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Temperate exoplanets between the sizes of Earth and Neptune, known as "sub-Neptunes", have emerged as intriguing targets for astrobiology. It is unknown whether these planets resemble Earth-like terrestrial worlds with a habitable surface, Neptune-like giant planets with deep atmospheres and no habitable surface, or something exotic in between. Recent JWST transmission spectroscopy observations of the canonical sub-Neptune K2-18 b revealed ~1% CH4, ~1% CO2, and a non-detection of CO in the atmosphere. While previous studies have proposed that the observed atmospheric composition could help constrain the lower atmosphere conditions and determine the interior structure of sub-Neptunes like K2-18 b, the possible interactions between the atmosphere and a hot, supercritical water ocean at its base remain unexplored. In this work, we investigate whether a global supercritical water ocean, resembling a planetary-scale hydrothermal system, can explain these observations on K2-18 b-like sub-Neptunes through equilibrium aqueous geochemical calculations. We find that the observed atmospheric CH4/CO2 ratio implies a minimum ocean temperature of ~715 K, whereas the corresponding CO/CO2 ratio allows ocean temperatures up to ~1060 K. These results indicate that a global supercritical water ocean on K2-18 b is plausible. While life cannot survive in this ocean, this work represents the first step towards understanding how a global supercritical water ocean may influence observable atmospheric characteristics on volatile-rich sub-Neptunes. Future observations with better constrained NH3 and CO mixing ratios could further help distinguish between possible interior compositions of K2-18 b., Comment: 15 pages, 5 figures, 1 table

We examine the role of refractory organics as a major C carrier in the outer protosolar nebula and its implications for the compositions of large Kuiper belt objects (KBOs) and CI chondrites. By utilizing Rosetta measurements of refractory organics in comet 67P/Churyumov-Gerasimenko, we show that they would make up a large fraction of the protosolar C inventory in the KBO-forming region based on the current widely adopted solar abundances. However, this would free up too much O to form water ice, producing solid material that is not sufficiently rock-rich to explain the uncompressed density of the Pluto-Charon system and other large KBOs; the former has been argued as the most representative value we have for the bulk composition of large KBOs (Barr & Schawmb 2016, Bierson & Nimmo 2019). This inconsistency further highlights the solar abundances problem - an ongoing challenge in reconciling spectroscopically determined heavy element abundances with helioseismology constraints. By employing a new dataset from solar CNO neutrinos and solar wind measurements of C, N, and O, we show that the uncompressed density of the Pluto-Charon system can be reproduced over a wide range of scenarios. We show that a lack of sulfates in Ryugu and Bennu samples implies a lower amount of water ice initially accreted into CI chondrite parent bodies than previously thought. These data are found to be consistent with the solar C/O ratio implied by the new dataset. Our predictions can be tested by future neutrino, helioseismology, and cosmochemical measurements., Comment: 32 pages, 10 figures, 3 tables, accepted for publication in ApJ

Quantifying disequilibria is important to understand whether an environment could be habitable. It has been proposed that the exoplanet K2-18b has a hydrogen-rich atmosphere and a water ocean, making it a "hycean world". The James Webb Space Telescope recently made measurements of methane, CO$_2$, and possibly dimethyl sulfide (DMS) in the atmosphere of this planet. The initial interpretation of these data is that they may support the occurrence of hycean conditions. Here, I attempt to take a next step in exploring the prospects for habitability. I use constraints on the abundances of atmospheric gases to calculate how much chemical disequilibrium there could be, assuming K2-18b is a hycean world. I find that the presence of oxidized carbon species coexisting with abundant H$_2$ (1-1000 bar) at cool to warm (25-120{\deg}C) conditions creates a strong thermodynamic drive for methanogenesis. More than ~75 kJ (mol C)$^{-1}$ of free energy can be released from CO$_2$ hydrogenation. Partially oxidized carbon compounds such as DMS (if present) also have potential to provide metabolic energy, albeit in smaller quantities. Because of the thermodynamic instability of CO$_2$ under hycean conditions, other reductive reactions of CO$_2$ are likely to be favored, including the synthesis of amino acids. Glycine and alanine synthesis can be energy-releasing or at least much less costly on K2-18b than in Earth's ocean, even when NH$_3$ is scarce but not totally absent. These first bioenergetic calculations for a proposed ocean-bearing exoplanet lay new groundwork for assessing exoplanetary habitability., Comment: To be published in The Astrophysical Journal Letters

