Implications of the Anomalous Cratering Record of the South Layered Deposits

Details Implications of the Anomalous Cratering Record of the South Layered Deposits Implications of the Anomalous Cratering Record of the South Layered Deposits

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p41a0889m&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #P41A-0889

Other

5420 Impact Phenomena (Includes Cratering), 5764 Surfaces, 6020 Ice, 6225 Mars

Scientific paper

The polar layered deposits are among the youngest/most-active surfaces on Mars and play a critical role in the transfer and storage of volatiles on a global scale. The surface exposure ages and modification rates and processes are key to understanding how these deposits respond over time to the independent changes in orbital elements. Through our cratering studies of the south polar layered deposits (SPLD) we have identified over 150 definite craters and characterized their distribution, depths and sizes. There are also three (and possibly four) distinct secondary fields on the SPLD that we have separately characterized. We have utilized the MOC and THEMIS visible datasets and the gridded MOLA products in this research, all referenced to a Mars GIS. Here, we present our findings concerning the isolated background craters on the SPLD which are well removed from those obvious secondary fields and whose morphology is suggestive of a hypervelocity origin. There are anomalous aspects to these SPLD craters. The population of craters greater than 1 km diameter exhibit generally different properties than the population of craters smaller than this. Anomalous and puzzling properties of these populations which must be recognized inorder to infer the exposure ages and modification processes which have characterized these surfaces include: 1. The distribution of craters with diameters > 1km is highly non-uniform with a distinct clustering of these larger craters over about half of the SPLD (especially longitudes 180-240E). Furthermore, that population of craters >1km diameter is extremely shallow with depths-to-diameter generally less than 0.02. The smaller-sized population, in contrast, is more uniformly distributed over the SPLD and exhibits a full spectrum of depth-to-diameters up to 0.25 These differences in geographic distribution and in morphology imply at least one quite recent resurfacing episode and at least one major unconformity in the lower portion of the SPLD. 2. Although some isolated small craters exhibit unusually low depth-to-diameter ratios, which may indicate they are low velocity secondaries in origin, many others exhibit clear hypervelocity impact morphology, e.g., circular outlines, raised rims, bowl shapes and, in some cases, even ejecta blankets and secondary craters. 3. Size-frequency distributions (SFDs) provide a means to date a surface if the crater production rate is known and follows a predictable power law relationship. The SFD of our SPLD craters however does not follow a predicted power law which may indicate an unmodeled size-dependant modification process. The Log Abundance/ Log Frequency exponent is only - 0.7 to -1.0, well below even the usually accepted "primary" production exponent of -2.0. 4. Our investigations into the northern polar layered deposits reveal far fewer craters than for the SPLD. We have located only 4 impact craters (all smaller than 450 meters across) despite searching a comparable geographic area. Based on the area we have searched so far we estimate that the total population (greater than a few 10s of meters across) will not exceed about 25 impact craters. We will elaborate the above points in more detail and use them to create a Preliminary Chronology of the Polar Layered Deposits.

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