Xenoliths of carbonaceous chondrites (CC) in meteoritic breccias can provide samples of primitive solar system materials that are not represented by individual meteorites and thus expand our knowledge of chemical and isotopic reservoirs in the early solar system and early geologic processes on CC parent bodies. The Almahata Sitta (AhS) polymict ureilite contains one such xenolith, referred to here as AhS 202. Hamilton et al. (2020a) discovered that, unlike any other known CC, the AhS 202 xenolith contains abundant (similar to 12-14 vol%) amphibole, a hydrous mineral that characteristically forms in greenschist to amphibolite facies metamorphism and requires a significantly larger parent body than typically inferred (<= 100 km diameter) for CC meteorite bodies. Building on that initial work, we report additional analyses of the mineralogy and petrology, and new analyses of the chemical composition, oxygen and chromium isotope compositions, and physical properties of this xenolith that further constrain its petrogenesis and provenance. Our results show that the AhS 202 precursor was chondritic and experienced aqueous alteration similar to many low petrologic type CC meteorites at temperatures of similar to 30-100 degrees C and fluid pressures of P-H2O < 0.1 kbar, leading to formation of serpentines, magnetite, and chlorite. However, unlike any known CC meteorite, AhS 202 was heated further under water-saturated conditions similar to prograde metamorphism of terrestrial serpentinites, leading to formation of chemically pure diopside, secondary olivine, and tremolite amphibole. Peak metamorphic conditions determined from thermodynamic modeling, constrained by olivine-magnetite oxygen isotope thermometry, were similar to 380-430 degrees C and similar to 0.5-2.25 kbar. Based on our measured density of 2.27 g/cc for AhS 202, these conditions imply parent body sizes of 600-1875 km diameter, confirming the previous estimate (640-1800 km) of Hamilton et al. (2020a). The fluid-assisted metamorphic conditions experienced by AhS 202 cannot be represented in current classification systems of meteorite petrologic type, which recognize only anhydrous metamorphism; we discuss an alternative approach to the classification of such materials. Oxygen and chromium isotope compositions show an affinity between AhS 202 and CR chondrites and/or CR-related achondrites, suggesting derivation from a common reservoir. However, petrology, refractory element composition, and extremely low carbon content indicate that it did not form on the same parent body as known CR chondrites or CR-related achondrites. The existence of this sample, in combination with several even higher-pressure clasts observed in CR chondrites (Kimura et al., 2013; Hiyagon et al., 2016), suggests that this reservoir contained multiple large planetesimals.

We investigated geothermal gases from Homa Hills, a carbonatitic complex situated along an adjacent branch of the Kenyan rift system, using neon, argon, krypton, xenon and nitrogen isotopes. Large quantities of gas were sampled in Giggenbach-type bottles (Giggenbach, 1975) and analyzed by dynamic mass spectrometry to resolve isotopic variations at high precision (0.01-0.1 parts per thousand; Seltzer and Bekaert, 2022; Bekaert et al., 2023; 2024). Neon and nitrogen isotope compositions are consistent with parental magmas being derived from the convecting mantle. Xenon isotopic data present ubiquitous enrichments (relative to air) of 129Xe from the decay of extinct 129I (T1/2 = 15.7 Myr) and 131-136Xef from fissions of 238U (T1/2 = 4.468 Myr) and/or 244Pu (T1/2 = 82 Myr). We also find slight excesses of 128Xe (relative to 130Xe and air), which could be due to subsurface isotopic fractionation during e.g., diffusive transport fractionation (DTF) and gravitational settling. However, the 128Xe excesses are not accompanied by correlated Kr isotope excesses and plot off the empirical fractionation line defined from several other locations worldwide (Bekaert et al., 2023). Instead, a detailed isotope deconvolution suggests the occurrence of either chondritic Xe (with mantle 130Xe consisting of up to 22 % of chondritic 130Xe) or recycled Xe from the Archean atmosphere could explain the observed Xe isotope signatures. The latter possibility would have profound implications for models of mantle-surface exchange throughout Earth history. The fission spectra indicate a predominantly 238U origin for fissiogenic Xe, with contribution of 244Pu-derived Xe being negligible within uncertainties, implying extensive mantle degassing during the Hadean and Archean eons. The 129Xe*/136Xe* ratio (where * indicates non-atmospheric excesses of Xe isotopes) of Homa Hills samples correlates with other tracers of mantle/crust contributions such as He, Ar and N isotopes. Variations in 129Xe*/136Xe* among the different gases sampled at Homa Hills is mainly the result of contribution from fissiogenic Xe produced in uranium-rich crustal material. Therefore, this ratio may constitute a robust tracer of mantle-crust interactions. Given available high precision data (Bekaert et al., 2023; 2024; this work) together with mantle-derived rock data, 129Xe*/136Xe* appears homogenous in the convecting mantle, and comparable to values observed at mantle plumes. Such homogeneity is in sharp contrast with light noble gas systematics and may call for whole mantle convection and a core origin for He and Ne..