We analyzed spectral cubes of Callisto's leading and trailing hemispheres, collected with the NIRSpec Integrated Field Unit (G395H) on the James Webb Space Telescope. These spatially resolved data show strong 4.25-micron absorption bands resulting from solid-state 12CO2, with the strongest spectral features at low latitudes near the center of its trailing hemisphere, consistent with radiolytic production spurred by magnetospheric plasma interacting with native H2O mixed with carbonaceous compounds. We detected CO2 rovibrational emission lines between 4.2 and 4.3 microns over both hemispheres, confirming the global presence of CO2 gas in Callisto's tenuous atmosphere. These results represent the first detection of CO2 gas over Callisto's trailing side. The distribution of CO2 gas is offset from the subsolar region on either hemisphere, suggesting that sputtering, radiolysis, and geologic processes help sustain Callisto's atmosphere. We detected a 4.38-micron absorption band that likely results from solid-state 13CO2. A prominent 4.57-micron absorption band that might result from CN-bearing organics is present and significantly stronger on Callisto's leading hemisphere, unlike 12CO2, suggesting these two spectral features are spatially anti-associated. The distribution of the 4.57-micron band is more consistent with a native origin and/or accumulation of dust from Jupiter's irregular satellites. Other, more subtle absorption features could result from CH-bearing organics, CO, carbonyl sulfide (OCS), and Na-bearing minerals. These results highlight the need for preparatory laboratory work and improved surface-atmosphere interaction models to better understand carbon chemistry on the icy Galilean moons before the arrival of NASA's Europa Clipper and ESA's JUICE spacecraft., Comment: Accepted in AAS Planetary Science Journal, January 2024

Dwarf planets Eris and Makemake have surfaces bearing methane ice of unknown origin. D/H ratios were recently determined from James Webb Space Telescope (JWST) observations of Eris and Makemake, giving us new clues to decipher the origin of methane. Here, we develop geochemical models to test if the origin of methane could be primordial, derived from CO$_2$ or CO ("abiotic"), or sourced by organics ("thermogenic"). We find that primordial methane is inconsistent with the observational data, whereas both abiotic and thermogenic methane can have D/H ratios that overlap the observed ranges. This suggests that Eris and Makemake either never acquired a significant amount of methane during their formation, or their original inventories were removed and then replaced by a source of internally produced methane. Because producing abiotic or thermogenic methane likely requires temperatures above ~150{\deg}C, we infer that Eris and Makemake have rocky cores that underwent substantial radiogenic heating. Their cores may still be warm/hot enough to make methane. This heating could have driven hydrothermal circulation at the bottom of an ice-covered ocean to generate abiotic methane, and/or metamorphic reactions involving accreted organic matter could have occurred in response to heating in the deeper interior, generating thermogenic methane. Additional analyses of relevant thermal evolution model results and theoretical predictions of the D/H ratio of methane in the solar nebula support our findings of elevated subsurface temperatures and an apparent lack of primordial methane on Eris and Makemake. It remains an open question whether their D/H ratios may have evolved subsequent to methane outgassing. Recommendations are given for future activities to further test proposed scenarios of abiotic and thermogenic methane production on Eris and Makemake, and to explore these worlds up close., Comment: Accepted by Icarus, 43 pages, 8 figures, 1 table

