P33B-01 INVITED 13:40h
Mars Exploration Rover Field Observations of Impact Craters at Gusev Crater and Meridiani Planum
The Mars Exploration Rovers have provided a field geologist's perspective of a number of impact craters in various states of degradation along their traverses at Gusev crater and Meridiani Planum. Spirit has traversed a generally low relief somewhat rocky plain dominated by shallow circular depressions called hollows. Hollows are typically 1-20 m in diameter, generally have rocky rims characterized by angular and fractured blocks, and smooth soil filled centers. Hollow morphology and size-frequency distribution strongly argue that they are impact craters filled in by eolian material. Three larger craters 100-200 m in diameter imaged by Spirit also have been filled in with soil (depths of only 3-6 m). Bonneville appears to be a relatively fresh 200 m diameter crater impacted into unconsolidated blocky debris (no outcrop). Rock abundance and the largest block size increases by a factor of 2-4 from the discontinuous ejecta, through the continuous ejecta to the rim, suggesting a relatively pristine ejecta blanket. The crater rim is $\sim$5 m high, the crater floor only 10 m deep, and the rubble walls are shallowly dipping ($\sim$$11\deg$). The walls show no signs of mass wasting and eolian material deposited inside is variable, but limited at the bottom to $\sim$1-2 m thickness by protruding boulders. The low depth to diameter ratio of Bonneville and other small fresh craters in and on its walls suggest that they formed as secondary craters, a conclusion also suggested by very low depth/diameter ratios measured from fresh craters in stereo high-resolution orbital images. Far fewer craters have been imaged by Opportunity at Meridiani Planum due to its relatively young surface age and all have been modified by eolian activity with sediment in their interiors and modified rims. Three well-characterized impact craters are Endurance, Eagle, and Fram, which are 150 m, 20 m and 10 m in diameter, respectively. Fram appears freshest with ejecta blocks on the surface, Endurance retains steep interior walls, and Eagle appears the most degraded with a highly modified shallow sand and granule filled interior. All expose sedimentary rocks in their walls. Many other shallow sandy circular depressions (1-10 m in diameter) have been imaged by Opportunity, but most lack raised rims and thus are difficult to distinguish from modified collapse pits along northeast-trending fractures.
P33B-02 14:10h
Comparing Styles of Crater Gradation in Gusev Crater, Meridiani Planum, and on the Earth
The Mars Exploration Rovers Spirit and Opportunity investigation of impact craters yields data enabling comparisons to terrestrial craters and helps constrain the processes responsible for gradation within Gusev crater and Meridiani Planum. In Gusev, the Spirit rover has imaged variably preserved craters ranging between 1 to approximately 200 meters in diameter. These craters and their associated ejecta deposits dominate the surficial landscape, possess raised rims, and are characterized by walls sloping at angles generally less than 10 degrees. By contrast, craters explored at Meridiani Planum are between 10 and 150 m in diameter, formed into bedrock, and possess variably sloped walls that locally exceed the repose angle in Endurance crater. Comparisons between craters in Meridiani and Gusev and simple terrestrial craters provide clues to the amount and processes responsible for their gradation. For example, modification of terrestrial craters in even the most arid environments involves an appreciable water-driven component that typically leads to notched rims and disparate interior and exterior drainage networks. Drainage features evolve and eventually outpace mass wasting as wall gradients are reduced by back wasting and down slope redistribution of debris via incised debris chutes that terminate in a mix of talus and increasing alluvium. Terrestrial craters can also form an efficient trap for eolian sediments that accumulate to 10's of meters in Roter Kamm crater in Namibia, but still do not mask features diagnostic of gradation by water. None of the Martian craters display landforms diagnostic of significant modification by water. Moreover, the absence of debris chutes or obvious talus along crater walls in Gusev coupled with generally well preserved ejecta deposits implies modification by mass wasting is limited and accompanied by local deflation and eolian deposition typically less than 1-2 meters. In Meridiani, some craters appear relatively pristine (Fram) whereas others are more modified by mass wasting and back wasting of the wall (Endurance) and significant deflation of ejecta and eolian infilling (Eagle and Endurance). Differences in gradational signatures associated with these Martian and terrestrial craters are due in part to the limited role played by water in post-Noachian gradation of the Martian craters. Although water may have contributed to local redistribution of fragments to form pavements or other small scale landforms, comparison with terrestrial craters and the predominance of eolian and mass wasting landforms demonstrates water was subordinate in crater modification on Mars.
