Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p13e..02o&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P13E-02
Physics
[5410] Planetary Sciences: Solid Surface Planets / Composition, [5421] Planetary Sciences: Solid Surface Planets / Interactions With Particles And Fields, [5422] Planetary Sciences: Solid Surface Planets / Ices, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
Results from the Moon Mineralogy Mapper (M3) on the Chandrayaan-1 spacecraft, the Visual and Infrared Mapping Spectrometer (VIMS) on Cassini during its flyby of the moon in 1999, and the extended mission for the Deep Impact Spacecraft (EPOXI) have recently implicated the existence of hydroxyl and water on the Moon. More recently, the potential presence of water on and within the surface regolith material of the Moon was somewhat validated by the Lunar Crater Observation and Sensing Satellite (LCROSS) impact event. LCROSS examined ejecta from a permanently-shadowed crater in the southern polar region, whereas the spacecraft observations characterized optical features in the 2-3.5 micron region throughout the polar and equatorial regions on the sunlit side. These optical features are thought to be indicative of hydroxyl and/or water-bearing materials with concentrations between 10 - 1000 parts per million and possibly higher. Though the source(s) of the hydroxyl and water is (are) not known, their formation via solar wind proton irradiation has been strongly suggested. There is precedent in the literature for proton irradiation-induced defect production and hydroxyl formation in silicates and some minerals. However, there are actually no reports that clearly indicate that proton irradiation leads to the formation of molecular water. We report our recent experimental probes of the desorption kinetics and binding energies of water adsorbed on minerals such as anorthosite, albite and a standard JSC1A lunar stimulant material. The temperature programmed desorption profiles clearly indicate the presence of chemisorbed water for all materials studied. The chemisorbed water requires intrinsic surface and grain boundary defects. We also report a combined low-energy (5-100 eV) electron and 5 keV proton (D+) beam bombardment study of anorthosite, albite and JSC-1A. Low-energy electron stimulated production and desorption of H3O+ (and DH2O+ after D+ bombardment) does occur, however, this involves the intrinsic water bound to surface defects and terminal -OH groups. Ion-beam bombardment does produce terminal -OH (OD) sites as expected, however, we do not observe ion-beam induced production of molecular water. Though molecular water formation is not induced via electronic excitations brought about by solar particle impact events, it may be produced via a thermally-induced recombinative desorption process. This seems to require temperatures well above the high temperatures (400K) reached as a result of solar irradiation and may also require trapped hydrogen.
Alexandrov Alexander
Darby Dyar M.
Greives G.
Hibbitts C.
McLain J.
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