Recent volcanic resurfacing of Venusian craters

Recent volcanic resurfacing of Venusian craters

Authors:

Whitten et al

Abstract:

Ejecta from impact craters on Venus are a major source of fine-grained materials across the planet, and crater spatial distribution has been studied as a guide to the relative age and resurfacing rates of large regions. Of particular interest is the potential intersection of distal crater deposits and tesserae, highly deformed landforms that may be the oldest materials on Venus. The composition of tesserae is unknown, but is key to understanding whether water played a role in crustal differentiation. Thus, tesserae are ideal sites for future landed missions to identify possible felsic materials, but the short lifespan of surface landers means that efforts must be made to avoid contaminating surface materials from craters. Here we develop a method to detect distal crater ejecta on tessera terrain across Venus using NASA Magellan radar data. Our results show that fine-grained ejecta are unevenly distributed in the tesserae with respect to nearby craters. Many tesserae within a few hundred kilometers of plains craters do not have evidence for thick (greater than 5–10 cm) mantling material, indicating that eolian or mass-wasting processes have moved the debris off the highland ridge slopes. At Sudenitsa Tesserae, within the young Beta-Atla-Themis region, we observe a radar signature of mantling debris, but there is no apparent source crater to which this material can be traced. We infer that the source crater has been resurfaced by volcanic activity within the past 80 m.y., and suggest that similar fine-grained ejecta deposits may have built up over time in other tesserae across Venus.

The Geology (Hadeology) of Pluto and Charon Through the Eyes of New Horizons

The Geology of Pluto and Charon Through the Eyes of New Horizons

Authors:

Moore et al

Abstract:

NASA's New Horizons spacecraft has revealed the complex geology of Pluto and Charon. Pluto's encounter hemisphere shows ongoing surface geological activity centered on a vast basin containing a thick layer of volatile ices that appears to be involved in convection and advection, with a crater retention age no greater than ≈10 Ma. Surrounding terrains show active glacial flow, apparent transport and rotation of large buoyant water-ice crustal blocks, and pitting, likely by sublimation erosion and/or collapse. More enigmatic features include tall mounds with central depressions that are conceivably cryovolcanic, and ridges with complex bladed textures. Pluto also has ancient cratered terrains up to ~4 Ga old that are extensionally fractured and extensively mantled and perhaps eroded by glacial or other processes. Charon does not appear to be currently active, but experienced major extensional tectonism and resurfacing (probably cryovolcanic) nearly 4 billion years ago. Impact crater populations on Pluto and Charon are not consistent with the steepest proposed impactor size-frequency distributions proposed for the Kuiper belt.

Fluvial erosion as a mechanism for crater modification on Titan

Fluvial erosion as a mechanism for crater modification on Titan

Authors:

Neish et al

Abstract:

There are few identifiable impact craters on Titan, especially in the polar regions. One explanation for this observation is that the craters are being destroyed through fluvial processes, such as weathering, mass wasting, fluvial incision and deposition. In this work, we use a landscape evolution model to determine whether or not this is a viable mechanism for crater destruction on Titan. We find that fluvial degradation can modify craters to the point where they would be unrecognizable by an orbiting spacecraft such as Cassini, given enough time and a large enough erosion rate. A difference in the erosion rate between the equator and the poles of a factor of a few could explain the latitudinal variation in Titan’s crater population. Fluvial erosion also removes central peaks and fills in central pits, possibly explaining their infrequent occurrence in Titan craters. Although many craters on Titan appear to be modified by aeolian infilling, fluvial modification is necessary to explain the observed impact crater morphologies. Thus, it is an important secondary modification process even in Titan’s drier equatorial regions.

