Robin Wordsworth

Model Outputs for: Mars Sample Return Missions Could Reveal Climate and Atmospheric Redox Changes on Mars Through Time. Submitted to JGR: Planets June 14, 2026.

Model Outputs for: Mars Sample Return Missions Could Reveal Climate and Atmospheric Redox Changes on Mars Through Time. Submitted to JGR: Planets June 14, 2026.

Geological evidence indicates that Mars experienced multiple lake-forming climates lasting longer than 100 years around 3–4 billion years ago. These early warm climates cannot be explained solely by the greenhouse effect of carbon dioxide and water vapor. Recently, a warming mechanism driven by high-altitude water-ice clouds has been proposed for early Mars under surface water-limited climatic conditions. Here, we develop a general circulation model for terrestrial planetary atmospheres capable of simulating both early and modern climates of Earth and Mars. Simulation results show that the radiative effect of clouds can lead to two distinct climate states: when low-latitude surface regions are relatively arid, cloud radiative effects are dominated by warming, which can sustain dry–wet cycles in early Mars climate; however, when surface meltwater in low-latitude regions exceeds a critical threshold, cloud radiative effects shift to cooling, maintaining the climate in a cold, stable state. This work provides a new perspective for studying the climate evolution of early Mars.

Abstract We present a new first-principles analytic approach to interpreting eclipses and phase curves of rocky planets. Observations with JWST have reported nondetections of atmospheres around the majority of hot rocky planets orbiting M dwarfs. However, most of these “bare rock” inferences are based on models that are ill-suited to many currently observable planets, as they were developed for use on cooler, slower-rotating bodies. In particular, these models rely on the weak temperature gradient assumption, in which rotation is neglected and temperature gradients can be simply related to wind speeds. We find that this assumption may not be valid for over 40% of terrestrials observed with JWST, including TRAPPIST-1 b, GJ 367 b, and TOI-2445 b. Our simple new four-box model does not rely on this assumption, and instead allows the heat transport efficiency to be specified or follow scalings derived herein. This method is fast, interpretable, physically motivated, reproduces previous general circulation model results, and can be used as a starting point for more detailed modeling. We observe that the longitudinal temperature structure of tidally locked terrestrials depends strongly on atmospheric circulation. Considering the applicable range of atmospheric dynamical regimes, we find that a given planet’s nightside temperature can plausibly vary by hundreds of kelvin (from detectable to undetectable). Furthermore, a planet’s dayside energy balance can display complex behavior, with degeneracies between surface pressure and dayside temperature. Illustrating an application to observations, we find that assumptions about atmospheric dynamics and longitudinal temperature structure can bias atmospheric constraints at the order-of-magnitude level.

We present a new first-principles analytic approach to interpreting eclipses and phase curves of rocky planets. Observations with JWST have reported nondetections of atmospheres around the majority of hot rocky planets orbiting M dwarfs. However, most of these "bare rock" inferences are based on models that are ill-suited to many currently observable planets, as they were developed for use on cooler, slower-rotating bodies. In particular, these models rely on the weak temperature gradient assumption, in which rotation is neglected and temperature gradients can be simply related to wind speeds. We find that this assumption may not be valid for over 40% of terrestrials observed with JWST, including TRAPPIST-1b, GJ 367b, and TOI-2445b. Our simple new four-box model does not rely on this assumption, and instead allows the heat transport efficiency to be specified or follow scalings derived herein. This method is fast, interpretable, physically motivated, reproduces previous general circulation model results, and can be used as a starting point for more detailed modeling. We observe that the longitudinal temperature structure of tidally locked terrestrials depends strongly on the atmospheric circulation. Considering the applicable range of atmospheric dynamical regimes, we find that a given planet's nightside temperature can plausibly vary by 100s of Kelvin (from detectable to undetectable). Furthermore, a planet's dayside energy balance can display complex behavior, with degeneracies between surface pressure and dayside temperature. Illustrating an application to observations, we find that assumptions about atmospheric dynamics and longitudinal temperature structure can bias atmospheric constraints at the order-of-magnitude level.

