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Giant impacts can generate transient hydrogen-rich atmospheres, reducing atmospheric carbon. The reduced carbon will form hazes that rain out onto the surface and can become incorporated into the crust. Once heated, a large fraction of the carbon would be converted into graphite. The result is that local regions of the Hadean crust were plausibly saturated with graphite. We explore the consequences of such a crust for a prebiotic surface hydrothermal vent scenario. We model a surface vent fed by nitrogen-rich volcanic gas from high-temperature magmas passing through graphite-saturated crust. We consider this occurring at pressures of 1-1000 bar and temperatures of 1500-1700 degC. The equilibrium with graphite purifies the left-over gas, resulting in substantial quantities of nitriles (0.1% HCN and 1 ppm HC3N) and isonitriles (0.01% HNC) relevant for prebiotic chemistry. We use these results to predict gas-phase concentrations of methyl isonitrile of ~ 1 ppm. Methyl isocyanide can participate in the non-enzymatic activation and ligation of the monomeric building blocks of life, and surface, or shallow, hydrothermal environments provide its only known equilibrium geochemical source., Comment: 26 pages, 17 figures, 2 tables. Accepted for publication in Life (MDPI Journal)

Long-standing unexplained Venus atmosphere observations and chemical anomalies point to unknown chemistry but also leave room for the possibility of life. The unexplained observations include several gases out of thermodynamic equilibrium (e.g. tens of ppm O2, the possible presence of PH3 and NH3, SO2 and H2O vertical abundance profiles), an unknown composition of large, lower cloud particles, and the "unknown absorber(s)". Here we first review relevant properties of the Venus atmosphere and then describe the atmospheric chemical anomalies and how they motivate future astrobiology missions to Venus., Comment: Published in Astrobiology (in press); Partially based on the text of the Venus Life Finder Mission Study report (arXiv:2112.05153)

As the characterization of exoplanet atmospheres proceeds, providing insights into atmospheric chemistry and composition, a key question is how much deeper into the planet we might be able to see from its atmospheric properties alone. For small planets with modest atmospheres and equilibrium temperatures, the first layer below the atmosphere will be their rocky surface. For such warm rocky planets, broadly Venus-like planets, the high temperatures and moderate pressures at the base of their atmospheres may enable thermochemical equilibrium between rock and gas. This links the composition of the surface to that of the observable atmosphere. Using an equilibrium chemistry code, we find a boundary in surface pressure-temperature space which simultaneously separates distinct mineralogical regimes and atmospheric regimes, potentially enabling inference of surface mineralogy from spectroscopic observations of the atmosphere. Weak constraints on the surface pressure and temperature also emerge. This regime boundary corresponds to conditions under which SO2 is oxidized and absorbed by calcium-bearing minerals in the crust, thus the two regimes reflect the sulphidation of the crust. The existence of these atmospheric regimes for Venus-like planets is robust to plausible changes in the elemental composition. Our results pave the way to the prospect of characterizing exoplanetary surfaces as new data for short period rocky planet atmospheres emerge., Comment: 12 pages, 9 figures, 2 tables. Accepted 17 Dec 2023 for publication in MNRAS

In this work we consider the potential of cometary impacts to deliver complex organic molecules and the prebiotic building blocks required for life to rocky exoplanets. Numerical experiments have demonstrated that for these molecules to survive, impacts at very low velocities are required. This work shows that for comets scattered from beyond the snow-line into the habitable zone, the minimum impact velocity is always lower for planets orbiting Solar-type stars than M-dwarfs. Using both an analytical model and numerical N-body simulations, we show that the lowest velocity impacts occur onto planets in tightly-packed planetary systems around high-mass (i.e. Solar-mass) stars, enabling the intact delivery of complex organic molecules. Impacts onto planets around low-mass stars are found to be very sensitive to the planetary architecture, with the survival of complex prebiotic molecules potentially impossible in loosely-packed systems. Rocky planets around M-dwarfs also suffer significantly more high velocity impacts, potentially posing unique challenges for life on these planets. In the scenario that cometary delivery is important for the origins of life, this study predicts the presence of biosignatures will be correlated with i) decreasing planetary mass (i.e. escape velocity), ii) increasing stellar-mass, and iii) decreasing planetary separation (i.e. exoplanets in tightly-packed systems)., Comment: Accepted by Proceedings A of the Royal Society. 17 pages, 5 figures

I show that exoplanets can be used to test origins scenarios. Origins scenarios start with certain initial conditions, proceed via a network of chemical reactions and, if successful, result in a chemistry that is closer to a living system than the initial conditions. Exoplanet environments can be applied to test each of these three aspects of origins scenarios. I show what tests can be applied to the UV-driven cyanosulfidic scenario and how the application of some of these tests has already falsified certain versions of this scenario. Testing initial conditions has replaced certain reactants with others and has affected the overall chemical network underlying the cyanosulfidic scenario. The sequence of reactions the scenario invokes provide a predicted upper limit on the ubiquity of life in the universe that has ample room for improvement. The outcome of the experiments in different environments is part of a predicted distribution of biosignature detections that can be compared to future observed distributions., Comment: Early uncorrected draft of: Rimmer, P.B., 2023. Origins of Life on Exoplanets. Conflicting Models for the Origin of Life, (Editors: Smoukov, S.K., Seckbach, J. and Gordon, R.) John Wiley & Sons. pp.407-424

We are of the opinion that several anomalies in the atmosphere of Venus provide evidence of yet-unknown processes and systems that are out of equilibrium. The investigation of these anomalies on Venus should be open to the wide range of explanations, including unknown biological activity. We provide an overview of two anomalies, the tentative detection of ammonia and phosphine in Venus's atmosphere. These anomalies fly in the face of the tacit assumption that the atmosphere of Venus must be in chemical redox equilibrium, an assumption connected to the belief that Venus is lifeless. We then discuss several major past discoveries in astronomy, biology and geology, which lead to the abandonment of certain assumptions held by many scientists as though they were well-established principles. The anomalies of ammonia and phosphine in the atmosphere of Venus are placed in the context of these historical discoveries. This context supports our opinion that persistence by the community in the exploration of these anomalies with a skeptical eye towards tacit assumptions will increase the chances of making profound discoveries about the atmosphere of Venus and the diverse and often strange nature of planetary environments., Comment: 10 pages, Opinion, Accepted to Aerospace (MDPI) Special Issue "The Search for Signs of Life on Venus: Science Objectives and Mission Designs"

