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Reduced surface environments are required for many prebiotic chemical pathways. Large impacts onto Hadean Earth have been suggested as scenarios able to generate such environments on a global scale. Physical and chemical processes that occur shortly after impact can, however, limit the reducing power that is available at planet surface. Here, we present impact simulations and chemical calculations that demonstrate the efficient loss of reducing power from the post-impact planetary surface. This loss occurs via the dynamic escape of impactor core material, or the sinking of this material to the planet core, during impact, and through interactions between the atmosphere and the impact-generated melt phase. We suggest that post-impact surface environments are sufficiently reduced that species important to prebiotic chemistry can form (e.g., HCN, HCCCN). Additionally, the formation of a reduced impact-generated melt phase allows for reducing power to be stored in the planet mantle, where it can degas to the surface on geologic timescales., {"references":["Wünnemann, K., Collins, G. S., & Osinski, G. R. (2008). Numerical modelling of impact melt production in porous rocks. Earth and Planetary Science Letters, 269(3-4), 530-539.","Bottke, W. F., Walker, R. J., Day, J. M., Nesvorny, D., & Elkins-Tanton, L. (2010). Stochastic late accretion to Earth, the Moon, and Mars. science, 330(6010), 1527-1530.","Day, J. M., Brandon, A. D., & Walker, R. J. (2016). Highly siderophile elements in Earth, Mars, the Moon, and asteroids. Reviews in Mineralogy and Geochemistry, 81(1), 161-238.","Genda, H., Brasser, R., & Mojzsis, S. J. (2017). The terrestrial late veneer from core disruption of a lunar-sized impactor. Earth and Planetary Science Letters, 480, 25-32.","Marchi, S., Canup, R. M., & Walker, R. J. (2018). Heterogeneous delivery of silicate and metal to the Earth by large planetesimals. Nature geoscience, 11(1), 77-81.","Benner, S. A., Bell, E. A., Biondi, E., Brasser, R., Carell, T., Kim, H. J., ... & Trail, D. (2020). When did life likely emerge on Earth in an RNA‐first process?. ChemSystemsChem, 2(2), e1900035.","Zahnle, K. J., Lupu, R., Catling, D. C., & Wogan, N. (2020). Creation and evolution of impact-generated reduced atmospheres of early Earth. The Planetary Science Journal, 1(1), 11.","Citron, R. I., & Stewart, S. T. (2022). Large impacts onto the early Earth: planetary sterilization and iron delivery. The Planetary Science Journal, 3(5), 116.","Itcovitz, J. P., Rae, A. S., Citron, R. I., Stewart, S. T., Sinclair, C. A., Rimmer, P. B., & Shorttle, O. (2022). Reduced atmospheres of post-impact worlds: The early Earth. The Planetary Science Journal, 3(5), 115."]}

The lavas associated with mantle plumes may sample domains throughout Earth’s mantle and probe its dynamics. However, plume studies are often only able to take snapshots in time, usually of the most recent plume activity, leaving the chemical and geodynamic evolution of major convective upwellings in Earth’s mantle poorly constrained. Here, we report the geodynamically key information of how the lithology and density of a plume change from plume head phase to tail. We use iron stable isotopes and thermodynamic modeling to show that the Galápagos plume has contained small, nearly constant, amounts of dense recycled crust over its 90-million-year history. Despite a temporal evolution in the amount of recycled crust-derived melt in Galápagos-related lavas, we show that this can be explained by plume cooling alone, without associated changes in the plume’s mantle source; results are also consistent with a plume rooted in a lower mantle low-velocity zone also sampling primordial components.

Funder: Embiricos Trust Scholarship, Jesus College Cambridge, Volcanism is a major and long-term source of volatile elements such as C and H to Earth’s atmosphere, likely has been to Venus’s atmosphere, and may be for exoplanets. Mod- els simulating volcanic growth of atmospheres often make one of two assumptions: ei- ther that atmospheric speciation is set by the high-temperature equilibrium of volcan- ism; or, that volcanic gases thermochemically re-equilibrate to the new, lower, temper- ature of the surface environment. In the latter case it has been suggested that volcanic atmospheres may create biosignature false positives. Here, we test the assumptions un- derlying such inferences by performing chemical kinetic calculations to estimate the re- laxation timescale of volcanically-derived atmospheres to thermochemical equilibrium, in a simple 0D atmosphere neglecting photochemistry and reaction catalysis. We demon- strate that for planets with volcanic atmospheres, thermochemical equilibrium over ge- ological timescales can only be assumed if the atmospheric temperature is above ∼700 K. Slow chemical kinetics at lower temperatures inhibit the relaxation of redox-sensitive species to low-temperature thermochemical equilibrium, precluding the production of two in- dependent biosignatures through thermochemistry alone: 1. ammonia, and 2. the co- occurrence of CO2 and CH4 in an atmosphere in the absence of CO. This supports the use of both biosignatures for detecting life. Quenched at the high temperature of their degassing, volcanic gases also have speciations characteristic of those produced from a more oxidized mantle, if interpreted as being at thermochemical equilibrium. This there- fore complicates linking atmospheres to the interiors of rocky exoplanets, even when their atmospheres are purely volcanic in origin.