We present a new first-principles analytic approach to interpreting eclipses and phase curves of rocky planets. Observations with JWST have reported nondetections of atmospheres around the majority of hot rocky planets orbiting M dwarfs. However, most of these "bare rock" inferences are based on models that are ill-suited to many currently observable planets, as they were developed for use on cooler, slower-rotating bodies. In particular, these models rely on the weak temperature gradient assumption, in which rotation is neglected and temperature gradients can be simply related to wind speeds. We find that this assumption may not be valid for over 40% of terrestrials observed with JWST, including TRAPPIST-1b, GJ 367b, and TOI-2445b. Our simple new four-box model does not rely on this assumption, and instead allows the heat transport efficiency to be specified or follow scalings derived herein. This method is fast, interpretable, physically motivated, reproduces previous general circulation model results, and can be used as a starting point for more detailed modeling. We observe that the longitudinal temperature structure of tidally locked terrestrials depends strongly on the atmospheric circulation. Considering the applicable range of atmospheric dynamical regimes, we find that a given planet's nightside temperature can plausibly vary by 100s of Kelvin (from detectable to undetectable). Furthermore, a planet's dayside energy balance can display complex behavior, with degeneracies between surface pressure and dayside temperature. Illustrating an application to observations, we find that assumptions about atmospheric dynamics and longitudinal temperature structure can bias atmospheric constraints at the order-of-magnitude level.

Abstract Detection of an atmosphere around a terrestrial exoplanet will be a major milestone in the field, but our observational capacities are biased towards tidally locked, close-in planets orbiting M dwarf stars. The atmospheres of these planets are vulnerable to atmospheric erosion and collapse due to condensation of volatiles on the nightside. However, these condensed volatiles constitute a stable reservoir that could be revaporized by meteorite impacts and reestablish the atmospheres. Through a simple energy balance model applied to atmospheric evolution simulations with stochastic impacts, we assess the viability and importance of this mechanism for CO 2 atmospheres. We find that moderate-sized impactors (5–10 km diameter) occurring at a frequency of 1–100 Gyr −1 can regenerate observable transient atmospheres on previously airless planets. We focus on specific targets from the James Webb Space Telescope Director’s Discretionary Time Rocky Worlds programme and compute the fraction of their evolution spent with a transient CO 2 atmosphere generated through this mechanism. We find this fraction can reach 70% for GJ 3929 b, 50% for LTT 1445 Ac, and 80% for LTT 1445 Ab at high impact rates and strong CO 2 outgassing over the planet’s lifetime. We also show that atmospheric collapse can shield volatiles from escape, particularly in the early, high X-ray and ultraviolet phase of M dwarf evolution. Overall, our work suggests that terrestrial planet atmospheres may not evolve monotonically but instead may be shaped by episodic external forcings.

