THE ROLE OF IMPACT MELTING IN THE EVOLUTION OF LUNAR WATER

Recent work by Saal et al. (2013) demonstrating the possibility that water sequestered in the lunar mantle was derived from the Earth during the Giant Impact triggered headlines that the findings cast doubt on that widely accepted mechanism of Moon formation. At issue is the question of how volatile compounds could survive the violent conditions of such an event.

We have shown through analysis of ejecta created during terrestrial and experimental impacts that volatiles can be trapped efficiently in the molten and quenched debris produced during hypervelocity collisions. Our results suggest that water derived from both wet target materials and water-bearing bolides can become trapped in aluminosilicate melts at very high concentrations, in some cases greater than 20 wt%. Although some of the water might be recondensed and absorbed into the melts from a vapor plume, our observations support the conclusion that much of the water is dissolved into the melt more directly from the superheated rocks or regolith. The unique transient high temperatures and pressures likely are required to allow these melts to form. Rapid quenching, usually within masses of nominally anhydrous impact glass, likely is a requisite condition of their long-term survival. If such melts were produced during a Moon-forming impact from wet terrestrial material and reaccreted, they should provide a robust internal reservoir of volatiles for much of lunar history.

Younger impacts of hydrous asteroids and comets also might be expected to contribute water-bearing glass to the lunar surface. These may contribute significantly to the inventory of water observed in the regolith and on the present surface, especially near fresh crater ejecta fields.