Nominally anhydrous minerals in rocky planet mantles can sequester oceans of water as a whole, giving a constraint on bulk water inventories. Here we predict mantle water capacities from the thermodynamically-limited solubility of water in their constituent minerals. We report the variability of mantle water capacity due to (i) host star refractory element abundances that set mineralogy, (ii) realistic mantle temperature scenarios, and (iii) planet mass. We find that planets large enough to stabilise perovskite almost unfailingly have a dry lower mantle, topped by a high-water-capacity transition zone which may act as a bottleneck for water transport within the planet's interior. Because the pressure of the ringwoodite-perovskite phase boundary defining the lower mantle is roughly insensitive to planet mass, the relative contribution of the upper mantle reservoir will diminish with increasing planet mass. Large rocky planets therefore have disproportionately small mantle water capacities. In practice, our results would represent initial water concentration profiles in planetary mantles where their primordial magma oceans are water-saturated. We suggest that a considerable proportion of massive rocky planets' accreted water budgets would form surface oceans or atmospheric water vapour immediately after magma ocean solidification, possibly diminishing the likelihood of these planets hosting land. This work is a step towards understanding planetary deep water cycling, thermal evolution as mediated by rheology and melting, and the frequency of waterworlds., Comment: 20 pages, 7 figures; this version accepted for publication in MNRAS

The Icelandic mantle contains a range of lithologies associated with the depleted upper mantle, a mantle plume, and recycled oceanic lithosphere but the precise nature of depleted and enriched components in the mantle and their relative contributions to melt production remain poorly constrained. In this study, we collect new olivine- and plagioclase-hosted melt inclusion data and compile this with existing literature data to investigate the relative contributions from different mantle lithologies to basaltic magmas erupted in Icelandic flank zones and neovolcanic zones by modelling the melting of a heterogeneous mantle and subsequent mixing of derived melts. We find that observed melt inclusion compositions from off-axis flank zones are best explained as homogenized mixtures of pyroxenite- and lherzolite-derived melts produced at depths around 80-93 km, by which point lherzolite has only experienced a low degree of melting whereas the pyroxenite lithology has melted extensively. These melts represent the onset of channelization in the mantle and are transported rapidly to the surface without input from shallower melts. Melt compositions from the on-axis neovolcanic zones and off-axis Öræfajökull, are produced by mixing this deep melt component with higher degree lherzolite melts produced at shallower depths, between 57-93 km. Proportions of shallow lherzolite-derived melts and deep homogenized melt vary, but the lowest contribution from the deep homogenized melt is seen in the Northern Volcanic Zone. Ourresults support a model whereby deep melts mix until melt channelization starts in the mantle, after which binary mixing between the homogenized deep melt and shallower fractional melts occurs.

In order to form a rocky, potentially habitable planet, like Earth, the planet’s building blocks must loose volatiles. White dwarfs that have accreted planetary material provide the perfect opportunity to study the volatile content of planetary building blocks. The unique behaviour of Mn and Na depending on the conditions under which volatiles are lost means that planetary material in the atmospheres of white dwarfs can tell us how planetary building blocks lost volatiles [1]. This talk will summarise some recent results regarding observations of volatiles in the atmospheres of white dwarfs, models that describe how white dwarfs accrete volatiles and evidence from white dwarfs for volatile loss both early, in the hot, inner regions of a planet-forming disc and late, when large-scale melting is induced following impacts.

Funder: Embiricos Trust Scholarship, Jesus College, Cambridge, The geology of Earth and super-Earth sized planets will, in many cases, only be observable via their atmospheres. Here, we investigate secondary volcanic atmospheres as a key base case of how atmospheres may reflect planetary geochemistry. We couple volcanic outgassing with atmospheric chemistry models to simulate the growth of C-O-H-S-N atmospheres in thermochemical equilibrium, focusing on what information about a planet's mantle fO$_2$ and bulk silicate H/C ratio could be determined by atmospheric observation. 800K volcanic atmospheres develop distinct compositional groups as the mantle fO$_2$ is varied, which can be identified using sets of (often minor) indicator species: Class O, representing an oxidised mantle and containing SO$_2$ and sulfur allotropes; Class I, formed by intermediate mantle fO$_2$'s and containing CO$_2$, CH$_4$, CO and COS; and Class R, produced by reduced mantles, containing H$_2$, NH$_3$ and CH$_4$. These atmospheric classes are robust to a wide range of bulk silicate H/C ratios. However, the H/C ratio does affect the dominant atmospheric constituent, which can vary between H$_2$, H$_2$O and CO$_2$ once the chemical composition has stabilised to a point where it no longer changes substantially with time. This final atmospheric state is dependent on the mantle fO$_2$, the H/C ratio, and time since the onset of volcanism. The atmospheric classes we present are appropriate for the closed-system growth of hot exoplanets, and may be used as a simple base for future research exploring the effects of other open-system processes on secondary volcanic atmospheres., The Embiricos Trust Scholarship, Jesus College Cambridge