Introduction Lyman-alpha transmission spectroscopy has been a powerful tool for observing hydrogen escape from close-in exoplanets. For instance, Ly-α observations of GJ 436b showed a maximum transit depth of 56% – compared to a 0.69% transit depth in optical wavelengths – due to the hydrodynamic escape of hydrogen from the planet (1). Ly-α observations assist in understanding the evolution of such exoplanets, including characterization of the atmosphere. To date, the Ly-α transit of terrestrial-sized exoplanets has yielded only non-detections for exoplanets such as Trappist 1b/c (2) and 55 Cn e (3) using the Hubble Space Telescope. These non-detections possibly indicate that terrestrial exoplanets do not have enough hydrogen escape to be observed in Ly-α. Despite these non-detections, Earth’s hydrogen exosphere has been shown to extend out past 38 Earth-radii, and modelling has suggested that an exoplanet with an Earth-like hydrogen exosphere orbiting an M-dwarf star would be observable with future space telescopes (4). This work models the Ly-α transit of varying terrestrial exoplanets to diagnose atmospheric composition. For example, how does the Ly-α transit of a desiccated planet like Venus compare to a water-rich planet like Earth? Can we discern these differences from future space telescope observations? To test this, we model the thermal escape of hydrogen from terrestrial exoplanets and compute the associated Ly-α transit depth. Atmospheric parameters in the upper atmosphere, such as the mixing ratio of hydrogen, are varied to analyze the resulting Ly-α transit depth. From this, we examine trends in the transit depth to characterize the atmospheres of terrestrial exoplanets. In addition, we derive key attributes of exoplanets that would be detectable with future space telescopes.Methods In this work, the hydrogen exosphere is modelled using the Chamberlain approach (5). Key input parameters needed are the exobase height, the number density of hydrogen, and the exobase temperature. The exobase height and hydrogen number density are calculated by setting up a 2-component atmosphere at the homopause (the altitude where different species can diffusively separate, situated at 100 km from the surface with a total species number density of 1019 molecules cm-3), with a given hydrogen mixing ratio. The species diffuse upward from the homopause until they reach the exobase where the mean free path of the atmosphere is equal to the scale height. We also account for diffusion-limited escape. In a scenario where Jean’s escape is larger than the diffusion-limited escape, the number density of hydrogen is scaled to the diffusion-limited value. The exobase temperature is a free parameter in our model, though observations from the solar system indicate that a CO2 dominated exobase is cooler than an atomic oxygen-dominated exobase such as Earth’s, due to infrared cooling. Once the hydrogen exosphere is modelled, the radiative transfer code from the open-source model Sunbather (6) is used to model the transit. By default, we consider the planet to be mid-transit with an impact factor of 0. Factors affecting the transit depth, such as thermal line broadening are included. We consider a wide parameter space of planets between 0.5 to 2 Earth radii, with atmospheric temperatures between 100 to 700 K, and hydrogen mixing ratios between 10-10 and 1. For each run of the model, the hydrogen exosphere is computed at several exospheric temperatures between 100 and 1000 K to span the range of exobase temperatures observed in the solar system.Results An example of the number density of hydrogen in the exosphere and the resulting Ly-α transit in shown in Figure 1. In this case, we have modelled an Earth-sized planet with a hydrogen mixing ratio of 10-6 at the homopause that diffuses through an upper atmosphere of atomic oxygen. The exobase is 228 km from the surface with a hydrogen number density of 3.4 x 104, limited by the diffusion. The exobase is dominated by atomic oxygen, and modelled to be at 1000 K. The Ly-α transit depth around a sun-sized star is 275 ppm, much smaller than observed for close-in Neptunes, but possibly observable with future space telescope technology at a distance where geocoronal contamination is minimal. Figure 1: (a) Hydrogen number density in the exosphere as a function of distance from the planet (b) The Ly-α transit in ppm. Figure 2 shows the transit depth of exoplanets with varying masses and hydrogen mixing ratios at the homopause. Here a fixed exobase temperature of 1000 K was used though the simulations were also run with other exobase temperatures (not shown here, though the Ly-α transit typically increases with increasing exobase temperature due to the hydrogen atoms escaping the exobase more easily). The transit depth shows a strong dependence on hydrogen mixing ratio, with higher hydrogen mixing ratios resulting in a larger transit depth. The mass of the planet also affects the transit depth, with lower mass planets allowing for hydrogen escape more easily than more massive planets.Figure 2: Ly-α transit depths for exoplanets with varying masses and hydrogen mixing ratios.We will also present the effects of photoionization, making the hydrogen no longer observable in Ly-α. Lastly, we will test the ability to detect these Ly-α transits with space telescopes. (1) Ehrenreich D, Bourrier V, Wheatley PJ, des Etangs AL, Hébrard G, Udry S, et al. A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b. Nature. 2015 Jun;522(7557):459–61.(2) Bourrier V, Ehrenreich D, Wheatley PJ, Bolmont E, Gillon M, Wit J de, et al. Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman-α line. Astron Astrophys. 2017 Mar 1;599:L3.(3) Salz M, Czesla S, Schneider PC, Schmitt JHMM. Simulating the escaping atmospheres of hot gas planets in the solar neighborhood. Astron Astrophys. 2016 Feb 1;586:A75.(4) Santos LA dos, Bourrier V, Ehrenreich D, Kameda S. Observability of hydrogen-rich exospheres in Earth-like exoplanets. Astron Astrophys. 2019 Feb 1;622:A46.(5) Chamberlain JW. Planetary coronae and atmospheric evaporation. Planet Space Sci. 1963 Aug 1;11(8):901–60.(6) Linssen D, Shih J, MacLeod M, Oklopčić A. The open-source sunbather code: Modeling escaping planetary atmospheres and their transit spectra. Astron Astrophys. 2024 Aug 1;688:A43.