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. Models simulating volcanic growth of atmospheres often make one of two assumptions: either that atmospheric speciation is set by the high-temperature equilibrium of volcanism; or, that volcanic gases thermochemically re-equilibrate to the new, lower, temperature 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 underlying such inferences by performing chemical kinetic calculations to estimate the relaxation timescale of volcanically-derived atmospheres to thermochemical equilibrium, in a simple 0D atmosphere neglecting photochemistry and reaction catalysis. We demonstrate that for planets with volcanic atmospheres, thermochemical equilibrium over geological timescales can only be assumed if the atmospheric temperature is above ~700K. Slow chemical kinetics at lower temperatures inhibit the relaxation of redox-sensitive species to low-temperature thermochemical equilibrium, precluding the production of two independent biosignatures through thermochemistry alone: 1. ammonia, and 2. the co-occurrence of CO$_2$ and CH$_4$ 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 therefore complicates linking atmospheres to the interiors of rocky exoplanets, even when their atmospheres are purely volcanic in origin., Comment: Accepted by JGR: Planets

Life in the clouds of Venus, if present in sufficiently high abundance, must be affecting the atmospheric chemistry. It has been proposed that abundant Venusian life could obtain energy from its environment using three possible sulfur energy-metabolisms. These metabolisms raise the possibility of Venus's enigmatic cloud-layer SO$_2$-depletion being caused by life. We here couple each proposed energy-metabolism to a photochemical-kinetics code and self-consistently predict the composition of Venus's atmosphere under the scenario that life produces the observed SO$_2$-depletion. Using this photo-bio-chemical kinetics code, we show that all three metabolisms can produce SO$_2$-depletions, but do so by violating other observational constraints on Venus's atmospheric chemistry. We calculate the maximum possible biomass density of sulfur-metabolising life in the clouds, before violating observational constraints, to be $\sim10^{-5}\,-\,10^{-3}\,{\rm mg\,m^{-3}}$. 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., Comment: Published in Nature Communications

The atmosphere of Venus remains mysterious, with many outstanding chemical connundra. These include: the unexpected presence of ~10 ppm O2 in the cloud layers; an unknown composition of large particles in the lower cloud layers; and hard to explain measured vertical abundance profiles of SO2 and H2O. We propose a new hypothesis for the chemistry in the clouds that largely addresses all of the above anomalies. We include ammonia (NH3), a key component that has been tentatively detected both by the Venera 8 and Pioneer Venus probes. NH3 dissolves in some of the sulfuric acid cloud droplets, effectively neutralizing the acid and trapping dissolved SO2 as ammonium sulfite salts. This trapping of SO2 in the clouds together with the release of SO2 below the clouds as the droplets settle out to higher temperatures, explains the vertical SO2 abundance anomaly. A consequence of the presence of NH3 is that some Venus cloud droplets must be semi-solid ammonium salt slurries, with a pH~1, which matches Earth acidophile environments, rather than concentrated sulfuric acid. The source of NH3 is unknown, but could involve biological production; if so, then the most energy-efficient NH3-producing reaction also creates O2, explaining the detection of O2 in the cloud layers. Our model therefore predicts that the clouds are more habitable than previously thought, and may be inhabited. Unlike prior atmospheric models, ours does not require forced chemical constraints to match the data. Our hypothesis, guided by existing observations, can be tested by new Venus in situ measurements.

The initial reports of the presence of phosphine in the cloud decks of Venus has led to the suggestion that volcanism was the source of phosphine, through volcanic phosphides ejected into the clouds. Here we examine the idea that mantle plume volcanism, bringing material from the deep mantle to the surface, could generate observed amounts of phosphine through interaction of explosively erupted phosphide with sulfuric acid clouds. Direct eruption of deep mantle phosphide is unphysical, but shallower material could contain traces of phosphide, and could be erupted to the surface. Explosive eruption that efficiently transported material to the clouds would require ocean:magma interactions or subduction of hydrated oceanic crust, neither of which occur on modern Venus. The transport of erupted material to altitudes coinciding with the observations of phosphine is consequently very inefficient. Using the model proposed by Truong and Lunine as a base case, we estimate that an eruption volume of at least 21,600 km3/year would be required to explain the presence of 1 ppb phosphine in the clouds. This is greater than any historical terrestrial eruption rate, and would have several detectable consequences for remote and in situ observations to confirm. More realistic lithospheric mineralogy, volcano mechanics or atmospheric photochemistry require even more volcanism., Comment: The paper expands on and confirms the results of Bains et al. 2021 arXiv:2009.06499 and provides further constrains on the model of the production of phosphine on Venus proposed by Truong and Lunine https://www.pnas.org/content/118/29/e2021689118 ; v2 is published in Universe

The potential detection of ppb levels phosphine (PH3) in the clouds of Venus through millimeter-wavelength astronomical observations is extremely surprising as PH3 is an unexpected component of an oxidized environment of Venus. A thorough analysis of potential sources suggests that no known process in the consensus model of Venus' atmosphere or geology could produce PH3 at anywhere near the observed abundance. Therefore, if the presence of PH3 in Venus' atmosphere is confirmed, it is highly likely to be the result of a process not previously considered plausible for Venusian conditions. The source of atmospheric PH3 could be unknown geo- or photochemistry, which would imply that the consensus on Venus' chemistry is significantly incomplete. An even more extreme possibility is that strictly aerial microbial biosphere produces PH3. This paper summarizes the Venusian PH3 discovery and the scientific debate that arose since the original candidate detection one year ago., Comment: A short overview of the Venusian PH3 discovery and the scientific debate that arose since the original candidate detection in September 2020. Additional discussion of possible non-canonical sources of PH3 on Venus is also included. arXiv admin note: text overlap with arXiv:2009.06499

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., Comment: Accepted for publication in JGR:Planets