Trace element and volatile heterogeneity in the Earth's mantle is in uenced by the recycling of oceanic lithosphere through subduction. Oceanic island basalts commonly have high concentrations of volatiles compared to mid-ocean ridge basalts, but the extent to which this enrichment is linked to recycled mantle domains remains unclear. Boron is an ideal tracer of recycled subducted material, since only a small percentage of a recycled component is required to modify the bulk X11B of the source mantle. Boron isotopic compositions of primary melts thus have potential to trace the fate of recycled subducted material in the deep mantle, and to constrain the lengthscales of lithologic and compositional heterogeneities in diverse tectonic settings. We present new measurements of volatiles, light elements and boron isotopic ratios in basaltic glasses and melt inclusions that sample the mantle at two endmember spatial scales. Submarine glasses from the Reykjanes Ridge sample long-wavelength mantle heterogeneity on the broad scale of the Iceland plume. Crystal-hosted melt inclusions from the Askja and Barðarbunga volcanic systems in North Iceland sample short-wavelength mantle heterogeneity close to the plume centre. The Reykjanes Ridge glasses record only very weak along-ridgeenrichment in B content approaching Iceland, and there is no systematic variability in X11B along the entire ridge segment. These observations constrain ambient Reykjanes Ridge mantle to have a X11B of -6.1h (2SD=1.5h, 2SE=0.3h). The North Iceland melt inclusions have widely variable X11B between -20.7 and +0.6h. We screen melt inclusions against in uence from crustal contamination, identifying high [B] and low X18O as ngerprints of assimilation processes. Only the most primitive melt inclusions with MgO 8 wt.% reliably record mantle-derived X11B. In North Iceland, incompatible trace element (ITE)-depleted primitive melt inclusions from Holuhraun record a X11B of -10.6h, a signal that has also been seen in melt inclusions from southwest Iceland (Gurenko and Chaussidon, 1997). In contrast, primitive ITE-enriched melt inclusions from nearby Askja volcano record a X11B of -5.7h, overlapping with our new constraint on the X11B of Reykjanes Ridge mantle. Coupled [B], X11B and X18O signatures of more evolved melt inclusions from North Iceland are consistent with primary melts assimilating Ÿ5-20% of hydrothermally altered basaltic hyaloclastite as they ascend through the upper crust. Our data reveal the presence of a depleted, low-X11B and an enriched, higher-X11B mantle component, both intrinsic to the Icelandic mantle source and distinct from Reykjanes Ridge mantle. Non-modal melting calculations suggest that the enriched and depleted mantle components both contain 0.085 g/g B, slightly lower than the 0.10-0.11 g/g calculated for Reykjanes Ridge mantle. These data are consistent with the Icelandic mantle containing B-depleted dehydrated recycled oceanic lithosphere, in keeping with the low B/Pr of Icelandic melt inclusions in comparison to Reykjanes Ridge glasses or MORB. Our new data provide strong support for the role of recycled subducted lithosphere in melt generation at ocean islands, and highlight the need for careful screening of melt inclusion compositions in order to study global volatile recycling in ocean island basalts.

Lithological heterogeneity is a widely accepted feature of the Earth's mantle, with recycled crustal material accounting for a significant part of heterogeneity in ocean island basalt (OIB) geochemistry. Fe isotopes have been used to link geochemical heterogeneity in OIB sources to distinct mantle lithologies due to their mineral-specific equilibrium fractionation effects, or their composition, such as incorporation of kinetically-fractionated core liquids entrained from the core-mantle boundary. Here we present Fe isotope data for Samoan shield, and Azores volcanoes, together with a combined phase-equilibria and equilibrium mineral-melt isotope fractionation model. These OIB lavas allow us to study the roles of core-derived and recy-cled mantle components in generating heavy delta Fe-57 melts. Heavy delta Fe-57 correlates with radiogenic isotope signatures of enrichment by a crustal component and not with Fe/Mn or any indicator of core involvement. However, single-stage melting of a MORB-like eclogitic pyroxenite cannot generate the heavy delta Fe-57 seen in Pitcairn, Azores, and rejuvenated Samoa lavas. Melts of a reaction-zone pyroxenite (commonly suggested to form part of the OIB source), derived from eclogite melts hybridised with peridotite, also fail to generate the heaviest Fe isotopic compositions seen in OIB. Instead, the generation of heavy delta Fe-57 melts in OIB requires: (1) processes that make subducted eclogite isotopically heavier than its pristine precursor MORB (e.g., hydrothermal alteration, metamorphism, sediment input); (2) lithospheric processing, such as remobilisation of previously frozen small-degree melts, or a contribution from lithospheric material metasomatised by silicate melts; and/or (3) melting conditions that limit the dilution of melts with heavy delta Fe-57 by ambient lower delta Fe-57 materials. No single process we consider can generate the heavy delta Fe-57 seen in the Azores, Pitcairn, and rejuvenated Samoan lavas. Therefore, it cannot be assumed that a pyroxenite lithology derived from recycled crustal material is the sole producer of heavy delta Fe-57 melts in OIB, nor can these signatures be related to contributions from the Earth's core. Instead, the observation of heavy delta Fe-57 OIB melts cannot be ascribed to a unique source or process. This ambiguity reflects the multitude of processes operating from the generation of recycled lithologies through to their mantle melting: from MORB generation, its low tem-perature alteration, through mantle heterogeneity development and lithospheric processing, to eruption at ocean islands. (C) 2020 Elsevier Ltd. All rights reserved.

The recently adopted Ariel ESA mission will measure the atmospheric composition of a large number of exoplanets. This information will then be used to better constrain planetary bulk compositions. While the connection between the composition of a planetary atmosphere and the bulk interior is still being investigated, the combination of the atmospheric composition with the measured mass and radius of exoplanets will push the field of exoplanet characterisation to the next level, and provide new insights of the nature of planets in our galaxy. In this white paper, we outline the ongoing activities of the interior working group of the Ariel mission, and list the desirable theoretical developments as well as the challenges in linking planetary atmospheres, bulk composition and interior structure., Experimental Astronomy, 53, ISSN:0922-6435, ISSN:1572-9508