The relationship between stars and planets provides important information for understanding the interior composition, mineralogy, and overall classification of small planets (R ≲ 3.5R_⊕). Since stars and planets are formed at the same time and from the same material, their compositions are inextricably linked to one another, especially with respect refractory elements like Mg, Si, and Fe. As a result, stellar elemental abundances can help break the degeneracy inherent to planetary mass-radius models and determine whether planets may be similar to the Earth in composition or if additional factors, such as formation near the host star or a giant impact, may have influenced the planet's make-up. To this end, we now have observations of the abundances of extrasolar rocks that were pulled onto the surfaces of a white dwarfs, whose compositions act as a direct insight into the interiors of small exoplanets. From measurements of ∼30 of these "polluted" white dwarfs, we have found that composition of the extrasolar rocks are similar to Solar System chondritic meteorites.

Recent claims of biosignature gases in sub-Neptune atmospheres have renewed interest in water-rich sub-Neptunes with surface oceans, often referred to as Hycean planets. These planets are hypothesized to form beyond the snow line, accreting large amounts of H _2 O (>10 wt%) before migrating inward. However, current interior models often neglect chemical equilibration between primordial atmospheres and molten interiors. Here, we compute global chemical equilibrium states for a synthetic population of sub-Neptunes with magma oceans. Although many initially accrete 5–30 wt% water, interior–atmosphere interactions destroy most of it, reducing final H _2 O mass fractions to below 1.5 wt%. As a result, none meet the threshold for Hycean planets. Despite that, we find H _2 O-dominated atmospheres exclusively on planets that accreted the least ice. These planets form inside the snow line, are depleted in carbon and hydrogen, and develop small envelopes with envelope mass fractions below 1%, dominated by endogenic water. In contrast, planets formed beyond the snow line accrete more volatiles, but their water is largely converted to H _2 gas or sequestered into the interior, resulting in low atmospheric H _2 O mass fractions. Most H _2 O-rich envelopes are also fully miscible with H _2 , making a separate water layer unlikely. Our results challenge the conventional link between ice accretion and water-rich atmospheres, showing instead that H _2 O-dominated envelopes emerge through chemical equilibration in hydrogen-poor planets formed inside the snow line.

Super-Earths are highly irradiated, small planets with bulk densities approximately consistent with Earth. We construct combined interior atmosphere models of super-Earths that trace the partitioning of water throughout a planet, including an iron-rich core, silicate-rich mantle, and steam atmosphere. We compare these models with exoplanet observations to infer a 1 σ upper limit on the total water mass fraction of ≲3% at the population level. We consider end-member scenarios that may change this value, including the efficiency of mantle outgassing, escape of high mean molecular weight atmospheres, and increased iron core mass fractions. Although our constraints are agnostic as to the origin of water, we show that our upper limits are consistent with its production via chemical reactions of primordial hydrogen-dominated atmospheres with magma oceans. This mechanism has also been hypothesised to explain Earth's water content, possibly pointing to a unified channel for the origins of water on small terrestrial planets.

