Physical State of the ``Bright'' South Seasonal Polar Cap From OMEGA Observations

Details Physical State of the ``Bright'' South Seasonal Polar Cap From OMEGA Observations Physical State of the ``Bright'' South Seasonal Polar Cap From OMEGA Observations

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p21b1216d&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P21B-1216

Physics

[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 composition, physical state and texture of the South Seasonal Polar Cap (SSPC) have important consequences on energy balance. The imaging spectrometer OMEGA on board Mars Express has acquired the most comprehensive set of observations to date in the near-infrared (0.93-5.1 microns) on the SSPC from mid-winter solstice (Ls=110° , December 2004) to the end of the recession at Ls=320° (November 2005) [1]. The time resolution is 3 days to one month and the spatial resolution ranges from 700m to 10 km/pixel. [1] showed that during southern spring and summer, there is a very complex evolution in terms of effective grain size of CO2 ice and contamination by dust or H2O ice. H2O ice does not play a significant role except close to the end of the recession. [2] systematically segmented the South Seasonal Polar Cap into different spectral units and tested diverse surface representations by the modeling of spectral end-members and average unit spectra. Here we focus on the “bright” part of the SSPC corresponding to spectral unit SSPC1 (I.b of [1]). Regions belonging to this unit have a very bright albedo (≈ 0.6-0.8) associated with strong CO2 ice absorption features. According to [1] such characteristics are compatible with granular CO2 deposits with grain size in the range of 5 cm implying an extremely low contamination by dust and water ice. Furthermore, from their study of a representative region at 34° E, 76° S, the albedo increases from Ls=223° up to a maximum at Ls=240° and then decreases until total disappearance of the ice. The albedo increase would require a decrease of grain size if the granular model is really relevant. However the mean free path within CO2 ice as well as its thermodynamic behavior would rather favor a porous CO2 slab model [3]. Then photometric effects could be expected that could explain the brightening. In this paper we propose to further study the physical state of the “bright” part of the SSPC and its evolution by monitoring two reference regions from Ls 223° to 260° of martian year 27. We remove the aerosol effects on the spectra using radiative properties and maps of total column optical depth evaluated by [4] for the aerosols over large portions of the SSPC. Spectral modeling suggests that the upper 1 to 4 centimeters of the CO2 deposits form a slab of very compact ice. The slab thickness increases with time until CO2 ice demise. Increase of albedo due to a decreasing dust contamination first comes with this evolution, before the appearance of ice-free patches within the pixels that reverses the trend. Existence of the slab implies photometric and radiative effects. We have not be able to detect unambiguously the former, likely because the range of phase angles covered by our selected observations is too limited and the sun illumination is rather diffuse. [1] Langevin, Y., et al., 2007, J. Geophys. Res., 112, [2] Douté, S., et al., 2008, in LPSC Abstracts, vol. 39, 1736-+.[3] Eluszkiewicz, et al., 2005, Icarus, 174, 524. [4] Vincendon, et al., 2008, Icarus 196, 488.

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