Sustaining life beyond Earth requires the creation of habitats, which is typically assumed to require costly transport of high-mass components from Earth. Here, we investigate an alternative approach based on in situ fabrication using biologically generated materials. We show that several common biomaterials are capable of blocking UV radiation, transmitting visible light, and maintaining pressure differences sufficient to permanently stabilize liquid H 2 O in a vacuum or low-pressure environment. As a proof of concept, we then demonstrate growth of eukaryotic green alga in a 3D printed PLA bioplastic habitat under Mars-relevant conditions of a 600 Pa CO 2 background atmosphere. Our results demonstrate that products of biology itself can be used to create habitats in extraterrestrial environments. This approach is scalable, sustainable, and plausibly could be extended to construction of human habitats in the future.

The causes of Snowball Earth events, rare global glaciations important for the evolution of life, are unknown. Recent geochronology of the Sturtian Snowball and Franklin Large Igneous Province (LIP) suggest that the Franklin caused the Sturtian via CO 2 drawdown by silicate weathering. By modeling the climate response to LIP weathering, we show that this initiation mechanism is feasible given a cold background climate, rapid chemical weathering and erosion over a large area, and a weak sensitivity of the silicate weathering feedback strength. Our model indicates that similar Phanerozoic LIPs may have failed to trigger Snowballs due to hot background climates and lower erosion rates caused by lower topography or vegetation, while high CO 2 suppressed LIP perturbations earlier in Earth history. We therefore argue that Snowball initiation by weathering of the Franklin is plausible and consistent with the lack of Snowballs in the Phanerozoic and mid-Proterozoic, given specific conditions that motivate future geologic observations.

Abstract The causes of Snowball Earth events, rare global glaciations important for the evolution of life, are unknown. Recent geochronology of the Sturtian Snowball and Franklin Large Igneous Province (LIP) suggest that the Franklin caused the Sturtian via drawdown by silicate weathering. By modeling the climate response to LIP weathering, we show that this initiation mechanism is feasible given a cold background climate, rapid chemical weathering and erosion over a large area, and a weak sensitivity of the silicate weathering feedback strength. Our model indicates that similar Phanerozoic LIPs may have failed to trigger Snowballs due to hot background climates and lower erosion rates caused by lower topography or vegetation, while high suppressed LIP perturbations earlier in Earth history. We therefore argue that Snowball initiation by weathering of the Franklin is plausible and consistent with the lack of Snowballs in the Phanerozoic and mid‐Proterozoic, given specific conditions that motivate future geologic observations.

Abstract We use a one-dimensional line-by-line radiative–convective model to simulate hot, dense terrestrial-planet atmospheres. We find that strong shortwave absorption by H 2 O and CO 2 inhibits near-surface convection, reducing surface temperatures by up to ∼2000 K compared to fully convective predictions. Pure-CO 2 atmospheres are typically 1000 K cooler than pure H 2 O, with only a few percent of H 2 O required to elevate surface temperatures by hundreds of kelvins for a fixed incident stellar radiation. We also show that minor greenhouse gases such as SO 2 and NH 3 have a limited warming effect when H 2 O is abundant. We find that even for insolation values as high as 12,500 W m −2 (37 × Earth’s) planets with mixed CO 2 –H 2 O envelopes have surface temperatures in the 1200–2000 K range, limiting surface melting. Our results highlight the critical role of shortwave heating on magma ocean planets and the need for improved high-temperature spectroscopy beyond 20,000 cm −1 .

