Crater Density Research Articles

Inversion of relief on Mars

Relief inversion has been invoked to explain a number of geomorphic features of the martian surface. Terrestrial relief inversion occurs when former depressions become elevated because their fill is more resistant to erosion than the surrounding terrain. It is a common product of long-term landscape evolution on Earth, especially in relatively stable intra-cratonic settings and flat, or near flat lying successions. The inverted relief will preserve relicts of former land surfaces and is therefore older than the surrounding terrain. Relief inversion can occur by a range of processes, including infill of depressions by intrinsically resistant material, selective secondary cementation via diagenesis and weathering, or surface armouring. We examine a number of possible cases of inverted relief on Mars that appear to have formed by these three processes. We suggest that the most likely cementing agents for surface induration are iron oxides, silica, and sulfates. Possible cementation mechanisms include fluid mixing during regional groundwater flow, cooling of hydrothermal or basinal fluids as they near the surface, and evaporation and sublimation of near surface water. Wind action appears the most common erosive process on Mars capable of the regional landscape lowering necessary for relief inversion to occur, unlike on Earth where both deflation and runoff are important. Preliminary crater densities of selected features show that the tops of the proposed inverted relief have considerably more craters than the surrounding plains, as is predicted by the inversion hypothesis. More accurate dating of inverted surfaces and the adjacent areas may provide a simple way of measuring the degree of erosion over time in at least some areas of Mars.

Titan's young surface: Initial impact crater survey by Cassini RADAR and model comparison

Cassini's Radar instrument has mapped over 10% of Titan's surface through the T18 flyby in August 2006 and has detected remarkably few impact craters. Three impact craters have been conclusively identified and named, and a handful of unconfirmed candidate impact structures are documented. These results indicate that the overall crater density is a factor of ∼5 higher than Earth, and has a size‐distribution slope very similar to Earth's indicating rapid obliteration of smaller structures by erosion and burial. The paucity of small craters was expected from atmospheric shielding considerations, but the rarity of medium (20–100 km diameter) craters – some 30–300 times less abundant than on nearby Saturnian satellites – implies rapid resurfacing. Erosion and burial appear to be the dominant means of crater obliteration, with viscous relaxation playing a lesser role. Models of the 20–100 km impact crater production rate suggest an overall crater retention age of 100 Myr–1 Gyr, the same as the corresponding age for the Earth.

MARSIS radar sounder evidence of buried basins in the northern lowlands of Mars

A hemispheric dichotomy on Mars is marked by the sharp contrast between the sparsely cratered northern lowland plains and the heavily cratered southern highlands. Mechanisms proposed to remove ancient crust or form younger lowland crust include one or more giant impacts, subcrustal transport by mantle convection, the generation of thinner crust by plate tectonics, and mantle overturn following solidification of an early magma ocean. The age of the northern lowland crust is a significant constraint on these models. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on the European Space Agency's Mars Express spacecraft is providing new constraints on the martian subsurface. Here we show evidence of buried impact basins ranging in diameter from about 130 km to 470 km found over approximately 14 per cent of the northern lowlands. The number of detected buried basins >200 km in diameter indicates that the lowland crust is ancient, dating back to the Early Noachian epoch. This crater density is a lower limit because of the likelihood that not all buried basins in the area surveyed by MARSIS have been detected. An Early Noachian age for the lowland crust has been previously suggested on the basis of a large number of quasi-circular topographic depressions interpreted to be evidence of buried basins. Only a few of these depressions in the area surveyed by MARSIS, however, correlate with the detected subsurface echoes. On the basis of the MARSIS data, we conclude that the northern lowland crust is at least as old as the oldest exposed highland crust. This suggests that the crustal dichotomy formed early in the geologic evolution of Mars.