Geomorphological map of the Afekan Crater region, Titan

Geomorphological map of the Afekan Crater region, Titan: Terrain relationships in the equatorial and mid-latitude regions

Authors:

Malaska et al

Abstract:

We carried out geomorphological mapping in a mid-latitude area surrounding the Afekan Crater region on Titan. We used Cassini RADAR (Synthetic Aperture Radar mode) data as the basemap, supplemented by Cassini RADAR microwave emissivity, Imaging Science Subsystem (ISS) infrared data, Visual and Infrared Mapping Spectrometer (VIMS) spectral images, and topography derived from Synthetic Aperture Radar (SAR). Mapping was done at a spatial scale of 300 m/pixel, which corresponds to a map scale of 1:800,000. We describe multiple terrain units and their spatial relations. We describe five broad classes of units that are in agreement with previous mapping efforts: crater, labyrinth, hummocky/mountainous, plains, and dune terrain classes. We subdivide these into seven crater units, four hummocky/mountainous units, six plains units, and three dunes units. Our results show that plains are the dominant class of terrain unit in Titan’s mid latitudes. Of the plains units, the undifferentiated plains are the largest by total areal extent in the mapped region, accounting for over 45% of the mapped area. We developed a stratigraphic sequence that has the hummocky/mountainous and labyrinth terrains as the oldest units. The observed properties of the hummocky/mountainous terrain are consistent with fractured water ice materials, while the labyrinth terrains are consistent with organic materials. The youngest units are the dune units and streak-like plains units, with the undifferentiated plains units being of intermediate age. The microwave emissivity of the undifferentiated plains and dune units are consistent with organic materials. Given their properties and stratigraphic placement, we conclude that the hummocky/mountainous terrains are most consistent with the presumed crustal materials of Titan. The plains materials are consistent with deposits resulting from the transport and emplacement of organic-rich materials predominantly by aeolian mechanisms. Our geomorphological mapping results are consistent with the equatorial and mid-latitudes of Titan being dominated by organic materials that have been deposited and emplaced by aeolian activity.

Formation of gravel pavements during fluvial erosion as an explanation for persistence of ancient cratered terrain on Titan and Mars

Formation of gravel pavements during fluvial erosion as an explanation for persistence of ancient cratered terrain on Titan and Mars

Authors:

Howard et al

Abstract:

In many terrestrial channels the gravel bed is only transported during rare floods (threshold channels), and rates of erosion are very slow. In this paper we explore how coarse debris delivered to channels on Mars and Titan from erosion may inhibit further erosion once a coarse gravel channel bed develops. Portions of the equatorial region of Titan are fluvially eroded into banded (crenulated) terrain, some of which contains numerous circular structures that are likely highly degraded large impact craters surviving from the late heavy bombardment. No mechanism that can chemically or physically break down ice (likely the most important component of Titans crust) has been unambiguously identified. This paper examines a scenario in which fluvial erosion on Titan has largely involved erosion into an impact-generated megaregolith that contains a modest component of gravel-sized debris. As the megaregolith is eroded, coarse gravel gradually accumulates as a lag pavement on channel beds, limiting further erosion and creating a dissected, but largely inactive, or senescent, landscape. Similar development of gravel pavements occur in ancient mountain belts on Earth, and partially explain the persistence of appreciable relief after hundreds of millions of years. Likewise, coarse gravel beds may have limited the degree to which erosion could modify the heavily cratered terrains on Mars, particularly if weathering were largely due to physical, rather than chemical weathering processes in a relatively cold and/or arid environment.

How Quickly do the Blast Zones of Martian Impact Craters Fade?

Changes in blast zone albedo patterns around new martian impact craters

Authors:

Daubar et al

Abstract:

“Blast zones” (BZs) around new martian craters comprise various albedo features caused by the initial impact, including diffuse halos, extended linear and arcuate rays, secondary craters, ejecta patterns, and dust avalanches. We examined these features for changes in repeat images separated by up to four Mars years. Here we present the first comprehensive survey of the qualitative and quantitative changes observed in impact blast zones over time. Such changes are most likely due to airfall of high-albedo dust restoring darkened areas to their original albedo, the albedo of adjacent non-impacted surfaces. Although some sites show drastic changes over short timescales, nearly half of the sites show no obvious changes over several Mars years. Albedo changes are more likely to occur at higher-latitude sites, lower-elevation sites, and at sites with smaller central craters. No correlation was seen between amount of change and Dust Cover Index, relative halo size, or historical regional albedo changes. Quantitative albedo measurements of the diffuse dark halos relative to their surroundings yielded estimates of fading lifetimes for these features. The average lifetime among sites with measurable fading is ∼15 Mars years; the median is ∼8 Mars years for a linear brightening. However, at approximately half of sites with three or more repeat images, a nonlinear function with rapid initial fading followed by a slow increase in albedo provides a better fit to the fading behavior; this would predict even longer lifetimes. The predicted lifetimes of BZs are comparable to those of slope streaks, and considered representative of fading by global atmospheric dust deposition; they last significantly longer than dust devil or rover tracks, albedo features that are erased by different processes. These relatively long lifetimes indicate that the measurement of the current impact rate by Daubar et al. (Daubar, I.J. et al. [2013]. Icarus 225, 506–516. http://dx.doi.org/10.1016/j.icarus.2013.04.009) does not suffer significantly from overall under-sampling due to blast zones fading before new impact sites can be initially discovered. However, the prevalence of changes seen around smaller craters may explain in part their shallower size frequency distribution.

