DPS 34th Meeting, October 2002
Session 8. Inner Planets
Oral, Chair(s): T. Widemann and R.M. Nelson, Monday, October 7, 2002, 4:00-5:00pm, Room M

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[8.02] Mercury's sodium exosphere: new insights

F. Leblanc (Service d'Aeronomie du CNRS), R.E. Johnson (University of Virginia)

We describe the formation of Mercury's sodium exosphere using a 3D Monte Carlo model in which we follow sodium atoms ejected from Mercury's surface by photo desorption, meteoritic vaporization and solar wind sputtering. Comparisons of our results with observations (Potter A.E., Killen R.M. and Morgan T.H., Meteoritics and Planetary Science, 37, 2002) are provided. Averaging over one Mercury year, the population of ejected Na atoms which are not reabsorbed by Mercury's surface is composed of ~20% lost as neutrals, ~20% ionized outside of Mercury's magnetosphere and ~60% ionized inside Mercury's magnetosphere. If the latter newly formed ions are all lost by Mercury, then the total sodium loss represents about 8 to 15% of the total amount of sodium ejected from Mercury's surface.

Since observations (Sprague et al., Icarus, 129, 506, 1997) have indicated a potential asymmetry of the surface ejecta distribution with respect to the subsolar longitude, different sources for this asymmetry are considered. We find that the ratio between the sodium column density on the morning side of Mercury's exosphere and on the afternoon side can be significantly different from the ratio between the sodium ejecta flux on the morning side of Mercury's surface and on the afternoon side. It can differ by up to a factor 4 because of the solar radiation pressure. An asymmetry of the ejecta flux with 1.5 to 2 times more Na atoms ejected on the morning side than Na atoms ejected on the afternoon side would be enough to produce a ratio of the column densities greater than 3 seen by Sprague et al. (1997).

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.