Rays and secondary craters of Tycho

The large, fresh crater Tycho in the nearside lunar highlands has an extensive system of bright rays covering approximately 560,000 km 2, containing dense clusters of secondary craters. Examination of crater densities in several clusters shows that Tycho produced almost 10 6 secondary craters larger than 63 m diameter. This is a lower limit, because small crater densities are reduced, most likely by mass wasting. We estimate a crater erasure rate of 2–6 cm/Myr, varying with crater size, and consistent with previous results. This process has removed many small craters, and it is probable that the original number of secondary craters formed by Tycho was higher. Also, we can only identify distant secondaries of Tycho where they occur in bright rays. Craters on Mars and Europa also formed large numbers of secondaries, but under possibly ideal conditions for spallation as a mechanism to produce high-velocity ejecta fragments. The results from Tycho show that large numbers of such fragments can be produced even from impact into a heavily fragmented target on which spallation is expected to be less important.

SAR mapping of Burfellshraun: A terrestrial analog for recent volcanism on Mars

Images of the Northern plain on Mars have revealed up to 1500 km long lava flows. The low density of impact craters on the lava flows implies that these lava flows were formed during the most recent volcanic activity on Mars. Estimates of the ages of the flows are controversial but generally between 10 and 100 Myr. Parts of the lava flows show smooth kilometer‐sized plates that appear to have rafted on the moving lava flow. We have found a terrestrial analogue with similar features, the Burfellshraun lava flow in Northern Iceland. This is the only known lava flow on Earth where kilometer‐sized rafting plates are observed and constitute the dominating feature. Using a combination of airborne remote sensing data and field observations we have studied the emplacement and physical properties of the flow. On the basis of our observations we have reconstructed the sequence of events that led to the disruption of the lava surface. Our observations show that a ∼45 km2 lava lake, holding about 0.7 km3 of lava, catastrophically collapsed releasing a flood of lava. During the collapse the crust of the lava lake was broken into plates up to a kilometer in width and a few meters thick that floated downstream for almost 10 km.

Stratigraphy and evolution of basalts in Mare Humorum and southeastern Procellarum

Abstract— We have studied the mare basalts of Mare Humorum and southeastern Procellarum (30°W–50°W, 0°–40°S). One hundred and nine basaltic units have been identified from differences in their FeO wt% and TiO2 wt% content, and variations in crater densities. Crater counting and reference to isotopically dated Apollo samples have provided an age for 33 major units. Some evidence for three distinct periods of volcanic activity has been found. We found that the large unit in the middle of Mare Humorum is the oldest in the basin. This supports the suggestion that the oldest central unit sank causing the lithosphere to bend and create dykes through which lava flowed to produce the outer units. No evidence of a trend in FeO wt% and TiO2 wt% content against time is found within Mare Humorum. There appears to be no lateral trend of basalts in terms of FeO and TiO2 wt% over the entire area with time. An increase in FeO content with time is found in the 33 major units and there is some evidence for an increase in TiO2 in the same units. A correlation between FeO wt% and TiO2 wt% content is evident when all 109 units are compared. A notable feature of this correlation is a sharp increase in gradient of TiO2 wt% content when the FeO wt% content rises above about 17%.

Secondary craters of Tycho: Size‐frequency distributions and estimated fragment size–velocity relationships

We examined the size‐frequency distribution and the spatial distribution of secondary craters around the lunar crater Tycho. Secondary crater diameters were found to range from 0.55 to 4.0 km, and their distance from Tycho to range from 130 to 370 km. The diameter and ejection velocity of the secondary‐forming fragments were also estimated from the crater size and the distance from the primary using the scaling relationships for cratering. The power law index of the size‐frequency distribution of the Tycho secondary craters ranges from 3.3 to 4.0, the same as the steep slope of the typical size‐frequency distribution for lunar craters of less than 4 km diameter. The decay index of secondary crater densities with distance from the primary craters is not constant and is small at large distances (less than 2.5 for r > 2.5R). The total number of secondaries forming ejecta fragments is more than twice larger than for other lunar primaries previously studied. These results imply that there are many small secondary craters in fields far from the primary crater. A dominant contribution of secondaries to the size‐frequency distribution of all lunar craters is thus strongly suggested. The index range of 2.6 to 3.0 for estimated fragment size distributions is not only close to other natural and experimental craters, but also to those of blocks on asteroids. The relative fragment size distributions are independent of the ejection velocity. This result is the first observational confirmation of the theoretical assumption, and agrees with a recent result from laboratory experiments.

