Physics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..782h&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Physics
Scientific paper
The similarities and differences of the escape mechanisms for H+ and D+ from Venus, H and D from Mars, and heavier ions (~ 17 and ~ 28 amu) from Titan are described. The dominant escape process for hydrogen and deuterium on Venus is thought to originate in the night side ionosphere, located in the night side H and D bulge region, where the polarization electric field is the dominant force accelerating ionospheric H+ and D+ upward into the induced magnetic tail of Titan [1]. The resulting loss rates ~ 8.6x1026 s-1 and ~ 3.2x1023 s-1 for H+ and D+, respectively, are consistent with the large observed D/H ratio ~ 160 times that of terrestrial water and an ancient ocean more than 10 m of liquid uniformly distributed on the surface. In contrast, Jeans escape is the dominant loss mechanism for H and D on Mars [2], which has a D/H ratio ~ 5.3 times that of terrestrial water. The resulting loss rates for H and D of ~ 3.7x1026 s-1 and ~ 1022 s-1, respectively, can be related to possible ancient water reservoirs below the surface. When horizontal atmospheric winds are taken into account, the Jeans escape rates for H and D are enhanced considerably [3], as are the corresponding water reservoirs. On Titan, 28 amu ions were observed to escape along its induced magnetic tail by the Voyager 1 Plasma Science Instrument (PLS). In analogy with Venus, the escaping ions were thought to originate in the ionosphere [4]. The Cassini mission permits a test of this principal due to the numerous flybys of Titan through both the ionosphere and the tail. A polarization electric field is obtained in the ionosphere of the TA flyby, yielding an upward acceleration of 17 and 28 amu ionospheric ions that is consistent with the flux of heavy ionospheric ions observed escaping along the magnetic tail by the Cassini Ion Mass Spectrometer (CAPS) during the T9 flyby [5]. References [1] R. E. Hartle, T. M. Donahue, et al., J. Geophys. Res., 101,4525, 1996. [2] T. M. Donahue, Icarus, 167, 225, 2004. [3] R. E. Hartle and H. G. Mayr, J. Geophys. Res., 81, 1207, 1976. [4] R. E. Hartle et al., J. Geophys. Res., 87, 1383, 1982. [5] E. C. Sittler et al., this conference, 2008.
Hartle Richard E.
Sittler Edward C.
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