Are Meteor Impacts on Mars Seasonal?

The current impact flux on Mars and its seasonal variation

Authors:

JeongAhn et al

Abstract:

We calculate the present-day impact flux on Mars and its variation over the martian year, using the current data on the orbital distribution of known Mars-crossing minor planets. We adapt the Öpik–Wetherill formulation for calculating collision probabilities, paying careful attention to the non-uniform distribution of the perihelion longitude and the argument of perihelion owed to secular planetary perturbations. We find that, at the current epoch, the Mars crossers have an axial distribution of the argument of perihelion, and the mean direction of their eccentricity vectors is nearly aligned with Mars’ eccentricity vector. These previously neglected angular non-uniformities have the effect of depressing the mean annual impact flux by a factor of about 2 compared to the estimate based on a uniform random distribution of the angular elements of Mars-crossers; the amplitude of the seasonal variation of the impact flux is likewise depressed by a factor of about 4–5. We estimate that the flux of large impactors (of absolute magnitude H less than 16h ) within ±30° of Mars’ aphelion is about three times larger than when the planet is near perihelion. Extrapolation of our results to a model population of meter-size Mars-crossers shows that if these small impactors have a uniform distribution of their angular elements, then their aphelion-to-perihelion impact flux ratio would be 11–15, but if they track the orbital distribution of the large impactors, including their non-uniform angular elements, then this ratio would be about 3. Comparison of our results with the current dataset of fresh impact craters on Mars (detected with Mars-orbiting spacecraft) appears to rule out the uniform distribution of angular elements.

Double Impact Crater Found in Sweden From Floian/Dapingian Ordovician, Possibly Linked to Proposed Ordovician Meteor Event

Double rainbows have nothing on Earth’s newest dynamic duo: the double crater.

Researchers from the University of Gothenburg have uncovered two impact craters in Jämtland, Sweden, which they believe to have occurred simultaneously around 460 million years ago. Double meteor impacts are rare events, and the discovery in Sweden is the first proven instance of its kind.

“Information from drilling operations demonstrates that identical sequences are present in the two craters, and the sediment above the impact sequences is of the same age. In other words, these are simultaneous impacts,” said Erik Sturkell, professor of geophysics at the University of Gothenburg and a member of the team that found the double crater, in a university statement.

Although these two meteorites struck at the same time, that does not mean they are physically alike, however. One crater measures a massive 4.7 miles in diameter, while the other, which was located nearly ten miles away, was a much a smaller 2,300 feet across.

As for how it happened, it all started in the stars.

“Around 470 million years ago, two large asteroids collided in the asteroid belt between Mars and Jupiter, and many fragments were thrown off in new orbits. Many of these crashed on Earth, such as these two in Jämtland,” said Sturkell.

link.

There are several impacts in North America and another in Estonia that roughly line up.  This hypothesized event is termed the "Ordovician Meteor Event."  They are all supposedly within a million years of each other and I wonder though whether or not they line up given the paleogeography. 