New analysis is presented of the 1.1 mm wavelength absorption lines in Venus' atmosphere that suggested the presence of phosphine. We retrieve a sulphur dioxide observation from the JCMT archive that was simultaneous within a few days of the PH3 1-0 spectrum obtained in June 2017, and demonstrate via a radiative transfer calculation that contamination of PH3 by SO2 was ~10 per cent. We also present ALMA 2019 spectra of PH3 1-0 and an SO2 transition acquired simultaneously, and infer that SO2 line-contamination was ~2 percent (for the least-noisy half of the planetary disc). The contamination-subtracted ALMA and JCMT spectra (of 6-8 sigma confidence) are now consistent with similar absorption-depths at the two epochs. The two values span -1.9(+/-0.2) 10-4 of the continuum signal (which was re-estimated for ALMA), albeit for differing planetary areas. This suggests that the abundance attributed to phosphine in Venus' atmosphere was broadly similar in 2017 and 2019., Comment: v3 is an proofread version of the paper that will appear in print in MNRAS

The study of origins of life on Earth and the search for life on other planets are closely linked. Prebiotic chemical scenarios can help prioritize planets as targets for the search for life as we know it and can provide informative priors to help us assess the likelihood that particular spectroscopic features are evidence of life. The prerequisites for origins scenarios themselves predict spectral signatures. The interplay between origins research and the search for extraterrestrial life must start with lab work guiding exploratory ventures in the solar system, and the discoveries in the solar system informing future exoplanet observations and laboratory research. Subsequent exoplanet research will in turn provide statistical context to conclusions about the nature and origins of life., Comment: To be published as article 7 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., Comment: 30 pages, 15 figures, 2 tables, accepted for publication in The Astrophysical Journal

We predict that cyanoacetylene (HC$_3$N) is produced photochemically in the atmosphere of GJ 1132 b in abundances detectable by the James Webb Space Telescope (JWST), assuming that the atmosphere is hydrogen dominated and rich in molecular nitrogen (N$_2$), methane (CH$_4$) and hydrogen cyanide (HCN), as described by Swain et al. (2021). First, we construct line list and cross-sections for HC$_3$N. Then we apply these cross-sections and the model atmosphere of Swain et al. (2021) to a radiative transfer model in order to simulate the transmission spectrum of GJ 1132 b as it would be seen by JWST, accounting for the uncertainty in the retrieved abundances. We predict that cyanoacetylene features at various wavelengths, with a clear lone feature at 4.5 $\mu$m, observable by JWST after one transit. This feature persists within the $1-\sigma$ uncertainty of the retrieved abundances of HCN and CH$_4$. The signal is detectable for stratospheric temperatures $\lesssim 600$ K and moderate stratospheric mixing ($10^6 \, {\rm cm^2 \, s^{-1}} \lesssim K_{zz} \lesssim 10^8 \, {\rm cm^2 \, s^{-1}}$). Our results also indicate that HC$_3$N is an important source of opacity that future retrieval models should consider., Comment: 11 pages, 5 figures, accepted for publication in ApJL, comments welcome

We recover PH3 in the atmosphere of Venus in data taken with ALMA, using three different calibration methods. The whole-planet signal is recovered with 5.4{\sigma} confidence using Venus bandpass self-calibration, and two simpler approaches are shown to yield example 4.5-4.8{\sigma} detections of the equatorial belt. Non-recovery by Villanueva et al. is attributable to (a) including areas of the planet with high spectral-artefacts and (b) retaining all antenna baselines which raises the noise by a factor ~2.5. We release a data-processing script that enables our whole-planet result to be reproduced. The JCMT detection of PH3 remains robust, with the alternative SO2 attribution proposed by Villanueva et al. appearing inconsistent both in line-velocity and with millimetre-wavelength SO2 monitoring. SO2 contamination of the ALMA PH3-line is minimal. Net abundances for PH3, in the gas column above ~55 km, are up to ~20 ppb planet-wide with JCMT, and ~7 ppb with ALMA (but with signal-loss possible on scales approaching planetary size). Derived abundances will differ if PH3 occupies restricted altitudes - molecules in the clouds will contribute significantly less absorption at line-centre than equivalent numbers of mesospheric molecules - but in the latter zone, PH3 lifetime is expected to be short. Given we recover phosphine, we suggest possible solutions (requiring substantial further testing): a small collisional broadening coefficient could give narrow lines from lower altitude, or a high eddy diffusion coefficient could allow molecules to survive longer at higher altitudes. Alternatively, PH3 could be actively produced by an unknown mechanism in the mesosphere, but this would need to be in addition to cloud-level PH3 detected retrospectively by Pioneer-Venus., Comment: Revised and updated re-analysis of the phosphine spectra in Venus' clouds; Submitted to Nature Astronomy "Matters Arising" on Apr/19/2021

The depletion of SO$_2$ and H$_2$O in and above the clouds of Venus (45 -- 65 km) cannot be explained by known gas-phase chemistry and the observed composition of the atmosphere. We apply a full-atmosphere model of Venus to investigate three potential explanations for the SO$_2$ and H$_2$O depletion: (1) varying the below-cloud water vapor (H$_2$O), (2) varying the below-cloud sulfur dioxide (SO$_2$), and (3) the incorporation of chemical reactions inside the sulfuric acid cloud droplets. We find that increasing the below-cloud H$_2$O to explain the SO$_2$ depletion results in a cloud top that is 20 km too high, above-cloud O$_2$ three orders of magnitude greater than observational upper limits and no SO above 80 km. The SO$_2$ depletion can be explained by decreasing the below-cloud SO$_2$ to $20\,{\rm ppm}$. The depletion of SO$_2$ in the clouds can also be explained by the SO$_2$ dissolving into the clouds, if the droplets contain hydroxide salts. These salts buffer the cloud pH. The amount of salts sufficient to explain the SO$_2$ depletion entail a droplet pH of $\sim 1$ at 50 km. Since sulfuric acid is constantly condensing out into the cloud droplets, there must be a continuous and pervasive flux of salts of $\approx 10^{-13} \, {\rm mol \, cm^{-2} \, s^{-1}}$ driving the cloud droplet chemistry. An atmospheric probe can test both of these explanations by measuring the pH of the cloud droplets and the concentrations of gas-phase SO$_2$ below the clouds., Comment: Accepted for publication in The Planetary Science Journal. We kindly ask the planetary science community for feedback and comments. (67 pages, 8 Figures, 7 Tables, Full Model Output Data: https://doi.org/10.7910/DVN/AKUTME)