Impact craters on regional plains on Venus: Age relations with wrinkle ridges and implications for the geological evolution of Venus

A global assessment of impact craters on Venus ≥30 km in diameter shows that 146 craters are completely or partially superposed on the regional plains that dominate the surface and typically are deformed by wrinkle ridges. In this study, we found that 10 of these craters are deformed by wrinkle ridges (pre‐WR), 93 craters are superposed on wrinkle ridges (post‐WR), and 43 craters were found to be ambiguous cases. We compared our findings with the results of a similar study by McGill (2004) done for craters of all sizes but for ∼1/3 of the surface of Venus. We found that the percentage of pre‐WR craters found by McGill (2004) among the unambiguous cases (15%) is close to our percentage (10%), but we are in disagreement on the classification of a significant number of specific craters and in this analysis we describe the major reason for the disagreements. We then summarize published data on the crater densities for different plains on Venus. We assess Monte Carlo modeling of volcanic resurfacing, which competed with the ongoing formation of impact craters. On the basis of these data and analyses, we conclude that the mean age of the regional plains is close to the mean global surface age of Venus and that the potential time span of regional plains emplacement was relatively short. We interpret our results as evidence that wrinkle ridging on Venus was a distinct tectonic phase that occurred rather early in post‐regional‐plains time, probably within the first 10%. Although the exact duration remains uncertain, regional and global stratigraphic relationships strongly support the hypothesis that wrinkle ridging on Venus predominantly occurred during a specific phase of history. The geologic record of wrinkle ridging and post‐plains tectonic evolution of volcanic plains on the Moon and Mars show interesting similarities to those of Venus.

Probing structural elements of small buried craters using ground-penetrating radar in the southwestern Egyptian desert: Implications for Mars shallow sounding

We report results from a field survey performed on a recently discovered impact field in the southwestern Egyptian desert, using a 270 MHz Ground-Penetrating Radar (GPR). This hyperarid region has significant similarities to the Martian heavily eroded mid-latitude cratered terrains in terms of crater density, size, and geomorphology. Profiles across small-buried craters revealed a coherent sequence of tilted layers constituting the cratonic infill resulting from aeolian deposits. In the intercrater areas the radargram revealed a poorly-defined subsurface stratigraphy and the presence of shallow structural elements associated with potential evidences of the consequences of the shock effects, i.e., faulting, fractures, and chaotic bedrock. The radar-penetration depth varied from 2 to 15 m, depending mainly on the amplitude of the volume and multiple scattering in the subsurface, caused by fractures and debris created by the impacts. We conclude that mid-frequency GPR onboard future Martian rovers can successfully perform similar structural mapping.

Seismic resurfacing by a single impact on the asteroid 433 Eros

Impact cratering creates a wide range of topography on small satellites and asteroids. The population of visible craters evolves with impacts, and because there are no competing endogenic processes to modify the surface, determining the various ways younger craters add to or subtract from the population is a fundamental aspect of small-body geology. Asteroid 433 Eros, the most closely studied small body, has regions of substantially different crater densities that remain unexplained. Here we show that the formation of a relatively young crater (7.6 km in diameter) resulted in the removal of other craters as large as 0.5 km over nearly 40 percent of the asteroid. Burial by ejecta cannot explain the observed pattern of crater removal. The limitation of reduced crater density to a zone within a particular straight-line distance through the asteroid from the centre of the large crater suggests degradation of the topography by seismic energy released during the impact. Our observations indicate that the interior of Eros is sufficiently cohesive to transmit seismic energy over many kilometres, and the outer several tens of metres of the asteroid must be composed of relatively non-cohesive material.

