solar wind sputtering and micrometeorite impact. Na and K are also present in the SBE atmospheres of Mercury, Io, and other Galilean satellites, thereby strengthening the utility of lunar SBE studies for enhancing knowledge of similar atmospheres across the solar system.

Science Goal 8d
—Learn how water vapor and other volatiles are released from the lunar surface and migrate to the poles where they are adsorbed in polar cold traps.

Evidence for volatile species, including H2O, CO, CO2, and CH4, was found sporadically by Apollo sensors, but these detections remain unconfirmed. The detection and study of volatiles are of great scientific interest and have obvious implications related to the trapping of ices that could represent resources to be exploited at the lunar poles. The expected sources of volatiles include comets, the solar wind, and meteoroids. Sinks include photodissociation, Jeans escape, solar wind pickup, and condensation. Once released from the surface by heating, sputtering, or other processes, the volatiles perform ballistic hops in a random walk across the surface. As the terminator is approached, the hops get smaller until the volatiles are adsorbed on the surface in darkness only to be released again at dawn. Modeling shows that there is a net migration toward the poles where the volatiles may be condensed in permanently shadowed depressions or craters. Future measurements should be designed to determine what processes (thermalization, release rate, and velocity) control atmospheric migration and what the efficiency of transport to the poles is.

Early observational studies to address these issues and the concern over human-induced modification of the ancient, native lunar environment should include the following:

  • A complete census and time variability of the composition of the lunar atmosphere;

  • Determination of the size, charge, and spatial distribution of electrostatically transported dust grains;

  • Determination of the time variability of indigenous (e.g., outgassing, sputtering) and exogenous (e.g., meteorite and solar wind) sources (see Figure 3.7);

FIGURE 3.7 Lunar volatile transport. SOURCE: D.H. Crider and R.R. Vondrak, The solar wind as a possible source of lunar polar hydrogen deposits, J. Geophys. Res. 105:26773, 2000, ©2000 American Geophysical Union. Reproduced by permission of American Geophysical Union.

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