arXiv subjects

Paul B. Rimmer

Publications and source records attributed to Paul B. Rimmer.

At least 19 recordsLinked to original sources

arXiv2026

A Model of UV-Blue Absorbance in Bulk Liquid of Venusian Cloud Aerosols Is Consistent with Efficient Organic Absorbers at High Concentrations

At visible wavelengths, Venus appears serene and pale-yellow, but since the 1920s, observers have noted high-contrast features in the ultraviolet. These features track the about 4-day superrotation of the upper cloud deck and vary widely over time and space. The identity of the UV absorber(s), active between at least 280 and 500 nm, remains unknown, as no proposed candidate fully matches all observational data. From remote observations of Venus, and accounting for light scattering by sub-micrometer droplets, we modeled the 365-455 nm absorbance per cm of the bulk liquids forming Venus's clouds. Assuming a uniform distribution in mode 1 and 2 particles across a 6 km layer below the cloud top at 65 km, we constrain the we constrained the decadic absorption coefficient within the modeled range to a peak at 375 nm to be 1278 per cm. This extremely high absorption coefficient implies the presence of a highly efficient absorber, for example, conjugated organics, at relatively high concentration, e.g., approx. 12 g/L for porphyrin-type pigments with molar absorption coefficient of approx. 100,000 per mole per cm. Inorganic absorbers, typically below 10,000 would either need to constitute a large portion of the aerosols or still not be sufficiently light-absorbing, even in pure form. We emphasize that all candidate absorbers must be evaluated against Venus's reflectance curve using (i) known molar absorption coefficients, (ii) realistic atmospheric distributions, and (iii) appropriate particle size distributions. The planned Rocket Lab mission will test the hypothesis of organics in Venus's clouds.

astro-ph.EP↗

arXiv2026

Chemical Tracers for 3D Atmospheric Asymmetries on WASP-69 b

Warm giant exoplanets exhibit strong three-dimensional temperature contrasts that can significantly alter atmospheric chemistry through quenching and photochemistry, yet transmission spectra are commonly interpreted using one-dimensional, limb-averaged models. Such simplifications may bias inferred atmospheric properties, particularly metallicity and C/O ratio. In this work we investigate the relative influence of atmospheric composition and three-dimensional thermal structure on atmospheric chemistry and transmission spectra using WASP-69b as a test case. WASP-69b is a ~900K warm Saturn, residing in a thermal regime especially sensitive to disequilibrium chemistry. We use three-dimensional general circulation model derived pressure-temperature profiles as inputs for a one-dimensional photochemical-kinetics model to resolve longitudinal and latitudinal chemical asymmetries across the atmosphere. We find that CH4 exhibits strong latitudinal variations linked to deep quench temperatures, while SO2 shows longitudinal asymmetries driven by upper-atmospheric photochemistry and irradiation geometry. In contrast, CO2 remains comparatively insensitive to spatial thermal variations and emerges as a robust tracer of atmospheric metallicity. Synthethic transmission spectra reveal that three-dimensional chemical asymmetries can produce spectral variations comparable to those induced by metallicity itself, demonstrating that limb-averaged interpretations can mask substantial spatial structure in warm giant exoplanet atmospheres.

astro-ph.EP↗

arXiv2026

New Hydrolysis Rate Constants Reveal Longest Cyanide Persistence in Cool, Neutral Waters

Hydrogen cyanide (HCN) is a key molecule in prebiotic chemistry, and its availability in water is limited by hydrolysis. In this work, we estimate the hydrolysis rates of \ce{HCN}, particularly the errors associated with the rates as a function of temperature, pH and in the presence of salts containing sulfite, sulfide and phosphate. For pure water, we find an acid-catalyzed hydrolysis rate constant at ${\rm 0 ^{\circ}C}$ of $\ln (k^+_{273}/1\,{\rm M^{-1} s^{-1}}) = -12.7 \pm 1.12$ with an activation energy of $67.1 \, {\rm kJ \, mol^{-1}}$. We find a base-catalyzed rate constant of $\ln (k_{273}^-/1\,{\rm M^{-1} s^{-1}}) = -9.0 \pm 1.27$ at ${\rm 0^{\circ}C}$ with an activation energy of $89.5 \, {\rm kJ \, mol^{-1}}$. These values are consistent with estimates from the literature within our uncertainties at ${\rm pH} > 8$ but diverge from literature values at lower pH. In the presence of salts, hydrolysis is accelerated under acidic conditions to 6--14$\times$ the acid-catalyzed rate without salts. However, at ${\rm pH \gtrsim 8}$ hydrolysis rates become $2.5\times$ times slower with sulfite and sulfide. These results demonstrate the importance of estimating uncertainties associated with rates when constraining the maximum concentrations of prebiotic molecules attainable in natural waters on the Earth and other planets.

