 400 and 700 km, the density on Mercury is only 21% of the lunar crater density. The size-frequency distribution curve for the large Mercurian craters follows the same cumulative -2 slope as the lunar curve but lies well below the 10% surface saturation level characteristic of the lunar curve. This is taken as evidence that the old heavily cratered terrain on Mercury is, at least presently, not in a state of cratering equilibrium. The reduced density of large craters and basins on Mercury relative to the moon could be either a function of the crater-production rates on these bodies or an effect of different crustal histories. Resurfacing of the planet after the basin-forming period is ruled out by the presence of 54 craters and basins 100 km in diameter and larger (on the imaged portion of Mercury) that have either well-defined or poorly-defined secondary-crater fields. Total isostatic compensation of impact craters ???800 km in diameter indicates that the average viscosity of the Mercurian crust over the past 4+ aeons was the same as that for the moon (???1026.5 P). This calculated viscosity and the distribution of large craters and basins suggest that either the very early crustal viscosity on Mercury was less than that of the moon and the present viscosity greater, or the differences in large crater populations on the two bodies is indeed the result of variations in rates of crater production. ?? 1977.

Martian impact craters - Correlations of ejecta and interior morphologies with diameter, latitude, and terrain

NASA Technical Reports Server (NTRS)

Barlow, Nadine G.; Bradley, Tracy L.

1990-01-01

An effort is made to establish the ability of a correlation between crater morphology and latitude, diameter, and terrain, to discriminate among the effects of impact energy, atmosphere, and subsurface volatiles in 3819 larger-than-8 km diameter craters distributed over the Martian surface. It is noted that changes in ejecta and interior morphology correlate with increases in crater diameter, and that while many of the interior structures exhibit distributions interpretable as terrain-dependent, central peak and peak ring interior morphologies exhibit minimal relationships with planetary properties.

Lunar Cratering Chronology: Calibrating Degree of Freshness of Craters to Absolute Ages

NASA Astrophysics Data System (ADS)

Trang, D.; Gillis-Davis, J.; Boyce, J. M.

2013-12-01

The use of impact craters to age-date surfaces of and/or geomorphological features on planetary bodies is a decades old practice. Various dating techniques use different aspects of impact craters in order to determine ages. One approach is based on the degree of freshness of primary-impact craters. This method examines the degradation state of craters through visual inspection of seven criteria: polygonality, crater ray, continuous ejecta, rim crest sharpness, satellite craters, radial channels, and terraces. These criteria are used to rank craters in order of age from 0.0 (oldest) to 7.0 (youngest). However, the relative decimal scale used in this technique has not been tied to a classification of absolute ages. In this work, we calibrate the degree of freshness to absolute ages through crater counting. We link the degree of freshness to absolute ages through crater counting of fifteen craters with diameters ranging from 5-22 km and degree of freshness from 6.3 to 2.5. We use the Terrain Camera data set on Kaguya to count craters on the continuous ejecta of each crater in our sample suite. Specifically, we divide the crater's ejecta blanket into quarters and count craters between the rim of the main crater out to one crater radii from the rim for two of the four sections. From these crater counts, we are able to estimate the absolute model age of each main crater using the Craterstats2 tool in ArcGIS. Next, we compare the degree of freshness for the crater count-derived age of our main craters to obtain a linear inverse relation that links these two metrics. So far, for craters with degree of freshness from 6.3 to 5.0, the linear regression has an R2 value of 0.7, which corresponds to a relative uncertainty of ×230 million years. At this point, this tool that links degree of freshness to absolute ages cannot be used with craters <8km because this class of crater degrades quicker than larger craters. A graphical solution exists for correcting the degree of freshness for craters <8 km in diameter. We convert this graphical solution to a single function of two independent variables, observed degree of freshness and crater diameter. This function, which results in a corrected degree of freshness is found through a curve-fitting routine and corrects the degree of freshness for craters <8 km in diameter. As a result, we are able to derive absolute ages from the degree of freshness of craters with diameters from about ≤20 km down to a 1 km in diameter with a precision of ×230 million years.

Distant Secondary Craters and Age Constraints on Young Martian Terrains

NASA Technical Reports Server (NTRS)

McEwen, A.; Preblich, B.; Turtle, E.; Studer, D.; Artemieva, N.; Golombek, M.; Hurst, M.; Kirk, R.; Burr, D.

2005-01-01

Are small (less than approx. 1 km diameter) craters on Mars and the Moon dominated by primary impacts, by secondary impacts of much larger primary craters, or are both primaries and secondaries significant? This question is critical to age constraints for young terrains and for older terrains covering small areas, where only small craters are superimposed on the unit. If the martian rayed crater Zunil is representative of large impact events on Mars, then the density of secondaries should exceed the density of primaries at diameters a factor of 1000 smaller than that of the largest contributing primary crater. On the basis of morphology and depth/diameter measurements, most small craters on Mars could be secondaries. Two additional observations (discussed below) suggest that the production functions of Hartmann and Neukum predict too many primary craters smaller than a few hundred meters in diameter. Fewer small, high-velocity impacts may explain why there appears to be little impact regolith over Amazonian terrains. Martian terrains dated by small craters could be older than reported in recent publications.

Hypervelocity impacts into ice-topped layered targets: Investigating the effects of ice crust thickness and subsurface density on crater morphology

NASA Astrophysics Data System (ADS)

Harriss, Kathryn H.; Burchell, Mark J.

2017-07-01

Many bodies in the outer solar system are theorized to have an ice shell with a different subsurface material below, be it chondritic, regolith, or a subsurface ocean. This layering can have a significant influence on the morphology of impact craters. Accordingly, we have undertaken laboratory hypervelocity impact experiments on a range of multilayered targets, with interiors of water, sand, and basalt. Impact experiments were undertaken using impact speeds in the range of 0.8-5.3 km s-1, a 1.5 mm Al ball bearing projectile, and an impact incidence of 45°. The surface ice crust had a thickness between 5 and 50 mm, i.e., some 3-30 times the projectile diameter. The thickness of the ice crust as well as the nature of the subsurface layer (liquid, well consolidated, etc.) have a marked effect on the morphology of the resulting impact crater, with thicker ice producing a larger crater diameter (at a given impact velocity), and the crater diameter scaling with impact speed to the power 0.72 for semi-infinite ice, but with 0.37 for thin ice. The density of the subsurface material changes the structure of the crater, with flat crater floors if there is a dense, well-consolidated subsurface layer (basalt) or steep, narrow craters if there is a less cohesive subsurface (sand). The associated faulting in the ice surface is also dependent on ice thickness and the substrate material. We find that the ice layer (in impacts at 5 km s-1) is effectively semi-infinite if its thickness is more than 15.5 times the projectile diameter. Below this, the crater diameter is reduced by 4% for each reduction in ice layer thickness equal to the impactor diameter. Crater depth is also affected. In the ice thickness region, 7-15.5 times the projectile diameter, the crater shape in the ice is modified even when the subsurface layer is not penetrated. For ice thicknesses, <7 times the projectile diameter, the ice layer is breached, but the nature of the resulting crater depends heavily on the subsurface material. If the subsurface is noncohesive (loose) material, a crater forms in it. If it is dense, well-consolidated basalt, no crater forms in the exposed subsurface layer.

Ancient fluvial processes in the equatorial highlands of Mars

NASA Technical Reports Server (NTRS)

Craddock, Robert A.; Maxwell, Ted A.

1991-01-01

Martian highland craters typically lack ejecta deposits, have no noticeable rim, and are flat floored. In addition, crater size frequency distribution curves show that highland craters have depleted populations less than 20 km in diameter. A variety of processes have been suggested to explain these observations including deposition of aeolian or volcanic materials up to the crater rim crests, thermal creep, terrain softening, and mass wasting. However, none of these processes adequately explains both the crater morphology and population distribution. In order to explain both the Martian highland crater morphology and population distribution, a fluvial process is proposed which is capable of removing the loose crater rim material. The resulting effect is to decrease the crater diameter, thereby causing the population curves to bendover. The eroded material is redistributed, burying or partially burying smaller diameter craters before complete erosion. This material may also be deposited into local topographic lows, creating the depositional basins observed. A fluvial process explains both sets of observations: crater morphology and crater population distribution curves.

Styles of crater gradation in Southern Ismenius Lacus, Mars: Clues from Meteor Crater, Arizona

NASA Technical Reports Server (NTRS)

Grant, J. A.; Schultz, P. H.

1992-01-01

Impact craters on the Earth and Mars provide a unique opportunity to quantify the gradational evolution of instantaneously created landforms in a variety of geologic settings. Unlike most landforms, the initial morphology associated with impact craters on both planets is uncomplicated by competition between construction and degradation during formation. Furthermore, pristine morphologies are both well-constrained and similar to a first order. The present study compares styles of graduation at Meteor Crater with those around selected craters (greater than 1-2 km in diameter) in southern Ismenius Lacus. Emphasis is placed on features visible in images near LANDSAT TM resolution (30-50 m/pixel) which is available for both areas. In contrast to Mars, vegetation on the Earth can modify gradation, but appears to influence overall rates and styles by 2X-3X rather than orders of magnitude. Further studies of additional craters in differing settings will refine the effects of this and other factors (e.g., substrate). Finally, by analogy with results from other terrestrial gradational surfaces this study should help provide constraints on climate over crater histories.

The Transition from Complex Crater to Peak-Ring Basin on the Moon: New Observations from the Lunar Orbiter Laser Altimeter (LOLA) Instrument

NASA Technical Reports Server (NTRS)

Baker, David M. H.; Head, James W.; Fassett, Caleb I.; Kadish, Seth J.; Smith, Dave E.; Zuber, Maria T.; Neumann, Gregory A.

