Computer Science – Sound
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p13b1277n&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P13B-1277
Computer Science
Sound
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6020] Planetary Sciences: Comets And Small Bodies / Ices, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Early in the exploration of Mars, Mariner 9 revealed a distinct morphologic class of craters that lie above the surrounding terrain atop steep-sided mesas roughly concentric to the craters. The predominant interpretation of these features argues that the crest of a pedestal represents the level of the surface at the time of the impact event that formed the central crater. Subsequent deflation lowered the surface pervasively, where as the pedestal remained due to increased resistance to removal via armoring or lag layers provided by the ejected blanket. Different models have been proposed to explain pedestal formation and persistence, and they generally involve the presence of a paleo-unit that was either friable or rich in volatiles. Given that pedestal craters occur dominantly in the mid-latitudes, with some cases also reported in the equatorial and polar regions, removal of extensive friable or volatile units at least as thick as the pedestals has occurred throughout Mars. We have analyzed data from the Shallow Radar (SHARAD) sounding instrument onboard the Mars Reconnaissance Orbiter to investigate the nature of the material composing the pedestals. Of the 60 pedestals examined, ranging a few to ~100 kilometers in diameter, we have identified several cases where the pedestal material is transparent to the SHARAD signal and a basal reflection is observed. These are cases where the pedestal is tens to ~ 100 kilometers in diameter. Subsurface reflections are not seen in every large pedestal, and for pedestals only a few kilometers wide we have not been able to unambiguously identify a basal reflector apart from surface clutter. In at least one case (in Malea Planum) internal layering between the surface and basal reflections is seen in SHARAD data. These layers appear to follow the shape of the basal topography and likely reflect past depositional events; layering is also seen at the edges of this pedestal in visible imagery from multiple orbiting cameras in this case. Assuming that the delay between the surface and basal reflections correspond to the pedestal height above the surrounding terrain, we estimate a bulk permittivity of 4.5 to 5.0 for all of the radar-transparent pedestals. Interpretation of these values is non-unique, as they are consistent with either a porous basaltic matrix or a mix of basaltic material and water ice. Contextual geologic analysis and examination of other remote sensing is ongoing with the aim of narrowing the interpretation of radar data, as determining whether a widespread volatile rich-layer once covered the Martian mid-latitudes is crucial to our understanding of Mars’ climatic history.
Fisher Benji
Head James W.
Holt Jeremy William
Nunes Daniel C.
Phillips James R.
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