Control of the radiative behavior of the Martian polar caps by surface CO₂ ice: Evidence from Mars Global Surveyor measurements

Abstract

Abstract. The seasonal polar caps of Mars are composed primarily of solid CO 2, and their growth and decay play a large part in the planer's atmospheric CO 2 cycle. The thermodynamic temperature of the caps is ~ 145 K, in equilibrium with the •-6 mbar atmosphere. The optical properties of CO 2 ice require that pure snow with a sufficiently small particle size has a high visible/near-infrared albedo and a low infrared emissivity in parts of the thermal infrared, particularly in the region between 20 and 50 gm. Dust mixed into or on top of the CO 2 ice will lower the visible albedo and bring the thermal infrared emissivity closer to 1. Water ice mixed with the CO 2 will have little effect in the visible and near-infrared to 1.4 gm but can raise the 20-50 gm emissivity if the particle size is >50 gm. Observations of both seasonal polar caps during two of the assessment orbits of the Mars Global Surveyor spacecraft are used to show that there is a strong correlation between visible brightness and low 20-50 gm emissivity. The infrared spectra from the Thermal Emission Spectrometer from regions with low 20-50 gm brightness temperature are consistent with surface deposits of CO 2 with millimeter-sized grains and containing varying small amounts of dust, and they are not consistent with the expected signature of water ice or clouds. Large regions of low emissivity in the spring seasonal caps have not been observed previously. They are correlated with visible bright regions which are known to become brighter as the spring progresses. The visible brightness of the south polar deposits examined is inferred from historical observations, while the brightness of the north polar region is determined from Mars Orbiter Laser Altimeter measurements at 1.06 gm. The model of seasonal CO 2 caps with bright, low-emissivity regions agrees with previous visible observations of bright crater rims, streaks, and other bright areas within the polar caps, some of which may evolve in time from dark, high-emissivity sheet ice to brighter, fractured, loweremissivity ice layers. It is also in agreement with models of the CO 2 cycle, which require average polar cap emissivities <0.9.

The State and Future of Mars Polar Science and Exploration

“…Observations of the north polar winter cap have also revealed sporadic "cold spots" similar to those observed in the south by the Viking IRTM (Kieffer 1979, Hansen 1999. Occasional "super-cold" spots have been seen with 25-pm brightness temperatures as low as 115 K (spectral emissivities -0.1); yet, brightness temperatures at 18 p m appear consistent with expected kinetic surface temperatures.…”

“…Laboratory studies, as well as TES and MOC observations, suggest that the thermophysical and radiative properties of COz are contributing to a seasonal polar cap behavior more complex than previously imagined (Hansen 1999, Kieffer et al 2000, James et al 2000. As the seasonal south polar cap receded during the first southern spring observed by the Mars Global Surveyor, a prominent and variegated dark region developed within the cap (--75"-85"S, 150"-310°W) that had an albedo similar to that of bare martian soil but a surface temperature that, while several degrees warmer than the surrounding cap, remained too cold to be ice-free.…”

“…One possible explanation for these observations is that atmospheric C02 becomes locally depleted near the surface, dramatically lowering its condensation temperature (Kieffer 1979, Weiss and. Alternatively, TES spectral observations suggest that these cold spots are largely caused by the radiative properties of unusually fine-grained C02, whose deep emission depression at 25 p m could well explain the less-than-unity effective emissivity required by modeling studies of the seasonal polar caps to match the observed annual surface pressure variation (Hansen 1999).…”

“…In order to match the Viking Lander pressure data, models need to suppress the emissivity of the north cap relative to the south cap. A reduced emissivity could be due to the unusual properties of pure C02 ice crystals on the surface, or to the observed presence of C02 ice clouds in the atmosphere (Bell et al 1996, Hansen 1999, Kieffer et al 2000.…”

Water and Brines on Mars: Current Evidence and Implications for MSL

“…This mechanism does not involve liquid water of any type. The proposed mechanism hypothesizes that solar radiation is efficiently transmitted through the translucent CO 2 ice layer (Hansen 1999), and is absorbed at the soil-ice interface. Then, the CO 2 ice sublimates creating high pressure pockets of CO 2 gas at the interface of the ice layer with the dusty soil.…”