Periodicity Rebuked: No Evidence Sun's Galactic Orbit Corrolates to Terrestrial Cratering, Glacial Events

Investigations into the impact of astronomical phenomena on the terrestrial biosphere and climate

Author:

Feng

Abstract:

This thesis assesses the influence of astronomical phenomena on the Earth's biosphere and climate. I examine in particular the relevance of both the path of the Sun through the Galaxy and the evolution of the Earth's orbital parameters in modulating non-terrestrial mechanisms. I build models to predict the extinction rate of species, the temporal variation of the impact cratering rate and ice sheet deglaciations, and then compare these models with other models within a Bayesian framework. I find that the temporal distribution of mass extinction events over the past 550 Myr can be explained just as well by a uniform random distribution as by other models, such as variations in the stellar density local to the Sun arising from the Sun's orbit. Given the uncertainties in the Galaxy model and the Sun's current phase space coordinates, as well as the errors in the geological data, it is not possible to draw a clear connection between terrestrial extinction and the solar motion. In a separate study, I find that the solar motion, which modulates the Galactic tidal forces imposed on Oort cloud comets, does not significantly influence this cratering rate. My dynamical models, together with the solar apex motion, can explain the anisotropic perihelia of long period comets without needing to invoke the existence of a Jupiter-mass solar companion. Finally, I find that variations in the Earth's obliquity play a dominant role in triggering terrestrial deglaciations over the past 2 Myr. The precession of the equinoxes, in contrast, only becomes important in pacing large deglaciations after the transition from the 100-kyr dominant periodicity in the ice coverage to a 41-kyr dominant periodicity, which occurred 0.7 Myr ago.

Counter argument.

Evidence of a Crater Lake in the Siberian Traps in During the Permian Extinction

An integrated carbon isotope record of an end-Permian crater lake above a phreatomagmatic pipe of the Siberian Traps

Authors:

Fristad et al

Abstract:

The largest mass extinction in Earth history occurred at the end-Permian (~ 252 million years ago) and is marked by a global negative carbon isotope excursion and the onset of Siberian Trap volcanism, prompting diverse hypotheses on the link between flood basalt volcanism, carbon cycle perturbations, and mass extinction. Phreatomagmatic pipes associated with Siberian Trap volcanism have been proposed as conduits for the release of 12C-enriched carbon gases from thermogenic and/or magmatic sources to the end-Permian atmosphere. Some of the pipes have preserved crater-lake sediments of volcaniclastic origin. This study examined the preserved evidence for 12C-enriched carbon release into the Western Oktyabrsk crater in east Siberia from the underlying volcanic basin. We find that the 13C/12C ratio of the carbonate cement, organic matter, and long-chain n-alkanes in the lacustrine crater sediments support the hypothesis that 12C-enriched carbon infiltrated the basal crater sediments and lake water immediately after crater formation. The values and trends of δ13CCarb, δ13CTOC, and δ13Cn-alkanes in the crater sediments are consistent with 12C-enriched carbon with isotopic values similar to that of carbon sourced from thermogenic and/or 12C-enriched magmatic sources. This implies that carbon release through the pipes in the Tunguska Basin may explain the source of the global negative carbon isotope perturbations, and their coincidence with Siberian Trap volcanism, at the end-Permian.

Central Pit Craters on Mars Formed Explosively?

Evidence for an explosive origin of central pit craters on Mars

Authors:

Williams et al

Abstract:

Kilometer-scale pits are nested in the centers of many impact craters on Mars as well as on icy satellites. They have been inferred to form in the presence of a water–ice rich substrate; however, the process(es) responsible for their formation is still debated. Previous models invoke origins by either explosive excavation of potentially water-bearing crustal material, or by subsurface drainage of meltwater and/or collapse. If explosive excavation forms central pits, pit-derived ejecta should be draped around the pits, whereas internal collapse should not deposit significant material outside pit rims. Using visible wavelength images from the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) instruments and thermal infrared images from the Odyssey Thermal Emission Imaging System (THEMIS) instrument, we conducted a survey to characterize, in detail, the global population of central pits in impact craters ⩾10 km in diameter. We specifically examined the morphology and thermophysical characteristics of the pits for evidence of pit ejecta. Our analysis of thermal images suggests that coarse-grained materials are distributed proximally around many central pits on the floors of their host craters. The decrease in average grain size with distance from pit rims is consistent with pit-derived ejecta. These observations and interpretations better support an explosive origin for central pits on Mars than they do an origin of subsurface meltwater drainage and collapse of the overlying substrate. A major weakness to previous explosive central pit formation models is the inability for them to form pits late enough in the impact process to be preserved. To address this, we present an alternative “melt contact model” where a central uplift brings ice-bearing substrate into contact with impact melt to generate steam explosions and excavate central pits during the impact modification stage. Theoretical calculations show that more than enough thermal energy is available via impact melt from the host crater to form central pits by steam explosions, and such explosions would require only a modest amount (2–6% by volume) of uplifted water–ice. We therefore propose that central pits on Mars could have formed explosively by the interaction of impact melt and subsurface water–ice.

