Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p21a1210b&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P21A-1210
Physics
[5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [6235] Planetary Sciences: Solar System Objects / Mercury
Scientific paper
Calcium in Mercury’s exosphere and neutral tail was measured with the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) visible channel during MESSENGER’s three Mercury flybys. These observations, which measured the atomic resonance line at 422 nm, show calcium to be extended in the anti-sunward direction with emission peaking near equatorial latitudes. While the spacecraft was in the planet’s shadow and moving closer to the planet, observations with the line of sight initially pointed toward the dawn hemisphere and then rotated ~ 180° to point toward the dusk hemisphere showed that calcium emission exhibits an enhancement on the dawn side. Previous ground-based observations have shown calcium to be highly energetic with temperatures in the range 12,000-20,000 K. The high equivalent temperature of the distribution and the short ionization time, which precludes significant acceleration by solar photons, imply that the source mechanism must be a high-energy process. A possible explanation is that calcium is ejected from the surface as a molecule by a lower-energy process and the molecule is then photo-dissociated while it is above the surface. The data from the extended tail region were compared with an analytic model to retrieve temperature and exospheric surface density under the assumption that calcium was photo-dissociated from CaO. The data were converted to column abundances along the line of sight by dividing the measured radiance by the g-value (number of photons scattered per atom per second) and correcting for ionization loss, both of which depend on the characteristic energy and initial height above the surface of the dissociated calcium atoms. Comparison of MASCS measurements to model simulations over a wide range of energy distributions supports the conclusion that the calcium emission reflects a very energetic distribution of calcium atoms with a characteristic temperature greater than 10,000 K, consistent with temperatures inferred from ground-based measurements.
Bradley Eric Todd
Burger Matthew Howard
Killen Rosemary Margaret
McClintock William E.
Mouawad Nelly
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