Abstract
The Apollo-derived tenet of an anhydrous Moon has been contested following measurement of water in several lunar samples that require water to be present in the lunar interior. However, significant uncertainties exist regarding the flux, sources and timing of water delivery to the Moon. Here we address those fundamental issues by constraining the mass of water accreted to the Moon and modelling the relative proportions of asteroidal and cometary sources for water that are consistent with measured isotopic compositions of lunar samples. We determine that a combination of carbonaceous chondrite-type materials were responsible for the majority of water (and nitrogen) delivered to the Earth–Moon system. Crucially, we conclude that comets containing water enriched in deuterium contributed significantly <20% of the water in the Moon. Therefore, our work places important constraints on the types of objects impacting the Moon ∼4.5–4.3 billion years ago and on the origin of water in the inner Solar System.
Highlights
The Apollo-derived tenet of an anhydrous Moon has been contested following measurement of water in several lunar samples that require water to be present in the lunar interior
Models for the differentiation of the Moon invoke the presence of a lunar magma ocean (LMO) and place the lifetime of this LMO to between B10 and 200 Ma after the Moon-forming event, which necessitates that volatile accretion occurred relatively early in the Moon’s geological history (Fig. 1)
We envisage two scenarios that can account for the bulk water inventory of the Moon, the first that water was accreted solely during the late accretion window (LAW) and no water was inherited from the Moon-forming material, and the second that a portion of the bulk silicate Moon (BSM) water was accreted with Moon-forming materials and was supplemented by water delivery during the LAW
Summary
The Apollo-derived tenet of an anhydrous Moon has been contested following measurement of water in several lunar samples that require water to be present in the lunar interior. It is likely that the lunar crust grew by continuous underplating of more anorthositic materials over a period of between B10 and B200 million years until the LMO completely solidified, leading to the mantle cumulate overturn (see review by Elkins-Tanton et al.[14]) This time period between lunar accretion and the overturn of mantle cumulates permits any water (and other volatiles) accreted to the LMO to be incorporated into the mantle source regions of mare basalts and picritic glasses (4B100 km deep15), and the urKREEP liquid provided that impactors were able to breach the lunar crust. We envisage two scenarios that can account for the bulk water inventory of the Moon, the first that water was accreted solely during the LAW and no water was inherited from the Moon-forming material (scenario 1), and the second that a portion (we consider up to 25%) of the BSM water was accreted with Moon-forming materials and was supplemented by water delivery during the LAW (scenario 2)
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