Mathematics – Probability
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.p24a..06c&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P24A-06
Mathematics
Probability
5405 Atmospheres (0343, 1060), 5421 Interactions With Particles And Fields, 5470 Surface Materials And Properties
Scientific paper
The surfaces of "airless" bodies in our solar system are covered by porous regoliths, granular surfaces generated by micrometeor impact. Europa's tenuous neutral atmosphere is generated by UV and plasma irradiation of and sublimation from this regolith. Therefore, in addition to the atmosphere above the surface, there is a substantial amount of gas in the porous regolith. The effect of the regolith on the source processes and sinks are typically neglected in modeling the spatial distribution and composition of the atmosphere. The regolith complicates processes such as sputtering, the ejection of mostly neutral atoms and molecules due to energetic ion flux, because the incident ions encounter surfaces at a variety of angles, rather than one angle as usually assumed. Also, most ejecta produced within a regolith no longer have a direct line to space. If ejecta do not stick to or react with grain surfaces, then it may be safely assumed that the majority of ejecta will interact with grain surfaces before leaving the regolith. Similarly, a returning non-sticking particle experiences numerous interactions with grains below the nominal surface. As compared to a flat, smooth planetary surface, these many interactions enhance the probability of chemical reactions or sticking. F. Leblanc and R.E. Johnson have shown that the sticking coefficient is critical in describing the alkali atmosphere at Mercury and likely Europa. The regolith will also affect the velocity distribution of non-sticking ejecta and atmospheric species, which will affect the population of the Europa neutral torus. In this presentation the effect of regolith on the source and sink processes is demonstrated by generating the gravitationally bound and escaping components of the ballistic Europan atmosphere with and without regolith effects. Assuming that O2 can react in the regolith where there is a high sulfur content, we can generate a morphology roughly consistent with HST observations by McGrath and coworkers. In this we also account for atmospheric sputtering using cross sections recently measured for O2 by H. C. Luna and coworkers. In addition, we show the effect of the sticking and adsorption probabilities on both the content and morphology of SO2 and CO2 components of Europa's atmosphere, species known to be trapped in the surface ice. Such a modeling effort is needed in order to determine the ability of an orbiting instrument to map the Europan atmosphere and to determine aspects of the surface composition, including the presence of organic molecules, which may also be sputtered into the atmosphere at detectable quantities.
Cassidy Timothy A.
Johnson Robert E.
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