Other
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.953r..11f&link_type=abstract
The First International Conference on Mars Polar Science and Exploration, Proceedings of the Conference held at Camp Allen, TX.
Other
Carbon Dioxide, Dust, Ice, Mars Surface, Polar Caps, Polar Regions, Sediments, Mars (Planet), Geology, Atmospheric General Circulation Models, Mariner 9 Space Probe, Thermal Mapping, Viking Mars Program
Scientific paper
One of the key processes controlling the geology of the martian polar regions is the seasonal condensation of the atmosphere into CO, ice caps. These polar caps mostly condense during the polar night, when surface and atmospheric temperature become cold enough to reach the frost point of CO2. Thus, almost all that is known about the formation of the polar caps has come from the Mariner 9 and Viking infrared measurements. These observations showed that the physical processes controlling the condensation are complex, because of the unique radiative and microphysical properties of CO2 ice condensing in a CO2 atmosphere. For instance, the Infrared Thermal Mapper (IRTM) instrument observed variable structures exhibiting brightness temperatures far below the physical temperature appropriate for condensed CO2 in vapor pressure equilibrium at the expected atmospheric pressure. A detailed analysis of the data suggests that these low brightness temperatures result from the radiative properties of the small CO2 ice particles that condense in the atmosphere rather than directly on the surface. Indeed, simulations performed with General Circulation Models have shown that a fraction of the total CO2 condensation can take place in the atmosphere. Atmospheric condensation can result from radiative cooling on the one hand (especially when the atmosphere is dust laden) and from adiabatic cooling in upward motions on the other . The resulting CO2 snowfalls could create the observed features, because the CO2 ice particles that condense in the atmosphere can be efficient scatterers at infrared wavelengths (whether they are airborne or have just fallen to the ground) carbon dioxide ice deposits composed of nonporous solid ice, however, having directly condensed on the ground or having undergone frost metamorphism should behave almost like blackbody emitters, or, more likely, be transparent in the infrared so that the ground beneath can radiate through. In fact, by simply parametrizing the radiative effects of the modeled CO2 snow fall and the "snow metamorphism," it has been possible to accurately reproduce the general behavior of the features observed by Viking in the thermal infrared, and, in turn, the global CO2 cycle. By scavenging airborne dust, the snow falls may dramatically increase the dust sedimentation rate in the polar region. How much dust can be trapped in the polar deposits by such a process? To answer this question, the scavenging of dust by CO2 has been simulated in a version of the LMD General Circulation Model that includes an interactive dust transport scheme and the CO2 snow fall parametrization. Also, because the CO2 clouds and snow falls strongly alter the radiative balance of the condensing polar caps, they affect the global CO2 cycle and thus the global climate. All these processes should be taken into account when studying past climates or the existence of permanent CO2 ice deposits near the south pole. New observations are being transmitted by Mars Global Surveyor. TES and MOLA data should greatly improve our understanding of what is really going on during the cap formation.
Chang Alice
Foster John Jr.
Hall Donavan
Klein Abel
Tait A.
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