Computer Science – Sound
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p34a..01p&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P34A-01
Computer Science
Sound
[5422] Planetary Sciences: Solid Surface Planets / Ices, [5462] Planetary Sciences: Solid Surface Planets / Polar Regions, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
The SHARAD instrument on the Mars Reconnaissance Orbiter (MRO) mission has carried out systematic radar soundings of the layered deposits at both martian polar regions. While well-organized sets of radar reflectors are ubiquitous in the North Polar Layered Deposits, those in the South Polar Layered Deposits (SPLD) are limited to specific regions, and it is difficult to map SPLD-wide radar stratigraphy. What is evident in the radar observations are four regional reflection-free zones (RFZ) distinguished qualitatively by their radar characteristics. They are up to a kilometer in thickness and extend downward from near the surface. One such zone (RFZ3) occurs beneath the South Polar Residual Cap (SPRC), which is composed of ~5 m of solid CO2 underlain by an apparently thin layer of water ice. Using a correlation technique, we inverted for the real permittivity, ɛ', on each of 41 usable SHARAD orbits over RFZ3. The results were mean values of ɛ' = 2.0 or 2.1, with a σ of 0.2. A secondary technique based on the “smoothest” solution gave similar results. These values are exceptionally close to the laboratory-measured permittivity value of bulk CO2 ice [Pettinelli et al., 2003] and distant from the bulk water ice value (ɛ' = 3.15); water ice is the dominant volatile in the SPLD. An alternative hypothesis for ɛ' = 2.0-2.1 is that the RFZ3 material is porous water ice, but this can be strongly discounted based on theoretical and empirical models of ɛ' of porous water ice vs. thickness. By the same arguments, the proposed CO2 material also cannot be very porous, and ɛ' should be close to the bulk value. With the permittivity estimates, radar time delays were converted to depth, and for RFZ3 a mean thickness of 210-220 m and a volume of 4,200-4,400 km3 result. This is unlikely to be the entire volume because MRO’s orbital inclination precludes SHARAD sounding poleward of ~87°S, where RFZ3 appears to extend. We do find a very good spatial correlation of RFZ3 with the stratigraphic unit (named “Aa3”) immediately beneath the SPRC [Kolb et al., 2006] and use this geologic unit as a basis for extrapolation, yielding a volume estimate range of 9,500 to 12,500 km3. For comparison, the CO2 in the SPRC is estimated to be < 380 km3 [Thomas et al., 2009]. The equivalent atmospheric pressure of the extrapolated RFZ3 volume is 4-5 mbar, competing in magnitude with the current atmospheric pressure of 6-7 mbar. We have searched the past million year orbit history of Mars for periods when insolation at the south pole would likely render the proposed CO2 mass unstable and are carrying out GCM simulations to evaluate the climate regime at those times with 10-12 mbar of CO2 in the atmosphere. The other three reflection-free zones may also contain a component of CO2, but the reflector geometry is not favorable for estimating permittivity.
Byrne Shane
Campbell Bruce A.
Carter Lynn Marie
Davis Brian J.
Haberle Robert M.
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