Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.6702b&link_type=abstract
American Astronomical Society, DPS meeting #41, #67.02
Astronomy and Astrophysics
Astronomy
Scientific paper
Mercury has a tenuous exosphere created by the combined effects of solar radiation and micrometeoroid bombardment on the surface and the interaction of the solar wind with Mercury's magnetic field and surface. Observations of this exosphere provide essential data necessary for understanding the composition and evolution of Mercury's surface, as well as the interaction between Mercury's magnetosphere and the solar wind.
The exosphere was observed during the MESSENGER flybys with the Mercury Atmospheric and Surface Composition Spectrometer (MASCS). The detections of sodium and calcium during the first two flybys, as well as magnesium during the second flyby, have been previously reported (McClintock et al. 2008,2009). Sodium and calcium showed significantly different distributions in both the tail and near-surface component of the exosphere suggesting they are ejected by different processes and from different regions.
We present a Monte-Carlo model of sodium and calcium in Mercury's exosphere. The important source mechanisms are sputtering by solar wind ions, photon-stimulated desorption, and micrometeoroid impact vaporization. Thermal desorption on the dayside does not supply enough energy to populate the exosphere, although it plays a role in redistributing volatiles over the surface. In addition, atomic calcium can be produced from the dissociation of Ca-bearing molecules, such as CaO, which can be formed by hypervelocity impacts.
We also consider the spatial distribution of the surface source. Impact vaporization is roughly isotropic over the surface, although there may be a leading/trailing asymmetry in the impact rate due to Mercury's orbital motion. Sputtering is confined to regions where the solar wind or magnetospheric plasma can impact the surface, which is shielded somewhat by the internal magnetic field. Differences in the magnetic field configuration of the solar wind between the first two flybys account for much of the variation in the sodium distribution observed in each encounter.
Benna Marcus
Bradley Eric Todd
Burger Matthew Howard
Killen Rosemary Margaret
McClintock William E.
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