Decades of study have documented several orders of magnitude variation in the oxygen fugacity (fO2) of terrestrial magmas and of mantle peridotites. This variability has commonly been attributed either to differences in the redox state of multivalent elements (e.g., Fe3+/Fe2+) in mantle sources or to processes acting on melts after segregation from their sources (e.g., crystallization or degassing). We show here that the phase equilibria of plagioclase, spinel, and garnet lherzolites of constant bulk composition (including whole-rock Fe3+/Fe2+) can also lead to systematic variations in fO2 in the shallowest ~100 km of the mantle.Two different thermodynamic models were used to calculate fO2 vs. pressure and temperature for a representative, slightly depleted peridotite of constant composition (including total oxygen). Under subsolidus conditions, increasing pressure in the plagioclase-lherzolite facies from 1 bar up to the disappearance of plagioclase at the lower pressure limit of the spinel-lherzolite facies leads to an fO2 decrease (normalized to a metastable plagioclase-free peridotite of the same composition at the same pressure and temperature) of ~1.25 orders of magnitude. The spinel-lherzolite facies defines a minimum in fO2 and increasing pressure in this facies has little influence on fO2 (normalized to a metastable spinel-free peridotite of the same composition at the same pressure and temperature) up to the appearance of garnet in the stable assemblage. Increasing pressure across the garnet-lherzolite facies leads to increases in fO2 (normalized to a metastable garnet-free peridotite of the same composition at the same pressure and temperature) of ~1 order of magnitude from the low values of the spinel-lherzolite facies. These changes in normalized fO2 reflect primarily the indirect effects of reactions involving aluminous phases in the peridotite that either produce or consume pyroxene with increasing pressure: Reactions that produce pyroxene with increasing pressure (e.g., forsterite + anorthite ⇄ Mg-Tschermak + diopside in plagioclase lherzolite) lead to dilution of Fe3+-bearing components in pyroxene and therefore to decreases in normalized fO2, whereas pyroxene-consuming reactions (e.g., in the garnet stability field) lead initially to enrichment of Fe3+-bearing components in pyroxene and to increases in normalized fO2 (although this is counteracted to some degree by progressive partitioning of Fe3+ from the pyroxene into the garnet with increasing pressure). Thus, the variations in normalized fO2 inferred from thermodynamic modeling of upper mantle peridotite of constant composition are primarily passive consequences of the same phase changes that produce the transitions from plagioclase → spinel → garnet lherzolite and the variations in Al content in pyroxenes within each of these facies. Because these variations are largely driven by phase changes among Al-rich phases, they are predicted to diminish with the decrease in bulk Al content that results from melt extraction from peridotite, and this is consistent with our calculations.Observed variations in FMQ-normalized fO2 of primitive mantle-derived basalts and peridotites within and across different tectonic environments probably mostly reflect variations in the chemical compositions (e.g., Fe3+/Fe2+ or bulk O2 content) of their sources (e.g., produced by subduction of oxidizing fluids, sediments, and altered oceanic crust or of reducing organic material; by equilibration with graphite- or diamond-saturated fluids; or by the effects of partial melting). However, we conclude that in nature the predicted effects of pressure- and temperature-dependent phase equilibria on the fO2 of peridotites of constant composition are likely to be superimposed on variations in fO2 that reflect differences in the whole-rock Fe3+/Fe2+ ratios of peridotites and therefore that the effects of phase equilibria should also be considered in efforts to understand observed variations in the oxygen fugacities of magmas and their mantle sources.

Conditions at the surface of terrestrial type worlds inhabitable by Earth-like life are hugely variable. Earth itself has explored much of this extensive parameter space over time, as evidenced via the rock record, which contains evidence of both multi-million year global glaciations as well as hot house conditions. The area of emergent land, the partial pressure of atmospheric carbon dioxide, and the geochemistry of crustal rocks have all evolved, and imply that terrestrial type exoplanets may be extremely diverse. Unfortunately, all of these parameters remain uncertain for Earth during the Era of Prebiotic chemistry. Understanding how the availability of critical molecules for prebiotic chemistry vary as a function of planetary conditions is therefore crucial for constructing self-consistent scenarios for the origin of life. We focus on phosphate, modelling 1) mineral hosts in crustal rocks, 2) the weathering of those minerals as a function of atmospheric composition, 3) the lithological composition of continental crust, 4) the ratio of continental crust to oceanic crust, 5) the ratio of emergent to submerged crust, and 6) the efficiency of sedimentary crustal reworking. Recent work has suggested that phosphate may be most available on worlds with high atmospheric pCO2, where abundant dissolved inorganic carbon in surface waters can help solubilise the P-bearing phase apatite. Provocatively, our modelling suggests that, on primitive worlds where apatite is rare, the weathering of rock-forming silicate and carbonate minerals may supply higher P fluxes - up to an order of magnitude higher than weathering of apatite rich crust at low pCO2, and roughly competitive with or, for mafic crust rich in basaltic glass, 1-2 orders of magnitue higher than the weathering of apatite rich crust at high pCO2. Finally, our results also strongly suggest that high rates of sedimentary reworking are needed to access the highest P weathering fluxes on Earth-like worlds. Our results point towards settings of active sedimentary cycling as crucial for fuelling prebiotic chemistry with endogenous P sources, and reveal a broad mineralogical and climatic parameter space for Earth-like worlds under which that chemistry may have plausibly taken place.