We use a one-dimensional line-by-line radiative-convective model to simulate hot, dense terrestrial-planet atmospheres. We find that strong shortwave absorption by H2O and CO2 inhibits near-surface convection, reducing surface temperatures by up to approximately 2000 K compared to fully convective predictions. Pure CO2 atmospheres are typically 1000 K cooler than pure H2O atmospheres, with only a few percent of H2O needed to elevate surface temperatures by hundreds of kelvin for a fixed incident stellar radiation. We also show that minor greenhouse gases such as SO2 and NH3 have a limited warming effect when H2O is abundant. Even at insolation values as high as 12,500 W/m2 (about 37 times Earth's current solar flux), planets with mixed CO2-H2O envelopes have surface temperatures in the 1200 to 2000 K range, limiting surface melting. Our results highlight the critical role of shortwave heating on magma ocean planets and the need for improved high-temperature spectroscopy beyond 20,000 cm-1.

Searching for extraterrestrial life and supporting human life in space are traditionally regarded as separate challenges. However, there are significant benefits to an approach that treats them as different aspects of the same essential inquiry: How can we conceptualize life beyond our home planet?

Searching for extraterrestrial life and supporting human life in space are traditionally regarded as separate challenges. However, there are significant benefits to an approach that treats them as different aspects of the same essential problem: How can we conceptualize life beyond our home planet?

Abstract We present a population-level view of volatile gas species (H 2 , He, H 2 O, O 2 , CO, CO 2 , CH 4 ) distribution during the sub-Neptune to rocky planet transition, revealing in detail the dynamic nature of small planet atmospheric compositions. Our novel model couples the atmospheric escape model IsoFATE with the magma ocean-atmosphere equilibrium chemistry model Atmodeller to simulate interior-atmosphere evolution over time for sub-Neptunes around G, K, and M stars. Chiefly, our simulations reveal that atmospheric mass fractionation driven by escape and interior-atmosphere exchange conspire to create a distinct oxidation gradient straddling the small-planet radius valley. We discover a key mechanism in shaping the oxidation landscape is the dissolution of water into the molten mantle, which shields oxygen from early escape, buffers the escape rate, and leads to oxidized secondary atmospheres following mantle outgassing. Our simulations reproduce a prominent population of He-rich worlds along the upper edge of the radius valley, revealing that they are stable on shorter timescales than previously predicted. Our simulations also robustly predict a broad population of O 2 -dominated atmospheres on close-in planets around low-mass stars, posing a potential source of false positive biosignature detection and marking a high-priority opportunity for the first-ever atmospheric O 2 detection. We motivate future atmospheric characterization surveys by providing a target list of planet candidates predicted to have O 2 -, He-, and deuterium-rich atmospheres.

ABSTRACT Cometary impacts play an important role in the early evolution of Earth, and other terrestrial exoplanets. Here, we present a numerical model for the interaction of weak, low-density cometary impactors with planetary atmospheres, which includes semi-analytical parametrizations for the ablation, deformation, and fragmentation of comets. Deformation is described by a pancake model, as is appropriate for weakly cohesive, low-density bodies, while fragmentation is driven by the growth of Rayleigh–Taylor instabilities. The model retains sufficient computational simplicity to investigate cometary impacts across a large parameter space, and permits simple description of the key physical processes controlling the interaction of comets with the atmosphere. We apply our model to two case studies. First, we consider the cometary delivery of prebiotic feedstock molecules. This requires the survival of comets during atmospheric entry, which is determined by three parameters: the comet’s initial radius, bulk density, and atmospheric surface density. There is a sharp transition between the survival and catastrophic fragmentation of comets at a radius of about 150 m, which increases with increasing atmospheric surface density and decreasing cometary density. Second, we consider the deposition of mass and kinetic energy in planetary atmospheres during cometary impacts, which determines the strength and duration of any atmospheric response. We demonstrate that mass loss is dominated by fragmentation, not ablation. Small comets deposit their entire mass within a fraction of an atmospheric scale height, at an altitude determined by their initial radius. Large comets lose only a small fraction of their mass to ablation in the lower atmosphere.