physics.chem-ph↗

arXiv2026

Organosulfur Chemistry on sub-Neptunes: Implications for hazes and biosignatures

The organosulfur biosignature gases dimethylsulfide (DMS) and dimethlydisulfide (DMDS) have recently been claimed to be present in the atmosphere of sub-Neptune exoplanet K2-18b, leading to the suggestion of possible extraterrestrial life. Abiotic formation pathways for DMS and DMDS in reducing atmospheres have also been proposed, raising concern over the use of DMS and DMDS as biosignature gases more generally. In this paper we independently test and contrast the proposed abiotic formation pathways for DMS and DMDS using K2-18b as a case study, and explore the wider implications for the atmospheric carbon and sulfur chemistry of hydrogen-rich sub-Neptunes. We demonstrate that one proposed formation pathway is capable of producing observable abundances of abiotic DMS and DMDS, however it depends sensitively on the energy barrier of the limiting step, which remains unmeasured experimentally. The formation of hydrocarbons including C2H6, however, occurs abundantly and offers a plausible alternative explanation to the reported suggestions of organosulfur compounds on K2-18b, having previously been shown to share similar spectral features with DMS and DMDS at near-IR wavelengths. Finally, we demonstrate that sulfur hazes form via the photochemistry of H2S and condense in the atmosphere of K2-18b even at trace abundances. We propose that variation in atmospheric sulfur abundance can explain the diversity of haziness observed across the sub-Neptune population so far with JWST.

astro-ph.EP↗

arXiv2026

Hydrocarbon complexity and photochemical shielding of prebiotic feedstock molecules in exoplanet atmospheres

The potential of prebiotic chemistry to propagate on an exoplanet fundamentally depends on whether the atmospheric conditions can facilitate the production of prebiotic feedstock molecules. Photochemical simulations of exoplanet atmospheres can be used to explore this potential atmospheric synthesis, but require a comprehensive chemical network. We present the implementation of the CRAHCN-O network, constructed to simulate the formation of feedstock molecules such as HCN, H$_2$CO, and simple hydrocarbons, into the VULCAN photochemical kinetics code. We investigate the production of feedstock molecules driven by M-star radiation and compare these to predictions by the N-C-H-O network in VULCAN, for N$_2$-dominated atmospheres with C/O ratios between 0.5-1.5. Predicted abundances are similar for C/O${=}$0.5. Once CH$_4$ is included (i.e., for C/O${>}$0.5), the abundance profiles diverge in the photochemical regions. By analysing the attenuation of UV radiation, we find that hydrocarbon photochemical shielding causes the diverging profiles. CRAHCN-O accumulates C$_2$H$_6$, while N-C-H-O accumulates C$_4$H$_3$ and C$_3$H$_4$. Importantly, C$_2$H$_6$ is photochemically active whereas C$_4$H$_3$ and C$_3$H$_4$ are assumed inactive. With mixing ratios up to a few percent in CRAHCN-O, C$_2$H$_6$ shields CH$_4$ and CO$_2$ from photodissociation and weakens the destruction of HCN and H$_2$CO. Maximum HCN mixing ratios reach 1000 ppm with CRAHCN-O compared to only 3 ppm with N-C-H-O. Other feedstock molecules like HC$_3$N and C$_2$H$_2$ form more efficiently in N-C-H-O. The shielding mechanism and its impact on feedstock molecules persist for radiation from distinct M-star types. These results demonstrate the crucial role of chemical kinetics in understanding prebiotic processes in exoplanet atmospheres, including important considerations for the construction and applicability of chemical networks.

astro-ph.EP↗

arXiv2026

Comparative biosignatures with systemic retrievals

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

astro-ph.EP↗

arXiv2025

A Fast, Parallelized, GPU-Accelerated Photochemical Model, XODIAC, with Built-in Equilibrium Chemistry and Multiple Chemical Networks for Exoplanetary Atmospheres

