P34A-01 INVITED 16:00h
Inferring Martian Surface Properties from Impact Crater Morphologies
The morphologies of Martian impact craters and ejecta reflect the dynamic strength, layered structure, and composition of the target materials. Using the Mars Orbiter Laser Altimeter data, significant differences in the geometrical properties of impact craters are resolved between lowland (Utopia Planitia and Isidis Planitia) and highland (Lunae Planum and Solis Planum) plains. Simple craters are deeper and rim uplifts are higher in the lowlands. The resulting cavity volume differences indicate that the lowland materials are a factor of two weaker than the highlands. Numerical simulations show that specific geometric markers, such as rim uplift height and cavity volumes, are sensitive to the abundance of ground ice, surface strength, and layered structures. I will present comparisons between the geometric observations and numerical simulations to constrain the properties of the surface materials in highland and lowland plains.
P34A-02 16:30h
The Role of Volatiles in Martian Oblique Impact Ejecta Emplacement
We have performed a detailed comparison of the geomorphology of oblique impacts in a dry vacuum (lunar and experimental) with those on Mars, a body with a tenuous atmosphere and apparently substantial volatiles in the crust. The ejecta from oblique impacts in a dry vacuum follow a consistent pattern as the impact angle decreases. Starting at angles of $\sim$20 degrees, an area uprange of the crater develops that has no ejecta (a "forbidden zone"). The shape of the forbidden zone is an outward curving "V" with its apex at the crater rim. As the impact angle becomes more oblique, the amount of ejecta downrange decreases. Between 5 and 10 degrees there is an abrupt transition to a complete lack of downrange ejecta (a second forbidden zone). This downrange forbidden zone is a straight-edged wedge that is always a wider angle than the uprange forbidden zone. There is also a narrow ray extending from the crater wall through the downrange forbidden zone that we interpret as ricocheted impactor material. All of the ejecta in the dry-vacuum craters was emplaced ballistically and exhibits no post-emplacement flow. While all but the smallest Martian craters have rampart ejecta, their ejecta planforms closely resemble those for craters in a dry vacuum. As impact angle decreases, the "curving V" forbidden zone appears uprange and the extent of downrange ramparts decreases. At the lowest impact angles there is an abrupt transition to a downrange straight-edged forbidden zone in the ramparts. Unlike the dry-vacuum craters, Martian craters maintain an elevated uprange rim at even the lowest impact angles. In all cases the downrange rim appears "blown out" by ricocheted material for the lowest angle impacts, but no Martian craters were observed that preserve a downrange ray, presumably because this feature is easily eroded on Mars. The similarity in ejecta patterns between the Martian rampart craters and the ballistically emplaced dry-vacuum craters suggests that Martian crater ejecta are first ballistically emplaced. Ramparts then form as a result of modest, post-emplacement lahar-like flows that preserve the basic ejecta planform.
P34A-03 16:45h
The Emplacement of the Rampart Ejecta Deposits of Mars Craters
We present a model for the emplacement of a single layer ejecta rampart deposit. The model assumes that the ejecta blanket and rampart were formed by a continuum surface flow beginning at or near the crater rim. There has been abundant evidence for this in the literature for many years and recent THEMIS and MOC imaging continue to support the validity of this assumption. We use this model to determine the basic physical processes necessary to form the distal ramparts in such craters. We find cylindrically symmetric solutions for the flow thickness and velocity of the advancing ejecta flow from volume and momentum conservation equations. The momentum equation has an inertial term and a frictional resistance term that is proportional to the local volumetric flow rate. Only a few basic physical processes are necessary to form sharp distal ramparts. First, there must be sufficient material to form a continuum overland flow. Then, the cylindrical geometry, the inertia of the flow, and local flow resistance combine naturally to form a distal rampart when volume conservation is imposed. We measured nine clearly expressed single layer ejecta deposits for craters with diameters of 3.4 to 17.0 km. The model provides estimates of the emplacement time, the shape of the radial flow depth profiles and the approximate width of the rampart. We find unexpectedly modest velocities of ~20 to 40 m s$^{-1}$, maximum crater rim flow depths of 10 to 30 m, and emplacement durations of 10 to 80 minutes adequately explain the morphologies of selected rampart craters.
P34A-04 17:15h
Characterizing Starting Conditions for Hydrothermal Systems Underneath Martian Craters
Mars is the first place to look for any sign of present or past extraterrestrial life. Its surface shows many features indicative of the presence of surface and sub-surface water, while impact cratering and volcanism have provided temporary and local surface heat sources throughout Mars geologic history. In particular, impact-generated hydrothermal systems could have been some of the most favorable sites for the origin of life on Mars. We present initial results of hydrocode simulations of impacts on Mars aimed at characterizing the initial conditions required for modeling the onset and evolution of a hydrothermal system on the red planet. High-resolution 3D SOVA simulations of the early stages of impact cratering indicate that the amount of melt generated in impacts is sensitive to the impact velocity. In particular, cometary impacts produce 2 to 3 times more melting than asteroidal impacts for roughly the same final crater. The particular distribution of ice in the target (mixed cells versus layering) does not appear to significantly affect the overall shock decay. However, the description of the mixed material target, mixed cells of individual materials (basalt/ice) versus mixed material equation of state, may affect the overall results and requires a more detailed investigation, both theoretical and experimental. Modeling crater collapse is a necessary step to determine the final thermal state of the target underneath. Crater collapse simulations carried out with SALEB show that the combination of shock/plastic heating and the structural uplift of initially deeper strata create a water-bearing zone at depths where water is in the liquid stability field. In the central uplift, the high temperatures cause water to evaporate (steam-driven circulation). The simulations indicate that for a mid-sized crater (rim diameter around 30 km) the hydrothermal circulation is probably restricted to a ``column'' contained well within the final crater.