Geological records indicate that the surface of ancient Mars harboured substantial volumes of liquid water, a resource gradually diminished by processes such as the chemical alteration of crustal materials by hydration and atmospheric escape. However, how a relatively warm climate existed on early Mars to support liquid water under a fainter young Sun is debated. Greenhouse gases such as H2 in a CO2-rich atmosphere could have contributed to warming through collision-induced absorption, but whether sufficient H2 was available to sustain warming remains unclear. Here we use a combined climate and photochemical model to simulate how atmospheric chemistry on early Mars responded to water–rock reactions and climate variations, as constrained by existing observations. We find that H2 outgassing from crustal hydration and oxidation, supplemented by transient volcanic activity, could have generated sufficient H2 fluxes to transiently foster warm, humid climates. We estimate that Mars experienced episodic warm periods of an integrated duration of ~40 million years, with each event lasting ≥105 years, consistent with the formation timescale of valley networks. Declining atmospheric CO2 via surface oxidant sinks or variations in the planet’s axial tilt could have led to abrupt shifts in the planet’s redox state and transition to a CO-dominated atmosphere and cold climate. Photochemical modelling suggests that H2 outgassing from crustal hydration could have supported transient warming episodes on early Mars in a CO2-rich atmosphere with abrupt transitions to cold climate states in a CO-rich atmosphere.

Abstract In hydrogen-rich atmospheres with low mean molecular weight (MMW), an air parcel containing a higher-molecular-weight condensible can be negatively buoyant even if its temperature is higher than the surrounding environment. This should fundamentally alter the dynamics of moist convection, but the low-MMW regime has previously been explored primarily via 1D theories that cannot capture the complexity of moist turbulence. Here, we use a 3D cloud-resolving model to simulate moist convection in atmospheres with a wide range of background MMWs and confirm that a humidity threshold for buoyancy reversal first derived by T. Guillot coincides with an abrupt change in tropospheric structure. Crossing the “Guillot threshold” in near-surface humidity causes the dry (subcloud) boundary layer to collapse and be replaced by a very cloudy layer with a temperature lapse rate that exceeds the dry adiabatic rate. Simulations with reduced surface moisture availability in the lower atmosphere feature a deeper dry subcloud layer, which allows the superadiabatic cloud layer to remain aloft. Our simulations support a potentially observable systematic trend toward increased cloudiness for atmospheres with near-surface moisture concentrations above the Guillot threshold. This should apply to H 2 O and potentially to other condensible species on hotter worlds. We also find evidence for episodic convective activity and associated variability in cloud cover in some of our low-MMW simulations, which should be investigated further with global-scale simulations.

In hydrogen-rich atmospheres with low mean molecular weight (MMW), an air parcel containing a higher-molecular-weight condensible can be negatively buoyant even if its temperature is higher than the surrounding environment. This should fundamentally alter the dynamics of moist convection, but the low-MMW regime has previously been explored primarily via one-dimensional theories that cannot capture the complexity of moist turbulence. Here, we use a three-dimensional cloud-resolving model to simulate moist convection in atmospheres with a wide range of background MMW, and confirm that a humidity threshold for buoyancy reversal first derived by Guillot (1995) coincides with an abrupt change in tropospheric structure. Crossing the "Guillot threshold" in near-surface humidity causes the dry (subcloud) boundary layer to collapse and be replaced by a very cloudy layer with a temperature lapse rate that exceeds the dry adiabatic rate. Simulations with reduced surface moisture availability in the lower atmosphere feature a deeper dry subcloud layer, which allows the superadiabatic cloud layer to remain aloft. Our simulations support a potentially observable systematic trend toward increased cloudiness for atmospheres with near-surface moisture concentrations above the Guillot threshold. This should apply to \ce{H2O} and potentially to other condensible species on hotter worlds. We also find evidence for episodic convective activity and associated variability in cloud cover in some of our low-MMW simulations, which should be investigated further with global-scale simulations.

