Comparison of Exospheric Observations by the Mercury Atmospheric and Surface Composition Spectrometer during the First and Second MESSENGER Flybys with Concurrent Ground-based Observations

Details Comparison of Exospheric Observations by the Mercury Atmospheric and Surface Composition Spectrometer during the First and Second MESSENGER Flybys with Concurrent Ground-based Observations Comparison of Exospheric Observations by the Mercury Atmospheric and Surface Composition Spectrometer during the First and Second MESSENGER Flybys with Concurrent Ground-based Observations

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u12a..07k&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #U12A-07

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0328 Exosphere

Scientific paper

Ground-based observations of Mercury's sodium exospheric emission were obtained at the McMath-Pierce Solar Telescope at Kitt Peak, Arizona, concurrently with the observations from the Ultraviolet and Visible Spectrometer (UVVS) on the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument onboard the MESSENGER spacecraft during the first flyby of the planet, January 14, 2008, and three weeks after the second flyby on October 6, 2008. During the first flyby the UVVS observed Na in the tail and nightside exosphere, and Ca in the dawn and nightside exosphere. Similar observations are planned for the second flyby. These will be compared with ground-based observations of calcium obtained at the Keck telescope with the HIRES spectrometer. Of particular interest is the morphology of the exosphere, which has been observed to be highly asymmetric and variable, at times to be peaked near the subsolar point and at other times peaked at one or both poles. The UVVS observations of the sodium tail revealed a N/S asymmetry near the planet during the first flyby. The cause of the high-latitude enhancements in the sodium exosphere has been variously ascribed to solar wind ion impacts, to radiation pressure, to inherent surface compositional and physical differences, and to cold trapping. The first and second flybys of MESSENGER are both near conditions of maximum radiation pressure for the inbound (towards the Sun) part of the orbit, while observations on October 27 - 30 span the maximum radiation pressure conditions for the outbound leg of the orbit. The effective radiation pressure is greatest on the outbound leg. Observations of the sodium tail will help determine the source of the energetic sodium. In-situ observations of the surface and magnetospheric environment will constrain the variations due to compositional differences and sputtering yield, respectively. Over the course of the mission, a goal is to determine the partitioning of Na and other species between the thermal and non-thermal components to determine or constrain the desorption processes from surface materials.

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