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S. Jurac, J. D. Richardson (MIT), M. A. McGrath (STSI), R. E. Johnson (Univ. Virginia), V. M. Vasyliunas (Max Planck Inst. Aeronomie), A. Eviatar (Tel Aviv Univ.)
The origin of the large hydroxyl radical (OH) cloud near the inner moons of Saturn is indicative of a surprisingly large water-vapor source and has represented a puzzle since its discovery in 1992. The water molecules presumably serve as a precursor for both water-group neutrals (O,OH,H2O,H) and plasma in Saturn's magnetosphere. We have developed a model for the dynamical evolution of the water-group neutrals which includes the effects of plasma chemistry and both plasma-neutral and neutral-neutral collisions to determine their equilibrium densities. We show that momentum transfer from these processes causes fast diffusion of neutrals away from the source region. Line-of-sight OH emission measurements by the Hubble Space Telescope are used to constrain the model densities and to pinpoint the source region.
Collisions between plasma ions and neutral molecules substantially inflate the OH cloud, and increase the OH loss rate, requiring a water source about three times larger than previous estimates. Our model indicates that the vast majority of the water vapor (>80%) originates from Enceladus's orbital distance (4 Rs). This result may indicate the presence of a dense population of small, as yet unseen, bodies concentrated near Enceladus; collisions between these fragments are the suggested mechanism for producing the necessary amounts of water vapor. The high OH content measured at 3 Rs indicates that about 15 % of water seems to originate from between 2-3.5 Rs. The work is supported by a NASA Planetary Atmospheres grant to MIT.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.