Investigating Creep of Ground ice as a Cause of Crater Relaxation in Martian High-latitude Softened Terrain

Details Investigating Creep of Ground ice as a Cause of Crater Relaxation in Martian High-latitude Softened Terrain Investigating Creep of Ground ice as a Cause of Crater Relaxation in Martian High-latitude Softened Terrain

May 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...p31a11t&link_type=abstract

American Geophysical Union, Spring Meeting 2001, abstract #P31A-11

Mathematics

Logic

1823 Frozen Ground, 3210 Modeling, 5420 Impact Phenomena (Includes Cratering), 5470 Surface Materials And Properties, 6225 Mars

Scientific paper

Ever since Mariner 9 mapped Mars in 1971-72, researchers have commented on the curious "softened terrain" that exists poleward of 35 - 40 deg. latitude. In impact craters this softening is expressed as flat crater profiles, degraded rims and floor infill among larger, older craters. It is linked with evidence of ground ice in the upper hundreds of meters of soil at higher latitudes, with depths to the top of the ice less than 100-200 m (Squyres et al., 1992). We have applied finite-element analysis to investigate the possible formation of softened craters by creep of ice-rich terrain. Our models simulate the relaxation of 2 km and 20 km diameter craters in layers of 30% ground ice and 70% soil between 0.5 and 2.0 km thick overlying bedrock. They incorporate recent laboratory measurements of the rheological parameters for dust/water ice mixtures undergoing both dislocation creep and grain size dependent creep (Durham et al., 1997; 2000). Our simulations produce features similar to those observed in craters in the softened terrain on Mars, especially rounded ledges at the base of the walls of large craters. These results are particularly striking because our models were for present martian ambient conditions, yet they produced the above deformation on timescales less than ~100,000 years. This implies that extreme softening would be produced even today if ground ice were present near the surface. Mangold et al. (1999) found that viscosity greatly increases when the ice content of dust/ice mixtures is less than 28%, thus, our results may imply an upper limit on the ice content in present day soils at non-polar latitudes. We will use our simulations to constrain the conditions under which softening could have occurred early in Mars' history and to establish current conditions under which crater morphologies are not degraded. Durham W.B. (1997), 16293-16302. Durham W.B. (2000) Second Intl. Conf. on Mars Polar Sci. and Exploration, LPI Contribution #1057, 28-29. Mangold N. (1999) LPSC, XXX, #1016. Squyres S. (1992) in Mars, Ed. H Kieffer, Univ. Arizona Press, Tucson, 523-554.

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