2012-01-01

Impact craters on planetary bodies transition with increasing size from simple, to complex, to peak-ring basins and finally to multi-ring basins. Important to understanding the relationship between complex craters with central peaks and multi-ring basins is the analysis of protobasins (exhibiting a rim crest and interior ring plus a central peak) and peak-ring basins (exhibiting a rim crest and an interior ring). New data have permitted improved portrayal and classification of these transitional features on the Moon. We used new 128 pixel/degree gridded topographic data from the Lunar Orbiter Laser Altimeter (LOLA) instrument onboard the Lunar Reconnaissance Orbiter, combined with image mosaics, to conduct a survey of craters >50 km in diameter on the Moon and to update the existing catalogs of lunar peak-ring basins and protobasins. Our updated catalog includes 17 peak-ring basins (rim-crest diameters range from 207 km to 582 km, geometric mean = 343 km) and 3 protobasins (137-170 km, geometric mean = 157 km). Several basins inferred to be multi-ring basins in prior studies (Apollo, Moscoviense, Grimaldi, Freundlich-Sharonov, Coulomb-Sarton, and Korolev) are now classified as peak-ring basins due to their similarities with lunar peak-ring basin morphologies and absence of definitive topographic ring structures greater than two in number. We also include in our catalog 23 craters exhibiting small ring-like clusters of peaks (50-205 km, geometric mean = 81 km); one (Humboldt) exhibits a rim-crest diameter and an interior morphology that may be uniquely transitional to the process of forming peak rings. Comparisons of the predictions of models for the formation of peak-ring basins with the characteristics of the new basin catalog for the Moon suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. In particular, a power-law model of growth of an interior melt cavity with increasing crater diameter is consistent with power-law fits to the peak-ring basin data for the Moon and Mercury. We suggest that the relationship between the depth of melting and depth of the transient cavity offers a plausible control on the onset diameter and subsequent development of peak-ring basins and also multi-ring basins, which is consistent with both planetary gravitational acceleration and mean impact velocity being important in determining the onset of basin morphological forms on the terrestrial planets.

Geology of the Side Crater of the Erebus volcano, Antarctica

NASA Astrophysics Data System (ADS)

Panter, Kurt S.; Winter, Brian

2008-11-01

The summit cone of the Erebus volcano contains two craters. The Main crater is roughly circular (˜ 500 m diameter) and contains an active persistent phonolite lava lake ˜ 200 m below the summit rim. The Side Crater is adjacent to the southwestern rim of the Main Crater. It is a smaller spoon-shaped Crater (250-350 m diameter, 50-100 m deep) and is inactive. The floor of the Side Crater is covered by snow/ice, volcanic colluvium or weakly developed volcanic soil in geothermal areas (a.k.a. warm ground). But in several places the walls of the Side Crater provide extensive vertical exposure of rock which offers an insight into the recent eruptive history of Erebus. The deposits consist of lava flows with subordinate volcanoclastic lithologies. Four lithostratigraphic units are described: SC 1 is a compound lava with complex internal flow fabrics; SC 2 consists of interbedded vitric lavas, autoclastic and pyroclastic breccias; SC 3 is a thick sequence of thin lavas with minor autoclastic breccias; SC 4 is a pyroclastic fall deposit containing large scoriaceous lava bombs in a matrix composed primarily of juvenile lapilli-sized pyroclasts. Ash-sized pyroclasts from SC 4 consist of two morphologic types, spongy and blocky, indicating a mixed strombolian-phreatomagmatic origin. All of the deposits are phonolitic and contain anorthoclase feldspar. The stratigraphy and morphology of the Side Crater provides a record of recent volcanic activity at the Erebus volcano and is divided into four stages. Stage I is the building of the main summit cone and eruption of lavas (SC 1 and SC 3) from Main Crater vent(s). A secondary cone was built during Stage II by effusive and explosive activity (SC 2) from the Side Crater vent. A mixed strombolian and phreatomagmatic eruption (SC 4) delimits Stage III. The final stage (IV) represents a period of erosion and enlargement of the Side Crater.

Effect of Cover Thickness on the Relationship of Surface Relief to Diameter of Northern Lowland QCDs on Mars

NASA Technical Reports Server (NTRS)

Buczkowski, D. L.; Frey, H. V.; McGill, G. E.

2005-01-01

Previous work has established that there is a relationship of surface relief to diameter for quasi-circular depressions (QCDs) around the Utopia Basin [1]. This relationship has been used to support the contention that the QCDs represent impact craters buried beneath a differentially compacting cover material. For any given regional cover thickness, total cover thickness is greater over the centers of completely buried craters than over their rims; thus total compaction is greater over the center of craters than their rims and topographic depressions will form. Since large craters are deeper than small craters, differential compaction models also predict that surface relief will be proportional to the diameter of the buried crater [2]. It is highly unlikely, however, that the material covering the QCD impact craters is a consistent thickness throughout the entire northern lowlands of Mars. We explore the effects that changes in cover thickness would have on the surface relief vs. diameter relationship of QCDs.

Evidence for self-secondary cratering of Copernican-age continuous ejecta deposits on the Moon

NASA Astrophysics Data System (ADS)

Zanetti, M.; Stadermann, A.; Jolliff, B.; Hiesinger, H.; van der Bogert, C. H.; Plescia, J.

2017-12-01

Crater size-frequency distributions on the ejecta blankets of Aristarchus and Tycho Craters are highly variable, resulting in apparent absolute model age differences despite ejecta being emplaced in a geologic instant. Crater populations on impact melt ponds are a factor of 4 less than on the ejecta, and crater density increases with distance from the parent crater rim. Although target material properties may affect crater diameters and in turn crater size-frequency distribution (CSFD) results, they cannot completely reconcile crater density and population differences observed within the ejecta blanket. We infer from the data that self-secondary cratering, the formation of impact craters immediately following the emplacement of the continuous ejecta blanket by ejecta from the parent crater, contributed to the population of small craters (< 300 m diameter) on ejecta blankets and must be taken into account if small craters and small count areas are to be used for relative and absolute model age determinations on the Moon. Our results indicate that the cumulative number of craters larger than 1 km in diameter per unit area, N(1), on the continuous ejecta blanket at Tycho Crater, ranges between 2.17 × 10-5 and 1.0 × 10-4, with impact melt ponds most accurately reflecting the primary crater flux (N(1) = 3.4 × 10-5). Using the cratering flux recorded on Tycho impact melt deposits calibrated to accepted exposure age (109 ± 1.5 Ma) as ground truth, and using similar crater distribution analyses on impact melt at Aristarchus Crater, we infer the age of Aristarchus Crater to be ∼280 Ma. The broader implications of this work suggest that the measured cratering rate on ejecta blankets throughout the Solar System may be overestimated, and caution should be exercised when using small crater diameters (i.e. < 300 m on the Moon) for absolute model age determination.

Crater size estimates for large-body terrestrial impact

NASA Technical Reports Server (NTRS)

Schmidt, Robert M.; Housen, Kevin R.

1988-01-01

Calculating the effects of impacts leading to global catastrophes requires knowledge of the impact process at very large size scales. This information cannot be obtained directly but must be inferred from subscale physical simulations, numerical simulations, and scaling laws. Schmidt and Holsapple presented scaling laws based upon laboratory-scale impact experiments performed on a centrifuge (Schmidt, 1980 and Schmidt and Holsapple, 1980). These experiments were used to develop scaling laws which were among the first to include gravity dependence associated with increasing event size. At that time using the results of experiments in dry sand and in water to provide bounds on crater size, they recognized that more precise bounds on large-body impact crater formation could be obtained with additional centrifuge experiments conducted in other geological media. In that previous work, simple power-law formulae were developed to relate final crater diameter to impactor size and velocity. In addition, Schmidt (1980) and Holsapple and Schmidt (1982) recognized that the energy scaling exponent is not a universal constant but depends upon the target media. Recently, Holsapple and Schmidt (1987) includes results for non-porous materials and provides a basis for estimating crater formation kinematics and final crater size. A revised set of scaling relationships for all crater parameters of interest are presented. These include results for various target media and include the kinematics of formation. Particular attention is given to possible limits brought about by very large impactors.

A search for Potential Impact Sites in Southern Argentina

NASA Astrophysics Data System (ADS)

Rocca, M. C. L.

The Southern part of Argentina is composed of five Provinces; Tierra del Fuego, Santa Cruz, Chubut, Rio Negro and Neuquen. A search for potential impact sites was performed by the author through the examination of 76 color LANDSAT satellite images ( 1:250,000 - resolution = 250 meters ) at the Instituto Geografico Militar ( IGM ) of Buenos Aires city. When a potential candidate was found a more detailed study of the site was done. If available the radar X-SAR satellite images of the Deutsche Forschungsanstalt fur Luft-und Raumfahrt, (DLR), Berlin, Germany , were also examined. The final step was to perform a review of the available published geologic information of each site at the Servicio Geologico y Minero Argentino ( SEGEMAR ), ( =Geological Survey of Argentina ), in Buenos Aires. The resulting catalogue contains information about sites where possible simple crater or complex impact structures could be present. Each case demands future detailed and `in situ' research by an impact cratering specialist. --Tierra del Fuego: TF1 ) Ushuaia 5569-II, No 218. Cerro Taarsh, Estancia San Justo. Possible complex structure. Semi-circular area of concentric low ridges. Estimated diameter : 12 km. Probably very eroded. --Santa Cruz: SC1 ) Gobernador Gregores 4969-I, No 127. Estancia La Aragonesa Possible eroded complex structure. Circular area of low ridges, estimated diameter: 10 km.. Bull's eye like morphology. SC2 ) Gobernador Gregores 4969-I, No 127. Gran Altiplanicie Central. Possible simple crater in basalts. Diameter: 1 km.. SC3 ) Tres Lagos 4972-IV, No 106. Meseta del Bagual Chico. Possible perfectly circular simple crater in basalts. Diameter: 1.0 km.. SC4 )Paso Rio Bote 5172-II, No 20. Rio Pelque, Ruta Provincial No 5. A circular bowl-shaped structure is present on fluvial deposits of pleistocenic age. Diameter: 3.5 km.. SC5 ) Caleta Olivia 4769-II, No 28. North of Cerro Doce Grande. Possible complex structure of concentric circular rings of ridges. SC6 ) Caleta Olivia 4769-II, No 28. NW shore of Laguna Sirven. Possible simple crater of 2.5 km. Most probably, the circular crater is a basaltic caldera of upper Miocene's age. SC7 ) Destacamento La Maria 4769-II, No 188. Estancia Los Mellizos, Ruta Provincial No 39. Possible eroded and covered complex structure. In this site there is a semi-circular feature of ridges and low hills. Diameter: 15 km. Topographic map shows the same pattern. The DLR's X-SAR images show a clear semi-circular feature of ridges and hills in this site. SC8 ) Hipolito Irigoyen 4772-IV, No 116. Meseta del Lago Buenos Aires. Possible perfectly circular simple crater of 1 Km. Neuquen: N1 ) Picun Leufu 3969-III No 14. Meseta de la Barda Negra. Nice perfectly circular possible simple crater in black Miocene's ( 14-10 Ma ) basaltic plateau. Diameter: 1.5 km.. Possible raised rim. Fresh aspect. No visible lava flows .

