While Earth locks much of its carbon in its crust as carbonates, Venus retains a comparable carbon inventory almost entirely in its atmosphere as CO2. On Earth, the geological carbon cycle that has produced this vast crustal carbonate inventory is regulated by biology, liquid water, and plate tectonics, which together have stabilized climate over geological time-scales. Venus presently lacks all these processes. We test whether Venus's massive CO(2)atmosphere is diagnostic of a specific evolutionary pathway by quantifying three routes: primary magma-ocean outgassing, secondary volcanic degassing in a stagnant-lid regime, and remobilization of crustal carbonates after climate destabilization. Using a coupled climate-weathering framework, we find that a past habitable Venus could have stored '20 bar of CO(2)as crustal carbonates. Following the transition to runaway conditions, crustal heating releases this reservoir over tens of Myr. In stagnant-lid secondary-degassing models with a MORB-like mantle, outgassing reaches only '25 bar CO2, limited by progressive mantle volatile depletion. However, Venus-like inventories can be achieved through: (i) magmatic carbon enrichment, (ii) increased magmatic delivery to the surface (high extrusion or melt production), and (iii) the recycling of undegassed carbon back into the planet's interior. Primary magma-ocean outgassing can generate > 10(2 )bar CO2, but the retained fraction after early escape remains uncertain. Ultimately, a Venus-like massive CO(2)atmosphere is an equifinal outcome and does not uniquely diagnose a temperate past.

The organosulfur biosignature gases dimethylsulfide (DMS) and dimethlydisulfide (DMDS) have recently been claimed to be present in the atmosphere of sub-Neptune exoplanet K2-18b, leading to the suggestion of possible extraterrestrial life. Abiotic formation pathways for DMS and DMDS in reducing atmospheres have also been proposed, raising concern over the use of DMS and DMDS as biosignature gases more generally. In this Letter we independently test and contrast the proposed abiotic formation pathways for DMS and DMDS using K2-18b as a case study, and explore the wider implications for the atmospheric carbon and sulfur chemistry of hydrogen-rich sub-Neptunes. We demonstrate that one proposed formation pathway is capable of producing observable abundances of abiotic DMS and DMDS; however, it depends sensitively on the energy barrier of the limiting step, which remains unmeasured experimentally. The formation of hydrocarbons including C _2 H _6 , however, occurs abundantly and offers a plausible alternative explanation to the reported suggestions of organosulfur compounds on K2-18b, having previously been shown to share similar spectral features with DMS and DMDS at near-IR wavelengths. Finally, we demonstrate that sulfur hazes form via the photochemistry of H _2 S and condense in the atmosphere of K2-18b, even at trace abundances. We propose that variation in atmospheric sulfur abundance can explain the diversity of haziness observed across the sub-Neptune population so far with JWST.

Static structure models, which map mass-radius constraints to bulk planet composition, are frequently used to categorise exoplanets due to their computational efficiency and the high-level insight they offer into planetary properties. However, static structure models typically have simplified atmospheric treatments, which may introduce systematic biases when interpreting the structures – and therefore the climates – of sub-Neptunes and super-Earths.We present a framework for recovering exoplanet properties using static structure models that accounts for necessary physical-chemical complexity in their atmospheres. We produce a comprehensive library of 504,000 exoplanet simulations that unify deep planetary interior structure with radiative-convective-chemical climate calculations. From these models we demonstrate that a planet's envelope mass fraction – a critical parameter to infer – is frequently degenerate with its instellation flux and atmospheric metallicity, and sensitive to the treatment of gravitational acceleration at the mbar level. Such uncertainties have significant implications for inferring planetary processes, as our modelling shows that habitable-zone sub-Neptunes readily host supercritical surfaces or deep magma oceans, despite their temperate irradiation regime. To marginalise over these uncertainties, we introduce a Bayesian retrieval tool that uses our library of self-consistent models. By applying this Bayesian approach to case-studies of Pi Men c and TOI-421 b, we show that robust physical interpretations are achievable through whole-planet mass-radius retrievals. While new data from JWST, Ariel, and PLATO will expand our observational horizon, physically-consistent modelling provides the means to transition from categorical interpretations toward a comprehensive picture of the exoplanet continuum.

