Physics
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusm.p42a..02k&link_type=abstract
American Geophysical Union, Spring Meeting 2005, abstract #P42A-02
Physics
2164 Solar Wind Plasma, 2780 Solar Wind Interactions With Unmagnetized Bodies, 5405 Atmospheres: Composition And Chemistry, 6225 Mars, 6295 Venus
Scientific paper
Long-exposure spectroscopy of Mars and Venus with the Extreme Ultraviolet Explorer (EUVE) has revealed emissions of He 584 Å on both planets and He 537 Å/O+ 539 Å and He+ 304 Å on Venus. Using the recent data on the solar emission at 584 Å and eddy diffusion in the upper atmospheres of both planets, the derived helium mixing ratios are 10 ± 6 and 9 ± 6 ppm in the lower atmospheres of Mars and Venus. Collisions with hot oxygen atoms, some ion reactions, sputtering by O+ pickup ions, and photo- and electron impact ionization followed by sweeping out of He+ by the solar wind result in a total loss of He from Mars at 8× 1023 s-1. There is no current volcanism on Mars, and seepage of volcanic gases is also very low. Therefore the current outgassing of He formed by radiactive decay of U and Th in the interior is negligible, and the loss is compensated by the capture of solar wind α-particles with an efficiency of 0.3. A similar analysis of the helium loss processes on Venus results in a total loss of 4.4× 1024 s-1. The corresponding lifetime of He on Venus is very long, 0.5 Byr, and helium released from the interior in the last resurfacing episode 0.6 Byr ago covers 0.3 of the loss. The most of the loss is compensated by the capture of solar wind α-particles with an efficiency of 0.1. We compare our derived α-particle capture efficiencies for Mars and Venus with observed x-ray emissions resulting from the charge exchange of solar wind heavy ions with the extended atmospheres on both planets (Dennerl et al. 2002, Dennerl 2002). (The most of the observed emissions are from scattering and fluorescence of the solar x-rays and not relevant to this discussion.) The emissions from both disk and halo on Mars agree with our calculated values; however, we do not see a reasonable explanation for the x-ray halo emission on Venus. An efficiency of the x-ray excitation in the martian halo is essentially similar to those in comets McNaught-Hartley and LINEAR S4. The ratio of the charge exchange efficiencies derived from the disk x-ray emissions of Mars and Venus is similar to the ratio of the capture efficiencies for these planets. Capture of the solar wind α-particles as well as charge exchange of the heavy ions impacting the atmospheres of Mars and Venus are kinetic effects that are proportional to ratios of gyroradii to the planet radii. This ratio is greater on Mars than on Venus by a factor of 3.8, in accord with the derived capture efficiencies and the measured x-ray emissions. The surprisingly bright emission of He+ at 304 Å observed by EUVE and Veneras 11 and 12 suggests that charge exchange in the flow of the solar wind α-particles around the ionopause is much stronger than in the flow of α-particles into the ionosphere.
Gladstone Randall G.
Krasnopolsky Vladimir A.
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