Morphology of Lonar Crater, India: Comparisons and implications

USGS Publications Warehouse

Fudali, R.F.; Milton, D.J.; Fredriksson, K.; Dube, A.

1980-01-01

Lonar Crater is a young meteorite impact crater emplaced in Deccan basalt. Data from 5 drillholes, a gravity network, and field mapping are used to reconstruct its original dimensions, delineate the nature of the pre-impact target rocks, and interpret the emplacement mode of the ejecta. Our estimates of the pre-erosion dimensions are: average diameter of 1710 m; average rim height of 40 m (30-35 m of rim rock uplift, 5-10 m of ejected debris); depth of 230-245 m (from rim crest to crater floor). The crater's circularity index is 0.9 and is unlikely to have been lower in the past. There are minor irregularities in the original crater floor (present sediment-breccia boundary) possibly due to incipient rebound effects. A continuous ejecta blanket extends an average of 1410 m beyond the pre-erosion rim crest. In general, 'fresh' terrestrial craters, less than 10 km in diameter, have smaller depth/diameter and larger rim height/diameter ratios than their lunar counterparts. Both ratios are intermediate for Mercurian craters, suggesting that crater shape is gravity dependent, all else being equal. Lonar demonstrates that all else is not always equal. Its depth/diameter ratio is normal but, because of less rim rock uplift, its rim height/diameter ratio is much smaller than both 'fresh' terrestrial and lunar impact craters. The target rock column at Lonar consists of one or more layers of weathered, soft basalt capped by fresh, dense flows. Plastic deformation and/or compaction of this lower, incompetent material probably absorbed much of the energy normally available in the cratering process for rim rock uplift. A variety of features within the ejecta blanket and the immediately underlying substrate, plus the broad extent of the blanket boundaries, suggest that a fluidized debris surge was the dominant mechanism of ejecta transportation and deposition at Lonar. In these aspects, Lonar should be a good analog for the 'fluidized craters' of Mars. ?? 1980 D. Reidel Publishing Co.

Variations in interior morphology of 15-20 km lunar craters - Implications for a major subsurface discontinuity

NASA Technical Reports Server (NTRS)

De Hon, R. A.

1980-01-01

Craters vary in morphology as a function of crater diameter, age, and mode of origin. This study concentrates on the morphology of young lunar impact craters within a limited size range. Elimination of morphologic variations generally attributed to crater size or age leaves a small population which should nearly reflect the varying properties of the lunar substrate. The sample consists of 17 craters 15-20 km in diameter with both simple and complex morphologies. While depth/diameter ratios do not obviously differ between mare and highland subsets, apparent depth, rim height, and profile data do differ distinctly. Highland craters tend to be deep, simple, and bowl-shaped. Mare craters tend to be shallow and flat-floored. Rim heights of complex mare craters are typically greater than those of simple craters. Differences of highland and mare crater morphologies are attributed to variations in the thickness of the lunar megaregolith. Highland craters in this size range do not penetrate the megaregolith. The depth and morphology of complex craters are controlled by the discontinuity at the transition from highly brecciated megaregolith to more coherent crystalline material of the upper crust.

High Resolution Digital Elevation Models of Pristine Explosion Craters

NASA Technical Reports Server (NTRS)

Farr, T. G.; Krabill, W.; Garvin, J. B.

2004-01-01

In order to effectively capture a realistic terrain applicable to studies of cratering processes and landing hazards on Mars, we have obtained high resolution digital elevation models of several pristine explosion craters at the Nevada Test Site. We used the Airborne Terrain Mapper (ATM), operated by NASA's Wallops Flight Facility to obtain DEMs with 1 m spacing and 10 cm vertical errors of 4 main craters and many other craters and collapse pits. The main craters that were mapped are Sedan, Scooter, Schooner, and Danny Boy. The 370 m diameter Sedan crater, located on Yucca Flat, is the largest and freshest explosion crater on Earth that was formed under conditions similar to hypervelocity impact cratering. As such, it is effectively pristine, having been formed in 1962 as a result of a controlled detonation of a 100 kiloton thermonuclear device, buried at the appropriate equivalent depth of burst required to make a simple crater. Sedan was formed in alluvium of mixed lithology and subsequently studied using a variety of field-based methods. Nearby secondary craters were also formed at the time and were also mapped by ATM. Adjacent to Sedan and also in alluvium is Scooter, about 90 m in diameter and formed by a high-explosive event. Schooner (240 m) and Danny Boy (80 m) craters were also important targets for ATM as they were excavated in hard basalt and therefore have much rougher ejecta. This will allow study of ejecta patterns in hard rock as well as engineering tests of crater and rock avoidance and rover trafficability. In addition to the high resolution DEMs, crater geometric characteristics, RMS roughness maps, and other higher-order derived data products will be generated using these data. These will provide constraints for models of landing hazards on Mars and for rover trafficability. Other planned studies will include ejecta size-frequency distribution at the resolution of the DEM and at finer resolution through air photography and field measurements, correlation of ejecta size and composition with radar and visible-thermal IR remote sensing signatures, and comparison of these results with similar measurements of Mars. The final DEMs, ancillary data sets, and derived data products will be made available to the community.

Small craters on the meteoroid and space debris impact experiment

NASA Technical Reports Server (NTRS)

Humes, Donald H.

1995-01-01

Examination of 9.34 m(exp 2) of thick aluminum plates from the Long Duration Exposure Facility (LDEF) using a 25X microscope revealed 4341 craters that were 0.1 mm in diameter or larger. The largest was 3 mm in diameter. Most were roughly hemispherical with lips that were raised above the original plate surface. The crater diameter measured was the diameter at the top of the raised lips. There was a large variation in the number density of craters around the three-axis gravity-gradient stabilized spacecraft. A model of the near-Earth meteoroid environment is presented which uses a meteoroid size distribution based on the crater size distribution on the space end of the LDEF. An argument is made that nearly all the craters on the space end must have been caused by meteoroids and that very few could have been caused by man-made orbital debris. However, no chemical analysis of impactor residue that will distinguish between meteoroids and man-made debris is yet available. A small area (0.0447 m(exp 2)) of one of the plates on the space end was scanned with a 200X microscope revealing 155 craters between 10 micron and 100 micron in diameter and 3 craters smaller than 10 micron. This data was used to extend the size distribution of meteoroids down to approximately 1 micron. New penetration equations developed by Alan Watts were used to relate crater dimensions to meteoroid size. The equations suggest that meteoroids must have a density near 2.5 g/cm(exp 3) to produce craters of the shape found on the LDEF. The near-Earth meteoroid model suggests that about 80 to 85 percent of the 100 micron to 1 mm diameter craters on the twelve peripheral rows of the LDEF were caused by meteoroids, leaving 15 to 20 percent to be caused by man-made orbital debris.

Low-velocity impact craters in ice and ice-saturated sand with implications for Martian crater count ages.

USGS Publications Warehouse

Croft, S.K.; Kieffer, S.W.; Ahrens, T.J.

1979-01-01

We produced a series of decimeter-sized impact craters in blocks of ice near 0oC and -70oC and in ice-saturated sand near -70oC as a preliminary investigation of cratering in materials analogous to those found on Mars and the outer solar satellites. Crater diameters in the ice-saturated sand were 2 times larger than craters in the same energy and velocity range in competent blocks of granite, basalt and cement. Craters in ice were c.3 times larger. Martian impact crater energy versus diameter scaling may thus be a function of latitude. -from Authors

Impact cratering on porous targets in the strength regime

NASA Astrophysics Data System (ADS)

Nakamura, Akiko M.

2017-12-01

Cratering on small bodies is crucial for the collision cascade and also contributes to the ejection of dust particles into interplanetary space. A crater cavity forms against the mechanical strength of the surface, gravitational acceleration, or both. The formation of moderately sized craters that are sufficiently larger than the thickness of the regolith on small bodies, in which mechanical strength plays the dominant role rather than gravitational acceleration, is in the strength regime. The formation of microcraters on blocks on the surface is also within the strength regime. On the other hand, the formation of a crater of a size comparable to the thickness of the regolith is affected by both gravitational acceleration and cohesion between regolith particles. In this short review, we compile data from the literature pertaining to impact cratering experiments on porous targets, and summarize the ratio of spall diameter to pit diameter, the depth, diameter, and volume of the crater cavity, and the ratio of depth to diameter. Among targets with various porosities studied in the laboratory to date, based on conventional scaling laws (Holsapple and Schmidt, J. Geophys. Res., 87, 1849-1870, 1982) the cratering efficiency obtained for porous sedimentary rocks (Suzuki et al., J. Geophys. Res. 117, E08012, 2012) is intermediate. A comparison with microcraters formed on a glass target with impact velocities up to 14 km s-1 indicates a different dependence of cratering efficiency and depth-to-diameter ratio on impact velocity.

