Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p41b1370c&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P41B-1370
Mathematics
Logic
5415 Erosion And Weathering, 5464 Remote Sensing, 5470 Surface Materials And Properties, 6225 Mars
Scientific paper
Dune fields on Mars offer an opportunity to investigate the nature of eroded sediments and their interactions with the atmosphere. We examined 20 dune fields in Valles Marineris (VM) from the Mars Global Digital Dune Database [Hayward et al., 2007] to identify significant trends in composition, thermophysical properties, morphology and origin. Dune fields were examined in terms of: slopes, albedo, dust index, thermal inertia and the corresponding derived particle size. We have used image data from the Mars Reconnaissance Orbiter (MRO) instruments CTX [McEwen et al., 2006] and HiRISE [Malin et al., 2007] to establish geologic context for the dune fields, and in particular, to examine their relationships to neighboring geologic units. In general, VM dune fields display greater topographic relief and closer proximity to their inferred source regions than is typical for dune fields elsewhere on Mars. These dunes have a relatively high TES-derived thermal inertia mean value (394 Jm-2K-1 s-1/2, units hereafter assumed), which corresponds to ~1000 μm grains [Pelkey et al., 2001] or very coarse sand sizes. In contrast, typical non-VM dunes have a lower thermal inertia value of ~250, corresponding to ~350 μm grains. To investigate this more closely, high-resolution THEMIS-derived thermal inertia maps were created [Putzig et al., 2004]. CTX and HiRISE visible images revealed that bedrock outcrops are commonly found within dune fields, erroneously elevating the TES thermal inertia values over the ~3x5-km TES footprint. However, even after excluding intra-dune outcrop areas using higher-resolution THEMIS data, several VM dune fields have anomalously high thermal inertia values (>500) compared with non-VM dune fields. It is possible that the high thermal inertia values are indicative of indurated (fossilized) dune surfaces, rather than large individual grain sizes. Coprates Chasma contains a concentration of 6 dune fields both within the main chasm and in depressions to the south. The southern fields are comprised of isolated barchanoid dunes, in close proximity to or atop wall material that has been deposited by mass wasting. In the main chasm, previously unidentified barchans composed of large grain sizes, as inferred from THEMIS thermal inertia, are found in CTX images within spur and gully wall units 2-3 km above the canyon floor. TES spectrum of these dunes indicates a basaltic composition, suggesting that the nearby wall units, also thought to be of a basaltic composition [McEwen et al., 1999], could be the source of the dune sediments. Future MRO observations of this area may resolve whether these dune sediments are locally derived. Ganges Chasma has the highest concentration of dunes in VM, including the largest (~6000 km2) non-polar dune field on Mars. These dunes are found surrounding the sulfate-bearing Ganges Mensa and other layered deposits. In one example, a light-toned yardang containing CRISM-detected hydrated sulfates [Pelkey et al., 2007] has shed fans of fine-grained material, contributing sediment to the area. Dune slipface orientation would suggest a dominant wind direction blowing to the west at the last time of dunes activity. This corresponds with the more recent deposit of lighter-toned material down-wind and atop the dark-toned sand sheets, as observed in HiRISE and THEMIS thermal inertia images. These lighter-toned materials, inferred to be composed of sulfate grains (~350 μm), form bright ripples which gradually disappear away from the yardang. Whether these sulfates constitute a significant percentage of the dune composition is currently under investigation.
Chojnacki Mikolaj
Moersch Jeffery E.
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