Sub-Neptunes are the most common type of detected exoplanet, yet their observed masses and radii are degenerate with several interior structures. One possibility is that sub-Neptunes have silicate/iron interiors and H-2-dominated atmospheres (mu < 3.8 g mol(-1)), i.e., they are 'gas dwarfs'. If gas dwarfs have molten interiors, interactions between their magma oceans and atmospheres will produce distinct observational signatures. These signatures may break the degeneracy in interior structure, while providing insight into their interior processes, history, and population trends. We expect all such planets are born molten, but under what conditions do they remain molten today? We use the coupled interior-climate evolution model, proteus, to estimate the 'solidification shoreline': the instellation flux boundary (as a function of stellar T-eff that separates molten gas dwarfs from solidified ones. Our results show that 98 per cent of detected sub-Neptunes occupy a region of parameter space consistent with their having permanent magma oceans, if they are gas dwarfs. While mantle fmO(2) and bulk volatile C/H ratio both influence magma ocean cooling, planets with oxidizing mantles and carbon-rich atmospheres are likely to have high mean-molecular weight atmospheres (mu > 3.8 g mol(-1)) and are thus outside the scope of this study. Therefore, most detected sub-Neptunes, if they are gas dwarfs, have permanent magma oceans. This result motivates further research into the interactions between molten interiors and overlying atmospheres, and campaigns to identify unambiguous signatures of these interactions.

Planet population synthesis is an integral tool for linking exoplanets to their formation environments. Most planet population synthesis studies have focused on the carbon-to-oxygen ratio in gas or solids, yet more insight into planet formation may be afforded by considering a wider suite of elements. Sulphur is one such key element. It has been assumed to be entirely refractory in population synthesis models, restricting it to being a tracer of accreted rocky solids. However, sulphur also has a volatile reservoir dominant at the onset of star and planet formation, which is then converted into refractories. We investigate sulphur's wider potential as a formation history tracer by implementing a gas-grain chemical conversion, the first multi-phase treatment of S in a planet population synthesis model. We also present the planet formation module of sponchpop and its first predicted planet growth tracks and populations. We apply these to explore the diversity of the planetary sulphur budget. We show that planets can inherit a wide range of core and envelope sulphur content, depending on the information environment and accretion history including late-stage infall, demonstrating sulphur's new potential as a diagnostic tool for planet formation. Our models predict that some rocky planets are born sulphur-poor, which may have significant implications for their geochemistry and habitability. Enhanced sulphur abundances in gas-giant atmospheres, such as in our Solar system, may result not only from accretion of rocky planetesimals, but also from formation beyond the H2S ice line.

A crucial factor governing the habitability of exoplanets is the availability of bioessential elements such as nitrogen (N) and phosphorous (P), which foster prebiotic chemistry and sustain life after its emergence. However, concentrations of P and N in planetary mantles vary, owing to initial availability and oxidation conditions during planet formation, and thus their characterization and availability in planetary environments are challenging. Here we use a core-formation model to show that moderate oxygen fugacity during core formation is the key parameter to the availability of these two elements, with the existence of a narrow 'chemical Goldilocks zone' that allows both P and N to be present with the right abundances in the mantle. Earth falls within this zone, whereas planets with more reducing/oxidizing conditions will sequester P/N into the core, hindering their availability for life. Future observations refining estimates of the oxygen fugacity prevalent during exoplanet core formation will be crucial to properly evaluate exoplanetary habitability and correctly interpret possible biosignatures.