Impacts into porous asteroids

NASA Astrophysics Data System (ADS)

Housen, Kevin R.; Sweet, William J.; Holsapple, Keith A.

2018-01-01

Many small bodies in the solar system have bulk density well below the solid density of the constituent mineral grains in their meteorite counterparts. Those low-density bodies undoubtedly have significant porosity, which is a key factor that affects the formation of impact craters. This paper summarizes the results of lab experiments in which materials with porosity ranging from 43% to 96% were impacted at ∼1800 m/s. The experiments were performed on a geotechnical centrifuge, in order to reproduce the lithostatic overburden stress and ejecta ballistics that occur in large-scale cratering events on asteroids or planetary satellites. Experiments performed at various accelerations, up to 514G, simulate the outcomes of impacts at size scales up to several tens of km in diameter. Our experiments show that an impact into a highly porous cohesionless material generates a large ovoid-shaped cavity, due to crushing by the outgoing shock. The cavity opens up to form a transient crater that grows until the material flow is arrested by gravity. The cavity then collapses to form the final crater. During collapse, finely crushed material that lines the cavity wall is carried down and collected in a localized region below the final crater floor. At large simulated sizes (high accelerations), most of the crater volume is formed by compaction, because growth of the transient crater is quickly arrested. Nearly all ejected material falls back into the crater, leaving the crater without an ejecta blanket. We find that such compaction cratering and suppression of the ejecta blankets occur for large craters on porous bodies when the ratio of the lithostatic stress at one crater depth to the crush strength of the target exceeds ∼0.005. The results are used to identify small solar system bodies on which compaction cratering likely occurs. A model is developed that gives the crater size and ejecta mass that would result for a specified impact into a porous object.

Microcraters on lunar samples

NASA Technical Reports Server (NTRS)

Fechtig, H.; Gentner, W.; Hartung, J. B.; Nagel, K.; Neukum, G.; Schneider, E.; Storzer, D.

1977-01-01

The lunar microcrater phenomenology is described. The morphology of the lunar craters is in almost all aspects simulated in laboratory experiments in the diameter range from less than 1 nu to several millimeters and up to 60 km/s impact velocity. An empirically derived formula is given for the conversion of crater diameters into projectile diameters and masses for given impact velocities and projectile and target densities. The production size frequency distribution for lunar craters in the crater size range from approximately 1 nu to several millimeters in diameter is derived from various microcrater measurements within a factor of up to 5. Particle track exposure age measurements for a variety of lunar samples have been performed. They allow the conversion of the lunar crater size frequency production distributions into particle fluxes. The development of crater populations on lunar rocks under self-destruction by subsequent meteoroid impacts and crater overlap is discussed and theoretically described. Erosion rates on lunar rocks on the order of several millimeters per 10 yr are calculated. Chemical investigations of the glass linings of lunar craters yield clear evidence of admixture of projectile material only in one case, where the remnants of an iron-nickel micrometeorite have been identified.

The nature of the gravity anomalies associated with large young lunar craters

NASA Technical Reports Server (NTRS)

Dvorak, J.; Phillips, R. J.

1977-01-01

The negative Bouguer anomalies (i.e., mass deficiencies) associated with four young lunar craters are analyzed. Model calculations based on generalizations made from studies of terrestrial impact structures suggest that the major contribution to the Bouguer anomaly for these lunar craters is due to a lens of brecciated material confined within the present crater rim crest and extending vertically to at least a depth of one-third the crater rim diameter. Calculations also reveal a systematic variation in the magnitude of the mass deficiencies with the cube of the crater diameter.

Geomechanical models of impact cratering: Puchezh-Katunki structure

NASA Technical Reports Server (NTRS)

Ivanov, B. A.

1992-01-01

Impact cratering is a complex natural phenomenon that involves various physical and mechanical processes. Simulating these processes may be improved using the data obtained during the deep drilling at the central mound of the Puchezh-Katunki impact structure. A research deep drillhole (named Vorotilovskaya) has been drilled in the Puchezh-Katunki impact structure (European Russia, 57 deg 06 min N, 43 deg 35 min E). The age of the structure is estimated at about 180 to 200 m.y. The initial rim crater diameter is estimated at about 40 km. The central uplift is composed of large blocks of crystalline basement rocks. Preliminary study of the core shows that crystalline rocks are shock metamorphosed by shock pressure from 45 GPa near the surface to 15-20 GPa at a depth of about 5 km. The drill core allows the possibility of investigating many previously poorly studied cratering processes in the central part of the impact structure. As a first step one can use the estimates of energy for the homogeneous rock target. The diameter of the crater rim may be estimated as 40 km. The models elaborated earlier show that such a crater may be formed after collapse of a transient cavity with a radius of 10 km. The most probable range of impact velocities from 11.2 to 30 km/s may be inferred for the asteroidal impactor. For the density of a projectile of 2 g/cu cm the energy of the impact is estimated as 1E28 to 3E28 erg. In the case of vertical impact, the diameter of an asteroidal projectile is from 1.5 to 3 km for the velocity range from 11 to 30 km/s. For the most probable impact angle of 45 deg, the estimated diameter of an asteroid is slightly larger: from 2 to 4 km. Numerical simulation of the transient crater collapse has been done using several models of rock rheology during collapse. Results show that the column at the final position beneath the central mound is about 5 km in length. This value is close to the shock-pressure decay observed along the drill core. Further improvement of the model needs to take into account the blocky structure of target rocks revealed by drilling.

The Mechanics of Peak-Ring Impact Crater Formation from the IODP-ICDP Expedition 364

NASA Astrophysics Data System (ADS)

Melosh, H.; Collins, G. S.; Morgan, J. V.; Gulick, S. P. S.

2017-12-01

The Chicxulub impact crater is one of very few peak-ring impact craters on Earth. While small (less than 3 km on Earth) impact craters are typically bowl-shaped, larger craters exhibit central peaks, which in still larger (more than about 100 km on Earth) craters expand into mountainous rings with diameters close to half that of the crater rim. The origin of these peak rings has been contentious: Such craters are far too large to create in laboratory experiments and remote sensing of extraterrestrial examples has not clarified the mechanics of their formation. Two principal models of peak ring formation are currently in vogue, the "nested crater" model, in which the peak ring originates at shallow depths in the target, and the "dynamic collapse" model in which the peak ring is uplifted at the base of a collapsing, over-steepened central peak and its rocks originate at mid-crustal depths. IODP-ICDP Expedition 364 sought to elucidate, among other important goals, the mechanics of peak ring formation in the young (66 Myr), fresh, but completely buried Chicxulub impact crater. The cores from this borehole now show unambiguously that the rocks in the Chicxulub peak ring originated at mid-crustal depths, apparently ruling out the nested crater model. These rocks were shocked to pressures on the order of 10-35 GPa and were so shattered that their densities and seismic velocities now resemble those of sedimentary rocks. The morphology of the final crater, its structure as revealed in previous seismic imaging, and the results from the cores are completely consistent with modern numerical models of impact crater excavation and collapse that incorporate a model for post-impact weakening. Subsequent to the opening of a ca. 100 km diameter and 30 km deep transient crater, this enormous hole in the crust collapsed over a period of about 10 minutes. Collapse was enabled by movement of the underlying rocks, which briefly behaved in the manner of a high-viscosity fluid, a brittle deformation state described by the process of "acoustic" fluidization initiated by strong elastic vibrations accompanying the opening and collapse of the crater. The shattered core, cut by both melt rock and clastic dikes, is consistent with the block model of acoustic fluidization supporting its application to crater collapse both on the Earth and on other planets.

Dawn Framing Camera: Morphology and morphometry of impact craters on Ceres

NASA Astrophysics Data System (ADS)

Platz, T.; A; Nathues; Schäfer, M.; Hoffmann, M.; Kneissl, T.; Schmedemann, N.; Vincent, J.-B.; Büttner, I.; Gutierrez-Marques, P.; Ripken, J.; Russell, C. T.; Schäfer, T.; Thangjam, G. S.

2015-10-01

In the first approach images of Ceres we tried to discern the simple-to-complex transition diameter of impact craters. Limited by spatial resolution we found the smallest complex crater without central peak development to be around 21.4 km in diameter. Hence, the transition diameter is expected to be between 21.4 km and 10.6 km, the predicted transition diameter for an icy target. It appears likely that either Ceres' surface material contains a rocky component or has a laterally inhomogeneous composition ranging from icy to ice-rocky

Effects of the Venusian atmosphere on incoming meteoroids and the impact crater population

NASA Technical Reports Server (NTRS)

Herrick, Robert R.; Phillips, Roger J.

1994-01-01

The dense atmosphere on Venus prevents craters smaller than about 2 km in daimater from forming and also causes formation of several crater fields and multiple-floored craters (collectively referred to as multiple impacts). A model has been constructed that simulates the behavior of a meteoroid in a dense planetary atmosphere. This model was then combined with an assumed flux of incoming meteoroids in an effort to reproduce the size-frequency distribution of impact craters and several aspects of the population of the crater fields and multiple-floored craters on Venus. The modeling indicates that it is plausible that the observed rollover in the size-frequency curve for Venus is due entirely to atmospheric effects on incoming meteoroids. However, there must be substantial variation in the density and behavior of incoming meteoroids in the atmosphere. Lower-density meteoroids must be less likely to survive atmospheric passage than simple density differences can account for. Consequently, it is likely that the percentage of craters formed by high-density meteoroids is very high at small crater diameters, and this percentage decreases substantially with increasing crater diameter. Overall, high-density meteoroids created a disproportionately large percentage of the impact craters on Venus. Also, our results indicate that a process such as meteoroid flattening or atmospheric explosion of meteoroids must be invoked to prevent craters smaller than the observed minimum diameter (2 km) from forming. In terms of using the size-frequency distribution to age-date the surface, the model indicates that the observed population has at least 75% of the craters over 32 km in diameter that would be expected on an atmosphereless Venus; thus, this part of the curve is most suitable for comparison with calibrated curves for the Moon.

