Geophysical constraints on the water content of the lunar mantle and its implications for the origin of the Moon

摘要

Although the Moon was considered to be "dry", recent measurements of hydrogen content in some of the lunar samples showed a substantial amount of water comparable to the water content in the Earth's asthenosphere. However, the interpretation of these observations in terms of the distribution of water in the lunar interior is difficult because the composition of these rocks reflects a complicated history involving melting and crystallization. In this study, I analyze geophysically inferred properties to obtain constraints on the distribution of water (and temperature) in the lunar interior. The electrical conductivity inferred from electromagnetic induction observations and the geodetically or geophysically inferred Q are interpreted in terms of laboratory data and the theoretical models on the influence of water (hydrogen) on these properties. Both electrical conductivity and Q are controlled by defect-related processes that are sensitive to the water (hydrogen) content and temperature but less sensitive to the major element chemistry. After a correction for the influence of the major element chemistry constrained by geophysical observations and geochemical considerations, I estimate the temperature-water content combinations that are consistent with the geophysically inferred electrical conductivity and Q. I conclude that the lunar interior is cooler than Earth (at the same depth) but the water content of the lunar mantle is similar to that of Earth's asthenosphere. A possible model is presented to explain the not-so-dry Moon where a small degree of water loss during the Moon formation is attributed to the role of liquid phases that play an important role in the Moon-forming environment.