In subduction zones, thermal springs release deeply-sourced volatiles from Earth's mantle, crust, and/or subducted slab-derived material. The origin and apparent ages of these volatiles are important for understanding the deep volatile cycle, which in turn affects the distribution of microbial life in the subsurface. Here, we report carbon (C-13, C-14), noble gas (He, Ne, Ar, Kr and Xe), and clumped nitrogen isotope data in gas and water samples from thermal springs within the Central Volcanic Zone (CVZ) of the Andean Convergent Margin (ACM). He isotopes show that CVZ gases are predominantly sourced from the crust (similar to 77 %), with smaller mantle contributions (similar to 23 %), consistent with previous studies from the CVZ. Thermal spring samples with non-atmospheric He-Ne characteristics have low C-14 activities, and are deeply derived (i.e., from the mantle and crust) and old (>22,000 years). To gain additional constraints on volatile sources, a gas sample from Pirquitas Argentina was analyzed using a new high-precision technique to reveal significant geogenic anomalies in argon (Ar-40/Ar-36 = 492), fissiogenic xenon (88 % crustal), and helium (84 % crustal) isotopes. Clumped N-2 isotopologue results also indicate that the N-2-rich Pirquitas sample is dominated by crustal and magmatic N-2, which was unambiguously released at high temperatures (indicated by Delta(30) of similar to 0 parts per thousand). When taken together, all carbon, noble gas and clumped N-2 isotope data from CVZ thermal springs point toward a predominantly crustal source of volatile elements, which is consistent with the thick crust beneath the arc. We conclude that thermal springs with noble gas isotopic evidence for minimal air contributions are old, suggesting that any microbial communities entrained in them are also supported by deeply-derived and old organic carbon.

Abstract. Measurements of methane (CH4) molecules containing two rare isotopes (13CH3D and 12CH2D2), also termed doubly substituted or ‘clumped’ isotopologues, have the potential to provide two additional isotopic dimensions to help investigating mechanisms producing the recent global trends and CH4 budget over decadal timescale. In this work, we summarise the current state of research on doubly substituted CH4 isotopologues, with an emphasis on compiling results of all relevant work. The database comprises 1475 records compiled from the literature published until April 2025 (https://dx.doi.org/10.5285/51ae627da5fb41b8a767ee6c653f83e6). For field samples, 40 % of records were sourced from natural gas reservoirs, while microbial terrestrial (e.g., agriculture, lake, wetland) samples account only for 12.5 %. Lakes samples contribute 75 % to collected microbial terrestrial samples. There is limited or no representation of samples coming from significant microbial CH4 sources to the atmosphere, like wetlands, agriculture and landfill. To date, laboratory experiments were mostly focused on microbial (28 % of samples from laboratory experiments) and pyrogenic (15 %) methanogenesis or anaerobic (16 %), and aerobic (8 %) CH4 oxidation, and only a single contribution to studies of photochemical oxidation via OH and Cl (5 %). The distinct ranges of Δ13CH3D and Δ12CH2D2 values measured in these studies suggests their potential to improve our understanding of atmospheric CH4. This work provides an overview of the major gaps in measurements and identifies where further studies should be focussed to enable the highest immediate impact on understanding global CH4.

Methane is a potent greenhouse gas, an important energy source, and an important part of the global carbon cycle. The relative abundances of doubly substituted ("clumped") methane isotopologues (13CH3D and 12CH2D2) offer important information on the sources and sinks of methane. However, the clumped isotope signatures of microbially produced methane from different methanogenic pathways lack a systematic investigation. In this study, we provide a data set encompassing isotopic signatures of hydrogenotrophic, methylotrophic, acetoclastic, and methoxydotrophic methanogenesis. We find that a statistical "combinatorial effect" generates significant differences in 12CH2D2 compositions between hydrogenotrophic methanogenesis and the other pathways, while variations in the fractionation factors of clumped isotopologues result in differences in 13CH3D compositions between the methylotrophic, acetoclastic, and methoxydotrophic pathways. The energy yield of methanogenesis and the energy conservation approaches implemented by different microbial strains may also influence the isotope values of methane. Further analysis suggests that previously observed isotopic signatures of methane in freshwater environments are potentially due to mixing between hydrogenotrophic and other methanogenesis pathways. This study provides new experimental constraints on the isotope signatures of different microbial methanogenic pathways and evidence of the mechanisms responsible for the observed differences. This enables a better understanding of the sources and sinks of methane in the environment.

