The origin of volatile elements in the Earth–Moon system

The origin of volatile species such as water in the Earth–Moon system is a subject of intense debate but is obfuscated by the potential for volatile loss during the Giant Impact that resulted in the formation of these bodies. One way to address these topics and place constraints on the temporal evolution of volatile components in planetary bodies is by using the observed decay of ⁸⁷Rb to ⁸⁷Sr because Rb is a moderately volatile element, whereas Sr is much more refractory. Here, we show that lunar highland rocks that crystallized ∼4.35 billion years ago exhibit very limited ingrowth of ⁸⁷Sr, indicating that prior to the Moon-forming impact, the impactor commonly referred to as “Theia” and the proto-Earth both must have already been strongly depleted in volatile elements relative to primitive meteorites. These results imply that 1) the volatile element depletion of the Moon did not arise from the Giant Impact, 2) volatile element distributions on the Moon and Earth were principally inherited from their precursors, 3) both Theia and the proto-Earth probably formed in the inner solar system, and 4) the Giant Impact occurred relatively late in solar system history.

Understanding the formation of the Moon has long been a topic of intense interest, although hard constraints on this event only developed after the Apollo program returned lunar samples to Earth. Based on the thousands of lunar rocks that have been studied to date, arguably one of the most stringent of these constraints is that the Moon is strongly depleted in volatile elements relative to the solar photosphere, primitive meteorites, and Earth. Recognition of such a depletion of volatile elements, combined with the orbital mechanics of the Moon and geochemical evidence that it differentiated from a mostly molten state, led to the now widely accepted “Giant Impact” hypothesis, in which the Moon accreted from a volatile element–depleted debris disk produced by an impact between a Mars-sized body (Theia) and the proto-Earth (1). Yet, the formation of the Moon through such an impact scenario raises questions about the composition of the proto-Earth and Theia and their respective contributions to the makeup and subsequent evolution of the Earth–Moon system. Of particular interest is how and when the Moon and Earth obtained their present allotments of volatile components, including, and most importantly, water. Did the Moon and Earth form with their current allotments of volatile elements, or were these elements lost during the Giant Impact and reintroduced to Earth by later accretion of volatile element–laden materials? Here, we address this issue using the Rb–Sr isotopic systematics of lunar samples to provide time constraints on the history and distribution of volatile elements in the Earth–Moon system.