Earth’s early collisions may have boiled away the very volatile elements—carbon, hydrogen, nitrogen—that later made the planet habitable. This consortium of seven UK institutions and three international partners is using computer models, high-pressure experiments, and diamond-encased mineral samples to reconstruct how Earth retained its “just right” mix of volatiles despite a violent formation. The core problem is a gap in knowledge: we do not know whether Earth’s volatile inventory was set by its position in the solar system, by the chemistry of its building blocks, or by the chaotic details of accretion and differentiation. The researchers will track the element selenium as a chemical fingerprint of volatile delivery, test how gases dissolve in magma oceans under extreme pressures, and probe the Moon’s volatile inventory for clues about Earth’s own history. This is fundamental science with no immediate practical application. But understanding how a planet becomes habitable directly informs the search for life on exoplanets—a field that could one day reshape our sense of Earth’s place in the cosmos. Past fundamental work on planetary formation, for instance, underpins the models now used to interpret telescope data from distant solar systems.
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In response to the NERC Theme Action (TA) we propose a consortium among scientists at seven UK institutions and with three international partners centred on the 'The Volatile Legacy of the Early Earth'. Earth's habitability is strongly linked to its inventory and cycling of volatiles, which today are coupled to plate tectonics, but we still have little notion as to how our planet found itself in this near-ideal 'Goldilocks' state where the volatile mix is 'just right'. Was it simply a matter of being at the right solar distance with the right supply of volatiles? Or were the details of the chemistry and dynamics of early accretion and differentiation crucial to the eventual outcome? Such questions are of critical importance for understanding our own planets development, and given the burgeoning field of exo-planet discovery, they gain extra piquancy for gauging the probability of life elsewhere. In this proposal we investigate how the early evolution of volatiles on Earth set the stage for habitability. Planets grow by collisions and these violent events may lead to loss of the volatiles carried within the impacting bodies. We will explore with numerical modeling the conditions under which the volatiles are retained or lost in planetesimal collisions. We will also assess the likelihood that volatiles were delivered to Earth 'late', namely after the maelstrom of major collisions was finished and the planet was largely constructed, by studying the element S and notably its geochemical twin, Se. We will constrain the process of loss to the core and the isotopic signature imparted by this process. We will further use isotopic measurements as finger-prints of the origin of modern Se, and will find out whether it corresponds to any known meteorite type, or if it was possibly delivered by comets. The Moon provides further clues to the origin of the Earth, and Interrogating the significance of the recently refined volatile inventory of the Moon requires new experiments under appropriate conditions. The energy generated by planetary collisions inevitably results in large-scale melting. The solubility and chemical nature of volatiles within a magma ocean controls whether or not gases are carried into the interior of the planet or left in the atmosphere. Volatiles retained in the magma ocean may become part of a deep mantle volatile cycle or become permanently sequestered in deep reservoirs. We will redress this issue with a series of experiments that simulate conditions of the early magma ocean. We will further investigate the stability of phases in the lower mantle that can potentially hold volatile elements if delivered to great depths by solubility in a convecting magma ocean. Using seismic and modeling techniques, we will assess if any remnants of such stored volatiles are currently 'visible' in the deepest mantle. The influence of the core on volatile budgets is potentially great because of its size, but volatile solubility is poorly known. We will examine the solubility of hydrogen, carbon and nitrogen in liquid metal at high pressures and temperatures. In this consortium we will also create a cohort of PhD students and supervisors who work as part of a large team to piece together the evidence for Earth's volatile evolution using inclusions trapped in diamonds. These may be the key 'space-time' capsules that can link experimental and theoretical work on early Earth evolution to present-day volatile budgets and fluxes in the deep Earth. The questions raised in this proposal are complex and require a wide range of information in order to provide meaningful answers. It is our goal to establish a much-improved understanding of how Earth initially became a habitable planet, and to build a solid foundation on which further UK research can continue to lead the way in this exciting field. This will be the ultimate legacy of this consortium, and through links to other consortia, of the entire Theme Action.
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