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

09:05-09:30 The origin of inner solar system water

Dr Conel Alexander, Carnegie Institute for Science, USA

Abstract

Midplane temperatures at ~1 AU in the solar protoplanetary disk were probably too warm for planetesimals to have accreted water-ice. In the classical picture, the terrestrial planets acquired their volatiles when planetesimals that formed in the outer asteroid belt (3-4 AU) were scattered into the terrestrial planet region. However, two recent models suggest that the volatile-rich asteroids formed in the outer Solar System and are the remnants of a population of planetesimals that were scattered into the inner Solar System by the radial migration of the giant planets. The H isotopes of water are predicted to show a strong radial gradient due to mixing between D-rich interstellar water and water that re-equilibrated with H₂ in the hot inner Solar System. The D/H of meteoritic water suggests that at most ~7-10% of it is interstellar in origin, and that the meteorites formed sunward of the formation locations of Saturn’s moons (~3-7 AU in the migration scenarios). The N isotopic compositions of meteorites are also consistent with this. The O isotopic compositions of water in outer Solar System objects are predicted to exhibit large mass independent anomalies due to formation by UV photodissociation of CO in the interstellar medium and/or outer Solar System. While meteoritic water is significantly different from the solar composition, like most known solar materials, it is far less anomalous than some predictions, again pointing to an origin in the inner Solar System.

09:30-10:00 Evaluating the significance of comets in relation to the Earth’s water budget – the perspective from the Rosetta space mission

Professor Ian Wright, Open University, UK

Abstract

The water that exists at the surface of the Earth, both today, and through most of geological history, ultimately had to come from somewhere. In principle, it could have arisen from a process whereby volatiles that were included into the planet at the time of its formation subsequently migrated to the surface during degassing. Alternatively, the water could have been added from external sources some time after the major planet-building process was complete. In either case, the constraint has to be that the surface was stable enough to allow (liquid) water to accumulate; in other words, the crust had to have solidified and cooled down from whatever state it was in at the time of accretion (since during this era, conditions at the surface were such that water of whatever provenance was largely lost to space). So, in simplistic terms, the water on Earth either came from below (inside), or above (outside). Or perhaps, more likely, it came from a combination of sources. The Rosetta space mission to 67P/Churyumov-Gerasimenko had, as one of its goals, to try and assess the relevance of cometary water to the overall budget on Earth. We will review the extent to which it was successful in this regard, and consider the role of comets in the delivery of water to Earth.

11:00-11:30 The search for surviving direct samples of early Solar System water

Dr Michael Zolensky, NASA Johnson Space Center, USA

Abstract

We have become increasingly aware of the fundamental importance of water, and aqueous alteration, on primitive solar-system bodies. All classes of astromaterials studied show some degree of interaction with aqueous fluids. Nevertheless, we are still lacking fundamental information such as the location and timing of the aqueous alteration and the detailed nature of the aqueous fluids. Halite crystals in two meteorite regolith breccias were found to contain aqueous fluid inclusions (brines) trapped ~4.5 BYBP. Heating/freezing studies of the aqueous fluid inclusions in these halites demonstrated that they were trapped near 25°C. The initial results of our O and H isotopic measurements on these brine inclusions can be explained by a simple model mixing asteroidal and cometary water. We have been analysing solids and organics trapped alongside the brines in the halites by FTIR, C-XANES, SXRD and Raman, as clues to the origin of the water. The organics show thermal effects that span the entire range witnessed by organics in all chondrite types. Since we identified water-soluble aromatics, including partially halogenated methanol, in some of the halite, we suspected amino acids were also present, but have thus far found that levels of amino acids were undetectable (which is very interesting). We have also been locating aqueous fluid inclusions in other astromaterials, principally carbonates in CI and CM chondrites. Although we have advanced slowly towards detailed analysis of these ancient brines, since they require techniques right at or just beyond current analytical capabilities, their eventual full characterisation will completely open the window on the origin and activity of early solar system water.

11:30-12:00 Professor Monica Grady, Open University, UK

13:30-14:00 What do recent orbital and rover observations tell us about the history of water flow and stability at the martian surface?

Professor Sanjeev Gupta, Imperial College London, UK

Abstract

Mars provides the only other evidence aside from Earth of the significant flow of water at a planetary surface. Four decades of orbital observations have revealed a rich record of erosional and depositional landforms that are inferred to have formed by fluvial processes. The ever increasing resolution of image and topographic datasets has provided significant confidence in our interpretation of past aqueous processes, in particular through comparison with Earth analogues. Moreover, the discovery in orbital images of sedimentary rock deposits exhumed at the martian surface clearly demonstrates, from their preserved morphology, deposition in ancient river and deltaic systems. These results provide vital information to reconstruct past climatic conditions on Mars and the evolution of palaeoclimates. Furthermore, they inform us about the potential for rocks on Mars to contain a record of ancient microbial life. Despite these advances, numerous questions remain about the timing, magnitude and duration of water activity on Mars and the sequence of events in the evolution of water flow at the martian surface. In this talk, I will review the latest findings from both orbital and rover (eg. Mars Science Laboratory Curiosity rover) investigations, and consider the current questions and controversies. I will conclude by highlighting the major questions that need to be addressed about the evolution of liquid water at the martian surface.

