SPECTRAL PROPERTIES OF THE MARTIAN POLAR REGIONS FROM CRISM AND OMEGA

Since the Viking era it has been known that the residual ice caps had differing compositions and that the seasonal caps are mostly composed of condensed CO₂ gas from the atmosphere. A long history of telescopic observations has shown that the seasonal advance and retreat of CO₂ surface frost closely repeats from year to year. It is this process of condensation and sublimation that drives the current martian climate. Roughly 25% of the atmosphere, which is 95% CO₂ by volume, is cycled through the seasonal caps annually. Both water and CO₂ ice have strongly varying spectral properties with wavelength. Small amounts of water or dust in CO₂ ice can dramatically change the reflectivity and emissivity, thus modifying the energy balance of the ice caps.

Both the Mars Express OMEGA instrument and the Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) observe these polar deposits in the wavelength range where multiple, diagnostic features can separate H₂O and CO₂ ices and allow the determination of effective grain size of ice and level of dust contamination using modern scattering models. Initial OMEGA results have shown evidence for the evolution of water frost grain size in the north residual ice cap and suggest that the late northern seasonal frost is dominated by water rather than CO₂ ice. There is active observation and analysis regarding the nature of the “Cryptic region” and other anomalous and rapidly varying albedo patterns in the southern seasonal frost. Persistent small high albedo patches in the north residual ice were noted by TES and OMEGA. Significant mobility and spatial variation in these patches are found in the northern summer. Examination of high spatial resolution CRISM data suggest these spots are fine-grained water frost and their motions may provide information on winds or local accumulation and ablation occurring in the permanent ice deposits. Spectral properties in exposed layers are more complicated. Large fractions of the polar layered deposits appear as transparent ice to MARSIS and SHARAD but have no infrared spectral absorption features of either ice. The modern view shows that both seasonal and permanent ice deposits are highly active with complex temporal patterns and understanding these dynamic phenomena are critical to understanding martian climate history.