Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u14a..08m&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #U14A-08
Mathematics
Logic
0710 Periglacial Processes, 6020 Ices, 6055 Surfaces, 6225 Mars
Scientific paper
The Phoenix spacecraft landed in a high-latitude region of Mars rich in subsurface water ice and geologic landforms that evolved in the presence of this ice. Ground ice (subsurface ice and icy soil) at depths of a few centimeters has been found by excavation at the landing site. These ice-table depths are largely in line with pre-landing predictions. As expected, the ice-table depth exhibits some variability due to topographic and surface material effects, though other processes may play a role. The frozen ground is generally densely cemented by ice and widespread under and around the spacecraft. These ice-table characteristics and the current martian climate are ideal for formation of polygonally-patterned ground by seasonal thermal- contraction cracking. Indeed, polygonal ground is ubiquitous throughout the region and northern plains of Mars as seen from orbit. Surface imaging by the Phoenix spacecraft provides the first close up view of these well developed features. These polygons are characterized by perimeter troughs and central mounds. The trough depths range from centimeter to a couple-of-decimeter scale. The dominant polygons outlined by these troughs are small relative to typical terrestrial forms, only a few meters across. As on Earth, small centimeter-scale furrows created by surface fines infiltrating into sub-surface cracks, strongly suggest that the polygon-forming processes are currently active. Variability of the depth along individual troughs and small furrows cross cutting polygon mounds indicate a complex history of polygon formation on more than one size scale. While rock abundances are low, heterogeneously-scattered small rocks are common. Rock distributions suggest that rocks are preferentially concentrated in polygon troughs, consistent with thermal- contraction-based cryoturbation. Few ventifacts are observed suggesting that aeolian erosion of the surface is slower than cryoturbation. The size, morphological characteristics, and development of polygons are consistent with the presence of shallow ground ice and permanently subfreezing temperatures throughout geologically recent times.
Arvidson Ray E.
Heet T.
Lemmon Mark Thomas
Malin Michael C.
Marshall James J.
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