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Mars Orbiter Laser Altimeter (MOLA) elevation measurements for 23 different impact craters and 12 different long runout landslides show rampart ridges on Martian fluidized ejecta flows are higher relief than those on Martian landslides. We propose a conceptual model to explain this height difference that is based on the effects of the impact and ejecta emplacement process on the development of ejecta ramparts. Our model explains the relatively high relief of the distal ramparts as well as the particle size distribution that is inferred from thermal inertia measurements. In this model, impact events produce ejecta curtains that advance radially at increasingly higher velocity outward excavating and roughening the surface as they impact. This produces an inertia-driven, ground-hugging ejecta flow composed of primary and secondary ejecta. This flow moves rapidly across the surface following closely behind the impacting ejecta curtain. The ejecta curtain continuously adds impact-generated debris to the flow front that includes large particles, some of which are overridden by the flow, but some accumulate at the flow front. These particles are pushed forward at the flow front as a high-friction "dam" with the accumulating material growing into a relatively high rampart as the ejecta curtain adds more large particles to it. In addition, the impact-roughened surface cause substantial vibrations and shear in the flows moving behind the ejecta curtain. This roughness results in kinetic sieving in the flows that brings large particles to the surface and transports them to the flow front where some are also overridden or accumulate to add the ones already at flow front. We propose that these processes combine to produce the observed high ejecta ramparts. The relatively high velocity of the ejecta flows pushes the load of coarse-grained debris to the top of even high developing ramparts. When the flow halts, it drains back from the accumulated coarse debris at the flow front, leaving a high rampart dominantly composed of large particles. • Ramparts on Martian ejecta flows are higher than ramparts on Martian landslides. • Martian ejecta flows are considerable thinner than Martian landslides of a given volume. • Ejecta emplacement processes are likely to be the cause the high relief ejecta ramparts. [ABSTRACT FROM AUTHOR]

Mons Hansteen is a relatively high-albedo, well-known lunar ``red spot'' located on the southern margin of Oceanus Procellarum (2.3°S, 50.2°W). It is an arrowhead-shaped (∼ 25 km on a side), two-layer mesa with a small cone-shaped massif on its north edge formed by three morphologically and compositionally distinct geologic units. These units were emplaced in three phases over nearly 200 million years. The oldest (∼3.74 Ga), Hilly–Dissected unit, composed of high-silica, and low-FeO content materials formed a low, steep sided mesa. The materials of this unit erupted mainly from vents along northeast- and northwest-trending sets of fractures. The Pitted unit, which comprises the upper-tier mesa, is composed of high-silica and even lower-FeO content materials. This material was erupted at ∼ 3.5 Ga from numerous closely spaced vents (i.e., pits) formed along closely spaced northeast-southwest-trending sets of fractures. At nearly the same time, eruptions of lower silica and higher FeO materials occurred on the north flank of Mons Hansteen at the intersection of two major fractures to produce the North Massif unit. The eruptions that created the North Massif units also produced materials that thinly blanketed small areas of the Hilly-Dissected and Pitted units on the north flank of Mons Hansteen. Also at nearly the same time (i.e., ∼ 3.5 Ga), basalt flows formed the surrounding mare. Each unit of Mons Hansteen appears to be mantled by locally derived ash, which only modestly contaminated the other units. The morphology of Mons Hansteen (especially the Pitted unit) suggests a style of volcanism where only a relatively small amount of material is explosively erupted from numerous, closely spaced vents. [ABSTRACT FROM AUTHOR]

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-explosive 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 characteristics 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. [ABSTRACT FROM AUTHOR]

Low-Aspect-Ratio Layered Ejecta (LARLE) craters are a unique landform found on Mars. LARLE craters are characterized by a crater and normal layered ejecta pattern surrounded by an extensive but thin outer deposit which terminates in a sinuous, almost flame-like morphology. We have conducted a survey to identify all LARLE craters ⩾1-km-diameter within the ±75° latitude zone and to determine their morphologic and morphometric characteristics. The survey reveals 140 LARLE craters, with the majority (91%) located poleward of 40°S and 35°N and all occurring within thick mantles of fine-grained deposits which are likely ice-rich. LARLE craters range in diameter from the cut-off limit of 1km up to 12.2km, with 83% being smaller than 5km. The radius of the outer LARLE deposit displays a linear trend with the crater radius and is greatest at higher polar latitudes. The LARLE deposit ranges in length between 2.56 and 14.81 crater radii in average extent, with maximum length extending up to 21.4 crater radii. The LARLE layer is very sinuous, with lobateness values ranging between 1.45 and 4.35. LARLE craters display a number of characteristics in common with pedestal craters and we propose that pedestal craters are eroded versions of LARLE craters. The distribution and characteristics of the LARLE craters lead us to propose that impact excavation into ice-rich fine-grained deposits produces a dusty base surge cloud (like those produced by explosion craters) that deposits dust and ice particles to create the LARLE layers. Salts emplaced by upward migration of water through the LARLE deposit produce a surficial duricrust layer which protects the deposit from immediate removal by eolian processes. [ABSTRACT FROM AUTHOR]