Control of impact crater fracture systems on subsurface hydrology, ground subsidence, and collapse, Mars

Noachian layered materials are pervasively exposed throughout the highlands of Mars. The layered deposits, in places many kilometers thick, exhibit impact craters of diverse morphologic characteristics, ranging from highly degraded to pristine, most of which formed during the period of heavy bombardment. In addition, exhumed impact craters, ancient channels, and fluvial and alluvial fans are visible in the layered deposits through MOC imagery. These features are more abundant in Noachian terrains, which indicates relatively high erosion rates during ancient Mars that competed with heavy meteoritic bombardment. The Noachian layered materials are thus expected to contain numerous buried impact craters in various states of preservation. Here, we propose that impact craters (buried and exposed) and associated fracture systems dominate the basement structural fabric of the ancient highlands and that they have significantly influenced the hydrogeology. Diversity in the occurrence of high and low densities of impact craters and associated fracture systems controls the magnitude of the local effects of magmatic‐driven hydrothermal activity. In and surrounding the Tharsis region, for example, the formation of chaotic terrains (the source regions of the circum‐Chryse outflow channel system) and a large diversity of collapse structures, including impact crater moats and pit chains, appear to be the result of enhanced hydrothermal activity.

A Contribution to the Understanding of Elementary Wear Mechanisms of Rubber Filled Compounds

Abstract Tire rubber treads wear patterns that are described as craters can appear in mild conditions. This article demonstrates that craters can be recreated on a laboratory tribometer, which has been developed especially for rubber filled compounds testing. A local analysis of the worn surfaces has shown that the dimension, density and location of craters are closely related to carbon black agglomerates found within compounds. The viscoelastic properties of the agglomerates have been found by carrying out nanoindentation tests: these give a Young's modulus of around 300 MPa and a elasto-plastic limit between 7% and 14%. Finally, an original system is proposed to explain the crater formation on the release of internal stresses due to an agglomerate mechanical plasticizing of the rubber surface. Electronic microscope observations and a finite element analysis show that the agglomerates are flattened by the indentor contact and tilted, to create craters on the worn surface.

Surface defects created by 20 keV Xe ion irradiation of Ge(1 1 1)

Surface defects created by 20 keV Xe ion irradiation of Ge(1 1 1) are investigated by scanning tunneling microscopy (STM). One bilayer deep vacancy islands and surface craters with a depth of more than two bilayers are observed. The creation of the surface craters, which is caused by thermal spike of 20 keV Xe ions, is an unusual event with a yield of ∼0.1%. The areal density of surface craters increases linearly with increasing ion fluence at 215 °C and increases sublinearly with increasing ion fluence at 275 °C.

THEMIS characterization of the MER Gusev crater landing site

Gusev crater, previously interpreted as the depocenter for the Gusev‐Ma'adim Vallis fluvio‐lacustrine system, is a proposed landing site for one of the Mars Exploration Rovers (MER). Here we use new remote‐sensing data from the Thermal Emission Imaging System (THEMIS) supplemented by data from the Thermal Emission Spectrometer (TES), Mars Orbiter Camera (MOC), and Mars Orbiter Laser Altimeter (MOLA) to characterize the geology of Gusev crater. Thermal infrared data from THEMIS and TES were used to map thermophysical units on the basis of relative albedos and diurnal temperature variations. THEMIS and MOC visible images were used to map unit morphologies and to estimate crater density ages. MOLA data were used to identify unit contacts and stratigraphic relationships. Various data were then combined to construct a new surface unit map and stratigraphy for units on the floor of Gusev. Seven surface units were identified in Gusev, mostly Hesperian in age, but with two showing evidence of later modification and redistribution. Five or more surface units and layering are present within the MER‐A landing ellipse, attesting to the geologic diversity of this site. Surface units show features that could be consistent with fluvio‐lacustrine, aeolian, and/or volcanoclastic deposition, but the spatial resolution of visible/infrared data does not allow for the identification of unambiguous volcanic or fluvio‐lacustrine textures. However, a MER landing in Gusev may provide the opportunity to analyze multiple units, distinguish rock types, examine stratigraphic relationships, and shed light on the ancient depositional environment.