MESSENGER observations revealed a primary graphite flotation crust on Mercury, implying substantial carbon retention in its magma ocean rather than sequestration into the core. To investigate the conditions enabling this retention, we conducted high-pressure, high-temperature metal-silicate partitioning experiments over a wide range of oxygen fugacities. Carbon behavior is strongly redox dependent: under relatively oxidizing conditions it is highly siderophile, whereas under the reducing conditions relevant to Mercury it becomes significantly less siderophile, promoting carbon retention in silicate melts and graphite crystallization. Modeling of carbon partitioning between the core, mantle, crust, and atmosphere indicates that oxygen fugacities of IW - 6 to IW - 6.5 best reproduce the graphite crust thickness inferred from MESSENGER data. Under these conditions, Mercury's core remains relatively carbon-poor ( < 5000 μ g/g), implying that its density deficit is primarily controlled by other light elements, most likely silicon and sulfur. These results link Mercury's extreme reduction to both its graphite crust and internal chemical structure.

The orbital elements of the solar system’s ancient asteroid families are highly dispersed, recording the last dynamical chaos in its history. However, dynamics alone cannot precisely date this when this terminal chaos occurred. Instead, we can precisely date the collisions triggered by such dynamical rearrangement to constrain this event. On planets, erosion, volcanism, and crustal recycling have removed almost all trace of rocks older than 4 Ga, erasing the archive of early solar system history. In contrast, the meteorite record generated by asteroid collisions represents a separate and more complete archive of the solar system’s early dynamical evolution. Here we build a record of in situ ordinary chondrite meteorite apatite U-Pb ages, sensitive to collisions that induce parent-body break-up events. We show that the U-Pb records of strongly shocked and weakly shocked meteorites are distinct. The U-Pb ages of weakly shocked meteorites record the decline of radiogenic heating in asteroidal bodies. Meanwhile, shocked meteorite ages record major collisions. All sampled ordinary chondrite bodies record collisions that occurred 4480 ± 20 Ma million years ago. No further multi-parent-body clusters of shock ages are found in this record until the very recent events that brought the meteorites to Earth. These ages constrain that the last date of major dynamical chaos to modify the orbital elements of asteroids occurred at around 4480 Ma. This date is relatively late in solar system history, possibly representing the timing of an orbital instability of the giant planets.

Magma-ocean crystallization sets up the early structure and long-term evolution of terrestrial planets. Recent seismic evidence signals the presence of a silicate layer at the base of Mars's mantle. Magma-ocean crystallization and subsequent overturn has been invoked as a hypothesis for this layer's origin. However, while a magma ocean existed on both Earth and Mars, there is no seismic evidence for a basal layer in present-day Earth. In this study, we apply a parameterized-convection model to study whether the effect of partial melting in the growing mantle on the overlying magma-ocean composition can explain this discrepancy. Melts from the mantle buffer the crystallizing magma ocean, limiting progressive differentiation, iron enrichment, and the density anomaly of the overturned layer. This buffering is more efficient for larger planets with more vigorous mantle convection and for planets that are originally less enriched in iron. Consequently, a shallow magma ocean is more iron enriched and denser on Mars than on Earth, providing an explanation for the Mars-Earth difference in the present-day structure of the mantle. We also predict a dichotomy in terrestrial-exoplanet interior structures, with a population with small, stratified mantles and another with large, mostly homogeneous mantles.

The discovery of inhabited exoplanets hinges on identifying biosignature gases. JWST can reveal biosignature gases, though current discoveries have yet to evidence life. The central challenge is attribution: how can we confidently identify biogenic sources while ruling out, or deeming unlikely, abiotic explanations? Attribution is particularly difficult for individual planets, especially given the stochastic abiotic processes that can set atmospheric conditions. To address this, we propose a comparative multi-planet approach centred on systemic retrievals: the analysis of multiple planets within a system to empirically define the `abiotic baseline'. This baseline, constructed from obligate uninhabited planets, serves as a local reference point. Systemic retrievals enable marginalisation over inaccessible, latent, shared abiotic parameters within planet evolution models. This is possible because planets within a system are linked by their birth in the same natal disk, have been irradiated by the same evolving star, and have a linked dynamical history. Observations aligning with the abiotic baseline, where the locally-informed abiotic planet evolution models demonstrate high out-of-sample predictive accuracy, are likely non-biological. Potentially biological anomalies are identified as statistical outliers from the abiotic baseline using Bayesian leave-one-out cross-validation. A comparative biosignature is thus defined: an anomaly where a biotic planetary evolution model provides a superior fit than its abiotic counterpart. Where both abiotic and biotic models yield poor predictive accuracy, the anomaly is flagged as an “unknown unknown"; a signature of either unconstrained abiotic processes, or life as we don't yet know it.

