Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p13b1281w&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P13B-1281
Other
[5419] Planetary Sciences: Solid Surface Planets / Hydrology And Fluvial Processes, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing
Scientific paper
A notable aspect of MARSIS data to date is the absence of any deep, continuous reflectors that might represent the interface between ice-rich rock and an aquifer. Detecting such boundaries was the primary motive behind creating MARSIS, and as such their complete absence is highly significant with respect to speculations about the water inventory of the Martian subsurface, as well as about the penetration capabilities of MARSIS. This study aims to estimate a theoretical limit on the aquifer detection depth beneath an overlying basaltic layer for MARSIS based on the strengths of reflections from the bases of the thickest parts of the South Polar Layered Deposits (SPLDs) and Medusae Fossae Formation (MFF) through which MARSIS has been observed to penetrate. The method is based on the hypothesis that if the radio waves transmitted by MARSIS can penetrate through a certain dielectric medium to reach an interface at a certain depth, characterized by a certain dielectric contrast, and return an echo of a certain strength to MARSIS, the depth at which the same echo strength would be received for a scenario featuring different dielectric media can be deduced. This is accomplished using formulae relating the power of echoes reflected from the interfaces between crustal layers to the dielectric properties of the crustal layers. It has been reported that MARSIS has penetrated to maximum depths of 3.7 km through the SPLDs and 3.5 km through the MFF, which have dielectric constants of 4 and 2.9, and loss tangents of 0.003 and 0.004, respectively. The basaltic substrate is regarded as having a dielectric constant of 8 in both cases. This information was used to calculate reflection coefficients for the surfaces and bases of the SPLDs and MFF, as well as maximum two-way time delays at which the basal reflections of the SPLDs and MFF extend to; these were in turn used to calculate the power of the energy reflected from the bases of the SPLDs and MFF. In order to calculate the depth at which the same power would be reflected from the top of a hypothetical aquifer, it is assumed that the aquifer comprises a layer of porous, wet basalt with a bulk dielectric constant of 36, overlain by a layer of porous basalt mixed with ice that has a bulk dielectric constant of 7 and a loss tangent of 0.014. The calculation process is then reversed in order to deduce the depth at which the same reflected powers would be received for the aquifer scenario. A potential source of inconsistency when extrapolating the penetration ability through the SPLDs and MFF to the rest of Mars includes differences in volume scattering, which is fairly negligible for the SPLDs and MFF when compared to other Martian environments. In order to account for this to an extent, the results of previous Mars-analogue GPR surveys are considered, which indicate that volume scattering in mafic environments can reduce the penetration depth by a quarter. Factoring this into the calculation leads to maximum aquifer detection depths of 642 m and 457 m for the SPLDs and MFF respectively.
Stofan Ellen
White Olivia L.
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