Quantitative estimation of helium-3 spatial distribution in the lunar regolith layer

Abstract

3He (helium-3) in the lunar regolith implanted by the solar wind is one of the most valuable resources because of its potential as a fusion fuel. The abundance of 3He in the lunar regolith is related to solar wind flux, lunar surface maturity and TiO 2 content, etc. A model of solar wind flux, which takes account of variations due to shielding of the nearside when the Moon is in the Earth's magnetotail, is used to present a global distribution of relative solar wind flux over the lunar surface. Using Clementine UV/VIS multispectral data, the global distribution of lunar surface optical maturity (OMAT) and the TiO 2 content in the lunar regolith are calculated. Based on Apollo regolith samples, a linear relation between 3He abundance and normalized solar wind flux, optical maturity, and TiO 2 content is presented. To simulate the brightness temperature of the lunar surface, which is the mission of the Chinese Chang-E project's multichannel radiometers, a global distribution of regolith layer thickness is first empirically constructed from lunar digital elevation mapping (DEM). Then an inversion approach is presented to retrieve the global regolith layer thickness. It finally yields the total amount of 3He per unit area in the lunar regolith layer, which is related to the regolith layer thickness, solar wind flux, optical maturity and TiO 2 content, etc. The global inventory of 3He is estimated as 6.50 × 10 8 kg , where 3.72 × 10 8 kg is for the lunar nearside and 2.78 × 10 8 kg is for the lunar farside.