1. Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
2. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington DC 20015, USA
3. Department of Geological Sciences Case Western Reserve University, Cleveland, Ohio 44106, USA

Correspondence to: Alberto E. Saal¹ Correspondence and requests for materials should be addressed to A.E.S. (Email: asaal@brown.edu).

The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events¹, during which most of the highly volatile elements were lost. Hydrogen, being the lightest element, is believed to have been completely lost during this period². Here we make use of considerable advances in secondary ion mass spectrometry³ to obtain improved limits on the indigenous volatile (CO₂, H₂O, F, S and Cl) contents of the most primitive basalts in the Moon—the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon's formation and its thermal and chemical evolution.

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