High-precision estimates of Fe valence (Fe3+/ΣFe, where ΣFe = Fe2++Fe3+) in glasses and isotropic minerals from Fe K-edge X-ray absorption near-edge structure spectroscopy (Fe-XANES) have greatly improved our understanding of magmatic fO2 in recent years. However, isotropic phases are not always present in the rock record, and our poor understanding of Fe3+/ΣFe in anisotropic minerals, including near-ubiquitous clinopyroxene, hampers our ability to use them to investigate magmatic fO2. Here we evaluate strategies for using pre-edge centroid positions obtained from Fe-XANES to determine Fe3+/ΣFe in clinopyroxene powders and oriented single crystals. First, we show that clinopyroxene Fe3+/ΣFe can be calibrated against pre-edge centroid positions collected from powdered reference materials characterised by Mössbauer spectroscopy, albeit with a precision of 11% (1σ absolute). Second, spectra collected from oriented crystals reveal that centroid positions depend not only on crystal orientation but also that the nature of this dependence varies with Fe3+/ΣFe. Nevertheless, we are able to determine Fe3+/ΣFe in unknown, but oriented, clinopyroxene crystals with precisions of 12–19% (1σ absolute). Applying clinopyroxene Fe-XANES to samples from Iceland and the Azores validates previously reported estimates of Fe3+/ΣFe from stoichiometry. However, our findings confirm that determining clinopyroxene Fe3+/ΣFe by Fe-XANES requires reference materials and unknowns to be reproducibly oriented to within a few degrees, a necessity that makes Fe-XANES ill-suited for routine analyses of clinopyroxene. We find that electron microprobe-based approaches readily and rapidly return more precise clinopyroxene Fe3+/ΣFe determinations than the Fe-XANES approaches we describe here, and are hence more appropriate for measuring the large numbers of samples required to investigate the nature and causes of fO2 variability in magmatic systems.

The excess abundance of highly siderophile elements (HSEs), as inferred for the terrestrial planets and the Moon, is thought to record a `late veneer' of impacts after the giant impact phase of planet formation. Estimates for total mass accretion during this period typically assume all HSEs delivered remain entrained in the mantle. Here, we present an analytical discussion of the fate of liquid metal diapirs in both a magma pond and a solid mantle, and show that metals from impactors larger than approximately 1 km will sink to Earth's core, leaving no HSE signature in the mantle. However, by considering a collisional size distribution, we show that to deliver sufficient mass in small impactors to account for Earth's HSEs, there will be an implausibly large mass delivered by larger bodies, the metallic fraction of which lost to Earth's core. There is therefore a contradiction between observed concentrations of HSEs, the geodynamics of metal entrainment, and estimates of total mass accretion during the late veneer. To resolve this paradox, and avoid such a mass accretion catastrophe, our results suggest that large impactors must contribute to observed HSE signatures. For these HSEs to be entrained in the mantle, either some mechanism(s) must efficiently disrupt impactor core material into ≤0.01 mm fragments, or alternatively Earth accreted a significant mass fraction of oxidised (carbonaceous chondrite-like) material during the late veneer. Estimates of total mass accretion accordingly remain unconstrained, given uncertainty in both the efficiency of impactor core fragmentation, and the chemical composition of the late veneer.

