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V.A. Krasnopolsky (1), S. Bowyer (2), S. Chakrabarti (3), G.R. Gladstone (4), and J.S. McDonald (5)
1) NASA/Goddard Space Flight Center, Greenbelt, MD 20771
2) Center for EUV Astrophysics, 2150 Kittredge Street, University of
California, Berkeley, CA 94720
3) Boston University, 725 Commonwealth Ave, Boston Ma. 02215
4)
Southwest Research Institute, Space Sciences Dept., 6220 Culebra Rd.
San Antonio, Tx. 78238-5166
5) Astronomy Dept., University of California, San Diego
One hundred-eight photons of the Martian He 584 Å airglow detected by the Extreme Ultraviolet Explorer satellite during a two-day exposure (1993 January 22-23) correspond to the effective disk average intensity of 43 +/ -10 Rayleigh. Radiative transfer calculations, using a model atmosphere appropriate to the conditions of the observation and having an exospheric temperature of 210 +/- 20 K, result in a He mixing ratio of 1.1 +/- 0.4 ppm in the lower atmosphere. Nonthermal escape of helium is due to electron impact ionization and pickup of He+ by the solar wind, to collisions with hot oxygen atoms, and to charge exchange with molecular species with corresponding column loss rates of 1.4E+05, 3E+04, and 7E+03 cm-2 s-1, respectively. The lifetime of helium on Mars is 5E+04/yr, and it appears that outgassing processes have been rather strong on Mars. The He outgassing rate, coupled with the (40)Ar atmospheric abundance and with the K:U:Th ratio measured in the surface rocks, is used as input to a single two-reservoir degassing model which presumes the loss of all argon accumulated in the atmosphere during the first Gyr by large-scale impacts. The model results in total planet mass ratios of 1E-05 g/g for K, 2.3E-09 g/g for U, 8.5E-09 g/g for Th, 4E-10 g/g for He, and 1.5E-09 g.g for (40)Ar. The predicted radiogenic heat flux is 2 erg cm-2 s-1. Similar modeling for Venus results in total planet mass ratios of 4.7E-05 g/g for K, 6.7E-09 g/g for U, 2.2E-08 g/g for Th, 1.3E-09 g.g for He, 6.7E-09 g/g for (40)Ar, and a radiogenic heat flux of 15 erg cm-2 s-1. The implications of these results are discussed.
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