Standard definitions of habitability assume that life requires the presence of planetary gravity wells to stabilize liquid water and regulate surface temperature. Here, the consequences of relaxing this assumption are evaluated. Temperature, pressure, volatile loss, radiation levels, and nutrient availability all appear to be surmountable obstacles to the survival of photosynthetic life in space or on celestial bodies with thin atmospheres. Biologically generated barriers capable of transmitting visible radiation, blocking ultraviolet, and sustaining temperature gradients of 25-100 K and pressure differences of 10 kPa against the vacuum of space can allow habitable conditions between 1 and 5 astronomical units in the solar system. Hence, ecosystems capable of generating conditions for their own survival are physically plausible, given the known capabilities of biological materials on Earth. Biogenic habitats for photosynthetic life in extraterrestrial environments would have major benefits for human life support and sustainability in space. Because the evolution of life elsewhere may have followed very different pathways from that on Earth, living habitats could also exist outside traditional habitable environments around other stars, where they would have unusual yet potentially detectable biosignatures.

Context . The increased brightness temperature of young rocky protoplanets during their magma ocean epoch makes them potentially amenable to atmospheric characterization at distances from the Solar System far greater than thermally equilibrated terrestrial exoplanets, offering observational opportunities for unique insights into the origin of secondary atmospheres and the near surface conditions of prebiotic environments. Aims . The Large Interferometer For Exoplanets (LIFE) mission will employ a space-based midinfrared nulling interferometer to directly measure the thermal emission of terrestrial exoplanets. In this work, we seek to assess the capabilities of various instrumental design choices of the LIFE mission concept for the detection of cooling protoplanets with transient high-temperature magma ocean atmospheres at the tail end of planetary accretion. In particular, we investigate the minimum integration times necessary to detect transient magma ocean exoplanets in young stellar associations in the Solar neighborhood. Methods . Using the LIFE mission instrument simulator (LIFEsim), we assessed how specific instrumental parameters and design choices, such as wavelength coverage, aperture diameter, and photon throughput, facilitate or disadvantage the detection of protoplan-ets. We focused on the observational sensitivities of distance to the observed planetary system, protoplanet brightness temperature (using a blackbody assumption), and orbital distance of the potential protoplanets around both G- and M-dwarf stars. Results . Our simulations suggest that LIFE will be able to detect (S/N ≥ 7) hot protoplanets in young stellar associations up to distances of 100 pc from the Solar System for reasonable integration times (up to a few hours). Detection of an Earth-sized protoplanet orbiting a Solar-sized host star at 1 AU requires less than 30 minutes of integration time. M-dwarfs generally need shorter integration times. The contribution from wavelength regions smaller than 6 µm is important for decreasing the detection threshold and discriminating emission temperatures. Conclusions . The LIFE mission is capable of detecting cooling terrestrial protoplanets within minutes to hours in several local young stellar associations hosting potential targets. The anticipated compositional range of magma ocean atmospheres motivates further architectural design studies to characterize the crucial transition from primary to secondary atmospheres.

Abstract Abundant geomorphological and geochemical evidence of liquid water on the surface of early Mars during the late Noachian and early Hesperian periods needs to be reconciled with a fainter young Sun. While a dense atmosphere and related warming mechanisms are potential solutions to the early Mars climate problem, further investigation is warranted. