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Analysis of Regional Variations in Martian Rampart Crater Morphology Purpose: The purpose of this original research project is to compare rampart craters and specifically rampart crater ejecta topography over several different Mars surface types, in order to determine what differences in crater morphology may indicate about surface and subsurface composition, specifically in areas which may have contained ice or liquid water. Background: The morphology of the ejecta blankets of many Martian impact craters exhibits a lobate structure, with pronounced distal ridges bordering the ejecta facies. These craters have been termed rampart craters. The specific process which emplaces this type of ejecta is uncertain, but it is likely a combination of some subsurface volatile component [1] and the Martian low pressure atmosphere [2]. Craters can provide valuable information to determine surface and subsurface properties. Since the cratering process itself is generally consistent and well understood, examining the differences in cavity and ejecta volumes (among other features) between craters of comparable size and age (estimated from state of erosion and frequency of superimposed craters) can elicit information about the surface in which the crater was formed. Rampart craters are especially appropriate for this type of analysis for several reasons. Firstly, ejecta volumes of non-rampart craters can be difficult or impossible to measure due to the mobility of the ejecta (creating problems differentiating between ejecta and surface features), whereas rampart crater ejecta is constrained into a single (in most cases) clearly bounded surface flow which protects the ejecta blanket from aeolian erosion. Previous research by the author [3] has shown that on average, the volume of this flow fully accounts for the cavity volume of the crater. Secondly, rampart craters are a current focus of Mars remote sensing [e.g. 4, 5] due to their potential to indicate areas which may have contained subsurface ice or liquid water at the time of impact. Previous research by the author has also shown that there is a relationship between surface composition and cratering efficiency, which can be quantified through measurement of crater diameter, cavity volume, and ejecta area and volume. Therefore, it should be possible to make inferences about the structure and/or composition of an impact surface based on comparative measurements of rampart crater features. Methodology: The control for this project will be a subset of the 120 rampart craters which were measured during a previous study by the author [3]. Although all of the craters in the study were located on a consistent geologic unit, it was found that regional differences within the surficial geology of this field area produced significant variations in ejecta volume ratios. Therefore, the control will be restricted to a subset of craters which exhibited ejecta volume to cavity volume ratios of 1.0 +/- 50%, which removes the most significant outliers while preserving a large enough population of craters to form an adequate control group (~80 craters). The craters from this study are being used as a control because the geologic unit into which they impacted is interpreted as consisting of volcanic flows [6, 7], more specifically low viscosity lavas, meaning that the surface is largely basaltic, which is a material factor in the ejecta bulking calculation used to determine ejecta volume. Furthermore, this unit has a wide latitudinal coverage, and closely resembles lunar maria in composition, topography, and crater density [7]. The field area which I plan to study will consist of areas such as outflow channels and lakebeds, and other regions where possible fluvial activity has been identified, especially in recent observations from the High Resolution Stereo Camera aboard the European Space Agency’s Mars Express Orbiter [e.g. 4], in the hope that significant differences in crater morphology may provide information about surface composition and the potential for subsurface water or ice. The measurement system for this project is based on a system which was developed previously by the author [3] to analyze rampart craters in three dimensions. However, the efficiency and consistency will be improved in this project through the use of new software, specifically ESRI ArcGIS, which can be adapted to correctly project a variety of areoreferenced planetary data sets. Craters in the field area will be identified for measurement on Viking image mosaics. For each crater, a shapefile will be created to delineate the rim of the crater and the perimeter of the ejecta rampart in ArcMap. These shapefiles will be used to catalog the location of each crater, as well as to perform area and length calculations including crater rim perimeter, crater diameter (derived from rim perimeter to compensate for irregularities), ejecta area, and rampart length (sinuosity). Crater cavity volume will be calculated using Mars Orbiter Laser Altimeter data though the 3D Analyst Extension for ArcMap, based on a definition of a preimpact surface derived from the altitudes of points outside of the ejecta blanket. Results of all of these measurements will be input into a spreadsheet template that has been pre-coded to perform calculations on the ejecta blanket profiles. To determine ejecta volume, a minimum of two representative profiles will be taken through the ejecta blanket using MOLA profiles of the crater downloaded through the USGS ISIS interface. The raw profile data can be imported directly into the spreadsheet, which automatically separates each profile into three sections: rim, ejecta blanket, and rampart, based on customizable definitions of minimum and maximum slopes. Total ejecta volume will be calculated by adding the cross sectional area of the rim multiplied by the crater rim perimeter to the cross sectional area of the rampart multiplied by the rampart perimeter to the average ejecta thickness multiplied by the ejecta area, correcting for radial proportionality in the average thickness. The measured ejecta and cavity volumes will then be corrected to account for ejecta bulking, structural rim uplift [8], and down-driven material [9] to determine the final volumes. Anticipated Results: The primary goal of the Mars Exploration Program as defined by NASA is to determine the current distribution and geologic history of water on Mars [10]. Based on analysis of the control group, I anticipate that the rampart craters measured in the field area of this project will exhibit characteristic trends and relationships which can be used as a proxy from which to interpolate surface composition. For example, best fit power regression equations of the control group reveal statistically significant trends which can be interpreted as defining rampart crater topographic geometry in the low viscosity volcanic basalt of the control area, since an integral part of the volumetric correction process is the density of the surface material. Using the bulking coefficient of basalt, over the 80 crater control data set, ejecta volume is related to apparent crater diameter (Df) by the function 0.0347 Df 2.504, with a correlation coefficient of 0.93. I anticipate that craters in other geologic areas will exhibit significantly different trends and relationships, especially when different bulking coefficients are used, derived from either best fit data or data from other sources such as multispectral or neutron imaging. In addition to forming more accurate and detailed geologic maps of the Martian surface, this will lead to a better understanding of the scale and evolution of Martian water, which will be an important consideration to future unmanned and manned missions to Mars. Citations: [1] Carr, M.H. et al. (1977) JGR, 82, 4055-4065. [2] Schultz, P.H. and Gault, D.E. (1977) JGR, 84, 7669-7687. [3] Ackerman, E.S. (2005) LPSC XXXVI, 2151. [4] Neukom, G. et al. (2005) GeoRL, 32, L10202. [5] Baloga, S.M. et al. (2006) LPSC XXXVII, 1309. [6] Scott, D.H. and Carr, M.H. (1978) USGS Map I-1083. [7] Greely, R. et al. (1987) USGS Map I-1802. [8] Melosh, H.J. (1989) Impact Cratering, |