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J. C. Pearl (NASA Goddard Space Flight Center), M. D. Smith (Catholic University of America), B. J. Conrath (Cornell University), P. R. Christensen (Arizona State University)
Although the residual martian north polar cap is water ice, the seasonal cap is known to be predominantly carbon dioxide frost (1). Analysis of Mariner 9 IRIS and Viking IRTM data (2), and theoretical models (3) have indicated that substantial CO2 condensation may occur in the atmosphere during the fall and winter seasons. The Mars Global Surveyor Thermal Emission Spectrometer (TES) instrument has been monitoring the north polar region through an entire fall and winter, and into spring (\( 180 < Ls < 10\)) (4). During this time, a strong polar vortex formed, poleward of which atmospheric temperatures dropped to very low values. On the basis of a time series of latitudinal cross sections of atmospheric thermal structure, we have found continual evidence for very low temperatures in the lower two scale heights of the atmosphere, over latitudes extending as far south as 60N. A limited number of limb measurements across the same region has provided independent sampling of the atmospheric thermal stucture up to 65 km. Within the low temperature region, the derived lapse rate exceeds that of the CO2 condensation curve, leading to supersaturation. The low temperature region is overlain by an optically thin water ice haze that may provide the nucleation source for CO2 condensation; this region, however, is remarkably free of atmospheric dust. Heterogeneous nucleation of CO2 on water ice has been observed in the laboratory to require a supersaturation ratio near 1.2. Substantial longitudinal variability indicates that the condensation phenomenon is highly dynamic in its distribution around the polar region.
References: (1) H.P. Larson and U. Fink, ApJ 171, L91 (1972). (2) D.A. Paige, et al., BAAS 22, 1075 (1990); F. Forget, et al., JGR 100, 21219 (1995); F. Forget and J.B. Pollack, JGR 101, 16865 (1996). (3) J.B. Pollack, et al., JGR 95, 1447 (1990); F. Forget, et al., Icarus 131, 302 (1998). (4) P.A. Christensen, et al., Science 279, 1692 (1998). (5) D. L. Glandorf, EOS 79 (Supplement), S24 (1998).