Small water budgets produce desert worlds and large water budgets produce water worlds, but there is a narrow range of water budgets that would grant a marbled surface to a rocky planet. A planet’s highest point can constrain this range in that it defines the minimum ocean volume to flood all land. Thus we take a first step in quantifying water world limits by estimating how minimum surface elevation differences scale with planetary bulk properties. Our model does not require the presence of plate tectonics, an assumption which has constricted the scope of previous studies on exoplanet land fractions. We focus on the amplitudes of dynamic topography created by rising and sinking mantle plumes—obtained directly from models of mantle convection—but also explore rough limits to topography by other means. Rocky planets several times more massive than Earth can support much less topographic variation due to their stronger surface gravity and hotter interiors; these planets’ increased surface area is not enough to make up for low topography, so ocean basin capacities decrease with planet mass. In cooler interior thermal states, dynamically-supported topography alone could maintain subaerial land on Earth-size stagnant lid planets with surface water inventories of up to approximately 100 ppm of their mass (or half Earth’s ocean mass fraction). Considering the overall cap to topography on such planets would raise this threshold ocean mass fraction by an order of magnitude. Current estimates of the surface water contents on TRAPPIST-1e to g place these planets near or above the ultimate topographic waterworld threshold, depending on their core masses.

Life in the clouds of Venus, if present, has been proposed to extract energy from its environment using sulfur-based metabolisms. These metabolisms link life to the chemistry of Venus's atmosphere and thus provide testable predictions of life's presence given current observations. In particular, these hypothetical metabolisms raise the possibility of Venus's enigmatic cloud-layer SO2-depletion being explained by life. We couple each proposed metabolic pathway to a photochemical-kinetics code and self-consistently predict the composition of Venus's atmosphere under the scenario that life produces the observed SO2-depletion. Using this photo-bio-chemical kinetics code, we show that all three metabolisms produce SO2-depletions which violate other observational constraints on Venus's atmospheric chemistry. For each metabolism, we estimate the maximum potential biomass density in the cloud layer before the observational constraints are violated. Our analysis shows that either the observed SO2-depletion is due to a currently unknown metabolism, or there is not a high-mass biosphere in Venus's clouds. The methods employed are equally applicable to aerial biospheres on Venus-like exoplanets, planets that are optimally poised for atmospheric characterisation in the near-future.

The Earth’s mantle holds more carbon than its oceans, atmosphere and continents combined, yet the distribution of carbon within the mantle remains uncertain. Our best constraints on the distribution of carbon within the upper mantle are derived from the carbon-trace element systematics of ultra-depleted glasses and melt inclusions from mid-ocean ridge basalts. However, carbon-trace element systematics are susceptible to modification by crustal processes, including concurrent degassing and mixing, and melt inclusion decrepitation. In this study we explore how the influence of these processes varies systematically with both the mantle source and melting process, thereby modulating both global and local carbon-trace element trends. We supplement the existing melt inclusion data from Iceland with four new datasets, significantly enhancing the spatial and geochemical coverage of melt inclusion datasets from the island. Within the combined Iceland dataset there is significant variation in melt inclusion C/Ba ratio, which is tightly correlated with trace element enrichment. The trends in C/Ba-Ba space displayed by our new data coincide with the same trends in data compiled from global ocean islands and mid-ocean ridges, forming a global array. The overall structure of the global C/Ba-Ba array is not a property of the source, instead it is controlled by CO2 vapour loss pre- and post-melt inclusion entrapment; i.e., the array is a consequence of degassing creating near-constant maximum melt-inclusion carbon contents over many orders of magnitude of Ba concentration. On Iceland, extremely high C/Ba ( > 100) and C/Nb ( > 1000) ratios are found in melt inclusions from the most depleted eruptions. The high C/Ba and C/Nb ratios are unlikely to be either analytical artefacts, or to be the product of extreme fractionation of the most incompatible elements during silicate melting. Whilst high C/Ba and C/Nb ratios could be generated by regassing of melt inclusions by CO2 vapour, or by mantle melting occurring in the presence of residual graphite, we suggest the high values most likely derive from an intrinsically high C/Ba and C/Nb mantle component that makes up a small fraction of the Icelandic mantle.

The composition of forming planets is strongly affected by the protoplanetary disc's thermal structure. This thermal structure is predominantly set by dust radiative transfer and viscous (accretional) heating and can be impacted by gaps - regions of low dust and gas density that can occur when planets form. The effect of variations in dust surface density on disc temperature has been poorly understood until now. In this work, we use the radiative transfer code MCMax to model the 2D dust thermal structure with individual gaps corresponding to planets with masses of 0.1 M$_J$ - 5 M$_J$ and orbital radii of 3, 5, and 10 AU. Low dust opacity in the gap allows radiation to penetrate deeper into the disc and warm the midplane by up to 16 K, but only for gaps located in the region of the disc where stellar irradiation is the dominant source of heat (here, a$\gtrsim$4 AU). In viscously-heated regions (a$\lesssim$4 AU), the midplane of the gap is relatively cooler by up to 100 K. Outside of the gap, broad radial oscillations in heating and cooling are present due to changes in disc flaring. These thermal features affect local segregation of volatile elements (H$_2$O, CH$_4$, CO2, CO) between the dust and gas. We find that icelines experience dramatic shifts relative to gapless models: up to 6.5 AU towards the star and 4.3 AU towards the midplane. While quantitative predictions of iceline deviations will require more sophisticated models which include transport and sublimation/condensation kinetics, our results provide evidence that planet-induced iceline variations represent a potential feedback from the planet onto the composition of material it is accreting., Comment: 11 pages, 5 figures. Submitted to MNRAS, preprint for reference at Exoplanets IV

