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. Our examples focus on Mercury and the BepiColombo instrument SERENA-Strofio, but these methods apply equally to other surface bounded exospheres like those of the Moon or other airless bodies. (C) 2015 Elsevier Inc. All rights reserved.

Helium is one of the first elements clearly identified in the lunar exosphere (Hoffman, J.H., Hodges, R.R., Johnson, F.S., Evans, D.E. [1973]. Proc. Lunar Sci. Conf. 3, 2865-2875). Apollo 17 measured the He density at the surface during four lunations. It confirmed the expected day to night asymmetry of the He exosphere with a maximum density near the dawn terminator on the nightside. Few years later, the first detection of Mercury's He exosphere was successfully obtained by Mariner 10 (Broadfoot, AL., Shemansky, D.E., Kumar, S.[1976]. Geophys. Res. Lett. 3, 577-580). These observations highlighted similar global distribution of the He exosphere at Mercury and at the Moon, but also significant differences that have never been convincingly explained. In this paper, we model the He exosphere at the Moon and Mercury with the same approach. The energy accommodation of the exospheric He particles interacting with the surface can be roughly constrained using Apollo 17 and Mariner 10 measurements. Neither a low energy accommodation, as suggested by Shemansky and Broadfoot (Shemansky, D.E., Broadfoot, A.L. [1977]. Rev. Geophys. 15, 491-499), nor a full energy accommodation, as suggested by Hodges (Hodges Jr., R.R.[1975]. The Moon, 14, 139-157), can fit all the observations. These observations and their modeling suggest a diurnal variation of the energy distribution of the He ejected from the surface that cannot be explained satisfactorily by any of the present theories on the gas-surface interaction in surface-bounded exospheres. (C) 2011 Elsevier Inc. All rights reserved.

To ascertain the importance of sputtering by solar wind ions on the formation of a sodium exosphere around Mercury and the Moon, we have irradiated with 4 keV He ions, the Na bearing tectosilicates: albite, labradorite, and anorthoclase, as well as adsorbed Na layers deposited on albite and on olivine (a neosilicate that does not contain Na). Sodium at the surface and near surface (<40 angstrom) was quantified with X-ray photoelectron spectroscopy before and after each irradiation to determine the depletion cross section. We measured a cross for sputtering of Na adsorbed on mineral surfaces, sigma(s) approximate to 1 x 10(-15) cm(2) atom(-1). In addition, mass spectrometric analyses of the sputtered flux show that a large fraction of the Na is sputtered as ions rather than as neutral atoms. These results have strong implications for modeling the sodium population within the mercurian and the lunar exospheres. (C) 2011 Elsevier Inc. All rights reserved.