P33B-03 14:25h
MGS MOC Perspectives on the Role of Fluids in Impact Crater Formation and Modification
The majority of the $ > $75,000 Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) high resolution (0.5-15 m/pix) narrow angle (NA) images acquired to date show impact craters. Many were specifically targeted to address the role of volatiles, particularly groundwater, ground ice and liquid water, in the formation and modification of impact craters. The lobate margins of ejecta associated with martian rampart craters have long been interpreted as resulting from a bolide impacting a fluid-laden substrate. However, meter-scale evidence for the role of fluids in ejecta emplacement is absent: there is no indication that boulders on the ejecta were transported in a flowing fluid nor are there channels on or beyond the ejecta indicating dewatering of a flow. Fluids may be involved in the generation of fan-shaped landforms on crater walls during or closely associated with the impact event in one exceptional Xanthe Terra crater. The fans in `Mojave' Crater (provisionally-accepted IAU name; $7.6\deg$N, $33.0\deg$W) have morphological characteristics consistent with fluid-driven alluvial sedimentation, including tributary and distributary channel networks. The MOC imaging and research effort reveals abundant geologic evidence supporting post-impact modification of martian craters by volatiles, particularly fluids. Meter-scale evidence for glaciation or ice-related modification of craters in the MOC dataset is controversial (e.g. rare tongue-shaped lobes on crater walls, polygonal patterns on high-latitude crater floors). Young gullies on middle- and high-latitude crater walls have attributes consistent with the flow of fluids. An extensive campaign to survey new crater walls, complete MOC NA coverage and monitor known gully sites has been ongoing for $ > $3 years. Examination of $ > $14,000 individual gullies yielded the following results: (a) no new gully formation observed, (b) gullies are located poleward of $30\deg$ (few examples poleward of $27\deg$S), and (c) contrary to the original report, gullies do not exhibit a pole-facing preferential orientation. Subaerial and subaqueous sedimentation occurred within many crater basins. Where a valley enters a crater, if a depositional apron is present it is one of two types: 1) single-lobe apron with concentric steps interpreted to result from mass movement, or 2) single and multiple-lobe aprons with distributary drainage patterns interpreted to be alluvial fans or deltas. Noteworthy is the `Eberswalde' Crater ($24.3\deg$S, $33.5\deg$W) fan complex, interpreted as a lithified and eroded deltaic deposit. Persistent fluvial activity is indicated by an inverted relief cut-off meander loop at this site. Finally, the MOC NA dataset has yielded insights into the `cratered volume' nature of the upper martian crust, including the recognition that craters can be filled, buried and exhumed. Fluvial processes and lacustrine sedimentation were involved in the development of layered strata within craters as well as strata that buries craters.