Depth-diameter ratios for Martian impact craters: Implications for target properties and episodes of degradation

NASA Technical Reports Server (NTRS)

Barlow, N. G.

1993-01-01

This study determines crater depth through use of photoclinometric profiles. Random checks of the photoclinometric results are performed using shadow estimation techniques. The images are Viking Orbiter digital format frames; in cases where the digital image is unusable for photoclinometric analysis, shadow estimation is used to determine crater depths. The two techniques provide depth results within 2 percent of each other. Crater diameters are obtained from the photoclinometric profiles and checked against the diameters measured from the hard-copy images using a digitizer. All images used in this analysis are of approximately 40 m/pixel resolution. The sites that have been analyzed to date include areas within Arabia, Maja Valles, Memnonia, Acidalia, and Elysium. Only results for simple craters (craters less than 5 km in diameter) are discussed here because of the low numbers of complex craters presently measured in the analysis. General results indicate that impact craters are deeper than average. A single d/D relationship for fresh impact craters on Mars does not exist due to changes in target properties across the planet's surface. Within regions where target properties are approximately constant, however, d/D ratios for fresh craters can be determined. In these regions, the d/D ratios of nonpristine craters can be compared with the fresh crater d/D relationship to obtain information on relative degrees of crater degradation. This technique reveals that regional episodes of enhanced degradation have occurred. However, the lack of statistically reliable size-frequency distribution data prevents comparison of the relative ages of these events between different regions, and thus determination of a large-scale episode (or perhaps several episodes) cannot be made at this time.

Characterization of the Morphometry of Impact Craters Hosting Polar Deposits in Mercury's North Polar Region

NASA Technical Reports Server (NTRS)

Talpe Matthieu; Zuber, Maria T.; Yang, Di; Neumann, Gregory A.; Solomon, Sean C.; Mazarico, Erwan; Vilas, Faith

2012-01-01

Earth-based radar images of Mercury show radar-bright material inside impact craters near the planet s poles. A previous study indicated that the polar-deposit-hosting craters (PDCs) at Mercury s north pole are shallower than craters that lack such deposits. We use data acquired by the Mercury Laser Altimeter on the MESSENGER spacecraft during 11 months of orbital observations to revisit the depths of craters at high northern latitudes on Mercury. We measured the depth and diameter of 537 craters located poleward of 45 N, evaluated the slopes of the northern and southern walls of 30 PDCs, and assessed the floor roughness of 94 craters, including nine PDCs. We find that the PDCs appear to have a fresher crater morphology than the non-PDCs and that the radar-bright material has no detectable influence on crater depths, wall slopes, or floor roughness. The statistical similarity of crater depth-diameter relations for the PDC and non-PDC populations places an upper limit on the thickness of the radar-bright material (< 170 m for a crater 11 km in diameter) that can be refined by future detailed analysis. Results of the current study are consistent with the view that the radar-bright material constitutes a relatively thin layer emplaced preferentially in comparatively young craters.

Expanded Craters on Mars: Implications for Shallow, Mid-latitude Excess Ice

NASA Astrophysics Data System (ADS)

Viola, Donna

Understanding the age and distribution of shallow ice on Mars is valuable for interpreting past and present climate conditions, and has implications on habitability and future in situ resource utilization. Many ice-related features, such as lobate debris aprons and concentric crater fill, have been studied using a range of remote sensing techniques. Here, I explore the distribution of expanded craters, a form of sublimation thermokarst where shallow, excess ice has been destabilized and sublimated following an impact event. This leads to the collapse of the overlying dry regolith to produce the appearance of diameter widening. The modern presence of these features suggests that excess ice has remained preserved in the terrain immediately surrounding the craters since the time of their formation in order to maintain the surface. High-resolution imagery is ideal for observing thermokarst features, and much of the work described here will utilize data from the Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO). Expanded craters tend to be found in clusters that emanate radially from at least four primary craters in Arcadia Planitia, and are interpreted as secondary craters that formed nearly simultaneously with their primaries. Crater age dates of the primaries indicate that the expanded secondaries, as well as the ice layer into which they impacted, must be at least tens of millions of years old. Older double-layer ejecta craters in Arcadia Planitia commonly have expanded craters superposed on their ejecta - and they tend to be more expanded (with larger diameters) in the inner ejecta layer. This has implications on the formation mechanisms for craters with this unique ejecta morphology. Finally, I explore the distribution of expanded craters south of Arcadia Planitia and across the southern mid-latitudes, along with scalloped depressions (another form of sublimation thermokarst), in order to identify the modern excess ice boundary in this region and any longitudinal variations. This study identifies some potential low-latitude locations with patchy excess ice, possibly preserved during a past climate. Through these studies, I will infer regions that contain abundant ice today and consider the implications that this ice has on both the martian climate and future exploration.

Morphologic Analysis of Lunar Craters in the Simple-to-Complex Transition

NASA Astrophysics Data System (ADS)

Chandnani, M.; Herrick, R. R.; Kramer, G. Y.

2015-12-01

The diameter range of 15 km to 20 km on the Moon is within the transition from simple to complex impact craters. We examined 207 well preserved craters in this diameter range distributed across the moon using high resolution Lunar Reconnaissance Orbiter Camera Wide Angle Camera Mosaic (WAC) and Narrow Angle Camera (NAC) data. A map of the distribution of the 207 craters on the Moon using the global LROC WAC mosaic has been attahced with the abstract. By examining craters of similar diameter, impact energy is nearly constant, so differences in shape and morphology must be due to either target (e.g., porosity, density, coherence, layering) or impactor (e.g., velocity, density) properties. On the basis of the crater morphology, topographic profiles and depth-diameter ratio, the craters were classified into simple, craters with slumped walls, craters with both slumping and terracing, those containing a central uplift only, those with a central uplift and slumping, and the craters with a central uplift accompanied by both slumping and terracing, as shown in the image. It was observed that simple craters and craters with slumped walls occur predominately on the lunar highlands. The majority of the craters with terraced walls and all classes of central uplifts were observed predominately on the mare. In short, in this size range craters in the highlands were generally simple craters with occasionally some slumped material in the center, and the more developed features (terracing, central peak) were associated with mare craters. This is somewhat counterintuitive, as we expect the highlands to be generally weaker and less consolidated than the mare. We hypothesize that the presence of rheologic layering in the mare may be the cause of the more complex features that we observe. Relatively weak layers in the mare could develop through regolith formation between individual flows, or perhaps by variations within or between the flows themselves.

Lunar crater arcs. [origins, distribution and age classification of Pre-Imbrian families

NASA Technical Reports Server (NTRS)

Jaffe, L. D.; Bulkley, E. O.

1976-01-01

An analysis has been made of the tendency of large lunar craters to lie along circles. A catalog of the craters at least 50 km in diameter was prepared first, noting position, diameter, rim sharpness and completion, nature of underlying surface, and geological age. The subset of those craters 50-400 km in diameter was then used as input to computer programs which identified each 'family' of four or more craters of selected geological age lying on a circular arc. For comparison, families were also identified for randomized crater models in which the crater spatial density was matched to that on the moon, either overall or separately for mare and highland areas. The observed frequency of lunar arcuate families was statistically highly significantly greater than for the randomized models, for craters classified as either late-pre-Imbrian (Nectarian), middle pre-Imbrian, or early pre-Imbrian, as well as for a number of larger age-classes. The lunar families tend to center in specific areas of the moon; these lie in highlands rather than maria and are different for families of Nectarian craters than for pre-Nectarian. The origin of the arcuate crater groupings is not understood.

Small Rayed Crater Ejecta Retention Age Calculated from Current Crater Production Rates on Mars

NASA Technical Reports Server (NTRS)

Calef, F. J. III; Herrick, R. R.; Sharpton, V. L.

2011-01-01

Ejecta from impact craters, while extant, records erosive and depositional processes on their surfaces. Estimating ejecta retention age (Eret), the time span when ejecta remains recognizable around a crater, can be applied to estimate the timescale that surface processes operate on, thereby obtaining a history of geologic activity. However, the abundance of sub-kilometer diameter (D) craters identifiable in high resolution Mars imagery has led to questions of accuracy in absolute crater dating and hence ejecta retention ages (Eret). This research calculates the maximum Eret for small rayed impact craters (SRC) on Mars using estimates of the Martian impactor flux adjusted for meteorite ablation losses in the atmosphere. In addition, we utilize the diameter-distance relationship of secondary cratering to adjust crater counts in the vicinity of the large primary crater Zunil.

Low-emissivity impact craters on Venus

NASA Technical Reports Server (NTRS)

Weitz, C. M.; Elachi, C.; Moore, H. J.; Basilevsky, A. T.; Ivanov, B. A.; Schaber, G. G.

1992-01-01

An analysis of 144 impact craters on Venus has shown that 11 of these have floors with average emissivities lower than 0.8. The remaining craters have emissivities between 0.8 and 0.9, independent of the specific backscatter cross section of the crater floors. These 144 impact craters were chosen from a possible 164 craters with diameters greater than 30 km as identified by researchers for 89 percent of the surface of Venus. We have only looked at craters below 6053.5 km altitude because a mineralogical change causes high reflectivity/low emissivity above the altitude. We have also excluded all craters with diameters smaller than 30 km because the emissivity footprint at periapsis is 16 x 24 km and becomes larger at the poles.