The geochemistry of global mantle melts suggests that both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) sample lithological and temperature heterogeneities originating in both the upper and lower mantle. Recently, non-traditional stable isotopes have been suggested as a new tool to complement existing tracers of mantle heterogeneity (e.g., major and trace elements, radiogenic isotopes), because mineral- and redox-specific equilibrium stable isotope fractionation effects can link the stable isotope ratios of melts to their source mineralogy and melting degree. Here, we investigate five stable isotope systems (Mg-Ca-Fe-V-Cr) that have shown promise in models or natural samples as tracers of mantle temperature and/or lithological heterogeneity. We use a quantitative model, combining thermodynamically self-consistent mantle melting and equilibrium isotope fractionation models, to explore the behaviour of the isotope ratios of these elements during melting of three mantle lithologies (peridotite, and silica-excess and silica-deficient pyroxenites), responding to changes in mantle mineralogy, oxygen fugacity, temperature and pressure.We find that, given current analytical precision, the stable isotope systems examined here are not predicted to be sensitive to mantle potential temperature variations through equilibrium isotope fractionation processes. By contrast, source lithological heterogeneity is predicted to be detectable in some cases in the stable isotope ratios of erupted basalts, although generally only at proportions of > 10% MORB-like pyroxenite in the mantle source, given current analytical precision. Magnesium and Ca stable isotopes show most sensitivity to a garnet-bearing source lithology, and Fe and Cr stable isotopes are potentially sensitive to the presence of MORB-like pyroxenite in the mantle source, although the behaviour of Cr isotopes is comparatively under-constrained and requires further work to be applied with confidence to mantle melts. When comparing the magnitude and direction of predicted equilibrium isotopic fractionation of peridotite and pyroxenite melts to natural MORB and OIB data, we find that aspects of the natural data (including the mean Mg-Ca-Fe-V isotopic composition of MORB, the range of Mg-Ca isotopic compositions seen in MORB data, the mean Mg-Ca-Cr isotopic composition of OIB, and the range of Mg-V-Cr isotopic compositions in OIB data) can be matched by equilibrium isotope fractionation during partial melting of peridotite and pyroxenite sources -- with pyroxenite required even for some MORB data. However, even when considering analytical uncertainty on natural sample measurements, the range in stable isotope compositions seen across the global MORB and OIB datasets suggests that kinetic isotope fractionation, or processes modifying the isotopic composition of recycled crustal material such that it is distinct from MORB, may be required to explain all the natural data. We conclude that the five stable isotope systems considered here have potential to be powerful complementary tracers to other geochemical tracers of the source lithology of erupted basalts. However, continued improvements in analytical precision in conjunction with experimental and theoretical predictions of isotopic fractionation between mantle minerals and melts are required before these novel stable isotopes can be unambiguously used to understand source heterogeneity in erupted basalts.

Impacts may have had a significant effect on the atmospheric chemistry of the early Earth. Reduced phases in the impactor (e.g., metallic iron) can reduce the planet's H$_2$O inventory to produce massive atmospheres rich in H$_2$. Whilst previous studies have focused on the interactions between the impactor and atmosphere in such scenarios, we investigate two further effects, 1) the distribution of the impactor's iron inventory during impact between the target interior, target atmosphere, and escaping the target, and 2) interactions between the post-impact atmosphere and the impact-generated melt phase. We find that these two effects can potentially counterbalance each other, with the melt-atmosphere interactions acting to restore reducing power to the atmosphere that was initially accreted by the melt phase. For a $\sim10^{22}\,\mathrm{kg}$ impactor, when the iron accreted by the melt phase is fully available to reduce this melt, we find an equilibrium atmosphere with H$_2$ column density $\sim10^4\,\mathrm{moles\,cm^{-2}}$ ($p\mathrm{H2}\sim120\,\mathrm{bars}\mathrm{,}~X_\mathrm{H2}\sim0.77$), consistent with previous estimates. However, when the iron is not available to reduce the melt (e.g., sinking out in large diameter blobs), we find significantly less H$_2$ ($7\times10^2-5\times10^3\,\mathrm{moles\,cm^{-2}}$, $p\mathrm{H2}\lesssim60\,\mathrm{bars}\mathrm{,}~X_\mathrm{H2}\lesssim0.41$). These lower H$_2$ abundances are sufficiently high that species important to prebiotic chemistry can form (e.g., NH3, HCN), but sufficiently low that the greenhouse heating effects associated with highly reducing atmospheres, which are problematic to such chemistry, are suppressed. The manner in which iron is accreted by the impact-generated melt phase is critical in determining the reducing power of the atmosphere and re-solidified melt pool in the aftermath of impact., Comment: 34 Pages, 11 Figures, 3 Appendices, Accepted for publication in PSJ

Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution., Comment: Ariel Definition Study Report, 147 pages. Reviewed by ESA Science Advisory Structure in November 2020. Original document available at: https://www.cosmos.esa.int/documents/1783156/3267291/Ariel_RedBook_Nov2020.pdf/

The collision history of asteroids is an important archive of inner Solar System evolution. Evidence for these collisions is brought to Earth by meteorites, which can preserve impact-reset radioisotope mineral ages. However, as meteorites often preserve numerous mineral ages, their interpretation is controversial. Here, we combine analysis of phosphate U-Pb ages and mineral microtextures to construct a collision history for the highly shocked Chelyabinsk meteorite. We show that phosphate U-Pb ages in the meteorite are independent of thermal history at macro-to-microscales, correlating instead with phosphate microtexture. Isotopic data from pristine phosphate domains is largely concordant, whereas fracture-damaged domains universally display discordance. Combining both populations best constrains upper (4,473 +/- 11 Ma) and lower intercept (-9 +/- 55 Ma, i.e., within error of the present day) U-Pb ages for Chelyabinsk phosphates. We conclude that all phosphate U-Pb ages were completely reset during an ancient high energy collision. Fracture-damaged phosphate domains experienced further Pb-loss during mild collisional heating in the geologically recent past, and must be targeted to properly constrain a lower intercept age. Targeting textural sub-populations of phosphate grains can significantly improve the calculation and interpretation of U-Pb ages, permitting more robust reconstruction of both ancient and recent asteroidal collision histories.

