Other
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.2901b&link_type=abstract
American Astronomical Society, DPS meeting #42, #29.01; Bulletin of the American Astronomical Society, Vol. 42, p.997
Other
Scientific paper
Although neutral calcium in Mercury's exosphere was discovered by ground-based observations, the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on MESSENGER has made the first high-spatial-resolution measurements of Ca in the tail region anti-sunward of Mercury and over both poles. We use a Monte Carlo model of the exosphere to track the trajectories of Ca atoms and molecules (e.g., CaO) ejected from the surface until they are photo-ionized or dissociated, escape from the system, or stick to the surface. This model allows us to understand the exospheric source processes and the interactions among neutrals, solar radiation, Mercury's magnetosphere, and the planetary surface. The MASCS data have suggested a stable, high-energy source of Ca that is enhanced in the dawn-side equatorial region. Although the origin of the asymmetry in the production flux is uncertain, high-energy calcium atoms can be supplied through a two-step process. In the first step, CaO may be ejected from the surface by impact vaporization, sputtering, or some other process. This molecule quickly photo-dissociates (within a few minutes), producing atomic Ca with 2 eV excess energy, equivalent to a speed of 3 km/s. This speed is less than Mercury's escape velocity (4.3 km/s), but radiation pressure can push Ca produced near the terminator into the tail. Simulations of possible source processes provide insight into these scenarios. A better understanding of the calcium source mechanism will be possible once MESSENGER enters orbit about Mercury and MASCS measures the global distribution of exospheric calcium.
Burger Matthew Howard
Killen Rosemary Margaret
McClintock William E.
Vervack Ronald J. Jr.
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