Study of Mercury's neutral molecular exosphere

Date
2015
Authors
Miles, Paul
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Abstract

A new era of exosphere study is beginning with observations being conducted by orbiting neutral mass spectrometers. We have developed a new analysis framework to improve our ability to invert the detection signal of these instruments into local and near-surface densities. By leveraging Liouville's theorem we are able to rapidly reach a source solution that best matches exosphere filling processes to an observable signal. Two approaches are developed in this work, the Liouville Algorithm and a traditional Forward Monte Carlo model that acts as validation. The Liouville Algorithm can be applied to the interpretation of photometric observations but is especially powerful for in situ study. This type of analysis is motivated by the velocity dependence of mass spectrometers, which demands that the velocity space be fully resolved. Several examples are described to demonstrate the consistency of the two approaches and highlight the strengths of the new algorithm when used in conjunction with or in lieu of Monte Carlo. When applied to the observed signal this algorithm can deduce the actual abundances of atomic species in the exosphere. Accurately interpreting the molecular abundances requires the additional step of inverting the fragmentation pattern in the mass spectrum. This is possible when the dissociative ionization cross sections for the molecule are known, however there are no reported data for most candidate species at Mercury. We establish a new method for experimentally determining the ionization cross sections of refractory molecules. This method and the preliminary results presented here will contribute to the effort of discovering molecules in Mercury's neutral exosphere. Species selection for cross section measurements performed here is based off predicted constituents whose atomic components have already been detected. Our technique though can be applied to a wide variety of other compounds that previously could never have been studied. Examples for the interpretation of exospheric data focus on Mercury and the BepiColombo instrument Strofio, but these methods apply equally to other surface bounded exospheres like those of the Moon or Europa.

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Keywords
Electron Impact Ionization, Exospheres, Mercury, Planetary Science
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Department
Physics and Astronomy