Context. Planets form from the same cloud of molecular gas and dust as their host stars. Confirming if planetary bodies acquire the same refractory element composition as their natal disk during formation and how efficiently volatile elements are incorporated into growing planets is key to linking the poorly constrained interior composition of rocky exoplanets to the observationally constrained composition of their host star. Such comparisons also afford insight into the planet formation process. Aims. This work compares planetary composition with host star composition using observations of a white dwarf that has accreted planetary material and its F-type star wide binary companion as a reference for the composition of the natal molecular gas and dust. Methods. Spectroscopic analysis reveals abundances of Fe, Mg, Si, Ca, and Ti in both stars. We used the white dwarf measurements to estimate the composition of the exoplanetary material and the F-type companion to constrain the composition of the material the planet formed from. Results. Our results from comparing planetary material to the composition of its natal cloud reveal that the planetary material is depleted in moderate refractories (Mg, Si, and Fe) relative to the refractory material (Ca, Ti). Grouping elements based on their condensation temperatures is key to linking stellar and planetary compositions. Conclusions. Fractionation during formation or subsequent planetary evolution leads to the depletion of moderate refractories from the planetary material accreted by the white dwarf. This signature, as seen for bulk Earth, will likely be present in the composition of many exoplanets relative to their host stars.

Signs of lightning on Venus have long been sought, including by space missions and ground-based telescopes searching for optical flashes, plasma waves, or radio signatures. These efforts have yielded conflicting findings regarding the presence or absence of lightning in Venus’s atmosphere. In this study, we adopt an indirect approach to constrain the prevalence of lightning on Venus using the chemical by-products it produces in Venus’s atmosphere. Nitric oxide (NO) is a key tracer species of lightning, being exclusively generated by lightning in Venus’s lower atmosphere. By calculating the present rate of atmospheric destruction of NO in Venus’s atmosphere through photochemical-kinetic modeling, we constrain the lightning power required to sustain the estimated NO abundances on modern Venus. The reported NO constraints require lightning to generate at least 3 times the power released on Earth, consistent with either a higher rate of strikes, greater energy per strike, or a combination of both. Limited detections of optical flashes within the clouds could point to lightning striking deeper in the atmosphere and nearer the surface—with the result that its optical flashes are obscured by the clouds—driven by triboelectric charging during volcanic eruptions or wind interactions with surface sediments. Our findings underscore the importance for future missions of confirming lightning on Venus, either by verifying the below-cloud NO abundance or by detecting another unambiguous lightning signature, to provide the first definitive evidence of lightning on a rocky planet other than Earth.

The phosphorus budget of planets is intertwined with their formation history and is thought to influence their habitability. The chemical reservoirs and volatile versus refractory budget of phosphorus in planet-forming environments have so far eluded empirical characterization. We employ high-resolution spectra from Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) in the ultraviolet and the PI230 receiver on the single-dish APEX telescope in the sub-mm to constrain the phosphorus budget in the well-characterized HD 100546 star and protoplanetary disc system. We measure log ( P /H )(*) = -7.50(-0.28)(+0 . 23) on the stellar surface, which traces the total inventory of P in accreting gas and dust from the inner disc. The inner disc gas, inside of the main dust trap, has log ( P /H)(in) less than or similar to-8.70, and the outer disc gas log ( P /H)(out) less than or similar to -9.30. Phosphorus in the disc is carried by a relatively refractory reservoir, consistent with minerals such as apatite or schreibersite, or with ammonium phosphate salts, in terms of sublimation temperature. We discuss the impact this might have on the two protoplanets around HD 100546. Our results contribute to our understanding of the chemical habitability of planetary systems and lay a foundation for future explorations, especially in the context of JWST and Ariel which can study phosphorus in exoplanet atmospheres.

We introduce three new synthetic basalt reference materials and a new high‐precision set‐up for stable carbon isotope measurement in basaltic glasses using a large‐geometry secondary ion mass spectrometry (SIMS) instrument. The new reference materials, characterised for carbon mass fraction and isotope composition, show homogeneity for in situ analysis for the reported set‐up. Their bulk hydrogen mass fraction and isotope ratios are reported. Our SIMS protocol uses multi‐collection, cycling between concurrent measurements of 12 C and 13 C on electron multipliers, and either 30 Si or 18 O, as a reference mass, on a 10 11 Ω resistor Faraday cup. This set‐up achieves high measurement repeatability for δ 13 C down to ± 0.35‰ 1RSE at 1706 +89 / ‐88 μg g ‐1 CO 2 , with ± 1.00‰ 1RSE or better between 163 +5.1 / ‐5.2 and 267 +8.9 / ‐8.9 μg g ‐1 CO 2 , using a 10 nA primary beam current and a 40 μm analytical pit over a 100 cycle analysis. Carbon blanks were characterised by measuring carbon‐free olivines, allowing for blank corrections on δ 13 C measurements. After blank and instrument mass fractionation corrections, we measure δ 13 C in glasses down to 26.16 +0.85 / ‐0.86 μg g ‐1 CO 2 with a final measurement standard sample deviation of ± 2.97‰ 1 s . We report in situ measurements on an ocean floor basaltic glass and a set of synthetic basaltic glasses to demonstrate our approach. Reference materials and the SIMS set‐up improve the accuracy and precision of δ 13 C measurements in natural basaltic glasses across a wide range of geologically relevant carbon contents.