Age Determination of Martian Linear Surface Features

Age determination of linear surface features using the Buffered Crater Counting approach – Case studies of the Sirenum and Fortuna Fossae graben systems on Mars

Authors:

Kneissi et al

Abstract:

Buffered Crater Counting (BCC) offers a possibility to determine ages of linear/curvilinear surface features that provide no or only very limited surface areas for the conventional crater counting approach. In this study we applied the BCC analysis to two tectonic fault systems, Fortuna Fossae and a subsection of Sirenum Fossae. We compared BCC results with age estimates derived from conventional crater counting on the surrounding geologic units and investigated to what extent crater ejecta blankets can be used for determining the stratigraphic placement of craters pre- or post-dating the formation of linear features. Furthermore, we introduce a new functionality of the CraterTools software for ArcGIS which allows for a user-friendly semi-automatic application of the otherwise time-consuming procedure of BCC analysis. The software provides the resulting crater size-frequency data in a standard format, which can be read and analyzed in the CraterStats analysis software.

Our case studies showed that the BCC approach provides equivalent or even more precise age results compared to the conventional stratigraphic approach. Here, we found that the investigated section of Sirenum Fossae is younger than previously thought. The derived formation age from the BCC analysis is View the MathML source3.44-0.25+0.1Ga which corresponds to Late instead of Early Hesperian. Fortuna Fossae formed shortly after the emplacement of its now-fractured geologic host unit (Late Hesperian). Ages derived from BCC analysis vary between View the MathML source3.53-0.11+0.06Ga and View the MathML source3.50-0.11+0.07Ga. Furthermore, we recommend the use of crater ejecta blankets to position them in the stratigraphic sequence in order to improve crater statistics. However, the accuracy of the results depends on the extent and preservation state of the continuous ejecta blankets in the region of interest. Thus, the applied buffer width has to be chosen carefully according to investigated crater sizes and local observations.

Volcanically Embayed Craters on Venus

Volcanically embayed craters on Venus: Testing the catastrophic and equilibrium resurfacing models

Authors:

Ivanov et al

Abstract:

Two major types of volcanic units, older regional plains and younger lobate plains, make up ∼50% of the surface of Venus and represent different epochs of volcanism. The abundance of impact craters partially embayed from the exterior by each of these two types of units permits the testing of the key points of the model of equilibrium resurfacing. The proportion of craters embayed by the older regional plains is ∼3%, which requires the typical size of a volcanic resurfacing event to be ∼2700 km (∼25° of angular diameter) in the framework of the equilibrium model. These event dimensions are inconsistent with the quasi-random spatial distribution of the craters. The proportion of craters embayed by younger lobate plains is 33%, which can be achieved if the characteristic size of the resurfacing event is less than ∼160 km (∼1.5° of angular diameter). Events of this size do not disturb the character of the spatial distribution of craters. We conclude that the style of volcanic resurfacing on Venus has changed significantly during its observable portion of the geologic history. During the global volcanic regime when regional plains were emplaced, volcanism acted in large regions and the process of formation of regional plains was more intensive than accumulation of impact craters. This led to the very small proportion of embayed craters (∼3%). Later, during the network-rifting and volcanism regime (emplacement of lobate plains), volcanic sources were localized at distinctive centers, the net volcanic intensity decreased and became comparable to the rate of accumulation of craters, which resulted in much larger percentage (33%) of craters embayed by lobate plains.

