Colloquium: Shun Karato

Water in the Moon: Geophysical constraints and implications for the origin of the Moon

Shun-ichiro Karato, Professor
Department of Geology & Geophysics, Yale University

Abstract: Although the Moon was considered to be “dry”, recent measurements of hydrogen content in some of the lunar rocks show a substantial amount of water. However, the interpretation of these observations in terms of distribution of water in the Moon’s interior is difficult because the composition of these rocks reflects a complicated history involving melting and crystallization. Here, I use a different approach, i.e., analyze geophysical observations including electrical conductivity and tidal dissipation to obtain constraints on the distribution of water in the deep lunar interior. The conductivity-depth profiles inferred from electromagnetic induction observations were interpreted in terms of temperature and water content. The geodetically inferred tidal Q values are interpreted in terms of water content using a model of depth distribution of tidal strain energy and the lab-based model of anelastic properties. I conclude that high electrical conductivity in the deep lunar mantle (~10-2-10-1 S/m) and low tidal Q (40-60) require a substantial amount of water in the deep mantle of the Moon similar to Earth’s asthenosphere (~0.01 wt%). The inferred relatively high water content of the deep lunar mantle is consistent with a giant impact hypothesis if lunar accretion occurred quickly relative to the cooling time-scale of the Moon-forming disk.