Computer Science – Sound
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p53a..11h&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P53A-11
Computer Science
Sound
[0305] Atmospheric Composition And Structure / Aerosols And Particles, [0321] Atmospheric Composition And Structure / Cloud/Radiation Interaction, [0343] Atmospheric Composition And Structure / Planetary Atmospheres, [5422] Planetary Sciences: Solid Surface Planets / Ices
Scientific paper
The present climate of Mars is strongly influenced by the energy balance at the planet’s poles, with ~30% of the atmospheric mass exchanged seasonally with the polar ice caps. While the spring and summer sublimation process is observable in sunlight, the deposition process occurs in the darkness of polar night. We present direct radiometric observations of carbon dioxide snow clouds from the Mars Climate Sounder (MCS) and estimate the rate of deposition due to snowfall. We also present radiative transfer models capable of reproducing the observations and providing constraints on the radiative and thermal properties of the cap-atmosphere system. Snow clouds display a multi-layered structure with greatest opacity near the surface and extending to typical altitudes of about 20 km, with equivalent normal visible optical depths of ~0.1. Our modeling suggests the observed carbon dioxide snow grains are ~10 μm in radius, implying modest deposition rates, and suggesting the majority of the seasonal cap is deposited in a vertical region within one MCS field of view (or ~1 km) of the surface. Models reproducing the MCS limb observations only reproduce the nadir observations if the surface (or near-surface) is an optically thick layer of small (< 100 μm radius) carbon dioxide grains, which are therefore the primary cause of radiometrically cold areas (“cold spots”) observed since the Viking era. For the extreme polar regions, a persistent, ~500 km diameter snow cloud is strongly coupled to the most active cold spots, and smaller clouds (< 50 km diameter) in the latitude range 60-80°, though unobserved, cannot be ruled out by the MCS data. Based on this correlation, and observations of cold spots recurring near topographic slopes, we conclude that deposition is indeed linked to cloud formation, with the majority of material condensing below ~1 km altitude. Optically thin water ice layers are necessary to accurately model the MCS spectrum, particularly at altitudes above 20 km. This suggests water ice functions as the required condensation nucleus, consistent with earlier laboratory and theoretical studies. Important hemispherical differences are observed in the deposition process: 1) northern clouds are optically thicker at middle altitudes, ~5-15 km; 2) southern clouds are more often “detached”, showing a local maximum opacity near 20-25 km altitude; 3) mode particle radii are larger (~100 μm versus ~10 μm) in the north. Total normal optical depths are typically higher by a factor of ~2 in the north, and water ice content is relatively higher. Energy balance constraints can be placed on the system by MCS observations of outgoing infrared flux, which we map through time as an effective emissivity by taking account of the topography from MOLA and the expected frost point temperature.
Hayne Paul O.
Paige David A.
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