P34A-05 17:30h
Regional Differences in Gully Occurrence on Mars: A Comparison Between the Hale and Bond Craters Using HRSC, THEMIS-IR and MOC Data
Gullies on Mars indicate liquid water in the recent past. The strong latitude-dependence suggests a climatic control on their formation. However, in some regions many gullies occur in one crater and do not in another crater nearby. This is the case for the Hale (gullies) and Bond (no gullies) crater, respectively. These regional differences have been interpreted as an argument against climatically controlled deposition and melting of volatiles. The formation of gullies on Earth depends on rainfall and/or melting of snow as well as on several parameters such as (1) the presence of steep slopes, (2) sufficient amounts of fines/debris and (3) low or no vegetation. We investigated the Hale/Bond region (325$^\circ$E and 35$^\circ$S) with High Resolution Stereo Camera (HRSC) images, which covers this region with a resolution of $\sim$25 m/pxl in the nadir channel, 100 m/pxl in color (blue, green, red, near infrared) and HRSC stereo data (100 m/pxl). The color images and derived products show regional variances in surface reflectance, which correlate well with differences in the thermophysical surface properties derived from nighttime THEMIS-IR images. The gully regions in the Hale crater show low nighttime temperatures (unconsolidated material), while higher temperature slopes (consolidated material) occur in the Bond Crater. These different surface properties of unconsolidated (gullies) and consolidated (no gullies) material is confirmed by the morphology as analysed in MOC--images. The morphology indicates debris slopes in the Hale crater in contrast to Bond crater where the material seems to consist of cemented mantle deposits. Furthermore, the Bond crater is highly degraded and the rim slopes derived from HRSC stereo data vary between 10$^\circ$ to 20$^\circ$ in contrast to the more pristine Hale crater with slopes in the range of 20$^\circ$ to 30$^\circ$. We conclude that the occurrence of gullies in the Hale/Bond region depends on the distribution of unconsolidated material and/or steep slopes. The regional and local gully distribution is likely to vary because of differences in topography and surface material composition.
P34A-06 17:45h
Hydrothermal Processes in Impact Craters on Mars: Implications From Lonar Crater, India, and Other Craters.
Impact craters played an important role in aqueous and geochemical processes in the near-surface environment of Mars, including chemical transport and soil formation. The formation of large craters on Mars resulted in hydrothermal systems that lasted for tens to hundreds of thousands of years and probably resulted in the mobilization of salts onto the surface of Mars. We have been carrying out extensive studies of impact deposits from several terrestrial analog craters, including Mistastin, Lonar, and Chicxulub using SIMS, XRF, XRD and EMP techniques. Even small craters may have been important for surface processes on Mars based on our recent work at the Lonar, India crater. The relatively small Lonar crater (1.8 km diameter) is one of only two known terrestrial craters to be emplaced in basaltic target rock [1], and our work has led to a new model for the rock component of the martian soil involving a component of hydrothermally altered basalt [2]. Based on the work of Hagerty and Newsom [1], this crater is the smallest known crater with a substantial post-impact hydrothermal system. Our work on the nature of hydrothermal alteration in drill cores from the crater floor of Lonar and Chicxulub includes mobilization of trace elements, such as lithium, beryllium, and boron, based on our new SIMS analyses. In January of 2004 during fieldwork at the Lonar crater we discovered previously unknown alteration zones in the ejecta blanket around the rim of the crater. The ejecta blanket at Lonar extends beyond 1350 m from the rim with discontinuous patches as far as 3000 m. These consist of areas in the ejecta blanket on the order of 20 to 50 meters in extent that are moderately to highly altered. Preliminary analysis shows a depletion of K2O, Na2O and Fe2O3 in the material from the altered zones compared to the fresher basalt blocks. The recent fieldwork at the crater and examination of drill core material from the ejecta blanket suggests that the ejecta blanket is far more important for geochemical transport and hydrothermal alteration in small craters than had previously been realized. The discovery of alteration zones in the ejecta blanket is consistent with evidence for fluid flow (carbonate deposition) in a drill core north of the crater in the ejecta blanket. [1] Hagerty, J.J. and Newsom, H.E. (2003) Meteoritics and Planetary Science, 38, 365-381, [2] Nelson M.J., Newsom, H.E. and Draper D. (2004), in review.