Oblique view of crater Theophilus at northwest edge of Sea of Nectar

NASA Technical Reports Server (NTRS)

1969-01-01

An Apollo 11 oblique view of the large crater Theophilus located at the northwest edge of the Sea of Nectar on the lunar nearside. Theophilus is about 60 statute miles in diameter. the smooth area is Mare Nectaris. The smaller crater Madler, about 14 statute miles in diameter, is located to the east of Theophilus. Visible in the background are the large crater Fracastorius and the smaller crater Beaumont. The coordinates of the center of this photograph are 29 degrees east longitude and 11 degrees south latitude.

An object-based classification method for automatic detection of lunar impact craters from topographic data

NASA Astrophysics Data System (ADS)

Vamshi, Gasiganti T.; Martha, Tapas R.; Vinod Kumar, K.

2016-05-01

Identification of impact craters is a primary requirement to study past geological processes such as impact history. They are also used as proxies for measuring relative ages of various planetary or satellite bodies and help to understand the evolution of planetary surfaces. In this paper, we present a new method using object-based image analysis (OBIA) technique to detect impact craters of wide range of sizes from topographic data. Multiresolution image segmentation of digital terrain models (DTMs) available from the NASA's LRO mission was carried out to create objects. Subsequently, objects were classified into impact craters using shape and morphometric criteria resulting in 95% detection accuracy. The methodology developed in a training area in parts of Mare Imbrium in the form of a knowledge-based ruleset when applied in another area, detected impact craters with 90% accuracy. The minimum and maximum sizes (diameters) of impact craters detected in parts of Mare Imbrium by our method are 29 m and 1.5 km, respectively. Diameters of automatically detected impact craters show good correlation (R2 > 0.85) with the diameters of manually detected impact craters.

The Morphology of Craters on Mercury: Results from MESSENGER Flybys

NASA Technical Reports Server (NTRS)

Barnouin, Oliver S.; Zuber, Maria T.; Smith, David E.; Neumann, Gregory A.; Herrick, Robert R.; Chappelow, John E.; Murchie, Scott L.; Prockter, Louise M.

2012-01-01

Topographic data measured from the Mercury Laser Altimeter (MLA) and the Mercury Dual Imaging System (MDIS) aboard the MESSENGER spacecraft were used for investigations of the relationship between depth and diameter for impact craters on Mercury. Results using data from the MESSENGER flybys of the innermost planet indicate that most of the craters measured with MLA are shallower than those previously measured by using Mariner 10 images. MDIS images of these same MLA-measured craters show that they have been modified. The use of shadow measurement techniques, which were found to be accurate relative to the MLA results, indicate that both small bowl-shaped and large complex craters that are fresh possess depth-to-diameter ratios that are in good agreement with those measured from Mariner 10 images. The preliminary data also show that the depths of modified craters are shallower relative to fresh ones, and might provide quantitative estimates of crater in-filling by subsequent volcanic or impact processes. The diameter that defines the transition from simple to complex craters on Mercury based on MESSENGER data is consistent with that reported from Mariner 10 data.

Numerical modeling of Stickney crater and its aftermath

NASA Astrophysics Data System (ADS)

Schwartz, Stephen R.; Michel, Patrick; Bruck Syal, Megan; Owen, J. Michael; Miller, Paul L.; Richardson, Derek C.; Zhang, Yun

2016-10-01

Phobos is characterized by a large crater called Stickney. Its collisional formation and its aftermath have important implications on the final structure, morphology, and surface properties of Phobos that still need further clarification. This is particularly important in the current environment, with space mission concepts to Phobos under active study by several space agencies. SPH hydrocode simulations of the impact that formed Stickney crater [1] have been performed. Using the Soft-Sphere Discrete Element Method (SSDEM) collisional routine of the N-body code pkdgrav [2], we take the outcome of SPH simulations as inputs and model the ensuing phase of the crater formation process and its ejecta evolution under the gravitational influence of Phobos and Mars. In our simulations, about 9 million particles comprise Phobos' shape [3], and the evolution of particles that are expected to form or leave the crater is followed using multiple plausible orbits for Phobos around Mars. We track the immediate fate of low-speed ejecta (~3-8 m/s), allowing us to test an hypothesis [4] that they may scour certain groove marks that have been observed on Phobos' surface and to quantify the amounts and locations of re-impacting ejecta. We also compute the orbital fate of ejecta whose speed is below the system escape speed (about 3 km/s). This allows us to estimate the thickness and distribution of the final ejecta blanket and to check whether crater chains may form. Finally, particles forming the crater walls are followed until achieving stability, allowing us to estimate the final crater depth and diameter. We will show examples of these simulations from a set of SPH initial conditions and over a range of parameters (e.g., material friction coefficients). Work ongoing to cover a larger range of plausible impact conditions, allowing us to explore different scenarios to explain Phobos' observed properties and to infer more, giving useful constraints to space mission studies. [1] Bruck Syal, M. et al. (this meeting); [2] Schwartz, S.R. et al. 2012, Granul. Matter 14, 363; [3] Willner, K. et al. 2010, E. Earth Planet. Sci. Lett. 294, 541; [4] Wilson, L. & Head, J.W. 2015, Planet. Space Sci. 105, 26.

A revised surface age for the North Polar Layered Deposits of Mars

USGS Publications Warehouse

Landis, Margaret E.; Byrne, Shane; Daubar, Ingrid J.; Herkenhoff, Kenneth E.; Dundas, Colin M.

2016-01-01

The North Polar Layered Deposits (NPLD) of Mars contain a complex stratigraphy that has been suggested to retain a record of past eccentricity- and obliquity-forced climate changes. The surface accumulation rate in the current climate can be constrained by the crater retention age. We scale NPLD crater diameters to account for icy target strength and compare surface age using a new production function for recent small impacts on Mars to the previously used model of Hartmann (2005). Our results indicate that ice is accumulating in these craters several times faster than previously thought, with a 100 m diameter crater being completely infilled within centuries. Craters appear to have a diameter-dependent lifetime, but the data also permit a complete resurfacing of the NPLD at ~1.5 ka.

The morphology of small fresh craters on Mars and the Moon

NASA Astrophysics Data System (ADS)

Daubar, Ingrid J.; Atwood-Stone, C.; Byrne, S.; McEwen, A. S.; Russell, P. S.

2014-12-01

The depth/diameter ratio for new meter- to decameter-scale Martian craters formed in the last ~20 years averages 0.23, only slightly deeper than that expected for simple primary craters on rocky surfaces. Large variations in depth/diameter (d/D) between impact sites indicate that differences between the sites such as target material properties, impact velocity, angle, and physical state of the bolide(s) are important in determining the depth of small craters in the strength regime. On the Moon, the d/D of random fresh small craters with similar diameters averages only 0.10, indicating that either the majority of them are unrecognized secondaries or some proportion are degraded primaries. Older craters such as these may be shallower due to erosional infilling, which is probably not linear over time but more effective over recently disturbed and steeper surfaces, processes that are not yet acting on the new Martian craters. Brand new meter- to decameter-scale craters such as the Martian ones studied here are statistically easily distinguishable as primaries, but the origins of older craters of the same size, such as the lunar ones in this study, are ambiguous.

Timing and Distribution of Single-Layered Ejecta Craters Imply Sporadic Preservation of Tropical Subsurface Ice on Mars

NASA Astrophysics Data System (ADS)

Kirchoff, Michelle R.; Grimm, Robert E.

2018-01-01

Determining the evolution of tropical subsurface ice is a key component to understanding Mars's climate and geologic history. Study of an intriguing crater type on Mars—layered ejecta craters, which likely form by tapping subsurface ice—may provide constraints on this evolution. Layered ejecta craters have a continuous ejecta deposit with a fluidized-flow appearance. Single-layered ejecta (SLE) craters are the most common and dominate at tropical latitudes and therefore offer the best opportunity to derive new constraints on the temporal evolution of low-latitude subsurface ice. We estimate model formation ages of 54 SLE craters with diameter (D) ≥ 5 km using the density of small, superposed craters with D < 1 km on their continuous ejecta deposits. These model ages indicate that SLE craters have formed throughout the Amazonian and at a similar rate expected for all Martian craters. This suggests that tropical ice has remained at relatively shallow depths at least where these craters formed. In particular, the presence of equatorial SLE craters with D 1 km indicates that ice could be preserved as shallow as 100 m or less at those locations. Finally, there is a striking spatial mixing in an area of highlands near the equator of layered and radial (lunar-like ballistic) ejecta craters; the latter form where there are insufficient concentrations of subsurface ice. This implies strong spatial heterogeneity in the concentration of tropical subsurface ice.

Geomorphology and Geology of the Southwestern Margaritifer Sinus and Argyre Regions of Mars. Part 4: Flow Ejecta Crater Distribution

NASA Technical Reports Server (NTRS)

Parker, T. J.; Pieri, D. C.

1985-01-01

Flow ejecta craters - craters surrounded by lobate ejecta blankets - are found throughout the study area. The ratio of the crater's diameter to that of the flow ejecta in this region is approximately 40 to 45%. Flow ejecta craters are dominantly sharply defined craters, with slightly degraded craters being somewhat less common. This is probably indicative of the ejecta's relatively low resistence to weathering and susceptibility to burial. Flow ejecta craters here seem to occur within a narrow range of crater sizes - the smallest being about 4km in diameter and the largest being about 27km in diameter. Ejecta blankets of craters at 4km are easily seen and those of smaller craters are simply not seen even in images with better than average resolution for the region. This may be due to the depth of excavation of small impacting bodies being insufficient to reach volatile-rich material. Flow ejecta craters above 24km are rare, and those craters above 27km do not display flow ejecta blankets. This may be a result of an excavation depth so great that the volatile content of the ejecta is insufficient to form a fluid ejecta blanket. The geomorphic/geologic unit appears also to play an important role in the formation of flow ejecta craters. Given the typical size range for the occurrence of flow ejecta craters for most units, it can be seen that the percentage of flow ejecta craters to the total number of craters within this size range varies significantly from one unit to the next. The wide variance in flow ejecta crater density over this relatively small geographical area argues strongly for a lithologic control of their distribution.