We investigate the consequences of non-ideal mixing between silicate, iron metal, and hydrogen for the structures of the cores of sub-Neptunes with implications for super-Earths, warm Neptunes, and ice giants. A method of extrapolating what we know about the miscibility in the three bounding binary systems MgSiO_3-H_2, MgSiO_3-Fe, and Fe-H_2 to the ternary composition space is used to deduce the phase equilibria of this system at relevant temperature and pressure conditions. We find that while separate silicate and metal phases can exist at shallow depths, the phases become entirely miscible deeper in the cores, thus altering the density structure of the cores. The assumption that the interiors of large rocky planets, either with extant magma oceans beneath H_2-rich envelopes, or evolved from such bodies, are composed of a differentiated metal core overlain by a silicate mantle is inconsistent with our understanding of the phase equilibria of these bodies.

Methane is a major greenhouse gas and a key component of global biogeochemical cycles. Microbial methane often deviates from isotope and isotopolog equilibrium in surface environments but approaches equilibrium in deep subsurface sediments. The origin of this near-equilibrium isotopic signature in methane, whether directly produced by methanogens or achieved through anaerobic oxidation of methane (AOM), remains uncertain. Here, we show that, in the absence of AOM, microbial methane produced from deep-sea sediments exhibits isotopolog compositions approaching thermodynamic equilibrium due to energy limitation. In contrast, microbial methane from salt marsh and thermokarst lakes exhibits significant hydrogen and clumped isotopic disequilibrium due to high free-energy availability. We propose that clumped isotopologs of methane provide a proxy for characterizing the bioenergetics of environments for methane production. Together, these observations demonstrate methane clumped isotopes as a powerful tool to better understand the relation between methane metabolisms and the energy landscape in natural environments.

Sub-Neptunes with substantial atmospheres may possess magma oceans in contact with the overlying gas, with chemical interactions between the atmosphere and magma playing an important role in shaping atmospheric composition. Early JWST observations have found high abundances of carbon- and oxygen-bearing molecules in a number of sub-Neptune atmospheres, which may result from processes including accretion of icy material at formation or magma–atmosphere interactions. Previous work examining the effects of magma–atmosphere interactions on sub-Neptunes has mostly been limited to studying conditions at the atmosphere–mantle boundary, without considering implications for the upper atmosphere, which is probed by spectroscopic observations. In this work, we present a modeling architecture to determine observable signatures of magma–atmosphere interactions. We combine an equilibrium chemistry code which models reactions between the core, mantle, and atmosphere with a radiative–convective model that determines the composition and structure of the observable upper atmosphere. We examine how different conditions at the atmosphere–mantle boundary and different core and mantle compositions impact the upper atmospheric composition. We compare our models to JWST NIRISS + NIRSpec observations of the sub-Neptune TOI-270 d, finding that our models can provide a good fit to the observed transmission spectrum with little fine-tuning. This suggests that magma–atmosphere interactions may be sufficient to explain the high abundances of molecules such as H _2 O, CH _4 , and CO _2 in sub-Neptune atmospheres, without additional accretion of icy material from the protoplanetary disk. Although other processes could lead to similar compositions, our work highlights the need to consider magma–atmosphere interactions when interpreting the observed atmospheric composition of a sub-Neptune.

Recently, the James Webb Space Telescope has enabled detailed spectroscopic characterization of sub-Neptune atmospheres. With detections of carbon- and oxygen-bearing species such as CO, CO _2 , CH _4 , and H _2 O, a central question is whether the atmospheric C/O ratio, commonly used to trace formation location in giant planets, can serve a similar diagnostic role for sub-Neptunes. We use the global chemical equilibrium framework of H. E. Schlichting & E. D. Young to quantify how magma ocean–atmosphere interactions affect the atmospheric C/O ratio. We find that the resulting C/O ratios range from several orders of magnitude below solar to a few times solar. The atmospheric C/O ratio in sub-Neptunes is therefore not inherited from the protoplanetary disk, but instead emerges from chemical equilibrium between the atmosphere and the underlying magma ocean. Planetary mass, atmospheric mass fraction, and thermal state all strongly influence the atmospheric C/O ratio. In addition, carbon partitioning into the metal phase typically reduces the atmospheric C/O ratio substantially, particularly for atmospheric mass fractions less than a few percent. Finally, we couple the deep equilibrium compositions to 1D atmospheric models that self-consistently solve for the pressure–temperature structure and chemical composition, including photochemistry. We find that the C/O ratio varies with altitude under low vertical mixing conditions ( K _zz = 10 ^4 cm ^2 s ^−1 ) but remains constant under strong mixing ( K _zz = 10 ^7 cm ^2 s ^−1 ). Our results imply that observed C/O ratios of sub-Neptunes can be used to probe their interiors. Specifically, C/O ratios much lower than host star values would imply an underlying magma ocean with iron metal having sequestered significant amounts of carbon.

