Exploring Mercury's Surface-bound Exosphere: An Overview of Observations During the MESSENGER Orbital Phase

Details Exploring Mercury's Surface-bound Exosphere: An Overview of Observations During the MESSENGER Orbital Phase Exploring Mercury's Surface-bound Exosphere: An Overview of Observations During the MESSENGER Orbital Phase

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p44a..01m&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P44A-01

Other

[5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [6235] Planetary Sciences: Solar System Objects / Mercury

Scientific paper

Prior to the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, Mercury's surface-bounded exosphere was known to contain H, He, Na, K, and Ca. During three flybys en route to orbit, the Ultraviolet and Visible Spectrometer (UVVS) channel of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on MESSENGER detected Ca+ in a narrow region 2.5 Mercury radii behind the planet's terminator, mapped Na, Ca, and Mg in the tail region, and measured altitude profiles of H on the dayside. UVVS began routine orbital observations of both the dayside and nightside exosphere on March 29, 2011, measuring altitude profiles for all previously detected neutrals except for He and K. The former has no emission features within the UVVS wavelength range (115 - 600 nm), and the latter has only one relatively weak feature there. Distributions of all species are variable and do not correlate with each other, consistent with flyby results. UVVS also scans for species that are known or are predicted to be present in the surface materials (e.g., Si, Al, S, Mn, Fe, and OH), but these emissions are not sufficiently bright for routine observation under current operational scenarios. Targeted sequences, scheduled for MESSENGER's third and fourth Mercury years, will measure the average exosphere content of species that are detected and substantially reduce the detection limits for the rest. The UVVS team uses a variety of techniques to relate exosphereic composition and structure to source processes, including tomographic inversion and Monte Carlo modeling. In addition, correlations of Mercury's neutral exosphere with measurements from MESSENGER's Magnetometer (MAG) and Energetic Particle and Plasma Spectrometer (EPPS) provide additional insight. Models incorporating MAG observations identify the location and area of the surface exposed to solar wind plasma, while EPPS observations reveal episodic populations of energetic electrons in the magnetosphere and the presence of planetary He+, O+, and Na+. The electrons impact the surface producing X-rays, which have been observed by MESSENGER's X-Ray Spectrometer, and likely triggering electron-stimulated desorption.

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