DELIVERY AND RECYCLING OF IMPACT GENERATED VOLATILES

Lunar missions reveal that the deep shadows at the lunar poles contain significant quantities of water. NASA’s LCROSS impact experiment, however, also revealed the presence of organic compounds and heavy species requiring low vaporization temperatures (Hg, Ag), which occur at very low levels (10 to 50ppb) in the lunar regolith. The combination of light and heavy species implicate the role of both impact delivery and regolith vaporization, in addition to processes related to solar wind bombardment. Small (<10cm) cometary relicts dominate the impact flux on the Moon, including cometary showers and storms. Because of their very high speed, retention of volatiles would seem unlikely. Although laboratory impact experiments at the NASA Ames Vertical Gun Range (AVGR) cannot achieve such speeds, they reveal fundamental processes that do apply. High-speed spectroscopy allows probing the vapor plume. We find that an impact-generated vapor from an oblique impact exhibits a wide range of conditions: low temperature components uprange, high-temperature gas filling the crater cavity, and a complexly evolving plume above the impact. Moreover, the expanding vapor fills the cavity (within a regolith) before condensing on regolith grains. It is proposed that this process results in temporary retention of volatiles on the lunar surface. Thermal recycling of the regolith release these trapped volatiles with much less internal energy. The direct trapping of volatiles in impact melts (Harris and Schultz, 2011) combined with vapor injected into the regolith provide a process by which volatiles can be trapped and released slowly to the cold traps at the poles. These products include delivered (trapped) volatiles as well as the build up of impact released heavier species, including Hg and Ag.