Nitrogen plays a critical role in maintaining Earth's hospitable surface environment over geological time. Despite our atmosphere being dominated by nitrogen, our understanding of how nitrogen was delivered to Earth and how subsequent planetary processes modified Earth's nitrogen budget through time is currently lacking. Here, we report measurements of isotopologues of N2 (Δ30), along with ultrahigh precision measurements of Ar, Kr, and Xe isotopes, of hydrothermal gas samples from Yellowstone National Park. We show that δ15N variations are correlated with nonradiogenic Ar, Kr, and Xe isotope ratios, indicating that groundwater-derived nitrogen and noble gases in hydrothermal samples are fractionated by the same process as they diffuse through a rising column of magmatic CO2. Notably, a similar correlation exists regardless of the degree of atmospheric contamination, suggesting that the δ15N of the Yellowstone mantle source is similar to the atmosphere (i.e., ∼0‰). Two component mixing models between Δ30 and noble gases demonstrate that N2/36Ar (5.3 ± 0.7 × 105) and 36Ar/130Xe (1611 ± 212) in the Yellowstone mantle source are lower and greater than the MORB mantle source, respectively, suggesting that contrary to previous findings, the plume mantle source has not been more efficiently overprinted by the addition of N2- and Xe-rich recycled material. Conversely, we suggest that the similarity in δ15N and N2/36Ar between the Yellowstone mantle source and chondritic meteorites indicates that nitrogen and noble gases in the deep mantle reflect the composition of the material that initially formed Earth.

We present the first evolving interior structure model for sub-Neptunes that accounts for the miscibility between silicate magma and hydrogen. Silicate and hydrogen are miscible above similar to 4000 K at pressures relevant to sub-Neptune interiors. Using the H-2-MgSiO3 phase diagram, we self-consistently couple physics and chemistry to determine the radial extent of the fully miscible interior. Above this region lies the envelope, where hydrogen and silicates are immiscible and exist in both gaseous and melt phases. The binodal surface, representing a phase transition, provides a physically/chemically informed boundary between a planet's 'interior' and 'envelope'. We find that young sub-Neptunes can store several tens of per cent of their hydrogen mass within their interiors. As the planet cools, its radius and the binodal surface contract, and the temperature at the binodal drops from similar to 4000 to similar to 3000 K. Since the planet's interior stores hydrogen, its density is lower than that of pure-silicate. Gravitational contraction and thermal evolution lead to hydrogen exsolving from the interior into the envelope. This process slows planetary contraction compared to models without miscibility, potentially producing observable signatures in young sub-Neptune populations. At early times (similar to 10-100 Myr), the high temperature at the binodal surface results in more silicate vapour in the envelope, increasing its mean molecular weight and enabling convection inhibition. After similar to Gyr of evolution, most hydrogen has exsolved, and the radii of miscible and immiscible models converge. However, the internal distribution of hydrogen and silicates remains distinct, with some hydrogen retained in the interior.

The elemental and isotopic abundances of major species in the Martian atmosphere have been determined, but analyses often lack sufficient precision, and those of minor and trace species are frequently not well known. Many important questions about the evolution and current state of Mars require the kind of knowledge that can be gained from analysis of a returned sample of the Martian atmosphere. Key target species include the noble gases, nitrogen, and various species containing carbon, hydrogen, and oxygen, such as methane. More detailed analyses will no doubt provide measurements of other species that will allow insights of their own. These volatiles can constrain the origin of the Martian atmosphere, exchange of volatiles between the surface and interior, polar processes, and (in the case of methane) the possibility of extant biology on Mars.