N$_2$ plays a profound role in supporting processes on the surface and in the atmosphere of Pluto, yet the origin of Pluto's N$_2$ remains a mystery. However, this may begin to change as the $^{14}$N/$^{15}$N ratio of N$_2$ was recently estimated based on a non-detection of HC15N in Pluto's atmosphere, while accounting for $^{14}$N/$^{15}$N fractionation between HCN and N$_2$. Here, I show that, if this latter step of translating isotope ratios is adequately understood, then the derived $^{14}$N/$^{15}$N ratio represents the first distinguishing constraint on the origin of N$_2$. One finding of the present study is that isotopic fractionation between atmospheric N$_2$ and N$_2$-rich ices on Pluto does not appear to be significant. I infer a lower limit of ~197 for the $^{14}$N/$^{15}$N ratio of the dominant reservoir of N$_2$ on Pluto; i.e., mostly contained in Sputnik Planitia. From this lower limit, an endmember ammonia source of Pluto's N$_2$ can be ruled out. I perform N isotope mixing calculations that enable quantitative understanding of the relationships between contributions by primordial N$_2$, NH$_3$, and nitrogen originally sourced in organic materials (Norg) to Pluto's observed N$_2$ inventory. These calculations address how uncertainties in the isotopic composition of Norg and the history of atmospheric escape affect the allowed ranges of primordial N$_2$, NH$_3$, and Norg contributions. While present uncertainties are substantial, I find that a contribution by primordial N$_2$, Norg, or both is implied, and the sum of their contributions should be at least ~45%. Hence, Pluto likely formed from building blocks that were cold enough to trap N$_2$, or Pluto has a thermally processed and dynamic interior that supports generation of N$_2$ from Norg and N$_2$ transport to the surface. The lower limit on $^{14}$N/$^{15}$N suggests that NH$_3$ has been a less significant contributor to the origin of N$_2$ on Pluto than on Titan., Comment: 23 pages, 3 figures, 3 tables

A key feature of the Galilean satellite system is its monotonic decrease in bulk density with distance from Jupiter, indicating an ice mass fraction that is zero in the innermost moon Io, and about half in the outer moons Ganymede and Callisto. Jupiter formation models, and perhaps the Juno spacecraft water measurements, are consistent with the possibility that the Jovian system may have formed, at least partly, from ice-poor material. And yet, models of the formation of the Galilean satellites usually assume abundant water ice in the system. Here, we investigate the possibility that the Jovian circumplanetary disk was populated with ice-depleted chondritic minerals, including phyllosilicates. We show that the dehydration of such particles and the outward diffusion of the released water vapor allow condensation of significant amounts of ice in the formation region of Ganymede and Callisto in the Jovian circumplanetary disk. Our model predicts that Europa, Ganymede and Callisto should have accreted little if any volatiles other than water ice, in contrast to the comet-like composition of Saturn's moon Enceladus. This mechanism allows for the presence of ice-rich moons in water-depleted formation environments around exoplanets as well., Comment: Accepted for publication in The Astrophysical Journal Letters

The Cassini mission to the Saturn system discovered a plume of ice grains and water vapor erupting from cracks on the icy surface of the satellite Enceladus. This moon has a global ocean in contact with a rocky core beneath its icy exterior, making it a promising location to search for evidence of extraterrestrial life in the solar system. The previous detection of H$_2$ in the plume indicates that there is free energy available for methanogenesis, the metabolic reaction of H$_2$ with CO$_2$ to form methane and water. Additional metabolic pathways could provide sources of energy in Enceladus' ocean, but require the use of other oxidants that have not been detected in the plume. Here, we perform chemical modeling to determine how the production of radiolytic O$_2$ and H$_2$O$_2$, and abiotic redox chemistry in the ocean and rocky core, contribute to chemical disequilibria that could support metabolic processes in Enceladus' ocean. We consider three possible cases for ocean redox chemistry: Case I in which reductants are not present in appreciable amounts and oxidants accumulate over time, and Cases II and III in which aqueous reductants or seafloor minerals, respectively, convert O$_2$ and H$_2$O$_2$ to SO$_4^{2-}$ and ferric oxyhydroxides. We calculate the upper limits on the concentrations of oxidants and chemical energy available for metabolic reactions in all three cases, neglecting additional abiotic reactions. For all three cases, we find that many aerobic and anaerobic metabolic reactions used by microbes on Earth could meet the minimum free energy threshold required for terrestrial life to convert ADP to ATP, as well as sustain positive cell density values within the Enceladus seafloor and/or ocean. These findings indicate that oxidant production and oxidation chemistry could contribute to supporting possible life and a metabolically diverse microbial community on Enceladus., Comment: Accepted to Icarus 2 December 2020