Possible Juventae Chasma subice volcanic eruptions and Maja Valles ice outburst floods on Mars: Implications of Mars Global Surveyor crater densities, geomorphology, and topography

This article discusses image, topographic, and spectral data from the Mars Global Surveyor (MGS) mission that provide new information concerning the surface age, geomorphology, and topography of the Juventae Chasma/Maja Valles system. Our study utilizes data from two instruments on board MGS: images from the Mars Orbiter Camera (MOC) and topography from the Mars Orbiter Laser Altimeter (MOLA). Within Maja Valles we can now observe depositional bars with megaripples that unequivocally show catastrophic floods occurred in the channel. Viking impact crater densities indicated the chasma and channel floor areas were all one age (late Hesperian to Amazonian); however, MOC data indicate a marked difference in densities of small craters between Juventae Chasma, Maja Valles, and the channel debouchment area in Chryse Planitia basin. Although other processes may contribute to crater variability, young resurfacing events in the chasma and episodes of recent erosion at Maja Valles channel head may possibly account for the disparate crater densities along the chasma/channel system. Relatively young volcanic eruptions may have contributed to resurfacing; as in Juventae Chasma, a small possible volcanic cone of young dark material is observed. MOC data also indicate previously unknown interior layered deposit mounds in the chasma that indicate at least two periods of mound formation. Finally, MOLA topography shows that the entire floor of the chasma lies at the same elevation as the channel debouchment area in Chryse basin, resulting in a 3‐km‐high barrier to water flow out of the chasma. Blocked ponded water would rapidly freeze in the current (and likely past) climate of Mars. For catastrophic flow to occur in Maja Valles, some process is required to melt ice and induce floods out of the chasma. We suggest subice volcanic eruption and calculate estimates of water discharges and volumes that these eruptions might have produced.

Utilization of the THEMIS visible and infrared imaging data for crater population studies of the Meridiani Planum landing site

The Mars Odyssey Thermal Emission Imaging System (THEMIS) acquires both visible and thermal infrared images of the Martian surface. This study shows that the daytime infrared images, as well as the visible images, may be utilized successfully for crater population studies. The THEMIS data bridge the resolution gap between higher‐resolution Mars Orbiter Camera narrow‐angle images and lower‐resolution Viking Orbiter images and THEMIS will provide unprecedented global coverage at 100‐m resolution by the end of the nominal Mars Odyssey mission. The crater‐population data in this work show evidence for the flattening of the mid‐sized (∼63 m < D < 1km) crater density curve due to erosion and/or deposition and the upturn of the smaller‐diameter (<∼90 m‐diameter) crater curve signaling the hiatus of major crater obliteration in Meridiani Planum about 1–10 Ma.

Asymmetrical distribution of rayed craters on the Moon

The synchronous rotation of the satellite ought to cause a spatial variation in the cratering rate over its surface. The crater density is expected to be maximum at the apex of the orbital motion and decrease with the increase of the angular distance from the apex. The ratio of the density at the apex (maximum) to that of the antapex (minimum) depends on the average encounter velocity of impactors to the satellite. Although the Moon is also in a state of the synchronous rotation, it has been supposed that the asymmetry in the crater density on the Moon can be hardly observed. We report here a spatial variation in the density of rayed craters on the Moon, which may be associated with the synchronous rotation. Since the lifetime of a ray is relatively short (<0.8 billion years), the results provide information on recent impacts. Rayed craters are identified on Clementine 750-nm mosaic images. We investigate craters in a lower latitude zone from 42°N to 42°S. To avoid an effect of material difference on the ray preservation, we analyze craters on the highland from 70°E to 290°E in east longitude. A total of 222 rayed craters larger than 5 km in diameter are identified in the study area of about 1.4×10 7 km 2. The average density of rayed craters on the leading side is substantially higher than that on the trailing side. The crater density decreases as a sinusoidal function of the angular distance from the apex. The observed ratio of the density at the apex to that at the antapex is about 1.5. The ratio suggests that recent craters on the Moon are formed mainly by near-Earth asteroids rather than comets with higher encounter velocities.

