The origin, history and role of water in the evolution of the inner Solar System

Water and/or hydroxyl detected remotely on the lunar surface may originate from several sources: 1) comets and other exogenous debris; 2) solar-wind implantation; 3) the lunar interior. While each of these sources are interesting in their own right, distinguishing among them is critical for testing hypotheses for the origin and evolution of the Moon and our solar system. Existing spacecraft observations are not of high enough spatial and spectral resolution to uniquely characterise the bonding energies of the hydroxyl molecules that have been detected. Nevertheless, the spatial distribution and associations of H, OH^-, or H₂O with specific lunar lithologies provide some insight into the origin of hydrous materials. Further inferences can be made on the basis of laboratory measurements and computational models that have been developed to constrain the production of solar wind hydroxyl and the migration and stability of any OH^- or H₂O molecules on the surface. The current understanding, limitations, and future outlook for mapping and characterising water and hydroxyl on the lunar surface will be discussed.

Recent sample studies have demonstrated that the lunar interior likely contains appreciable quantities of water, although estimates for the bulk-water content of the Moon vary considerably. The origin and evolution of this water remain unresolved with a range of possibilities, including retention of primordial water to a later addition of water following the crystallisation of the putative Lunar Magma Ocean.

In addition to water, recent investigations focussing on other volatiles (e.g., C, N, Cl), have brought about a paradigm shift in our understanding of the history of lunar volatiles and their wider implications for volatiles in the inner Solar System. 

We have been investigating the volatile inventory of the Moon through laboratory analyses of lunar samples. Our main target has been measuring volatiles, such as OH, F and Cl, in apatite using NanoSIMS. These studies are complemented by bulk analyses of lunar samples for their C, N and noble gas signatures using stepped-combustion.

Some highlights from our work carried out on lunar apatite include the revelation of significant effects of degassing on the initial magmatic H- and Cl-isotopic compositions, and the possibility of a common source of water in the Earth-Moon system. Whilst the dominant source for lunar water appears to be carbonaceous chondrite-type material, it is becoming apparent that the Moon may have received volatiles from multiple sources at different stages of its geological history. Unravelling this history will require further study of existing lunar samples, but also ‘new’ samples from regions of the Moon not sampled previously.

Based on meteorite measurements, the ratio of hydrogen (¹H) to deuterium (²H or D) differs for each type of rocky body in the inner Solar System. In astrophysical environments water is D-rich, but exchange with D-poor H₂ gas can decrease the D/H ratio. Thus, water close to the Sun, where higher temperatures drive ion exchange, has low D/H ratios relative to water further away from the Sun, where colder temperatures mean ion exchange is sluggish. Therefore, a meteorites D/H ratio can indicate how and where its parent body formed in the Solar System. For example, could Earth have formed from carbonaceous chondrites? However, hydrogens volatility and incompatibility in silicate melts means that separate water and hydrogen reservoirs can form on the same planetary bodies (e.g., surface vs. interior), with processes such as evaporation, sublimation, atmospheric stripping, impact shock events, aqueous alteration and plate tectonic recycling (on Earth) all affecting the D/H ratio of different reservoirs and/or individual meteorites over geological time. For example, water on the surface of Mars and Earth appears to be more D-rich than water from the interior of these planets. Therefore, if D/H ratios are to tell us anything about the Solar System’s formation it is important to unravel the processes each meteorite has experienced, and how these processes are likely to have affected the D/H ratio.

The bulk Earth composition contains only less than 1% of water, but this trace amount of water can affect the long-term evolution of the Earth in a number of different ways. The foremost issue is the occurrence of plate tectonics, which governs almost all aspects of the Earth system, and the presence of water could both promote and prevent the operation of plate tectonics, depending on where water resides. Global water cycle, which circulates surface water into the deep mantle and back to the surface again, is thus suggested to have played a critical role in the Earth history. In this contribution, we first review the present-day water cycle and discuss its uncertainty as well as its secular variation. The evolution of mantle dynamics and surface environment are linked through global water cycle, and we explore a range of co-evolutionary scenarios to take into account various uncertainties. We survey relevant geological and geochemical observations to distinguish between different possibilities. The initiation of plate tectonics in the early Earth is suggested to be possible with abundant surface water and dry mantle, and the gradual introduction of water into the mantle by subduction could potentially explain a major geological transition from the Archean to the Proterozoic and even the uprise of oxygen in the early Proterozoic.