Hot Jupiters provide valuable natural laboratories for studying potential contributions of high-energy radiation to prebiotic synthesis in the atmospheres of exoplanets. In this fourth paper of the MOVES (Multiwavelength Observations of an eVaporating Exoplanet and its Star) programme, we study the effect of different types of high-energy radiation on the production of organic and prebiotic molecules in the atmosphere of the hot Jupiter HD 189733b. Our model combines X-ray and UV observations from the MOVES programme and 3D climate simulations from the 3D Met Office Unified Model to simulate the atmospheric composition and kinetic chemistry with the STAND2019 network. Also, the effects of galactic cosmic rays and stellar energetic particles are included. We find that the differences in the radiation field between the irradiated dayside and the shadowed nightside lead to stronger changes in the chemical abundances than the variability of the host star's XUV emission. We identify ammonium (NH4+) and oxonium (H3O+) as fingerprint ions for the ionization of the atmosphere by both galactic cosmic rays and stellar particles. All considered types of high-energy radiation have an enhancing effect on the abundance of key organic molecules such as hydrogen cyanide (HCN), formaldehyde (CH2O), and ethylene (C2H4). The latter two are intermediates in the production pathway of the amino acid glycine (C2H5NO2) and abundant enough to be potentially detectable by JWST., Comment: 15 pages, 11 figures, 4 tables

We published spectra of phosphine molecules in Venus' clouds, following open-science principles in releasing data and scripts (with community input leading to ALMA re-processing, now benefiting multiple projects). Some misconceptions about de-trending of spectral baselines have also emerged, which we address here. Using the JCMT PH3-discovery data, we show that mathematically-correct polynomial fitting of periodic ripples does not lead to "fake lines" (probability < ~1%). We then show that the ripples can be characterised in a non-subjective manner via Fourier transforms. A 20 ppb PH3 feature is ~5{\sigma} compared to the JCMT baseline-uncertainty, and is distinctive as a narrow perturber of the periodic ripple pattern. The structure of the FT-derived baseline also shows that polynomial fitting, if unguided, can amplify artefacts and so artificially reduce significance of real lines., Comment: Response to: Snellen I. et al. Astron. Astroph., in press, arXiv:2010:09761 (2020); Villanueva G. et al. Nat. Ast. Matters Arising; arXiv:2010.14305 (2020); Thompson M. Mon. Not. Roy. Astron. Soc., in press, arXiv:2010.15118 (2020); Submitted to Nature Astronomy "Matters Arising" on Dec/10/2020

We first respond to two points raised by Villanueva et al. We show the JCMT discovery spectrum of PH3 can not be re-attributed to SO2, as the line width is larger than observed for SO2 features, and the required abundance would be an extreme outlier. The JCMT spectrum is also consistent with our simple model, constant PH3-abundance with altitude, with no discrepancy in line profile (within data limits); reconciliation with a full photochemical model is the subject of future work. Section 2 presents initial results from re-processed ALMA data. Villanueva et al. noted an issue with bandpass calibration. They have worked on a partially re-processed subset of the ALMA data, so we note where their conclusions, and those of Greaves et al., are now superseded. To summarise: we recover PH3 in Venus' atmosphere with ALMA (~5{\sigma} confidence). Localised abundance appears to peak at ~5-10 parts-per-billion (ppb), with suggestions of spatial variation. Advanced data-products suggest a planet-averaged PH3 abundance ~1-4 ppb, lower than from the earlier ALMA processing (which indicated 7+ ppb). The ALMA data are reconcilable with the JCMT detection (~20 ppb) if there is order-of-magnitude temporal variation; more advanced processing of the JCMT data is underway to check methods. Independent PH3 measurements suggest possible altitude dependence (under ~5 ppb at 60+ km, up to ~100 ppb at 50+ km; see Section 2: Conclusions.). Given that both ALMA and JCMT were working at the limit of observatory capabilities, new spectra should be obtained. The ALMA data in-hand are no longer limited by calibration, but spectral ripples still exist, probably due to size and brightness of Venus in relation to the primary beam. Further, spatial ripples are present, potentially reducing significance of real narrow spectral features., Comment: Response to the Matters Arising: "No phosphine in the atmosphere of Venus" by G. Villanueva et al., preprint at arXiv:2010.14305; v2 contains updated values and additional technical details

Measurements of trace-gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbor, Venus, has cloud decks that are temperate but hyper-acidic. We report the apparent presence of phosphine (PH3) gas in Venusian atmosphere, where any phosphorus should be in oxidized forms. Single-line millimeter-waveband spectral detections (quality up to ~15 sigma) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH3 at ~20 parts-per-billion abundance is inferred. The presence of phosphine is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently-known abiotic production routes in Venusian atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. Phosphine could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of phosphine on Earth, from the presence of life. Other PH3 spectral features should be sought, while in-situ cloud and surface sampling could examine sources of this gas.

The recent candidate detection of ~1 ppb of phosphine in the middle atmosphere of Venus is so unexpected that it requires an exhaustive search for explanations of its origin. Phosphorus-containing species have not been modelled for Venus' atmosphere before and our work represents the first attempt to model phosphorus species in the Venusian atmosphere. We thoroughly explore the potential pathways of formation of phosphine in a Venusian environment, including in the planet's atmosphere, cloud and haze layers, surface, and subsurface. We investigate gas reactions, geochemical reactions, photochemistry, and other non-equilibrium processes. None of these potential phosphine production pathways are sufficient to explain the presence of ppb phosphine levels on Venus. If PH3's presence in Venus' atmosphere is confirmed, it therefore is highly likely to be the result of a process not previously considered plausible for Venusian conditions. The process could be unknown geochemistry, photochemistry, or even aerial microbial life, given that on Earth phosphine is exclusively associated with anthropogenic and biological sources. The detection of phosphine adds to the complexity of chemical processes in the Venusian environment and motivates in situ follow up sampling missions to Venus. Our analysis provides a template for investigation of phosphine as a biosignature on other worlds., Comment: v2 is in press in Astrobiology, Special Collection: Venus; v2 also expands on the potential of phosphides from the deep mantle volcanism as a source of PH3 (as suggested by Truong and Lunine 2021: https://www.pnas.org/content/118/29/e2021689118) and shows the volcanic source of PH3 to be unlikely