On estimating contributions of basin ejecta to regolith deposits at lunar sites

Abstract— We have developed a quantitative model for predicting characteristics of ejecta deposits that result from basin‐sized cratering events. This model is based on impact crater scaling equations (Housen, Schmitt, and Holsapple 1983; Holsapple 1993) and the concept of ballistic sedimentation (Oberbeck 1975), and takes into account the size distribution of the individual fragments ejected from the primary crater. Using the model, we can estimate, for an area centered at the chosen location of interest, the average distribution of thicknesses of basin ejecta deposits within the area and the fraction of primary ejecta contained within the deposits. Model estimates of ejecta deposit thicknesses are calibrated using those of the Orientale Basin (Moore, Hodges, and Scott 1974) and of the Ries Basin (Hörz, Ostertag, and Rainey 1983). Observed densities of secondary craters surrounding the Imbrium and Orientale Basins are much lower than the modeled densities. Similarly, crater counts for part of the northern half of the Copernicus secondary cratering field are much lower than the model predicts, and variation in crater densities with distance from Copernicus is less than expected. These results suggest that mutual obliteration erases essentially all secondary craters associated with the debris surge that arises from the impacting primary fragments during ballistic sedimentation; if so, a process other than ballistic sedimentation is needed to produce observable secondary craters. Regardless, our ejecta deposit model can be useful for suggesting provenances of sampled lunar materials, providing information complementary to photogeological and remote sensing interpretations, and as a tool for planning rover traverses (e.g., Haskin et al. 1995, 2002).

An Engineering Model for the Phobos Surface

We give a summary of data on the characteristics of the surface relief and physical properties of the Phobos soil (regolith). This summary formed the basis for the engineering model recommended for the Phobos-Grunt engineering activities. Apart from the selection, systematization, and analysis of published data, we carried out special investigations. We (1) studied the characteristics of the surface relief within the potential landing ellipse, (2) analyzed the crater distribution over the prospective landing site, (3) estimated the size-frequency distribution of rocky blocks on the surface, and (4) calculated the thermal regime of the regolith surface layer at the prospective landing site. The paper consists of two sections: (1) a description of the planned landing site and (2) a description of the Phobos regolith. The first section contains data on the gravitational field and dynamical characteristics of Phobos; it also provides the currently available photomaps and digital topographic maps of Phobos that characterize the surface of the planned landing site. Then, the morphological characteristics of the surface, including the impact crater density distribution, are given. The second section provides current estimates of the structure and thickness of the Phobos regolith, including estimates of the size-frequency distribution of rocky blocks along the surface, as well as calculations of the thermal regime of the surface regolith layer at the prospective landing site.

The geology of 433 Eros

Abstract— The global high‐resolution imaging of asteroid 433 Eros by the Near‐Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft has made it possible to develop the first comprehensive picture of the geology of a small S‐type asteroid. Eros displays a variety of surface features, and evidence of a substantial regolith. Large scale facets, grooves, and ridges indicate the presence of at least one global planar structure. Directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object. As with other small objects, impact craters dominate the overall shape as well as the small‐scale topography of Eros. Depth/diameter ratios of craters on Eros average ∼0.13, but the freshest craters approach lunar values of ∼0.2. Ejecta block production from craters is highly variable; the majority of large blocks appear to have originated from one 7.6 km crater (Shoemaker). The interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of Phobos. This lack of mechanical boundaries, and the abundant evidence of regolith in nearly every high‐resolution image, suggests a gradation in the porosity and fracturing with depth. The density of small craters is deficient at sizes below ∼200 m relative to predicted slopes of empirical saturation. This characteristic, which is also found on parts of Phobos and lunar highland areas, probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith. Eros displays a variety of regolith features, such as debris aprons, fine‐grained “ponded” deposits, talus cones, and bright and dark streamers on steep slopes indicative of efficient downslope movement of regolith. These processes serve to mix materials in the upper loose fragmental portion of the asteroid (regolith). In the instance of “ponded” materials and crater wall deposits, there is evidence of processes that segregate finer materials into discrete deposits. The NEAR observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic. Future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the NEAR Shoemaker spacecraft.