Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.2102v&link_type=abstract
American Astronomical Society, DPS meeting #42, #21.02; Bulletin of the American Astronomical Society, Vol. 42, p.985
Astronomy and Astrophysics
Astronomy
Scientific paper
The Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MESSENGER spacecraft observed Mercury's exosphere during each of the probe's three Mercury flybys (M1 on 14 January 2008, M2 on 6 October 2008, and M3 on 29 September 2009). Two previously observed species, Na and Ca, were seen in unprecedented spatial resolution, and two species - Mg and Ca+ - were discovered in the exosphere by MASCS. A dawn/dusk asymmetry in the Ca exosphere with a dawnside enhancement was found to be a persistent feature that was not previously known to exist. Surprisingly, a corresponding feature was not seen in Mg or the more volatile species Na. A dawn/dusk asymmetry in Na cannot necessarily be ruled out because MASCS did not observe the dayside. Most surprising was the observation of a region of intense concentration of Ca+ about 2-3 planetary radii anti-sunward from the planet's center. This measurement provides evidence for tailward magnetospheric convection of photoions produced inside the magnetosphere. Observations of neutral sodium, calcium, and magnesium above the planet's north and south poles revealed altitude distributions that are distinct for each species. Strong emission was seen above the poles in each of these species, but the altitude distribution was most unusual for Mg and is inconsistent with an exponentially decreasing density with altitude. A two-component exosphere was clearly measured for Na, indicating that a combination of processes is at work in producing and maintaining the exosphere. Whereas the Na distribution is consistent with ground-based observations, the differences in spatial distributions for the chemically similar elements, Ca and Mg, are unexpected and remain unexplained.
Anderson Benjamin J.
Benna Marcus
Bradley Eric Todd
Burger Matthew Howard
Izenberg Noam R.
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