The atmospheric chemical composition of a hot Jupiter can lead to insights into where in its natal protoplanetary disk it formed and its subsequent migration pathway. We use a 1-D chemical kinetics code to compute a suite of models across a range of elemental abundances to investigate the resultant abundances of key molecules in hot jupiter atmospheres. Our parameter sweep spans metallicities between 0.1x and 10x solar values for the C/H, O/H and N/H ratios, and equilibrium temperatures of 1000K and 2000K. We link this parameter sweep to the formation and migration models from previous works to predict connections between the atmospheric molecular abundances and formation pathways, for the molecules \ce{H2O}, \ce{CO}, \ce{CH4}, \ce{CO2}, \ce{HCN} and \ce{NH3}. We investigate atmospheric \ce{H2O} abundances in eight hot Jupiters reported in the literature. All eight planets fall within our predicted ranges for various formation models, however six of them are degenerate between multiple models and, hence, require additional molecular detections for constraining their formation histories. The other two planets, HD 189733~b and HD 209458~b, have water abundances that fall within ranges expected from planets that formed beyond the \ce{CO2} snowline. Finally, we investigate the detections of \ce{H2O}, \ce{CO}, \ce{CH4}, \ce{CO2}, \ce{HCN} and \ce{NH3} in the atmosphere of HD 209458~b and find that, within the framework of our model, the abundances of these molecules best match with a planet that formed between the \ce{CO2} and \ce{CO} snowlines and then underwent disk-free migration to reach its current location., 14 pages, 11 figures

The origin of life occurred by a series of prebiotic reaction pathways (collectively a system) hosted in one or more geochemical environments (together forming an origin of life scenario). State-of-the-art prebiotic chemistry links together reactions to create systems, intended to be more representative of the diverse chemical pathways that may have proceeded on early Earth. By practical necessity, prebiotic systems chemistry must be investigated under simplified conditions in comparison to likely natural environments. The mismatch in complexity between lab and environment poses a challenge: how to build systems chemistry that is robust not only in the idealised conditions of a lab, but also under natural levels of environmental stress? Here, we propose and formalise a conceptual framework for such work: interference chemistry. We define interference chemistry as the interaction between prebiotic systems chemistry and the environmental scenarios proposed to host it. Natural environments in which prebiotic chemistry could have occurred are messy, containing many spectator ions, mineral phases, and spatially and temporally variable physical processes, e.g., wet/dry cycles. Each of these environmental variables may interfere either constructively or destructively with prebiotic pathways, respectively aiding or inhibiting their efficacy. Exploring interference chemistry for a reaction system will point towards favoured or disfavoured regions of environmental parameter space. To do so, innovation is needed in both the investigation of early planetary environmental conditions, and the continued incorporation of these constraints into experimental systems chemistry. We argue that interference chemistry provides a compelling way to assess combinations of system and environment, leading the way to increasingly prebiotically plausible scenarios for the origin of life on Earth.

The study of planets outside our solar system may lead to major advances in our understanding of the Earth, and provide insight into the universal set of rules by which planets form and evolve. To achieve these goals requires applying geoscience's wealth of Earth observations to fill in the blanks left by the necessarily minimalist exoplanetary observations. In turn, Earth's many one-offs, e.g., plate tectonics, surface liquid water, a large moon, and life - which have long presented chicken and egg type conundrums for geoscientists - may find resolution in the study of exoplanets possessing only a subset of these phenomena., To be published as article 1 in the "Geoscience Beyond the Solar System" issue of Elements magazine, v17 No4

Compared to the diversity seen in exoplanets, Venus is a veritable astrophysical twin of the Earth, however its global cloud layer truncates features in transmission spectroscopy, masking its non-Earth-like nature. Observational indicators that can distinguish an exo-Venus from an exo-Earth must therefore survive above the cloud layer. The above-cloud atmosphere is dominated by photochemistry, which depends on the spectrum of the host star and therefore changes between stellar systems. We explore the systematic changes in photochemistry above the clouds of Venus-like exoplanets orbiting K-Dwarf or M-Dwarf host stars, using a recently validated model of the full Venus atmosphere (0-115 km) and stellar spectra from the MUSCLES Treasury survey. SO2, OCS and H2S are key gas species in Venus-like planets that are not present in Earth-like planets, and could therefore act as observational discriminants if their atmospheric abundances are high enough to be detected. We find that SO2, OCS and H2S all survive above the cloud layer when irradiated by the coolest K-Dwarf and all seven M-Dwarfs, whereas these species are heavily photochemically depleted above the clouds of Venus. The production of sulfuric acid molecules that form the cloud layer decreases for decreasing stellar effective temperature. Less steady-state photochemical oxygen and ozone forms with decreasing stellar effective temperature, and the effect of chlorine-catalysed reaction cycles diminish in favour of HOx and SOx catalysed cycles. We conclude that trace sulfur gases will be prime observational indicators of Venus-like exoplanets around M-Dwarf host stars, potentially capable of distinguishing an exo-Venus from an exo-Earth., 30 pages, 15 figures, 2 tables, accepted for publication in The Astrophysical Journal

