An asteroidal origin for water in the Moon

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.

Figure 1. Cartoon showing the possible time windows and scenarios for the accretion of volatiles to the lunar interior.

Volatiles were accreted to the Moon during its formation (a) and/or continuously delivered by impacting bodies during the ca. 10–200 million years of crystallization of the lunar magma ocean (b). This graphic is not to scale.

Figure 2. Hydrogen isotope signatures for different objects in the Solar System.

The grey bar indicates the range of δD values predicted for the lunar interior based on the previous studies of water and its H-isotopic composition in lunar samples. (a) The H-isotopic compositions of comets, where the data for comets 1P/Halley, Hyakutake, Hale-Bopp, C/2002 T7 (ref. 63), C/2009 P1 (ref. 64), 8P/Tuttle, and 153P/Ikeya-Zhang, 45P/Honda-Mrkos-Pajdusakova, 103P/Hartley 2 (ref. 56) and 67P/Churyumov-Gerasimenko. (b) The average H-isotopic composition of apatite grains from eucrites, and the range in values for interplanetary dust particles (IDPs). (c) The bulk H-isotope data for bulk Tagish Lake (TL) and other carbonaceous and ordinary chondrites (CC and OC, respectively), carbonaceous and ordinary chondrite hydroxyl and organic matter. (d) Hydrogen isotope data for Enceladus and the jovian planets. (e) The H-isotopic compositions of martian apatite, martian melt inclusions, martian meteorite groundmass, martian atmosphere and martian crust and mantle. (f) Data for lunar apatite, lunar picritic glasses and lunar olivine-hosted melt-inclusions in picritic glass beads. (g) The range in H-isotopic compositions of H₂O on Earth. (h) The H-isotopic composition of Proto-solar and interstellar medium. For carbonaceous chondrites, CI-, CM-, CV-, CO- and CR- refer to the different groups, named according to one prominent meteorite of the group, respectively Ivuna, Mighei, Vigorano, Ornans and Renazzo. OC stands for ordinary chondrites. Error bars indicate measured analytical uncertainties, please see original references for more information (Encleadus, Jovian planets, and comets error bars, 1 s.d., and Eucrite data, 2 s.d.).

Figure 3. Mass of chondritic material added during late accretion compatible with BSM water estimates.

This figure shows the mass of the different types of chondritic material accreted (kg) to the Moon to add 10 (circles), 100 (squares) and 300 p.p.m. H₂O (diamonds), respectively, to a 400-km-deep LMO, and the corresponding amount in terms of lunar mass. The mass constraints imposed by HSE (0.02% lunar mass added) and highly volatile element abundances (up to 0.4% lunar mass added) are indicated by the dashed lines. Where: CI-type carbonaceous chondrites (CCs) are Ivuna-like, CO are Ornans-like, CV are Vigorano-like, CM are Mighei-like and CR are Renazzo-like, respectively.

Figure 4. An example of a two-component mixing model for scenario 1 considering that CI type CCs were dominant during the LAW.

This model assumes 100 p.p.m. H₂O in BSM, equivalent to 3.94 × 10¹⁸ kg of H₂O for a 400-km-deep LMO. The plot shows the resultant δD value and D/H ratios of the water mixture versus the amount of water (kg) supplied by CI-type carbonaceous chondrites as an example (results in Table 3). The bar underneath the x-axis shows how the mass of water is related to % of water mixed. Only final H-isotopic compositions below +100‰ (denoted by the blue box) are acceptable within the constraints of the model (see Methods). Where: CI-type carbonaceous chondrites (CCs) are Ivuna-like, CV are Vigorano-like CO are Ornans-like, CM are Mighei-like and CR are Renazzo-like, respectively. Note that it only takes a couple of per cent contribution of water from Oort or DEK comets to produce H-isotope compositions outside of the model limits. DDK, deuterium-depleted Kuiper belt comets; DEK, deuterium-enriched Kuiper belt comets; Oort, average H-isotopic composition of Oort cloud comets.

Figure 5. Two-component mixing models for scenario 2 considering that CI or CM or CO-type CCs dominated the impactor population during the LAW.

The resultant δD value of each water mixture (‰) versus the amount of water (kg) supplied by (a) CI-, (b) CM- and (c) CO-type carbonaceous chondrites. Table 3 also gives results from mixes with CV-type CCs. This model assumes that the LMO initially contained between 1% (dashed black lines) and 25% (solid black lines) of the BSM water (100 p.p.m. H₂O) with a δD value of −200‰. Only final H-isotopic compositions <+100‰ are acceptable within the model constraints (denoted by the black boxes). Where: CI-type carbonaceous chondrites (CCs) are Ivuna-like, CO are Ornans-like, CV are Vigorano-like, CM are Mighei-like and CR are Renazzo-like, respectively.