Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p41a1596b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P41A-1596
Physics
[2784] Magnetospheric Physics / Solar Wind/Magnetosphere Interactions, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [5421] Planetary Sciences: Solid Surface Planets / Interactions With Particles And Fields, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
Mercury is surrounded by a surface-bounded exosphere known to contain hydrogen, sodium, potassium, calcium, and magnesium. Because the exosphere is collisionless, its composition represents a balance of active source and loss processes. The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has made high-spatial-resolution observations of sodium, calcium, and magnesium near Mercury's surface and in the extended, anti-sunward direction. The most striking feature of these data is the substantial differences among species, which was detected during three close flybys of the planet and has been persistantly present during MESSENGER's orbital phase. Our modeling demonstrates that these differences are not because of post-ejection dynamics such as differences in photo-ionization rate and radiation pressure, but rather result from differences in the source mechanisms and regions on the surface from which each species is ejected. The observations of calcium have revealed a strong dawn/dusk asymmetry, with the abundance over the dawn hemisphere substantially greater than that on the dusk side. To understand this asymmetry, we use a Monte Carlo model of Mercury's exosphere that we developed to track the motions of exospheric neutrals under the influence of gravity and radiation pressure. In this model, Ca atoms can be ejected directly from the surface or produced by ejection of CaO followed by dissociation to produce Ca and O. Particles are removed from the system if they stick to the surface or escape from the model region of interest (within 15 Mercury radii). Photoionization reduces the final weighting given to each particle when simulating the Ca radiance. Data from the flybys are consistent with a high temperature (~1-2 x 104 K) source of atomic Ca concentrated over the dawn hemisphere. Such a high temperature resutls from dissociation of CaO in a near-surface exosphere with scale height ~100 km; such dissociation imparts ~2 eV to each freshly produced Ca atom. We are currently investigating whether this source region and energy fit the data obtained in orbit.
Burger Matthew Howard
Killen Rosemary Margaret
McClintock William E.
Merkel Aimee W.
Sarantos Menelaos
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