Prospecting For Helium-3 On The Moon

Drs. Jeffrey R. Johnson of the U.S. Geological Survey in Flagstaff, Arizona; Timothy S. Swindle of the University of Arizona's Lunar and Planetary Laboratory in Tucson; and Paul G. Lucey of the University of Hawaii's Institute of Geophysics and Planetology in Honolulu have developed a helium-3 map of the Moon based on a combination of factors they have analyzed. Their research will be published in a forthcoming issue of Geophysical Research Letters, a publication of the American Geophysical Union, as "Estimated Solar Wind-Implanted Helium-3 Distribution On The Moon."

The factors taken into account by the researchers in mapping the likely abundance of helium-3 in a given area are the exposure age of the Moon's surface matter, or regolith; the relative amount of charged particles, including helium-3, arriving from the Sun (the solar wind); and the titanium content of the lunar soil. The mineral ilmenite [FeTiO3], composed of iron, titanium, and oxygen, retains helium much better than other major lunar materials. The older soils should be better sources of helium-3, they report, because they have been exposed to the solar wind longer and contain greater amounts of fine-grained aggregates that absorb helium-3. Also, solar wind-implanted particles are more abundant on the far side, because the Earth shields the Moon's near side from the solar wind for a portion of each solar orbit.

The scientists estimate that the greatest amounts of helium-3 will be found on the far side maria, or "seas," of the Moon, due to the higher solar wind, and in nearside areas with high concentrations of titanium dioxide [TiO2]. Their hypothesis is based on analysis of rock samples brought back by Apollo astronauts and mineralogic maps produced by the Clementine spacecraft. They expect to refine their maps with new elemental composition maps produced by the Lunar Prospector spacecraft.