An empirical model for transient crater growth in granular targets based on direct observations

NASA Astrophysics Data System (ADS)

Yamamoto, Satoru; Barnouin-Jha, Olivier S.; Toriumi, Takashi; Sugita, Seiji; Matsui, Takafumi

2009-09-01

The present paper describes observations of crater growth up to the time of transient crater formation and presents a new empirical model for transient crater growth as a function of time. Polycarbonate projectiles were impacted vertically into soda-lime glass sphere targets using a single-stage light-gas gun. Using a new technique with a laser sheet illuminating the target [Barnouin-Jha, O.S., Yamamoto, S., Toriumi, T., Sugita, S., Matsui, T., 2007. Non-intrusive measurements of the crater growth. Icarus, 188, 506-521], we measured the temporal change in diameter of crater cavities (diameter growth). The rate of increase in diameter at early times follows a power law relation, but the data at later times (before the end of transient crater formation) deviates from the power law relation. In addition, the power law exponent at early times and the degree of deviation from a power law at later times depend on the target. In order to interpret these features, we proposed to modify Maxwell's Z-model under the assumption that the strength of the excavation flow field decreases exponentially with time. We also derived a diameter growth model as: d(t)∝[1-exp(-βt)]γ, where d(t) is the apparent diameter of the crater cavity at time t after impact, and β and γ are constants. We demonstrated that the diameter growth model could represent well the experimental data for various targets with different target material properties, such as porosity or angle of repose. We also investigated the diameter growth for a dry sand target, which has been used to formulate previous scaling relations. The obtained results showed that the dry sand target has larger degree of deviation from a power law, indicating that the target material properties of the dry sand target have a significant effect on diameter growth, especially at later times. This may suggest that the previously reported scaling relations should be reexamined in order to account for the late-stage behavior with the effect of target material properties.

Analytical Model for Mars Crater-Size Frequency Distribution

NASA Astrophysics Data System (ADS)

Bruckman, W.; Ruiz, A.; Ramos, E.

2009-05-01

We present a theoretical and analytical curve that reproduces essential features of the frequency distributions vs. diameter of the 42,000 impact craters contained in Barlow's Mars Catalog. The model is derived using reasonable simple assumptions that allow us to relate the present craters population with the craters population at each particular epoch. The model takes into consideration the reduction of the number of craters as a function of time caused by their erosion and obliteration, and this provides a simple and natural explanation for the presence of different slopes in the empirical log-log plot of number of craters (N) vs. diameter (D). A mean life for martians craters as a function of diameter is deduced, and it is shown that this result is consistent with the corresponding determination of craters mean life based on Earth data. Arguments are given to suggest that this consistency follows from the fact that a crater mean life is proportional to its volumen. It also follows that in the absence of erosions and obliterations, when craters are preserved, we would have N ∝ 1/D^{4.3}, which is a striking conclusion, since the exponent 4.3 is larger than previously thought. Such an exponent implies a similar slope in the extrapolated impactors size-frequency distribution.

Terrace width variations in complex Mercurian craters and the transient strength of cratered Mercurian and lunar crust

NASA Technical Reports Server (NTRS)

Leith, Andrew C.; Mckinnon, William B.

1991-01-01

The effective cohesion of the cratered region during crater collapse is determined via the widths of slump terraces of complex craters. Terrace widths are measured for complex craters on Mercury; these generally increase outward toward the rim for a given crater, and the width of the outermost major terrace is generally an increasing function of crater diameter. The terrace widths on Mercury and a gravity-driven slump model are used to estimate the strength of the cratered region immediately after impact (about 1-2 MPa). A comparison with the previous study of lunar complex craters by Pearce and Melosh (1986) indicates that the transient strength of cratered Mercurian crust is no greater than that of the moon. The strength estimates vary only slightly with the geometric model used to restore the outermost major terrace to its precollapse configuration and are consistent with independent strength estimates from the simple-to-complex crater depth/diameter transition.

Melt production in large-scale impact events: Implications and observations at terrestrial craters

NASA Technical Reports Server (NTRS)

Grieve, Richard A. F.; Cintala, Mark J.

1992-01-01

The volume of impact melt relative to the volume of the transient cavity increases with the size of the impact event. Here, we use the impact of chondrite into granite at 15, 25, and 50 km s(sup -1) to model impact-melt volumes at terrestrial craters in crystalline targets and explore the implications for terrestrial craters. Figures are presented that illustrate the relationships between melt volume and final crater diameter D(sub R) for observed terrestrial craters in crystalline targets; also included are model curves for the three different impact velocities. One implication of the increase in melt volumes with increasing crater size is that the depth of melting will also increase. This requires that shock effects occurring at the base of the cavity in simple craters and in the uplifted peaks of central structures at complex craters record progressively higher pressures with increasing crater size, up to a maximum of partial melting (approx. 45 GPa). Higher pressures cannot be recorded in the parautochthonous rocks of the cavity floor as they will be represented by impact melt, which will not remain in place. We have estimated maximum recorded pressures from a review of the literature, using such observations as planar features in quartz and feldspar, diaplectic glasses of feldspar and quartz, and partial fusion and vesiculation, as calibrated with estimates of the pressures required for their formation. Erosion complicates the picture by removing the surficial (most highly shocked) rocks in uplifted structures, thereby reducing the maximum shock pressures observed. In addition, the range of pressures that can be recorded is limited. Nevertheless, the data define a trend to higher recorded pressures with crater diameter, which is consistent with the implications of the model. A second implication is that, as the limit of melting intersects the base of the cavity, central topographic peaks will be modified in appearance and ultimately will not occur. That is, the peak will first develop a central depression, due to the flow of low-strength melted materials, when the melt volume begins to intersect the transient-cavity base.

Multivariate analyses of crater parameters and the classification of craters

NASA Technical Reports Server (NTRS)

Siegal, B. S.; Griffiths, J. C.

1974-01-01

Multivariate analyses were performed on certain linear dimensions of six genetic types of craters. A total of 320 craters, consisting of laboratory fluidization craters, craters formed by chemical and nuclear explosives, terrestrial maars and other volcanic craters, and terrestrial meteorite impact craters, authenticated and probable, were analyzed in the first data set in terms of their mean rim crest diameter, mean interior relief, rim height, and mean exterior rim width. The second data set contained an additional 91 terrestrial craters of which 19 were of experimental percussive impact and 28 of volcanic collapse origin, and which was analyzed in terms of mean rim crest diameter, mean interior relief, and rim height. Principal component analyses were performed on the six genetic types of craters. Ninety per cent of the variation in the variables can be accounted for by two components. Ninety-nine per cent of the variation in the craters formed by chemical and nuclear explosives is explained by the first component alone.

Martian planetwide crater distributions - Implications for geologic history and surface processes

NASA Technical Reports Server (NTRS)

Soderblom, L. A.; Condit, C. D.; West, R. A.; Herman, B. M.; Kreidler, T. J.

1974-01-01

Three different diameter size ranges are considered in connection with the Martian crater distribution, taking into account small craters from 0.6 to 1.2 km, intermediate-sized craters from 4 to 10 km, and large craters with diameters exceeding 20 km. One of the objectives of the investigation reported is to establish the effects of eolian processes in the modification of craters in the different size ranges. Another objective is concerned with a description of the genetic relationships among the three size ranges of craters. Observables related to the relative age of geologic provinces are to be separated from observables related to geographic variations in eolian transport and deposition. Lunar and Martian cratering histories are compared as a basis for establishing relative and absolute time scales for the geological evolution of Mars.

Computer simulations of large asteroid impacts into oceanic and continental sites--preliminary results on atmospheric, cratering and ejecta dynamics

USGS Publications Warehouse

Roddy, D.J.; Schuster, S.H.; Rosenblatt, M.; Grant, L.B.; Hassig, P.J.; Kreyenhagen, K.N.

1987-01-01

Computer simulations have been completed that describe passage of a 10-km-diameter asteroid through the Earth's atmosphere and the subsequent cratering and ejecta dynamics caused by impact of the asteroid into both oceanic and continental sites. The asteroid was modeled as a spherical body moving vertically at 20 km/s with a kinetic energy of 2.6 ?? 1030 ergs (6.2 ?? 107 Mt ). Detailed material modeling of the asteroid, ocean, crustal units, sedimentary unit, and mantle included effects of strength and fracturing, generic asteroid and rock properties, porosity, saturation, lithostatic stresses, and geothermal contributions, each selected to simulate impact and geologic conditions that were as realistic as possible. Calculation of the passage of the asteroid through a U.S. Standard Atmosphere showed development of a strong bow shock wave followed by a highly shock compressed and heated air mass. Rapid expansion of this shocked air created a large low-density region that also expanded away from the impact area. Shock temperatures in air reached ???20,000 K near the surface of the uplifting crater rim and were as high as ???2000 K at more than 30 km range and 10 km altitude. Calculations to 30 s showed that the shock fronts in the air and in most of the expanding shocked air mass preceded the formation of the crater, ejecta, and rim uplift and did not interact with them. As cratering developed, uplifted rim and target material were ejected into the very low density, shock-heated air immediately above the forming crater, and complex interactions could be expected. Calculations of the impact events showed equally dramatic effects on the oceanic and continental targets through an interval of 120 s. Despite geologic differences in the targets, both cratering events developed comparable dynamic flow fields and by ???29 s had formed similar-sized transient craters ???39 km deep and ???62 km across. Transient-rim uplift of ocean and crust reached a maximum altitude of nearly 40 km at ???30 s and began to decay at velocities of 500 m/s to develop large-tsunami conditions. After ???30 s, strong gravitational rebound drove both craters toward broad flat-floored shapes. At 120 s, transient crater diameters were ???80 km (continental) and ???105 km (oceanic) and transient depths were ???27 km; crater floors consisting of melted and fragmented hot rock were rebounding rapidly upward. By 60 s, the continental crater had ejected ???2 ?? 1014 t, about twice the mass ejected from the oceanic crater. By 120 s, ???70,000 km3 (continental) and ???90,000 km3 (oceanic) target material were excavated (no mantle) and massive ejecta blankets were formed around the craters. We estimate that in excess of ???70% of the ejecta would finally lie within ???3 crater diameters of the impact, and the remaining ejecta (???1013 t), including the vaporized asteroid, would be ejected into the atmosphere to altitudes as high as the ionosphere. Effects of secondary volcanism and return of the ocean over hot oceanic crater floor could also be expected to contribute substantial material to the atmosphere. ?? 1987.