Hydrogen in rocky planet atmospheres has been invoked in arguments for extending the habitable zone via N2-H2 and CO2-H2 greenhouse warming, and providing atmospheric conditions suitable for efficient production of prebiotic molecules. On Earth and Super-Earth-sized bodies, where H2-rich primordial envelopes are quickly lost to space, volcanic outgassing can act as a hydrogen source, provided it balances with the loss rate from the top of the atmosphere. Here, we show that both Earth-like and Mars-like planets can sustain atmospheric H2 fractions of several percent across relevant magmatic fO2 ranges. In general this requires hydrogen escape to operate somewhat less efficiently than the diffusion limit. We use a thermodynamical model of magma degassing to determine which combinations of magma oxidation, volcanic flux, and hydrogen escape efficiency can build up appreciable levels of hydrogen in a planet's secondary atmosphere. On a planet similar to the Archean Earth and with a similar magmatic fO2, we suggest that the mixing ratio of atmospheric hydrogen could have been in the range 0.2-3%. A planet erupting magmas around the Iron-Wustite (IW) buffer (i.e., ~3 log fO2 units lower than Archean Earth's), but with otherwise similar volcanic fluxes and H2 loss rates to early Earth, could sustain an atmosphere with approximately 10-20% H2. For an early Mars-like planet with magmas around IW, but a lower range of surface pressures and volcanic fluxes compared to Earth, an atmospheric H2 mixing ratio of 2-8% is possible. On early Mars, this H2 mixing ratio could be sufficient to deglaciate the planet. However, the sensitivity of these results to primary magmatic water contents and volcanic fluxes show the need for improved constraints on the crustal recycling efficiency and mantle water contents of early Mars., Comment: Submitted with reviewer comments to Earth and Planetary Science Letters

Using Gaussian Process regression to analyze the Martian surface methane Tunable Laser Spectrometer (TLS) data reported by Webster (2018), we find that the TLS data, taken as a whole, are not statistically consistent with seasonal variability. The subset of data derived from an enrichment protocol of TLS, if considered in isolation, are equally consistent with either stochastic processes or periodic variability, but the latter does not favour seasonal variation., Comment: 13 pages, 2 figures, 1 table, accepted for publication as a Note in Icarus

The study of the composition of brown dwarf atmospheres helped to understand their formation and evolution. Similarly, the study of exoplanet atmospheres is expected to constrain their formation and evolutionary states. We use results from 3D simulations, kinetic cloud formation and kinetic ion-neutral chemistry to investigate ionisation processes which will affect their atmosphere chemistry: The dayside of super-hot Jupiters is dominated by atomic hydrogen, and not H$_2$O. Such planetary atmospheres exhibit a substantial degree of thermal ionisation and clouds only form on the nightside where lightning leaves chemical tracers (e.g. HCN) for possibly long enough to be detectable. External radiation may cause exoplanets to be enshrouded in a shell of highly ionised, H$_3^+$-forming gas and a weather-driven aurora may emerge. Brown dwarfs enable us to study the role of electron beams for the emergence of an extrasolar, weather-system driven aurora-like chemistry, and the effect of strong magnetic fields on cold atmospheric gases. Electron beams trigger the formation of H$_3^+$ in the upper atmosphere of a brown dwarf (e.g. LSR-J1835) which may react with it to form hydronium, H$_3$O$^+$, as a longer lived chemical tracer. Brown dwarfs and super-hot gas giants may be excellent candidates to search for H$_3$O$^+$ as an H$_3^+$ product., Comment: 16 pages, accepted for publication in the Philosophical Transactions A of the Royal Society, (some typos corrected)

Hydrogen cyanide (HCN) is a key feedstock molecule for the production of life's building blocks. The formation of HCN in an N$_2$-rich atmospheres requires first that the triple bond between N$\equiv$N be severed, and then that the atomic nitrogen find a carbon atom. These two tasks can be accomplished via photochemistry, lightning, impacts, or volcanism. The key requirements for producing appreciable amounts of HCN are the free availability of N$_2$ and a local carbon to oxygen ratio of C/O $\geq 1$. We discuss the chemical mechanisms by which HCN can be formed and destroyed on rocky exoplanets with Earth-like N$_2$ content and surface water inventories, varying the oxidation state of the dominant carbon-containing atmospheric species. HCN is most readily produced in an atmosphere rich in methane (CH$_4$) or acetylene (C$_2$H$_2$), but can also be produced in significant amounts ($> 1$ ppm) within CO-dominated atmospheres. Methane is not necessary for the production of HCN. We show how destruction of HCN in a CO$_2$-rich atmosphere depends critically on the poorly-constrained energetic barrier for the reaction of HCN with atomic oxygen. We discuss the implications of our results for detecting photochemically produced HCN, for concentrating HCN on the planet's surface, and its importance for prebiotic chemistry., Comment: 20 pages, 8 figures, accepted for publication in Icarus

A key challenge in origins-of-life studies is estimating the abundances of species relevant to the chemical pathways proposed to have contributed to the emergence of life on early Earth. Dissolved nitrogen oxide anions (NO$_{X}^{-}$), in particular nitrate (NO$_{3}^{-}$) and nitrite (NO$_{2}^{-}$), have been invoked in diverse origins-of-life chemistry, from the oligomerization of RNA to the emergence of protometabolism. Recent work has calculated the supply of NO$_{X}^{-}$ from the prebiotic atmosphere to the ocean, and reported steady-state [NO$_{X}^{-}$] to be high across all plausible parameter space. These findings rest on the assumption that NO$_{X}^{-}$ is stable in natural waters unless processed at a hydrothermal vent. Here, we show that NO$_{X}^{-}$ is unstable in the reducing environment of early Earth. Sinks due to UV photolysis and reactions with reduced iron (Fe$^{2+}$) suppress [NO$_{X}^{-}$] by several orders of magnitude relative to past predictions. For pH$=6.5-8$ and $T=0-50^\circ$C, we find that it is most probable that NO$_{X}^{-}$]$<1~\mu$M in the prebiotic ocean. On the other hand, prebiotic ponds with favorable drainage characteristics may have sustained [NO$_{X}^{-}$]$\geq 1~\mu$M. As on modern Earth, most NO$_{X}^{-}$ on prebiotic Earth should have been present as NO$_{3}^{-}$, due to its much greater stability. These findings inform the kind of prebiotic chemistries that would have been possible on early Earth. We discuss the implications for proposed prebiotic chemistries, and highlight the need for further studies of NO$_{X}^{-}$ kinetics to reduce the considerable uncertainties in predicting [NO$_{X}^{-}$] on early Earth., Comment: In review for publication at Geochemistry, Geophysics, and Geosystems (G-cubed). Comments, questions, and criticism solicited; please contact corresponding author at sukrit@mit.edu. SI at: https://web-cert.mit.edu/sukrit/Public/nox_si.pdf. GitHub at: https://github.com/sukritranjan/nox