Abstract: We propose a numerical model for the formation of the closely-spaced pits found in the thin, ejecta-related deposits superposed on the floors, interior terrace blocks, and near-rim ejecta blankets of well-preserved martian impact craters. Our model predicts the explosive degassing of water from this pitted material, which is assumed to originally be water-bearing, impact melt-rich breccia at the time of deposition. This process is analogous to what occurred in the fall-out suevite deposits at the Ries impact structure in Germany. At Ries, impact heating of water-bearing target material resulted in the rapid degassing of its water and other volatiles. The martian environment plays an important role in enhancing the effects of this degassing by increasing the flow-speed of the escaping gas. The high flow-rate of gas through particulate materials, such as suevite, tends to quickly form segregation channels or vent pipes, similar to those found in the Ries deposits. These pipes act as conduits for the efficient high-speed escape of the gas and small clasts that it entrains. Escaping gas and entrained clasts abraded and eroded the conduit walls, flaring them to form pits above a network of pipes. [Copyright &y& Elsevier]

The Northeastern Oceanus Procellarum (NE-OP) study area is a patchwork of lava flows that range in model age from 1.4–3.5 Ga (average age for all count areas is 2.3 Ga), but whose FeO and TiO 2 contents deviate little. The intermediate TiO 2 content values (4.0–6.8 wt%) exhibited by the mare in this region represent material that is underrepresented in the current lunar sample collection. The model ages in the study region are bimodal (~2.2 Ga and ~3.0+ Ga), with eruption of lava flows at the Chang'E-5 landing site occurring at ~3.0 Ga. By comparison, other investigators estimate the model age of the Chang'E-5 site to be ~1.2 to 1.6 Ga. We find preliminary evidence that differences in measurement methodology may lead to disparate model ages and explain the difference in predicted model age of the Chang'E-5 site. • Northeastern Oceanus Procellarum is a patchwork of basaltic flows with a wide range of ages & relatively uniform geochemistry. • Chang'E-5 radiometric sample age will serve to ascertain sources of crater count errors, and explain disparate model ages. • Chang'E-5 landing site crater density is unknown, indicating a fundamental problem with uncodified measurement methodology [ABSTRACT FROM AUTHOR]

Detailed morphologic and morphometric analysis of the radial grooves on Martian layered ejecta indicates that the grooves fall into three groups that correlate with different types of layered ejecta craters. We collected over 16,000 width and length measurements of 2402 radial grooves on the inner ejecta layers of 36 relatively fresh, layered ejecta craters. These include seven single layered ejecta craters (SLE), seven double layered Type-2 (DLE Type-2), ten double ejecta layer Type-1 (DLE-Type-1), and twelve multiple ejecta layer (MLE) test craters. These test craters are in the diameter range 8.3 km – 38.8 km for latitudes ranging from 45oS to 54oN. Radial grooves resembling longitudinal grooves in landslides throughout the solar system occur on all ejecta layers of SLE, DLE Type-2, and MLE craters (i.e., SDM grooves). This leads us to propose that SDM grooves are structures formed in flowing ejecta by the same processes that produce longitudinal grooves in landslides (e.g., shear and divergent flow). In contrast, the radial grooves on the inner and outer ejecta layers of DLE Type-1 are morphologically and morphometrically different from one another and from SDM grooves. The only type of geophysical flows that form grooves similar to grooves on the inner ejecta layers of DLE Type-1 craters are ones produced by high velocity (i.e., supersonic) blast surges associated with explosive volcanic eruptions (e.g., that occurred during the May 18th1980 eruption of Mount St. Helens USA). This leads us to propose that these Martian grooves are produced by high-speed outflow of particles and gas towards the last phase of crater formation. Furthermore, we propose that this high speed flow is generated when volatile-rich target materials slide into the crater cavity to mix with hot impactite. This generates a fuel-coolant explosion that creates a blast surge that subsequently produces the grooves on the inner ejecta layer. In addition, as the velocity of the outflow drops to subsonic outward of the inner ejecta layer, both erosion and deposition processes operate that result in deposition of a thin layer of material that contains channel-like grooves that are unique to this ejecta layer of this type of crater. This thin deposit is found on the outer ejecta layers, and beyond (on the terrain surrounding fresh DLE Type-1 craters). The production of these grooves on DLE Type-1 ejecta layers likely destroys and/or masks SDM grooves that may have initially formed on DLE Type-1 ejecta layers when they were initially emplaced as dense ground hugging flows. • We identify three types of radial grooves on Martian layered ejecta craters. • One type is like grooves on landslides, the others arelikely formedlike volcanic blast surge grooves. • Blast surge-like grooves result from rim collapse in volatile-rich terrain.. • The collapse causes mixing and a fuel-coolant explosion like at Ries Crater, and a blast surge. [ABSTRACT FROM AUTHOR]