Population characteristics of submicrometer-sized craters on regolith particles from asteroid Itokawa

NASA Astrophysics Data System (ADS)

Matsumoto, Toru; Hasegawa, S.; Nakao, S.; Sakai, M.; Yurimoto, H.

2018-03-01

We investigated impact crater structures on regolith particles from asteroid Itokawa using scanning electron microscopy. We observed the surfaces of 51 Itokawa particles, ranging from 15 μm to 240 μm in size. Craters with average diameters ranging from 10 nm to 2.8 μm were identified on 13 Itokawa particles larger than 80 μm. We examined the abundance, spatial distribution, and morphology of approximately 900 craters on six Itokawa particles. Craters with sizes in excess of 200 nm are widely dispersed, with spatial densities from 2.6 μm2 to 4.5 μm2; a fraction of the craters was locally concentrated with a density of 0.1 μm2. The fractal dimension of the cumulative crater diameters ranges from 1.3 to 2.3. Craters of several tens of nanometers in diameter exhibit pit and surrounding rim structures. Craters of more than 100 nm in diameter commonly have melted residue at their bottom. These morphologies are similar to those of submicrometer-sized craters on lunar regolith. We estimated the impactor flux on Itokawa regolith-forming craters, assuming that the craters were accumulated during direct exposure to the space environment for 102 to 104 yr. The range of impactor flux onto Itokawa particles is estimated to be at least one order of magnitude higher than the interplanetary dust flux and comparable to the secondary impact flux on the Moon. This indicates that secondary ejecta impacts are probably the dominant cratering process in the submicrometer range on Itokawa regolith particles, as well as on the lunar surface. We demonstrate that secondary submicrometer craters can be produced anywhere in centimeter- to meter-sized depressions on Itokawa's surface through primary interplanetary dust impacts. If the surface unevenness on centimeter to meter scales is a significant factor determining the abundance of submicrometer secondary cratering, the secondary impact flux could be independent of the overall shapes or sizes of celestial bodies, and the secondary impact flux could have similar values on Itokawa and the Moon.

Preliminary Geological Map of the Ac-H-2 Coniraya Quadrangle of Ceres: An Integrated Mapping Study Using Dawn Spacecraft Data

NASA Astrophysics Data System (ADS)

Hiesinger, H.; Pasckert, J. H.; Williams, D. A.; Crown, D. A.; Mest, S. C.; Buczkowski, D.; Schenk, P.; Scully, J. E. C.; Jaumann, R.; Roatsch, T.; Preusker, F.; Platz, T.; Nathues, A.; Hoffmann, M.; Marchi, S.; De Sanctis, M. C.; Russell, C. T.; Raymond, C. A.

2015-12-01

To better understand the geologic history of dwarf planet Ceres, the surface has been divided into 15 quadrangles that are systematically mapped on the basis of images obtained by NASA's Dawn spacecraft, which began orbiting Ceres in April 2015. We will report on preliminary mapping results for the Ac-H-2 Coniraya Quadrangle based on Framing Camera (FC) mosaics from the Dawn Approach (1.3 km/px) and Survey (415 m/px) orbits. This quadrangle is located between 21-66°N and 0-90°E and is dominated by mostly highly degraded impact craters of diameters between 50 and 200 km and clusters of small- to midsize impact craters. Color data show that this quadrangle is generally darker than most regions of the southern hemisphere. Two prominent impact craters in this quadrangle have been named Coniraya and Gaue crater, respectively. Coniraya is the largest more or less intact impact crater with a diameter of 136 km, centered at 65.8°N/40.5°E. It appears shallow and its crater rim is heavily degraded but still continuous. At the current image resolution, textural differences between the interior and exterior of the crater are not visible. With a diameter of 84 km, Gaue crater appears to be the freshest large impact crater in this quadrangle. It is located at the eastern border of the Coniraya Quadrangle with a small central peak at 30°N/85.7°E. The crater rim is quite sharp and the ejecta blanket can be traced around the crater to a distance of ~200km from the crater center. Most of the crater floor around the central peak is covered by a smooth uniform unit with a lower impact crater population than the surrounding surfaces. Color data show that this smooth unit is darker than the surrounding surfaces. A similar unit can be found on the floor of a complex cluster of 10-56 km diameter craters at 32°N/40°E. With upcoming higher resolution data we will refine our geologic map and will specifically investigate possible formation processes of these smooth units.

The intercrater plains of Mercury and the Moon: Their nature, origin and role in terrestrial planet evolution. Cratering histories of the intercrater plains. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Leake, M. A.

1982-01-01

The intercrater plains of Mercury and the Moon are defined, in part, by their high densities of small craters. The crater size frequency statistics presented in this chapter may help constrain the relative ages and origins of these surfaces. To this end, the effects of common geologic processes on crater frequency statistics are compared with the diameter frequency distributions of the intercrater regions of the Moon and Mercury. Such analyses may determine whether secondary craters dominate the distribution at small diameters, and whether volcanic plains or ballistic deposits form the intercrater surface. Determining the mass frequency distribution and flux of the impacting population is a more difficult problem. The necessary information such as scaling relationships between projectile energy and crater diameter, the relative fluxes of solar system objects, and the absolute ages of surface units is model dependent and poorly constrained, especially for Mercury.

Craters of the Pluto-Charon system

NASA Astrophysics Data System (ADS)

Robbins, Stuart J.; Singer, Kelsi N.; Bray, Veronica J.; Schenk, Paul; Lauer, Tod R.; Weaver, Harold A.; Runyon, Kirby; McKinnon, William B.; Beyer, Ross A.; Porter, Simon; White, Oliver L.; Hofgartner, Jason D.; Zangari, Amanda M.; Moore, Jeffrey M.; Young, Leslie A.; Spencer, John R.; Binzel, Richard P.; Buie, Marc W.; Buratti, Bonnie J.; Cheng, Andrew F.; Grundy, William M.; Linscott, Ivan R.; Reitsema, Harold J.; Reuter, Dennis C.; Showalter, Mark R.; Tyler, G. Len; Olkin, Catherine B.; Ennico, Kimberly S.; Stern, S. Alan; New Horizons Lorri, Mvic Instrument Teams

2017-05-01

NASA's New Horizons flyby mission of the Pluto-Charon binary system and its four moons provided humanity with its first spacecraft-based look at a large Kuiper Belt Object beyond Triton. Excluding this system, multiple Kuiper Belt Objects (KBOs) have been observed for only 20 years from Earth, and the KBO size distribution is unconstrained except among the largest objects. Because small KBOs will remain beyond the capabilities of ground-based observatories for the foreseeable future, one of the best ways to constrain the small KBO population is to examine the craters they have made on the Pluto-Charon system. The first step to understanding the crater population is to map it. In this work, we describe the steps undertaken to produce a robust crater database of impact features on Pluto, Charon, and their two largest moons, Nix and Hydra. These include an examination of different types of images and image processing, and we present an analysis of variability among the crater mapping team, where crater diameters were found to average ± 10% uncertainty across all sizes measured (∼0.5-300 km). We also present a few basic analyses of the crater databases, finding that Pluto's craters' differential size-frequency distribution across the encounter hemisphere has a power-law slope of approximately -3.1 ± 0.1 over diameters D ≈ 15-200 km, and Charon's has a slope of -3.0 ± 0.2 over diameters D ≈ 10-120 km; it is significantly shallower on both bodies at smaller diameters. We also better quantify evidence of resurfacing evidenced by Pluto's craters in contrast with Charon's. With this work, we are also releasing our database of potential and probable impact craters: 5287 on Pluto, 2287 on Charon, 35 on Nix, and 6 on Hydra.

Craters of the Pluto-Charon System

NASA Technical Reports Server (NTRS)

Robbins, Stuart J.; Singer, Kelsi N.; Bray, Veronica J.; Schenk, Paul; Lauer, Todd R.; Weaver, Harold A.; Runyon, Kirby; Mckinnon, William B.; Beyer, Ross A.; Porter, Simon;

Evidence for rapid topographic evolution and crater degradation on Mercury from simple crater morphometry

NASA Astrophysics Data System (ADS)

Fassett, Caleb I.; Crowley, Malinda C.; Leight, Clarissa; Dyar, M. Darby; Minton, David A.; Hirabayashi, Masatoshi; Thomson, Bradley J.; Watters, Wesley A.

2017-06-01

Examining the topography of impact craters and their evolution with time is useful for assessing how fast planetary surfaces evolve. Here, new measurements of depth/diameter (d/D) ratios for 204 craters of 2.5 to 5 km in diameter superposed on Mercury's smooth plains are reported. The median d/D is 0.13, much lower than expected for newly formed simple craters ( 0.21). In comparison, lunar craters that postdate the maria are much less modified, and the median crater in the same size range has a d/D ratio that is nearly indistinguishable from the fresh value. This difference in crater degradation is remarkable given that Mercury's smooth plains and the lunar maria likely have ages that are comparable, if not identical. Applying a topographic diffusion model, these results imply that crater degradation is faster by a factor of approximately two on Mercury than on the Moon, suggesting more rapid landform evolution on Mercury at all scales.