Paper No. 3
Presentation Time: 1:40 PM
SOLAR WIND BOMBARDMENT OF LUNAR SOIL: FTIR STUDIES OF OH (OD) AFTER H+ (D+) IMPLANTATION
The origin of water on the Moon is a persistent question and sources such as trapped volcanic gas, comet and meteor impacts, and the interaction of the solar wind with lunar soil have been suggested. It is well known that proton bombardment of oxides such as silica, alumina, and titania produces characteristic hydroxyl IR signals at 3 μm by reaction with oxygen. The solar wind is a constant source of low energy 1 – 3 keV protons that bombards the lunar surface. Ion-beam and IR experiments (Ichimura et al
., EPSL, 345
(2012) 90-94) employed lunar highland and mare soils, in addition to <45 μm Stillwater plagioclase. These experiments were the first to use lunar soils in H+
) ion-beam experiments and thus connect the formation of OH (OD) to the 3 μm band observed by remote sensing of the lunar regolith. The solar wind hypothesis is thereby supported by these experiments. However, one of the limitations of the previous study was the high temperature treatment (500 o
C) of the lunar soil, which was used to deplete the soil of ambient water and intrinsic solar wind derived OH before ion-beam bombardment.
In the present work, the bake-out temperature of the lunar soil is varied in order to drive off adsorbed water, but retain some subsurface OH located within the amorphous lunar rims prior to D+ implantation. IR spectra of D+ implanted specimens at 150 oC < T < 500 oC show a loss of OH and growth of OD signals for both highland and mare lunar soils. In addition to OD formation, substantial sputtering and concomitant reduction of the specimen surfaces took place during these D+ bombardment experiments. Additional experiments probed the possibility that diffusion of H2 and D2 gas could account for the OH and OD signals due to the pressure 10-5 torr in our setup. It was found that H2 uptake by the lunar soil and concomitant OH formation was significantly less than that formed by the H+ or D+ ion-beams. Our work supports the solar wind formation of OH in the lunar regolith. If OH is removed by sputtering, then there is the possibility for redistribution of solar wind produced OH to cold traps and subsequent formation and preservation of water.