The formation of the Earth is now understood as a rapid transition from dust and gas to planetesimals, followed by a more protracted period of gravitationally-driven growth through planetary embryos the size of Mars, and finally to a completed planet. In tracing water through this process we find increasing evidence that while each heating stage involves the loss of water, none are sufficient to dry the planetary material. Internal heat of planetesimals by short-lived radioisotopes likely lead to some water loss, but impacts into planetesimals were insufficiently energetic to produce further drying. In contrast, the giant impacts of late accretion created magma lakes and oceans, which degassed during solidification to produce a heavy atmosphere.

On an Earth-sized planet a magma ocean would solidify to produce dense near-surface solids that contain the bulk of the interior water, held in the solid state. During gravitationally-driven overturn these solids sink deeper into the mantle and dewater. This event would have the potential to partially melt the upper mantle and to produce a damp asthenosphere. The transition from magma ocean to modern-day plate tectonics is very poorly understood. The earliest part of this transition, however, was likely dominated by an enormous flux of extra-terrestrial bodies. The surface of the Hadean Earth was likely widely reprocessed by impacts through mixing and melting, and the earliest atmosphere would have been highly altered by this process.

The presence of solar nebular gases in the earliest stages of terrestrial planet accretion provide a starting point for considering the origin of volatiles and water on and in Earth. Nevertheless, Earth likely accreted most of its mass in the absence of solar nebular gases. Planetary volatile acquisition after nebular dispersion is sourced from volatiles trapped within accreting meteoritic or cometary material, surviving on accreting planetisimals, or as part of a later veneer of similar material associated with the late heavy bombardment ending at ca3.8Ga. Noble gases provide a view on these processes. The extreme deficit of terrestrial Xe isotopes derived from ¹²⁹I and ²⁴⁴Pu has been used to argue for complete planetary loss of all gaseous species until closure to loss at ca100Myr after Solar Nebular formation. This closure ages appears to be the same for the Earth’s interior and atmosphere and points to a process capable of efficiently removing volatiles, including water, from the whole early planet. Such a loss process would point to a ‘dry’ accretion. Complete loss of early planetary volatiles is supported by the shielded isotopes of Ne, Ar and Kr within the Earth’s mantle (He and Xe cannot distinguish between sources) which are dominated by a meteoritic volatile source, albeit with a subducted atmosphere overprint. Whether or not some part of the Earth’s deep mantle, sampled by mantle plumes, preserves a remnant of Solar Nebular volatiles remains uncertain. There are no viable processes to link the noble gases in the Earth’s mantle alone to the Earth’s atmosphere. The Earth’s atmosphere requires a Solar-like noble gas component; one that has either mixed with earth interior meteoritic gases or mass fractionated to form the composition seen today. The most likely delivery of these Solar-like noble gases are cometary material. Although noble gas concentrations within cometary material are poorly defined, calculations suggest that the amount of cometary material to have supplied the atmosphere noble gases may provide an insignificant amount of water to Earth. Earth’s surface water is then most likely meteoritic (consistent with hydrogen and nitrogen isotopes) and delivered late to the Earth (>100Ma), most likely in the late heavy bombardment.

Geochemists have long relied on fluid inclusions, microscopic time capsules of fluid and gas encased in host rocks and fracture minerals, to access preserved samples of ore-forming fluids, metamorphic fluids, and remnants of the ancient atmosphere and hydrosphere. Until recently, groundwaters were thought to reflect only much younger periods of water-rock interaction (WRI) and Earth history, due to dilution with large volumes of yournger fluids recharging from surface (terrestrial) or mixing from the over-lying oceans. The earliest periods of Earth’s fluid history were largely thought to have been overprinted by mixing, and/or geochemically reset, at least on a macroscopic scale.

Global investigations in the world’s oldest rocks have recently revealed groundwaters flowing at rates > L/min from fractures at km depth in Precambrian cratons. Rich in reduced dissolved gases such as CH₄ and H₂, these fracture waters have been shown to host extant microbial communities of chemolithoautotrophs dominated by H₂-utilising sulfate reducers and, in some cases, methanogens. With mean residence times ranging from Ma to Ga at some sites, and in the latter case, geochemical components of Archean provenance, not only do these groundwaters provide unprecedented samples for investigation of the Earth’s ancient hydrosphere and atmosphere, they are opening up new lines of exploration of the history and biodiversity of extant life in the Earth’s subsurface.