Giant impacts have been suggested as promising scenarios under which reduced species important to prebiotic chemistry can form (e.g., CH4, NH3, HCN, PH3). This scenario relies on the ability of the impactor iron core to chemically reduce the planet's H2O inventory, producing significant quantities of H2 gas. Previous studies have focussed solely on the internal atmospheric chemistry of this process [1, 2]. The influence of the impact-generated melt phase on the atmospheric composition has been neglected. Further, the impactor’s iron inventory has been assumed to be fully available to reduce the atmosphere. Here, we examine the effects of these two assumptions, and hence the ability of giant impacts to produce atmospheres suitable for subsequent prebiotic chemistry.MethodWe evolve our systems, comprised of a target and impactor, from their pre-impact states to a post-impact state prior to interactions between the melt and atmosphere (Figure 1). Impactors have a mass ratio of iron core to silicate mantle of approximately 30:70. A range of impactor masses are considered, although a typical mass of 2x1022 kg can be defined based off of Earth’s mantle excesses in highly siderophilic elements [3]. The target has a pre-impact atmosphere of 100 bars CO2 and 2 bars N2, and a surface ocean of 1.85 Earth Oceans, considering estimates for early Earth. The target mantle oxygen fugacity is at the fayalite-magnetite-quartz buffer. As a result of the impact, the target’s initial atmosphere is eroded, its oceans are vaporised, and its surface is partially melted. The iron core of the impactor breaks up [4], and is either accreted by the target or escapes the system. Iron made available to the atmosphere during this accretion acts to reduce the vaporised oceans and form H2.To calculate the silicate melt mass produced from impact, we use the iSALE shock physics code [5, and references therin]. Impact melt masses are determined and parametrised as a function of specific impact energy (Figure 2). To calculate the distribution of impactor iron, we use the GADGET2 smooth particle hydrodynamics code [6]. The distribution of the impactor iron (between the atmosphere, the mantle, and escaping the system) is determined and parametrised as a function of impactor mass, impact velocity, and impact angle (Figure 3, see also [7]).After impact processing, we evolve our system to equilibrium via melt-atmosphere interactions. We define equilibrium as when both the oxygen fugacities of the atmosphere and melt phase, and simultaneously the partial pressure of H2O in each phase, are equal. Melt-atmosphere interactions include redox chemistry in an H2-H2O-Fe2O3-FeO-Fe system, and water partitioning between the atmosphere and the melt phase.ResultsWe find that for larger impactor masses, both the inclusion of the impact-generated melt phase and the iron distribution individually act to decrease the abundance of H2 in the post-impact atmosphere. Together, their effects compound one another to produce a large decrease in H2 compared to the fiducial case without either effect. This change holds over a range of initial conditions.We find that the greatest change is caused by the presence of the melt phase. Interactions between the atmosphere and melt phase alone (i.e., following previous models’ assumptions of all impactor iron being available to reduce the atmosphere [1]) can decrease the atmospheric H2 abundance by up to an order of magnitude, with greater change at greater impactor masses (Figure 4, left).The distribution of the impactor's iron affects results in 2 ways. Firstly, some of the reducing power of the impactor is lost, either through iron being buried in regions of the mantle not able to take part in melt-atmosphere interactions (e.g., solid mantle or rapidly solidifying melt), or through iron escaping the system during breakup of the impactor. Secondly, at large impactor masses, the reduction of the atmosphere by the impactor iron leads to a mass loss from the atmosphere that decreases atmospheric pressure. The decreased system pressure then influences the oxygen fugacity of the melt phase and affects the partitioning of H2O, both of which affect the H2 abundance of the atmosphere at equilibrium. These effects can further decrease the atmospheric H2 by up to a factor of 3 (Figure 4, right).ConclusionsIncluding equilibration between the impact-processed atmosphere and the impact-generated melt phase, as well as distribution of the impactor’s iron inventory, can decrease atmospheric H2 by up to an order of magnitude compared to the fiducial model not considering these effects. Overestimated H2 abundances can produce atmospheres suitable for subsequent formation of reduced species important for prebiotic chemistry. However, these atmospheres are also problematic for prebiotic chemistry in terms of surface temperatures and the blocking of Solar UV radiation by reduced carbon species in the atmosphere.Atmospheres that are in equilibrium with the impact-generated melt phases below them, and that have been formed under distribution of the impactor iron inventory, are more oxidised and less massive than the fiducial case without either effect. These atmospheres are thus less likely to encounter issues surrounding surface temperature and UV blocking. Importantly, despite the decreases in H2 from the fiducial case, these atmospheres still host H2 abundances sufficient for subsequent prebiotic chemistry to take place.References [1] Zahnle K. J., Lupu R., Catling D. C., Wogan N., 2020, The Planetary Science Journal, 1, 11[2] Benner S. A., et al., 2019, ChemSystemsChem, 2[3] Bottke W. F., Walker R. J., Day J. M., Nesvorny D., Elkins-Tanton L., 2010, Science, 330, 1527[4] Genda H., Brasser R. and Mojzsis S. J., 2017, Earth and Planetary Science Letters, 480, p.25-32.[5] Wünnemann K., Collins G. S., Melosh H. J., 2006, Icarus, 180, 514[6] Springel V., 2005, Monthly Notices of the Royal Astronomical Society, 364, 1105[7] Citron R. I., Stewart S.T., 2021, Lunar Planet. Sci., No. 2548, p. 1621