The Pluto-Charon system provides a broad variety of constraints on planetary formation, composition, chemistry, and evolution. Pluto was the first body to be discovered in what is now known as the Kuiper belt, its orbit ultimately becoming a major clue that the giant planets underwent substantial orbital migration early in Solar System history. This migration has been linked to an early instability in the orbits of the giant planets and the formation of the Kuiper belt itself, from an ancestral trans-Neptunian planetesimal disk that included Pluto. Pluto-Charon is emblematic of what are now recognized as small or dwarf planets. Far from being a cold, dead, battered icy relic, Pluto displays evidence of a complex geological history, with ongoing processes including tectonism, cryovolcanism, solid-state convection, glacial flow, atmospheric circulation, surface-atmosphere volatile exchange, aeolian processes, and atmospheric photochemistry, microphysics, and haze formation. Despite Pluto's relatively modest scale, the combination of original accretional heat, long-term internal radiogenic heat release, and external solar forcing, when combined with sufficiently volatile (and thus mobile) materials, yields an active world. Pluto may have inherited a large organic mass fraction during accretion, which may responsible, in part, for its surface and atmospheric volatiles. Charon, Pluto's major moon, displays evidence of extensive early tectonism and cryovolcanism. Dwarf planets are thus truly planetary in terms of satellite systems and geological and atmospheric complexity (if not ongoing activity). What they may lack in mass is made up in number, and the majority of the Solar System's dwarf planets remain undiscovered., Comment: 94 pages, 13 figures; will appear as a chapter/conference proceedings in the Space Science Series volume "The Pluto System After New Horizons," S.A. Stern et al., University of Arizona Press, Tucson, AZ

To test the potential of using amino acid abundances as a biosignature at icy ocean worlds, we investigate whether primordial amino acids (accreted or formed by early aqueous processes) could persist until the present time. By examining the decomposition kinetics of amino acids in aqueous solution based on existing laboratory rate data, we find that all fourteen proteinogenic amino acids considered in this study decompose to a very large extent (>99.9%) over relatively short lengths of time in hydrothermally active oceans. Therefore, as a rule of thumb, we suggest that if amino acids are detected at Enceladus, Europa, or other hydrothermally active ocean worlds above a concentration of 1 nM, they should have been formed recently and not be relicts of early processes. In particular, the detection of aspartic acid (Asp) and threonine (Thr) would strongly suggest active production within the ocean, as these amino acids cannot persist beyond 1 billion years even at the freezing point temperature of 273K. Identifying amino acids from the oceans of icy worlds can provide key insight into their history of organic chemistry., Comment: Accepted for publication in Icarus

The presence of N2 in the surface environment of Pluto is critical in creating Pluto's richness of features and processes. Here, we propose that the nitrogen atoms in the N2 observed on Pluto were accreted in that chemical form during the formation of Pluto. We use New Horizons data and models to estimate the amounts of N2 in the following exterior reservoirs: atmosphere, escape, photochemistry, and surface. The total exterior inventory is deduced to be dominated by a glacial sheet of N2-rich ices at Sputnik Planitia, or by atmospheric escape if past rates of escape were much faster than at present. Pluto's atmosphere is a negligible reservoir of N2, and photochemical destruction of N2 may also be of little consequence. Estimates are made of the amount of N2 accreted by Pluto based on cometary and solar compositions. It is found that the cometary model can account for the amount of N2 in Sputnik Planitia, while the solar model can provide a large initial inventory of N2 that would make prodigious atmospheric escape possible. These consistencies can be considered preliminary evidence in support of a primordial origin of Pluto's N2. However, both models predict accreted ratios of CO/N2 that are much higher than that in Pluto's atmosphere. Possible processes to explain "missing CO" that are given quantitative support here are fractional crystallization from the atmosphere resulting in CO burial at the surface, and aqueous destruction reactions of CO subject to metastable thermodynamic equilibrium in the subsurface. The plausibility of primordial N2 as the primary source of Pluto's nitrogen (vs. NH3 or organic N) can be tested more rigorously using future constraints on the 14N/15N ratio in N2 and the 36Ar/N2 ratio., Comment: 37 pages, 7 figures, 2 tables