Evidence suggests that the lobate flow feature that extends ~72 km outward from the western rim of Tsiolkovskiy crater is a long runout landslide. This landslide exhibits three (possibly four) morphologically different parts, likely caused by local conditions. All of these, plus the ejecta of Tsiolkovskiy crater, and its mare fill are approximately of the same crater model age, i.e., ~3.55 ± 0.1 Ga. The enormous size of this landslide is unique on the Moon and is a result of a combination of several geometric factors (e.g., its location relative to Fermi crater), and that Tsiolkovskiy crater was an oblique impact that produced an ejecta forbidden zone on its western side (Schultz, 1976). The landslide formed in this ejecta free zone as the rim of Tsiolkovskiy collapsed and its debris flowed across the relatively smooth, flat floor of Fermi crater. In this location, it could be easily identified as a landslide and not ejecta. Its mobility and coefficient of friction are similar to landslides in Valles Marineris on Mars, but less than wet or even dry terrestrial natural flows. This suggests that the Mars landslides may have been emplaced dry. The high density of small craters on the landslide is likely an illusion caused by the effects of age related differences in regolith thickness on crater morphology, and the presence of the abundant young, circular secondary craters produce by debris ejected from distant fresh craters. • The huge flow associated with the Tsiolkovskiy is a long runout landslide. • This landslide results from a combination of factors. • The landslide formed at ~3.55 ± 0.1 Ga, immediately after Tsiolkovskiy crater. • M and μ are similar to Valles Marineris landslides. • The high density of small craters on the landslide is likely an illusion. [ABSTRACT FROM AUTHOR]

We use topographic data to show that impact craters with pitted floor deposits are among the deepest on Mars. This is consistent with the interpretation of pitted materials as primary crater-fill impactite deposits emplaced during crater formation. Our database consists of 224 pitted material craters ranging in size from ∼1 to 150 km in diameter. Our measurements are based on topographic data from the Mars Orbiter Laser Altimeter (MOLA) and the High-Resolution Stereo Camera (HRSC). We have used these craters to measure the relationship between crater diameter and the initial post-formation depth. Depth was measured as maximum rim-to-floor depth, ( d r ), but we also report the depth measured using other definitions. The database was down-selected by refining or removing elevation measurements from “problematic” craters affected by processes and conditions that influenced their d r /D, such as pre-impact slopes/topography and later overprinting craters. We report a maximum (deepest) and mean scaling relationship of d r = ( 0.347  ±  0.021 ) D 0.537  ±  0.017 and d r = ( 0.323  ±  0.017 ) D 0.538  ±  0.016 , respectively. Our results suggest that significant variations between previously-reported MOLA-based d r vs. D relationships may result from the inclusion of craters that: 1) are influenced by atypical processes ( e.g. , highly oblique impact), 2) are significantly degraded, 3) reside within high-strength regions, and 4) are transitional (partially collapsed). By taking such issues into consideration and only measuring craters with primary floor materials, we present the best estimate to date of a MOLA-based relationship of d r vs. D for the least-degraded complex craters on Mars. This can be applied to crater degradation studies and provides a useful constraint for models of complex crater formation. [ABSTRACT FROM AUTHOR]

Abstract: Recently acquired high-resolution images of martian impact craters provide further evidence for the interaction between subsurface volatiles and the impact cratering process. A densely pitted crater-related unit has been identified in images of 204 craters from the Mars Reconnaissance Orbiter. This sample of craters are nearly equally distributed between the two hemispheres, spanning from 53°S to 62°N latitude. They range in diameter from ∼1 to 150km, and are found at elevations between −5.5 to +5.2km relative to the martian datum. The pits are polygonal to quasi-circular depressions that often occur in dense clusters and range in size from ∼10m to as large as 3km. Pit sizes scale with both the host crater’s diameter and the host deposit size. These pits have subtle raised rims, and unlike primary and secondary impact craters, they lack well-defined ejecta deposits and overlapping stratigraphic relationships. They also lack any sign of any preferential alignment expected of volcanic or tectonic collapse features. Morphologic and stratigraphic evidence in support of an impact origin includes the observation that pitted materials primarily occur as ponded and flow-like deposits on crater floors, behind terraces, and infilling the lowest local topographic depressions atop the ejecta blanket—similar to the distribution of impact melt-bearing bodies on the Moon. Based on the observations and comparisons to terrestrial and lunar analogs, we conclude that the pit-bearing materials are impactite deposits. The presence of these deposits in older craters, where preserved, suggests that they have formed on Mars throughout most of its geologic history; thus, understanding their origin may help to constrain the hydrological and climate history of Mars. [Copyright &y& Elsevier]