Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p31d..07b&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P31D-07
Physics
5420 Impact Phenomena, Cratering (6022, 8136), 5460 Physical Properties Of Materials, 5470 Surface Materials And Properties, 6221 Europa
Scientific paper
Researchers have used the few, large Europan primary craters to provide strong constraints on the bulk characteristics of the icy shell. Our approach employs this historical and well-proven technique, but with a novel variation, namely using the extensive secondary crater population to probe lateral and vertical variations in surface properties. Several impact parameters are known, or at least well-constrained, for a secondary impact, e.g. impact angle (which, on an airless world such as Europa, is close to the ejection angle of 45 degrees), impactor speed (either known exactly because distance between primary and secondary is known, or well-constrained because the impact speed cannot be greater than Europa's escape velocity), and the impactor source (a ballistic piece of Europa's surface). Furthermore, secondaries are ubiquitous across Europa's surface, occurring in all terrains visible in the high-resolution images. This enables us to investigate what variations between the surfaces exist, if any, at the sensitivity allowed by the data. We discuss our morphology measurements of small craters on Europa (nearly all of which are secondary craters), and the implications those morphologies have to physical properties of the upper tens of meters of Europa's icy shell. To date we've obtained 65 crater profiles on two different terrain types, "background ridged plains" (BRP) and "wide band" (WB), in two different locales. There is scatter within the data, but the best fit depth/diameter (d/D) ratio for the two terrains is 0.17 ± 0.01 for WB, and 0.14 ± 0.01 for BRP. The error only reflects the variance fit to a linear d to D plot, and does not include systematic errors, which we are still quantifying. These errors, plus additional measurements, will reveal whether or not there truly are systemmatic strength differences between these two terrains. Our initial results, however, at least demonstrate that Europa's small craters (diameters between roughly 300~m and 1~km) have a shallow ratio relative to the non-complex Europan primaries (diameters between 2~km and 9~km), which have d/D ~ 2 for all terrain types.
Bierhaus Edward B.
Chapman Clark R.
Schenk Paul
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