Estimating mantle temperature is essential for understanding mantle convection and circulation. One route to constraining mantle temperature is via petrological observations, from combining estimates of magmatic temperature with models of melt generation. However, a key factor that has been less emphasised in previous work is the interplay between the temperature of primitive magmas and the composition of the mantle they are derived from. In practice, both of these are unknown and require simultaneous inference from the data. Here we report new estimates of magmatic temperatures for 17 ocean islands and a mid-ocean ridge setting (Siqueiros Fracture Zone) using olivine-spinel thermometry. With the acquired crystallisation temperature estimates, we calculate primary magmatic temperatures using a reverse fractionation model based on the most forsteritic populations of olivine and then apply a multi-lithological mantle melting model to invert for mantle potential temperature (T-p). We find that the most forsteritic olivines investigated in this study have forsterite (Fo) contents between 83.5 and 88.5 and crystallisation temperatures (T-cry) between 1130 degrees C and 1340 degrees C (+/- 23-43 degrees C). Calculations using a multi-element diffusion model show that diffusive resetting of olivine Fo content during magma storage may be prevalent under ocean islands. Considering that the observed maximum Fo contents are several units lower than those of the presumed primary mantle olivines (Fo > 90), a correction on T(cry )is required to calculate that of the primary melts and T-p. Here we calculate an olivine-controlled liquid line of descent (LLD) with its one end fixed by the average Fo-T-cry (considering possible diffusive resetting) of the most forsteritic population at individual islands, and the other end lying on the (unknown) primary olivine Fo and crystallisation temperature. Mantle potential temperatures calculated from a fixed primary Fo(91) (=1326-1661 degrees C) show overlap with values reported by previous studies and are regarded as reliable temperature estimates. Using T-p = 1350 (+/- 12) degrees C calculated for Siqueiros using the same approach (but with additional constraints from crustal thickness and magma chemistry), we obtain the plume excess temperature Delta T-p for ocean islands with high-Fo olivines (Fo > 85) as -23 degrees C to 202 degrees C, which are comparable with results reported by recent seismic tomography studies and show correlations with plume buoyancy fluxes from the literature (especially at hotspots providing olivines Fo > 87). We obtain Delta T-p=229-311 degrees C for localities having lower olivine Fo (83-85), which likely indicates an overestimation of plume temperatures. The petrological approaches in this study can be applied to other ocean islands to constrain mantle temperature and identify key geophysical and petrological constraints that may contribute to more reliable mantle temperature estimates in future.

The evolution of planetary systems around white dwarfs is crucial to understanding the presence of planetary material in the atmospheres of white dwarfs. These systems uniquely probe exoplanetary compositions. Periodic signals in the photometry of a handful of white dwarfs suggest material blocking the star, potentially from disintegrating planetesimals. Tidal evolution followed by scattering can bring planetesimals onto close-in orbits that would have been within the envelope of the white dwarf progenitor. The orbital period distribution of planetesimals undergoing tidal evolution will peak at short-period (nearly) circularized orbits (~ 10 hour-1 day), with a rising tail towards long-period highly eccentric orbits (~ 100 day). This prediction is generally consistent with the observed white dwarf transiting systems. In order for the planetesimal on the 4.5 hour period around WD 1145+017 to be explained by the tidal evolution of a planetesimal, that planetesimal must have an ultimate tensile strength comparable to that of iron meteorites.