There are two dominant and contrasting classes of origin of life scenarios: those predicting that life emerged in submarine hydrothermal systems, where chemical disequilibrium can provide an energy source for nascent life; and those predicting that life emerged within subaerial environments, where UV catalysis of reactions may occur to form the building blocks of life. Here, we describe a prebiotically plausible environment that draws on the strengths of both scenarios: surface hydrothermal vents. We show how key feedstock molecules for prebiotic chemistry can be produced in abundance in shallow and surficial hydrothermal systems. We calculate the chemistry of volcanic gases feeding these vents over a range of pressures and basalt C/N/O contents. If ultra-reducing carbon-rich nitrogen-rich gases interact with subsurface water at a volcanic vent they result in 1 mM to 1 M concentrations of diacetylene, acetylene, cyanoacetylene, hydrogen cyanide, bisulfite, hydrogen sulfide and soluble iron in vent water. One key feedstock molecule, cyanamide, is not formed in significant quantities within this scenario, suggesting that it may need to be delivered exogenously, or formed from hydrogen cyanide either via organometallic compounds, or by some as yet-unknown chemical synthesis. Given the likely ubiquity of surface hydrothermal vents on young, hot, terrestrial planets, these results identify a prebiotically plausible local geochemical environment, which is also amenable to future lab-based simulation., Comment: 21 pages, 7 figures, 2 tables, accepted for publication in Life

We perform a study of stellar flares for the 24,809 stars observed with 2 minute cadence during the first two months of the TESS mission. Flares may erode exoplanets' atmospheres and impact their habitability, but might also trigger the genesis of life around small stars. TESS provides a new sample of bright dwarf stars in our galactic neighborhood, collecting data for thousands of M-dwarfs that might host habitable exoplanets. Here, we use an automated search for flares accompanied by visual inspection. Then, our public allesfitter code robustly selects the appropriate model for potentially complex flares via Bayesian evidence. We identify 1228 flaring stars, 673 of which are M-dwarfs. Among 8695 flares in total, the largest superflare increased the stellar brightness by a factor of 16.1. Bolometric flare energies range from 10^31.0 to 10^36.9 erg, with a median of 10^33.1 erg. Furthermore, we study the flare rate and energy as a function of stellar type and rotation period. We solidify past findings that fast rotating M-dwarfs are the most likely to flare, and that their flare amplitude is independent of the rotation period. Finally, we link our results to criteria for prebiotic chemistry, atmospheric loss through coronal mass ejections, and ozone sterilization. Four of our flaring M dwarfs host exoplanet candidates alerted on by TESS, for which we discuss how these effects can impact life. With upcoming TESS data releases, our flare analysis can be expanded to almost all bright small stars, aiding in defining criteria for exoplanet habitability., Comment: Published in The Astronomical Journal, 159, 60. 21 pages, 11 figures, 2 tables. This is the authors' version of the manuscript

The chemical composition of an exoplanet is a key ingredient in constraining its formation history. Iron is the most abundant transition metal, but has never been directly detected in an exoplanet due to its highly refractory nature. KELT-9b (HD 195689b) is the archetype of the class of ultra-hot Jupiters that straddle the transition between stars and gas-giant exoplanets and serve as distinctive laboratories for studying atmospheric chemistry, because of its high equilibrium temperature of 4050 +/- 180 K. These properties imply that its atmosphere is a tightly constrained chemical system that is expected to be nearly in chemical equilibrium and cloud-free. It was previously predicted that the spectral lines of iron will be detectable in the visible range of wavelengths. At these high temperatures, iron and several other transition metals are not sequestered in molecules or cloud particles and exist solely in their atomic forms. Here, we report the direct detection of atomic neutral and singly-ionized iron (Fe and Fe+), and singly-ionized titanium (Ti+) in KELT-9b via the cross-correlation technique applied to high-resolution spectra obtained during the primary transit of the exoplanet., Comment: 7 pages, 4 figures

The atmospheric temperatures of the ultra-hot Jupiter KELT-9b straddle the transition between gas giants and stars, and therefore between two traditionally distinct regimes of atmospheric chemistry. Previous theoretical studies assume the atmosphere of KELT-9b to be in chemical equilibrium. Despite the high ultraviolet flux from KELT-9, we show using photochemical kinetics calculations that the observable atmosphere of KELT-9b is predicted to be close to chemical equilibrium, which greatly simplifies any theoretical interpretation of its spectra. It also makes the atmosphere of KELT-9b, which is expected to be cloudfree, a tightly constrained chemical system that lends itself to a clean set of theoretical predictions. Due to the lower pressures probed in transmission (compared to emission) spectroscopy, we predict the abundance of water to vary by several orders of magnitude across the atmospheric limb depending on temperature, which makes water a sensitive thermometer. Carbon monoxide is predicted to be the dominant molecule under a wide range of scenarios, rendering it a robust diagnostic of the metallicity when analyzed in tandem with water. All of the other usual suspects (acetylene, ammonia, carbon dioxide, hydrogen cyanide, methane) are predicted to be subdominant at solar metallicity, while atomic oxygen, iron and magnesium are predicted to have relative abundances as high as 1 part in 10,000. Neutral atomic iron is predicted to be seen through a forest of optical and near-infrared lines, which makes KELT-9b suitable for high-resolution ground-based spectroscopy with HARPS-N or CARMENES. We summarize future observational prospects of characterizing the atmosphere of KELT-9b., Comment: Accepted by ApJ. 9 pages, 6 figures. Corrected minor errors in Figures 1a and 1b (some line styles were switched by accident), text and conclusions unchanged, these minor changes will be updated in final ApJ proof