Evidence of ice From the Secondary Craters on Arcadia Planitia, Mars

Expanded secondary craters in the Arcadia Planitia region, Mars: Evidence for tens of Myr-old shallow subsurface ice

Authors:

Viola et al

Abstract:

A range of observations indicates widespread subsurface ice throughout the mid and high latitudes of Mars in the form of both pore-filling and excess ice. It is generally thought that this ice was recently emplaced and is not older than a hundred thousand to a few millions of years old based on ice stability and orbital-induced climate change. We analyze the distribution of subsurface ice in Arcadia Planitia, located in the northern mid latitudes, by mapping thermokarstically expanded secondary craters, providing additional evidence for extensive excess ice down to fairly low latitudes (less than 40°N). We further infer the minimum age of this subsurface ice based on the ages of the four primary craters that are thought to be the source of a large portion of these secondaries, which yields estimates on the order of tens of millions of years old – much more ancient than anticipated. This estimated ancient age suggests that ice can be preserved in the shallow subsurface for long periods of time, at least in some parts of Arcadia Planitia where expanded secondary craters are especially abundant. We estimate the amount of ice lost to sublimation during crater expansion based on measurements of expanded secondary craters in HiRISE Digital Terrain Models. The loss is equivalent to a volume of ice between ∼140 and 360 km3, which would correspond to a global layer of 1–2.5 mm thick. We further argue that much more ice (at least 6000 km3) is likely preserved beneath the un-cratered regions of Arcadia Planitia since significant loss of this excess ice would have caused extensive terrain dissection and the removal of the expanded secondary craters. Both the loss of ice due to secondary crater expansion and the presence of this ice today have implications for the martian climate.

Sudbury Crater in Canada Confirmed to be Orosirian/Statherian Paleoproterozoic Cometary Impact

On the track of the elusive sudbury impact: geochemical evidence for a chondrite or comet bolide

Authors:

Petrus et al

Abstract:

Siderophile and lithophile trace element data for 69 samples from the Sudbury impact crater fill (Onaping Formation) and quartz diorite offset dikes help constrain the sources of the established moderately elevated platinum group element signature associated with the impact structure. The siderophile element distribution of the crater fill requires contributions from bulk continental crust, mafic rocks and a chondritic component. A mantle component is absent, but the involvement of mid to lower crust is implied. After considering post-impact hydrothermal alteration, melt heterogeneity, and mafic target admixture, the projectile elemental ratios were determined on a more robust data subset. Chondrite discrimination diagrams of these ratios identify an ordinary or enstatite chondrite as the most probable source of meteoritic material in the Sudbury crater fill. However, the relative and absolute siderophile element distributions within the impact structure as well as bolide size models are congruent with the bolide being a comet that had a chondritic refractory component.

How Formation of Gullies on Martian Craters

Quantitative analysis of the morphology of martian gullies and insights into their formation

Authors:

Yue et al

Abstract:

The process of formation of observed geologically recent gully features on Mars has remained a topic of intense debate since their discovery. In this study, we performed quantitative morphological analysis on certain parameters of gullies from different settings, such as crater walls, terraces, and sand dunes, on the martian surface in addition to the Meteor and Xiuyan craters on the Earth. The morphometric parameters were measured for cross profiles, which were extracted along each gully at certain intervals. Some interesting relationships among the parameters were determined, which could provide us a comprehensive understanding of the morphologies of the gullies’. The results show that strong correlations exist among those parameters, and the gullies are morphometrically similar, except for a scale difference in different geologic settings. The morphometric similarity implies that they were probably formed by some common processes. On the other hand, the morphometric differences indicate that the processes may have played different roles in the formation of the gullies. The formation of gullies on the Earth crater walls was heavily affected by surface flow and slippage, and pre-existing fractures and faults were also very influential in their formation. We propose that gullies in martian crater walls and terraces should have a similar formation mechanism, and they can probably account for most of gullies appearing on crater walls. The morphometric differences between the gullies in sand dunes and other gully types are probably a result of the disparity in lithological settings, which have significant influence on erosion ability even for the same agents.

Hunting for Iron From the South Pole-Aitken Basin Impactor

Surveying the South Pole-Aitken basin magnetic anomaly for remnant impactor metallic iron

Authors:

Cahill et al

Abstract:

The Moon has areas of magnetized crust (“magnetic anomalies”), the origins of which are poorly constrained. A magnetic anomaly near the northern rim of South Pole-Aitken (SPA) basin was recently postulated to originate from remnant metallic iron emplaced by the SPA basin-forming impactor. Here, we remotely examine the regolith of this SPA magnetic anomaly with a combination of Clementine and Lunar Prospector derived iron maps for any evidence of enhanced metallic iron content. We find that these data sets do not definitively detect the hypothesized remnant metallic iron within the upper tens of centimeters of the lunar regolith.

