Magnesium Atoms in the Exosphere Above the Volatile-rich Persistently Shadowed Region of Cabeus

We discuss properties of the lunar exosphere based on analyses of UV-visual spectrometer (VSP) (260--650 nm) data collected from the 1-degree nadir field-of-view, which narrowed in on the persistently shadowed region (PSR) of Cabeus as the spacecraft descended towards the surface. The LCROSS Mission consisted of a ~2000 kg Centaur impactor and a shepherding spacecraft (S-SC) that impacted the Moon 4 minutes after the Centaur. The LCROSS VSP spectra of the nadir-view of Cabeus PSR spectra reveal a forest of UV emission lines. Most of these emission lines were previously predicted [1,2,3] but not yet measured because the Moon has yet to been investigated at these wavelengths at such close distance. In the LCROSS UV spectra, we have identified Mg I (285.2 nm) [4] and tentatively identified lines of Ca, Fe, Ti, and Al; most emission lines appear to be from neutral atoms, with a few possible singly-ionized species, fluorescing in sunlight from near their ground states. The temporal evolution of the lines, derived after subtracting a solar reference spectrum scaled to the scattered light continuum, probe the altitude-dependence of exosphere species. Only a few species appear transient, such as the Na D lines that were released by impact [5] and expelled quickly to high altitudes (45 km, [6]). We are working to derive the column densities of the various species, with particular emphasis on MgI, by application of a fluorescence model. Some of the work involves careful re-calibration of the VSP at wavelengths shortward of 380 nm. We will report and compare the column density of magnesium over Cabeus PSR to the column density expected from exosphere models. Sputtering of materials from the floor of Cabeus may be enhanced because of the particular geometry of the solar wind flowing over polar craters acts to deflect passing ions into the polar craters and enhance sputtering rates [7,8]. Studying the lunar exosphere can tell us generally about volatile transport processes. These data stimulate interest in the comparison of exosphere observations by LRO LAMP in its polar-orbit versus the near-equatorial orbit of the future LADEE UVS. [1] Morgan T. H. and Killen R. M. (1997) Planet. Space Sci. 45, 81-94. [2] Sarantos M. et al. (2010) LEAG LPI Contrib. No. 1595, p. 62 [3] Killen, R. M. et al. (2010) AGU Abstract #P42A-04 [4] Wooden, D. H. et al. (2011) Wet vs Dry Moon, Abstract 6067. (http://www.lpi.usra.edu/meetings/volatiles2011/pdf/6067.pdf) [5] Colaprete A. et al. (2010) Science, 330, 463-468 [6] Killen, R. M. et al. (2010) Geophys. Res. Lett., 37, Issue 23, CiteID L23201 [7] Farrell, W. M. et al. (2011) Wet vs Dry Moon, Abstract 6015. (http://www.lpi.usra.edu/meetings/volatiles2011/pdf/6015.pdf) [8] Farrell W. M. et al. (2010), J. Geophys. Res. 115, E03004