Buried CO2 Ice traces in South Polar Layered Deposits of Mars detected by radar sounder

Abstract:

SHARAD (SHAllow RADar) is the subsurface sounding radar provided by the Italian Space Agency (ASI) as a facility instrument to NASA’s 2005 Mars Reconnaissance Orbiter (MRO). The Reduced Data Record of SHARAD data covering the area of the South Polar Layered Deposits (SPLD), has been used. The elaboration and interpretation of the data, aimed to estimate electromagnetic properties of surface layers, has been performed in terms of permittivity. The theory of electromagnetic scattering from fractal surfaces, and the estimation of geometric parameters from topographic data by Mars Orbiter Laser Altimeter (MOLA) which was one of five instruments on board the Mars Global Surveyor (MGS) spacecraft, has been used. A deep analysis of inversion has been made on all Mars and extended to the South Polar Caps in order to extract the area with a permittivity constant of CO₂ ice. Several corrections have been applied to the data, moreover the calibration of the signal requires the determination of a constant that takes into account the power gain due to the radar system and the surface in order to compensate the power losses due to the orbitographic phenomena. The determination of regions with high probability of buried CO₂ ice in the first layer of the Martian surface, is obtained extracting the real part of the permittivity constant of the CO2 ice (~2), estimated by other means. The permittivity of CO₂ice is extracted from the Global Permittivity Map of Mars using the global standard deviation of itself as following:

ε_CO2ice=ε_CO2ice+ Σ (1)

where Σ=±std(ε_MapMars)/2

Figure 1(a) shows the south polar areas where the values of the permittivity point to the possibility of a CO₂ ice layer. Figure 1(b) is the corresponding geologic map. The comparison between the two maps indicates that the area with probable buried CO₂ overlaps Hesperian and Amazonian polar units (Hp, Hesperian plains-forming deposits marked by narrow sinuous, anabranching ridges and irregular depressions, and Apu, Amazonian layered plateaus). From this analysis, the south polar cap could be covered by a thin frozen carbon dioxide coating. The perennial south polar cap is probably made of frozen carbon dioxide ca. 8 meters thick.