Other
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p24a..04m&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P24A-04
Other
[5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [5443] Planetary Sciences: Solid Surface Planets / Magnetospheres
Scientific paper
Mercury’s exosphere is the interface between the planet’s surface and the surrounding space environment. The exosphere’s composition and structure are controlled by interactions among the surface, magnetosphere, solar wind, and sunlight. Prior to the MESSENGER mission the exosphere was known to contain H and He, observed by Mariner 10, as well as Na, K, and Ca, observed from the ground. When species are liberated from the surface with sufficient energy they can be accelerated by solar radiation pressure to form an anti-sunward tail. During the MESSENGER flybys, the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) mapped Na, Ca, and Mg in the planet’s tail region as well as on the night-side portion of the dawn hemisphere. Na was seen at greater distance (up to 56,000 km) than Ca and Mg, which were first detected with statistical significance starting approximately 22,000 km behind the planet. Because radiation pressure is relatively large for Na, atoms released from the surface with anti-sunward velocities as small as 1.4 km s-1 (corresponding to 0.23 electron volts (eV) of kinetic energy) can be accelerated down the tail and escape. On the other hand, Ca and Mg atoms must be released anti-sunward with energies greater than 5 eV and 2.5 eV, respectively, in order to reach distances of 20,000 km. If these atoms originate from ion sputtering or impact vaporization, they must come from the high-energy tails of these sources. Emission maps of the three species also reveal differing spatial distributions close to the planet. Na exhibits north-south enhancements that differ in strength from flyby to flyby. In contrast, Ca shows enhancements in the dawn hemisphere near the equator while Mg appears to be more uniformly distributed than either Na or Ca. The north-south enhancements are correlated with the distribution of solar wind plasma reaching Mercury’s surface, suggesting that ion sputtering is an important source of Na. High-energy processes are required to deliver Ca and Mg to the extended tail. One hypothesis is that Ca and Mg are released from the surface as molecules that are subsequently photo-dissociated. While the Na distribution is consistent with ground-based observations, the differences in spatial distributions for the chemically similar elements of Ca and Mg are unexpected and remain unexplained.
Benna Marcus
Bradley Eric Todd
Burger Matthew Howard
Izenberg Noam R.
Killen Rosemary Margaret
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