The launch of the James Webb Space Telescope (JWST) has delivered high-quality atmospheric observations and expanded the known chemical inventory of exoplanetary atmospheres, opening new avenues for atmospheric chemistry modeling to interpret these data. Here, we present XODIAC, a fast, GPU-accelerated, one-dimensional photochemical model with a built-in equilibrium chemistry solver, an updated thermochemical database, and three chemical reaction networks. This framework enables comparative atmospheric chemistry studies, including the newly developed XODIAC-2025 network, a state-of-the-art C-H-O-N-P-S-Metals network, linking 594 species through 7,720 reactions. The other two are existing, publicly available C-H-O-N-S and C-H-O-N-S-Metals networks, from the established photochemical models VULCAN and ARGO, respectively, which are commonly used in the community. The XODIAC model has been rigorously benchmarked on the well-studied hot Jupiter HD 189733 b, with results compared against these two models. Benchmarking shows excellent agreement and demonstrates that, when the same chemical network and initial conditions are used, the numerical scheme for solving atmospheric chemistry does not significantly affect the results. We also revisited the atmospheric chemistry of HD 189733 b and performed a comparative analysis across the three networks. Sulfur chemistry shows the least variation across networks, carbon chemistry shows slightly more, and phosphorus chemistry varies the most, primarily due to the introduction of unique PHO and PN pathways comprising 390 reactions in the XODIAC-2025 network. These findings highlight XODIAC's capability to advance exoplanetary atmospheric chemistry and provide a robust framework for comparative exoplanetology.

astro-ph.EP↗

arXiv2025

Powerful lightning on Venus constrained by atmospheric NO

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

astro-ph.EP↗

arXiv2025

A Full-Atmosphere Model of Jupiter

This paper presents a combined 1D photochemical-thermochemical kinetics model of Jupiter's deeper atmosphere, troposphere and stratosphere. The model covers atmospheric pressure range from $1.1 \times 10^{3}$ bar to $7.4 \times 10^{-11}$ bar and is the first model that incorporates sulfur chemistry when spanning an atmospheric region of this extent. This model incorporates a new version of the STAND reaction network with updated NH4SH chemistry, and updated Antoine equation parameters for NH4SH and H2S. Validation against current models of Jupiter's atmosphere as well as recent observational data shows that our model successfully describes Jupiter's main observed chemical features. Since one of the focuses of the model is the chemistry on nitrogen, it correctly predicts the formation of a mixed NH3-NH4SH cloud layer between 0.1 and 1 bar. It also describes the chemistry of HCN throughout the atmosphere and discovers a region in the stratosphere between $1 \times 10^{-6}$ and $6.76 \times 10^{-8}$ bar, where HCN forms through radical chemistry with maximum mixing ratio 33 ppb at $2.94 \times 10^{-7}$ bar -- a prediction testable by observations. At the same time, our model predicts a quenched N2 mixing ratio 490 ppm up to 10$^{-6}$ bar. The model therefore successfully bridges the gap between existing models of separate regions of Jupiter's atmosphere and makes new testable predictions of several chemical species.

astro-ph.EP↗

arXiv2025

Nitrogen chemistry of hycean worlds on the example of K2-18b

A recent observation of the exoplanet K2-18b sparked interest among scientists - large amounts of carbon dioxide and methane were detected in an H2-rich background atmosphere. If the planet is a hycean world (liquid water ocean + hydrogen-dominated atmosphere), it could be habitable under certain conditions. The presence of carbon, hydrogen and oxygen was already confirmed, however, there was no detection of nitrogen or its compounds. Molecular nitrogen is difficult to detect directly. This study concentrates on possible photochemical products of N2 such as HCN, NH3 and HC3N. We set approximate limits on the amount of nitrogen bearing species by varying atmospheric parameters, such as the Eddy Diffusion coefficient and the amount of N2 present from 10 ppm to 10%. If the bulk nitrogen-containing gas in the atmosphere is N2, photochemistry produces only trace amounts of the aforementioned species. However, if ammonia is the main source of nitrogen, then the quantities of NH3, CH5N and HCN approach detectable range. HC3N and NO are bad tracers of the nitrogen source in the atmosphere, because they are produced in similar amounts in all tested scenarios. Assuming equilibrium chemistry at the surface of K2-18b results in underprediction of CO2 abundance. This result combined with the non-detection of ammonia by JWST suggests the planet is not a typical sub-Neptune, but could be indeed a hycean world or magma ocean planet. We also found that C2H6 is produced in significant amounts - if it is detected in the future, it could serve as a proxy for DMS presence.