P33B-04 14:55h
The Unique Characteristics of Double Layered Ejecta Craters on Mars
THEMIS VIS images reveal several unique characteristics of double layered ejecta (DLE) craters on Mars that suggest a strikingly different mode of formation from single layered ejecta (SLE) or multi-layered ejecta (MLE) craters. DLE craters are typically 15 to 25 km in diameter and differ from the other types of Martian craters in the following ways: (1) DLE craters lack secondary craters; (2) ejecta layers of DLE craters lack distal ramparts; (3) flow features within the outer layer of DLE craters suggest a very low emplacement velocity; and (4) radial striations exist only within DLE ejecta, and that these striations cross both the inner and outer ejecta layers. The interior morphology of DLE is also less complex than SLE or MLE layered ejecta craters; DLE craters lack wall terraces and, where present, have only simple central peaks. Previous morphologic analyses of DLE craters proposed that they might have formed in the volatile-rich sediments that are believed to infill areas such as Utopia, Arcadia and Acidalia Planitiae. But our inspection of the THEMIS VIS data set confirms the Viking-based results of Barlow and Perez (JGR-Planets, vol. 108 (E8), doi 10.1029/2002JE002036, 2003) that DLE craters are not uniquely located in the northern plains. We find that DLE craters with nearly identical morphologies also occur within the highlands of Mars, including Hesperia Planum, Icaria Planum, Arabia Terra, Noachis Terra, and Terra Sirenum. A few examples of DLE craters are found at a range of elevations between -5.8 km to +2.7 km relative to the MOLA datum, and within two latitudes belts between 23$\deg$ to 52$\deg$N, and between 29$\deg$ to 46$\deg$S. Thus some other mode of formation apart from impact into volatile-rich sediments of the northern plains needs to be identified. Through our on-going characterization of DLE craters with THEMIS VIS data, we hope to identify the attributes of these craters to help identify their unique mode of formation.
P33B-05 15:10h
Characteristics of Martian Layered Ejecta Craters and Their Implications for Near-Surface Volatiles
Fresh martian impact craters are typically surrounded by layered ejecta blankets which display one of three main morphologies: single layer ejecta (SLE), double layer ejecta (DLE), and multiple layer ejecta (MLE). Analyses of MOC and THEMIS (both visible and infrared) data are providing new insights into the characteristics and distributions of these ejecta morphologies. The SLE morphology is the most common type of ejecta morphology seen on Mars and recent numerical studies are able to replicate many of its observed features through impact into ice-rich targets. DLE craters display two layers of ejecta material with the outer layer emplaced after the inner, as indicated by scour marks across the inner layer. The inner layer is thicker, has lower sinuosity than the outer layer, and does not usually display an obvious distal rampart. The SLE pancake (Pn) ejecta morphologies occur in the same regions as the DLE craters and may simply represent DLE craters whose outer layer has been obliterated. The DLE outer layer may or may not be edged by a distal rampart, and the extent of the outer ejecta layer is among the greatest of any ejecta morphology, indicating a very fluid ejecta flow at the time of emplacement. DLE craters are concentrated in the 40 to 55 degree latitude zone in both hemispheres. In the northern hemisphere, DLE craters are located in topographic lows where layered sedimentary materials may have been deposited. The MLE morphology is usually associated with larger craters than the SLE or DLE morphology, is concentrated along the dichotomy boundary, and displays the highest sinuosity of any ejecta morphology. Central pits are commonly, but not always, associated with MLE craters and suggest the presence of subsurface volatiles. Pedestal (Pd) craters, where the crater and ejecta are perched above the surrounding terrain, are usually small craters found in many of the same locations as DLE craters. We propose that they form by impact into ice-rich fine-grained materials. The surroundings subsequently lose their ice through sublimation, leaving Pd craters elevated above the surroundings.
P33B-06 15:25h
Impact Induced Fluvial Erosion and Ponding on Early Mars
Impacts of moderate sized asteroids result in changes to the early Martian climate which would allow for short-lived, vigorous hydrological cycles. The extent, volume and duration of precipitation, surface runoff and "ponding" which occurs during these impact driven hydrologic cycles varies with the size of the impactor and the solar insolation at the site of the impact. Presented in this paper are simulation results from the NASA Ames Mars Impact General Circulation Model (MIGCM). The MIGCM simulates the effects of impacts on the climate of early Mars and includes a hydrological cycle from which precipitation, runoff rates and ponding can be predicted. Using the precipitation and runoff rates, total fluvial erosion rates are estimated and compared with observations of fluvial crater modification. It is concluded that the transient climates associated with impacts constitute a significant, if not dominate, source of early Martian fluvial erosion.