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N. G. Barlow (NAU)
Most models of Mars' history suggest that it has become colder and drier over time. Much of the water that the planet originally contained on its surface has migrated to subsurface reservoirs of ice and probably liquid at greater depth. The amount of volatiles is believed to strongly influence the emplacement of the layered (fluidized) ejecta morphologies, with higher concentrations of volatiles giving rise to larger radial extents of the ejecta blankets. To determine if the concentrations of volatiles in these subsurface reservoirs have varied over time, we are conducting a study to determine if the radial extent of these ejecta morphologies correlate with the preservational state of the crater. The revised ``Catalog of Large Martian Impact Craters" contains a new scale for crater preservation (ranging from ``ghost" craters at 0 to pristine craters at 7) and information on the ejecta mobility (EM) ratio (EM = maximum radial extent of ejecta/crater radius). At present we have analyzed 570 craters with preservational categories 4 through 7 in quadrangles MC 8 through 11 (0 to 30 N, 0 to 180 W). The analysis has looked at terrain units from throughout Martian history and has considered the data as a group as well as for separate terrain units. Craters with preservational categories 4 through 5 show a slight decrease in EM, but erosional effects are quite severe for this group and the results are likely not providing statistically valid information about volatile concentration during this time. Craters in higher preservational categories show a relatively constant EM ratio regardless of terrain, averaging between 1.5 and 1.6. Since these ejecta morphologies are not severely affected by erosion, this suggests that the concentration of subsurface volatiles at the depths sampled by the impact craters has not decreased substantially since these craters formed (primarily since Late Noachian, according to crater statistical studies).
This work was supported by NASA Mars Data Analysis Grant NAG58265.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.