astro-ph.EP↗

arXiv2025

Prediction of sulphate hazes in the lower Venus atmosphere

We study the amount, size distribution and material composition of (sub-)mic aerosol particles in the lower Venus atmosphere < 50 km. Our GGchem phase-equilibrium model predicts metal-chloride and metal-fluoride molecules to be present in the gas over the Venus surface in trace concentrations < 2.E-12, in particular FeCl2, NaCl, KCl and SiF4. Using an improved version of the DiffuDrift model developed by Woitke et al.2020, we find that these molecules deposit to form solid potassium sulphate K2SO4, sodium sulphate Na2SO4, and pyrite FeS2 above about 15.5 km, 9.5 km and 2.4 km, respectively. These heights coincide well with the three potential haze layers found in the Pioneer Venus Large Probe neutral mass spectrometer data by Mogul et al.2023. The particles with radius < 0.3 mic can be dredged up from the ground to reach the sulphuric acid cloud base from below by diffusion. The particle density decreases from ~ 5000/cm3 at ground level to ~100/cm3 at a height of 45 km. Particles larger than about 1 mic are found to stay confined to the ground < 10 km, indicating that the larger, so-called mode 3 particles, if they exist, cannot originate from the surface. All particles are expected to be coated by a thin layer of FeS2, Na2SO4 and K2SO4. We have included the repelling effect of particle charges on the coagulation, without which the model would predict much too steep gradients close to the surface, which is inconsistent with measured opacity data. Our models suggest that the particles must have at least 100 negative charges per micron of particle radius at ground level, and > 50/mic at a height of 45 km.

astro-ph.EP↗

arXiv2025

Constraining the survival of HCN during cometary impacts

Cometary impacts have been invoked as an atmosphere-independent method of stockpiling hydrogen cyanide (HCN), a key prebiotic feedstock molecule, into environments favourable for the onset of prebiotic chemistry on the early Earth. This work revisits the prospects for cometary delivery of HCN through new impacts simulations of idealised cometary bodies using the shock physics code iSALE combined with simple chemical modelling. Using temperature and pressure profiles for material within spherical, non-porous comets with a high resolution of Lagrangian tracer particles, we assess the survival rate of HCN across a range of impact velocities, sizes and angles, assuming both steady state and equilibrium chemistry. We find that HCN survival is extremely limited at impact velocities above the escape velocity of the Earth, unless the impact occurs at extreme obliquity ($θ\sim 15^\circ$). We present a parametrisation of the survival of HCN as a function of impact velocity, angle, and cometary diameter, which provides an upper limit to survival in more realistic scenarios to aid with future studies investigating the role of comets in the origins of life. Although successful HCN delivery may be possible in our idealised model, we neglect to consider the effect of atmospheric passage and our results suggest that delivery alone is not likely to be sufficient for the onset of prebiotic chemistry.

astro-ph.EP↗

arXiv2025

Tracing the Inner Edge of the Habitable Zone with Sulfur Chemistry

The circumstellar liquid-water habitable zone guides our search for potentially inhabited exoplanets, but remains observationally untested. We show that the inner edge of the habitable zone can now be mapped among exoplanets using their lack of surface water, which, unlike the presence of water, can be unambiguously revealed by atmospheric sulfur species. Using coupled climate-chemistry modelling we find that the observability of sulfur-gases on exoplanets depends critically on the ultraviolet (UV) flux of their host star, a property with wide variation: most M-dwarfs have a low UV flux and thereby allow the detection of sulfur-gases as a tracer of dry planetary surfaces; however, the UV flux of Trappist-1 may be too high for sulfur to disambiguate uninhabitable from habitable surfaces on any of its planets. We generalise this result to show how a population-level search for sulfur-chemistry on M-dwarf planets can be used to empirically define the Habitable Zone in the near-future.

astro-ph.EP↗

arXiv2024

A dry Venusian interior constrained by atmospheric chemistry

Venus's climatic history provides powerful constraint on the location of the inner-edge of the liquid-water habitable zone. However, two very different histories of water on Venus have been proposed: one where Venus had a temperate climate for billions of years, with surface liquid water, and the other where a hot early Venus was never able to condense surface liquid water. Here we offer a novel constraint on Venus's climate history by inferring the water content of its interior. By calculating the present rate of atmospheric destruction of H$_2$O, CO$_2$ and OCS, which must be restored by volcanism to maintain atmospheric stability, we show Venus's interior is dry. Venusian volcanic gases have at most a 6% water mole fraction, substantially drier than terrestrial magmas degassed at similar conditions. The dry interior is consistent with Venus ending its magma ocean epoch desiccated and thereafter having had a long-lived dry surface. Volcanic resupply to Venus's atmosphere therefore indicates that the planet has never been `liquid-water' habitable.

astro-ph.EP↗

arXiv2024

A Surface Hydrothermal Source of Nitriles and Isonitriles

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.

astro-ph.EP↗

arXiv2024

Astrobiological Potential of Venus Atmosphere Chemical Anomalies and Other Unexplained Cloud Properties

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.

astro-ph.EP↗

arXiv2023

Atmospheres as a Window to Rocky Exoplanet Surfaces

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.

astro-ph.EP↗

arXiv2023

Can comets deliver prebiotic molecules to rocky exoplanets?

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).

astro-ph.EP↗