Lightning induced radio emission has been observed on solar system planets. Lecavelier des Etangs et al. [2013] carried out radio transit observations of the exoplanet HAT-P-11b, and suggested a tentative detection of a radio signal. Here, we explore the possibility of the radio emission having been produced by lightning activity on the exoplanet, following and expanding the work of Hodos\'an et al. [2016a]. After a summary of our previous work [Hodos\'an et al. 2016a], we extend it with a parameter study. The lightning activity of the hypothetical storm is largely dependent on the radio spectral roll-off, $n$, and the flash duration, $\tau_\mathrm{fl}$. The best-case scenario would require a flash density of the same order of magnitude as can be found during volcanic eruptions on Earth. On average, $3.8 \times 10^6$ times larger flash densities than the Earth-storms with the largest lightning activity is needed to produce the observed signal from HAT-P-11b. Combined with the results of Hodos\'an et al. [2016a] regarding the chemical effects of planet-wide thunderstorms, we conclude that future radio and infrared observations may lead to lightning detection on planets outside the solar system., Comment: Accepted to the Conference Proceedings of the 8th International Workshop on Planetary, Solar and Heliospheric Radio Emissions (PRE 8), held in Seggauberg near Leibnitz/Graz, Austria, October 25-27, 2016. 12 pages, 2 figures

We present a model for lightning shock induced chemistry that can be applied to atmospheres of arbitrary H/C/N/O chemistry, hence for extrasolar planets and brown dwarfs. The model couples hydrodynamics and the STAND2015 kinetic gas-phase chemistry. For an exoplanet analogue to the contemporary Earth, our model predicts NO and NO2 yields in agreement with observation. We predict height-dependent mixing ratios during a storm soon after a lightning shock of NO ~ 1e-3 at 40 km and NO2 ~ 1e-4 below 40 km, with O3 reduced to trace quantities (<< 1e-10). For an Earth-like exoplanet with a CO2/N2 dominated atmosphere and with an extremely intense lightning storm over its entire surface, we predict significant changes in the amount of NO, NO2, O3, H2O, H2, and predict significant abundance of C2N. We find that, for the Early Earth, O2 is formed in large quantities by lightning but is rapidly processed by the photochemistry, consistent with previous work on lightning. The effect of persistent global lightning storms are predicted to be significant, primarily due to NO2, with the largest spectral features present at ~3.4 {\mu}m and ~6.2 {\mu}m. The features within the transmission spectrum are on the order of 1 ppm and therefore are not likely detectable with JWST. Depending on its spectral properties, C2N could be a key tracer for lightning on Earth-like exoplanets with a N2/CO2 bulk atmosphere, unless destroyed by yet unknown chemical reactions., Comment: 11 pages, 8+1 figures, 1+1 tables, accepted for publication in MNRAS

We present an open-source and validated chemical kinetics code for studying hot exoplanetary atmospheres, which we name VULCAN. It is constructed for gaseous chemistry from 500 to 2500 K using a reduced C-H-O chemical network with about 300 reactions. It uses eddy diffusion to mimic atmospheric dynamics and excludes photochemistry. We have provided a full description of the rate coefficients and thermodynamic data used. We validate VULCAN by reproducing chemical equilibrium and by comparing its output versus the disequilibrium-chemistry calculations of Moses et al. and Rimmer & Helling. It reproduces the models of HD 189733b and HD 209458b by Moses et al., which employ a network with nearly 1600 reactions. We also use VULCAN to examine the theoretical trends produced when the temperature-pressure profile and carbon-to-oxygen ratio are varied. Assisted by a sensitivity test designed to identify the key reactions responsible for producing a specific molecule, we revisit the quenching approximation and find that it is accurate for methane but breaks down for acetylene, because the disequilibrium abundance of acetylene is not directly determined by transport-induced quenching, but is rather indirectly controlled by the disequilibrium abundance of methane. Therefore, we suggest that the quenching approximation should be used with caution and must always be checked against a chemical kinetics calculation. A one-dimensional model atmosphere with 100 layers, computed using VULCAN, typically takes several minutes to complete. VULCAN is part of the Exoclimes Simulation Platform (ESP; http://www.exoclime.net/) and publicly available at http://github.com/exoclime/VULCAN ., Comment: Accepted for publication in ApJS http://www.exoclime.net/

Clouds form on extrasolar planets and brown dwarfs where lightning could occur. Lightning is a tracer of atmospheric convection, cloud formation and ionization processes as known from the Solar System, and may be significant for the formation of prebiotic molecules. We study lightning climatology for the different atmospheric environments of Earth, Venus, Jupiter and Saturn. We present lightning distribution maps for Earth, Jupiter and Saturn, and flash densities for these planets and Venus, based on optical and/or radio measurements from the WWLLN and STARNET radio networks, the LIS/OTD satellite instruments, the Galileo, Cassini, New Horizons and Venus Express spacecraft. We also present flash densities calculated for several phases of two volcano eruptions, Eyjafjallaj\"okull's (2010) and Mt Redoubt's (2009). We estimate lightning rates for sample, transiting and directly imaged extrasolar planets and brown dwarfs. Based on the large variety of exoplanets, six categories are suggested for which we use the lightning occurrence information from the Solar System. We examine lightning energy distributions for Earth, Jupiter and Saturn. We discuss how strong stellar activity may support lightning activity. We provide a lower limit of the total number of flashes that might occur on transiting planets during their full transit as input for future studies. We find that volcanically very active planets might show the largest lightning flash densities. When applying flash densities of the large Saturnian storm from 2010/11, we find that the exoplanet HD 189733b would produce high lightning occurrence even during its short transit., Comment: References updated, typos fixed. 24 pages, 7 figures, 7 tables, accepted for publication in MNRAS

Lightning induced radio emission has been observed on Solar system planets. There have been many attempts to observe exoplanets in the radio wavelength, however, no unequivocal detection has been reported. Lecavelier des Etangs et al. carried out radio transit observations of the exoplanet HAT-P-11b, and suggested that a small part of the radio flux can be attributed to the planet. Here, we assume that this signal is real, and study if this radio emission could be caused by lightning with similar energetic properties like in the Solar system. We find that a lightning storm with 3.8 x $10^6$ times larger flash densities than the Earth-storms with the largest lightning activity is needed to produce the observed signal from HAT-P-11b. The optical emission of such thunderstorm would be comparable to that of the host star. We show that HCN produced by lightning chemistry is observable 2-3 yr after the storm, which produces signatures in the L (3.0-4.0 \mu m) and N (7.5-14.5 \mu m) infrared bands. We conclude that it is unlikely that the observed radio signal was produced by lightning, however, future, combined radio and infrared observations may lead to lightning detection on planets outside the Solar system., Comment: Typos fixed from v2. 6 pages, 3 figures. Published in MNRAS

