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R.N. Nelson (GaSouU), B.P. Michael (NASA-NRC/GSFC), J.E. Allen, Jr. (NASA/GSFC)
Carbon dioxide (CO{2}) is a major constituent in the Martian atmosphere and its abundance is controlled by surface condensation primarily at the poles. Because the sublimation temperature is determined by the vapor pressure curve, the saturation law is arguably the most important physical property of CO{2} for Mars. A number of different representations have been used for the vapor pressure of CO{2}; however, they are all based on data taken sixty-five years ago (Meyers and Van Dusen 1933) or calculations and extrapolations based on that data (e.g., Brown and Ziegler 1980). Using our apparatus specifically designed for low-temperature measurements of thermodynamic properties, we have experimentally determined the vapor pressure of CO{2} from 167 to 87 K, corresponding to a pressure range of 100 to 1.8x10{-6} Torr and set by our lowest measurable pressure. Our preliminary data have been fitted with a simple Clausius-Clapeyron representation and compared with an extrapolation of the form recommended by Brown and Ziegler (1980). For comparison the extrapolation predicts a pressure of ~1.1x10{-6} Torr at 87 K, whereas our measured value is ~1.8x10{-6} Torr at this temperature. Further refinement of the data to account for thermal transpiration and fitting with a more comprehensive three-parameter model are in progress.
References:
Brown, G. N., Jr. and Ziegler, W. T. 1980. In Advances in Cryogenic Engineering, vol. 25 (K. Timmerhaus and H. A. Snyder, Eds.), pp. 662-670. (New York: Plenum Press).
Meyers, C. H. and Van Dusen, M. S. 1933, J. Res. Natl. Bur. Stndrds. 84, 2843.
{\em Support from NASA's Planetary Atmospheres Program is gratefully acknowledged}.