Understanding Mars' LARLE Craters

Origin of the outer layer of Martian low-aspect ratio layered ejecta craters

Authors:

Boyce et al

Abstract:

Low-aspect ratio layered ejecta (LARLE) craters are one of the most enigmatic types of Martian layered ejecta craters. We propose that the extensive outer layer of these craters is produced through the same base surge mechanism as that which produced the base surge deposits generated by near-surface, buried nuclear and high-explosion detonations. However, the LARLE layers have higher aspect ratios compared with base surge deposits from explosion craters, a result of differences in thicknesses of these layers. This is probably caused by the addition of large amounts of small particles of dust and ice derived from climate-related mantles of snow, ice and dust in the areas where LARLE craters form. These deposits are likely to be quickly stabilized (order of a few days to a few years) from eolian erosion by formation of duricrust produced by diffusion of water vapor out of the deposits.

High Resolution Crater Counting of Martian Terrains

Minimum Effective Area for High Resolution Crater Counting of Martian Terrains

Authors:

Warner et al

Abstract:

The acquisition of high-resolution imagery for the surface of Mars has enabled mapping of spatially limited (order of less than 103 km2) landforms such as alluvial fans, deltas, and lacustrine deposits that are targets for exploration due to their association with liquid water. It is essential for our understanding of the planet’s geologic and climate history therefore to place these landforms within the global chronostratigraphic context. Here, we analyze both the statistical variability in the cratering pattern as well as the influence of small crater resurfacing on crater counting small landforms. We identified and counted craters (diameter (D) > 200 m) on four type terrains using Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) imagery that span the Noachian, Hesperian, and Amazonian epochs. The counts from each location include a region covering 10,000 km2, ten 1,000 km2 subsets of that larger area, and approximately one hundred 100 km2 samples. The data demonstrate significant variation in the crater size frequency and derived model ages across a single terrain type for the 100 km2 samples. The crater size frequency at this area scale varies across a single, uniform geologic unit by up to a factor of 2 to 3 on the four different terrains. At 1,000 km2, the local pattern variations that are relevant at the 100 km2 scale become less important and the age variations are tighter. In all four terrain cases, the 10,000 km2 and 1,000 km2 samples capture distinct crater populations (km-sized craters) that formed before and after resurfacing event(s). However, due to the relatively high mean distance between km-sized craters, the 100 km2 size area samples more commonly than not exclude a statistically significant sample at the kilometer size range, masking important information about the pre-resurfacing history of the terrain. We therefore suggest that due to the effect of pattern variability in cratering over 100 km2 and the susceptibility of smaller craters to resurfacing, crater counts derived from small area samples are suspect to major uncertainties.

Lunar Polar Craters may NOT Have Much Ice

Lunar polar craters – Icy, rough or just sloping?

Authors:

Eke et al

Abstract:

Circular Polarisation Ratio (CPR) mosaics from Mini-SAR on Chandrayaan-1 and Mini-RF on LRO are used to study craters near to the lunar north pole. The look direction of the detectors strongly affects the appearance of the crater CPR maps. Rectifying the mosaics to account for parallax also significantly changes the CPR maps of the crater interiors. It is shown that the CPRs of crater interiors in unrectified maps are biased to larger values than crater exteriors, because of a combination of the effects of parallax and incidence angle. Using the LOLA Digital Elevation Map (DEM), the variation of CPR with angle of incidence has been studied. For fresh craters, CPR ∼0.7 with only a weak dependence on angle of incidence or position interior or just exterior to the crater, consistent with dihedral scattering from blocky surface roughness. For anomalous craters, the CPR interior to the crater increases with both incidence angle and distance from the crater centre. Central crater CPRs are similar to those in the crater exteriors. CPR does not appear to correlate with temperature within craters. Furthermore, the anomalous polar craters have diameter-to-depth ratios that are lower than those of typical polar craters. These results strongly suggest that the high CPR values in anomalous polar craters are not providing evidence of significant volumes of water ice. Rather, anomalous craters are of intermediate age, and maintain sufficiently steep sides that sufficient regolith does not cover all rough surfaces.