Water in the Lunar Interior: Truth and Consequences

Abstract:

Geochemical data for H₂O and other volatiles, as well as major and trace elements, in 384 samples of lunar volcanic glass from three chemical groups (A15 green, A15 yellow, A17 orange 74220) constrain the behavior of volatile elements during formation and evolution of the Moon. These data are combined with published data to estimate the composition of the bulk silicate Moon (BSM). Previous estimates of the volatile element budget of the BSM, constrained by the compositions of mare basalts, are biased to low concentrations due to degassing of volatiles during mare basalt eruption and cooling; lunar picritic glasses, which have very short cooling times, are much less depleted in volatiles compared with mare basalts. The estimated BSM composition for volatile elements, constrained by H₂O/Ce ratios and S contents in melt inclusions from orange glass sample 74220, are only moderately depleted compared with the bulk silicate Earth (0.1 – 0.3X BSE) and essentially overlap the composition of the terrestrial depleted MORB source. In a single giant impact scenario for the origin for the Moon, the Moon-forming material experiences three distinct stages of evolution characterized by very different timescales. Impact mass ejection (hours to days) and proto-lunar disk evolution (10s to 100s of years), both produce conditions that separate volatile elements into an atmospheric volume much larger than the Moon-forming magma disk, even under conditions in which hydrodynamic escape of hydrogen is difficult; this inevitably results in accretion of the Moon in a volatile-depleted state while the majority of the vapor surrounding the disk becomes incorporated into the Earth’s atmosphere. Only the extended evolution of the lunar magma ocean (LMO) presents a time window sufficiently long (10-200 Ma), and at the right time (4.30 – 4.50 Ga), for the Moon to gain water during the tail end of accretion. Yet there exists little evidence that the Moon formed in a singular event, as all detailed accretion models predict several giant impacts in the terrestrial planet region in which the Earth forms. It is thus likely that the Moon, like the Earth, experienced a history of heterogeneous accretion, with the possibility that the Moon may have inherited material directly from the Earth prior to the final giant impact.