14:00-14:30 Water on Mars: what we have learnt from the mineralogy of Martian meteorites and the Martian surface

Professor John Bridges, University of Leicester, UK

Abstract

Mineralogical traces of ancient water on Mars have been shown through Near IR observations by orbiters, which reveal – in gaps through the Martian dust – clays and carbonate often associated with large impact craters, originating either through excavation of ancient altered crust or impact-induced hydrothermal activity. Clays have also been identified in fine-grained sedimentary terrains and are one of the targets for landers. In Gale Crater, the Curiosity Rover has studied such sediments and, in the first 1000 sols of the MSL mission, shown a secondary assemblage of clay and magnetite with later sulphate veins. These are the result of low temperature alteration of basaltic to trachybasalt material by brines. The Martian meteorites have preserved a record of short-lived, near-surface alteration producing oxidised, low temperature ≤150 oC, metastable carbonate and clay (ferric saponite) dominated assemblages. In contrast, a meteorite fragment of an impact regolith records the presence of higher temperature water-rock reaction.

Much of the mineralogical record of water on Mars can be understood through the reaction of basaltic or moderately fractionated igneous assemblages with dilute, neutral pH, near surface fluids including groundwater. However, sulphate mineralogy at the MER Opportunity landing site suggests that sometimes more acidic, fluid activity occurred through evaporation of brines.

A major unresolved question that the observations of Mars’ mineralogy raise is whether extensive chemical weathering took place on the ancient martian surface, perhaps associated with the presence of large standing bodies of water in impact craters,  or whether water-rock reaction was a largely subsurface, closed system process. 

15:30-16:00 Accretion processes and the implications for inner Solar System volatile budgets

Professor Alex Halliday FRS, Vice President (Physical Secretary), the Royal Society

Abstract

The budgets of water, carbon and nitrogen in the Earth can be determined from comparisons with the noble gases. The concentrations of these are constrained by the age of the Earth and the budget of potassium and the resultant argon-40. However, the potassium budget is dependent on knowing the K/U ratio and the amount of uranium. Recent models of collisional stripping and volcanic losses of incompatible elements from accreting planetesimals and planetary embryos mean that the amount of water in the Earth’s interior is highly uncertain between one and ten ocean masses. What is clear however is that nitrogen is far more depleted than hydrogen in the combined silicate Earth, hydrosphere and atmosphere, than what is expected from a late veneer of accreted materials. Furthermore, cometary water is insignificant even though a major fraction of the heavy noble gases probably have such an origin.

16:00-16:30 Origin and delivery of water to the forming inner solar system

Dr Erik Hauri, Carnegie Institution of Washington, USA

Abstract

The origin of water in the planetary bodies of the inner solar system is fundamental to the potential and practical viability for the sustenance of life, as well as for the development of observable geophysical phenomena such as atmospheric composition, surface volcanism and plate tectonics. While important observations on the origins of water in the inner solar system have been obtained from primitive carbonaceous chondrites, the systematics of H₂O in these objects is often dominated by gas-ice-water-rock interactions at low temperatures where isotopic fractionations can be large and dominated by non-equilibrium processes. Our recent work on H₂O has focused on differentiated solar system objects, as exemplified not only by the Earth and Mars, but also achondrites (e.g. 4 Vesta) and the Moon, where volcanism has provided a record of the abundance and isotopic composition of H₂O preserved in volcanic glasses, melt inclusions and magmatic minerals. The age of water delivery to these various objects ranges widely, from ~8 Myr after CAIs to the formation age of the Moon 50-90 Myr after CAIs (i.e. 4560 – 4470 Ma). Mars, Earth, Moon and Vesta display a wide range of volatile-element depletion in their geochemistry, for example displaying a range of K/Th ratios from 360 (Moon) to 1000 (Vesta) to 5500 (Earth, Mars). Yet the abundance of H₂O in primitive magmas from these bodies, and the relative abundances of S/H₂O, F/H₂O and Cl/H₂O, appear to be rather similar to each other and to the Earth. This similarity extends to the D/H ratio which is solidly within the range of carbonaceous chondrites, as are the isotopic compositions of nitrogen and carbon (but not oxygen). The systematics of H₂O and other similarly volatile elements thus provides evidence for the presence of a ubiquitous and homogenous chondritic volatile reservoir in the inner solar system very shortly after the first planetary embryos began to form, and extending for nearly 100 Myr afterward. Cosmochemical and dynamical scenarios consistent with these observations will be discussed.

17:00-18:00 Poster session

09:00-09:30 Remotely distinguishing and mapping endogenic water on the Moon

Dr Rachel Klima, Johns Hopkins University Applied Physics Laboratory, USA

Abstract

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.

09:30-10:00 History of water and other volatiles in the Moon and implications for the evolution of the inner Solar System

Dr Mahesh Anand, Open University, UK

Abstract

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.

11:00-11:30 Hydrogen isotope ratios in the inner Solar System

Dr Lydia Hallis, University of Glasgow, UK

Abstract

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.

11:30-12:00 Global water cycle and the coevolution of Earth’s interior and surface environment

Professor Jun Korenga, Yale University, USA

Abstract

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.

13:30-14:00 Born wet: how the Earth gained and lost water during formation

Professor Lindy Elkins-Tanton, Arizona State University, USA

Abstract

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.

14:00-14:30 Relating the record preserved by the terrestrial noble gases to the processes controlling the accretion of water to Earth

Professor Chris Ballentine, University of Oxford, UK

Abstract

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.

15:30-16:00 Water on Earth – extending our knowledge of the deep hydrosphere and implications for subsurface life

Professor Barbara Sherwood Lollar, University of Toronto, Canada

Abstract

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.

16:00-17:00 Panel discussion - future directions