There are many open questions about prebiotic chemistry in both planetary and exoplanetary environments. The increasing number of known exoplanets and other ultra-cool, substellar objects has propelled the desire to detect life and prebiotic chemistry outside the solar system. We present an ion-neutral chemical network constructed from scratch, Stand2015, that treats hydrogen, nitrogen, carbon and oxygen chemistry accurately within a temperature range between 100 K and 30000 K. Formation pathways for glycine and other organic molecules are included. The network is complete up to H6C2N2O3. Stand2015 is successfully tested against atmospheric chemistry models for HD209458b, Jupiter and the present-day Earth using a simple 1D photochemistry/diffusion code. Our results for the early Earth agree with those of Kasting (1993) for CO2, H2, CO and O2, but do not agree for water and atomic oxygen. We use the network to simulate an experiment where varied chemical initial conditions are irradiated by UV light. The result from our simulation is that more glycine is produced when more ammonia and methane is present. Very little glycine is produced in the absence of any molecular nitrogen and oxygen. This suggests that production of glycine is inhibited if a gas is too strongly reducing. Possible applications and limitations of the chemical kinetics network are also discussed., Comment: 138 pages, 19 figures, accepted to ApJS

We explore the feasibility of using the atmosphere of Jupiter to detect Ultra-High-Energy Cosmic Rays (UHECR's). The large surface area of Jupiter allows us to probe cosmic rays of higher energies than previously accessible. Cosmic ray extensive air showers in Jupiter's atmosphere could in principle be detected by the Large Area Telescope (LAT) on the Fermi observatory. In order to be observed, these air showers would need to be oriented toward the Earth, and would need to occur sufficiently high in the atmosphere that the gamma rays can penetrate. We demonstrate that, under these assumptions, Jupiter provides an effective cosmic ray "detector" area of $3.3 \times 10^7$ km$^2$. We predict that Fermi-LAT should be able to detect events of energy $>10^{21}$ eV with fluence $10^{-7}$ erg cm$^{-2}$ at a rate of about one per month. The observed number of air showers may provide an indirect measure of the flux of cosmic rays $\gtrsim 10^{20}$ eV. Extensive air showers also produce a synchrotron signature that may be measurable by ALMA. Simultaneous observations of Jupiter with ALMA and Fermi-LAT could be used to provide broad constraints on the energies of the initiating cosmic rays., Comment: 8 pages, 5 figures. Accepted for publication in the Astrophysical Journal Letters

Charged dust grains in the atmospheres of exoplanets may play a key role in the formation of prebiotic molecules, necessary to the origin of life. Dust grains submerged in an atmospheric plasma become negatively charged and attract a flux of ions that are accelerated from the plasma. The energy of the ions upon reaching the grain surface may be sufficient to overcome the activation energy of particular chemical reactions that would be unattainable via ion and neutral bombardment from classical, thermal excitation. As a result, prebiotic molecules or their precursors could be synthesised on the surface of dust grains that form clouds in exoplanetary atmospheres. This paper investigates the energization of the plasma ions, and the dependence on the plasma electron temperature, in the atmospheres of substellar objects such as gas giant planets. Calculations show that modest electron temperatures of $\approx 1$ eV ($\approx 10^{4}$ K) are enough to accelerate ions to sufficient energies that exceed the activation energies required for the formation of formaldehyde, ammonia, hydrogen cyanide and the amino acid glycine., Comment: 18 pages, 3 figures. Accepted, The International Journal of Astrobiology

Observations of continuous radio and sporadic X-ray emission from low-mass objects suggest they harbour localized plasmas in their atmospheric environments. For low-mass objects, the degree of thermal ionization is insufficient to qualify the ionized component as a plasma, posing the question: what ionization processes can efficiently produce the required plasma that is the source of the radiation? We propose Alfv\'{e}n ionization as a mechanism for producing localized pockets of ionized gas in the atmosphere, having sufficient degrees of ionization ($\geq10^{-7}$) that they constitute plasmas. We outline the criteria required for Alfv\'{e}n ionization and demonstrate it's applicability in the atmospheres of low-mass objects such as giant gas planets, brown dwarfs and M-dwarfs for both solar and sub-solar metallicities. We find that Alfv\'{e}n ionization is most efficient at mid to low atmospheric pressures where a seed plasma is easier to magnetize and the pressure gradients needed to drive the required neutral flows are the smallest. For the model atmospheres considered, our results show that degrees of ionization of $10^{-6}-1$ can be obtained as a result of Alfv\'{e}n ionization. Observable consequences include continuum Bremsstrahlung emission, superimposed with spectral lines from the plasma ion species (e.g. He, Mg, H$_{2}$ or CO lines). Forbidden lines are also expected from the metastable population. The presence of an atmospheric plasma opens the door to a multitude of plasma and chemical processes not yet considered in current atmospheric models. The occurrence of Alfv\'{e}n ionization may also be applicable to other astrophysical environments such as protoplanetary disks., Comment: 53 pages, 18 figures, Accepted to ApJ

We report the detection of absorption lines by the reactive ions OH+, H2O+ and H3O+ along the line of sight to the submillimeter continuum source G10.6$-$0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH+ at 971 GHz, H2O+ at 1115 and 607 GHz, and H3O+ at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6$-$0.4 as well as in the molecular gas directly associated with that source. The OH+ spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O+ lines remains lower than 1 in the same velocity range, and the H3O+ line shows only weak absorption. For LSR velocities between 7 and 50 kms$^{-1}$ we estimate total column densities of $N$(OH+) $> 2.5 \times 10^{14}$ cm$^{-2}$, $N$(H2O+) $\sim 6 \times 10^{13}$ cm$^{-2}$ and $N$(H3O+) $\sim 4.0 \times 10^{13}$ cm$^{-2}$. These detections confirm the role of O$^+$ and OH$^+$ in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH+ by the H2O+ column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.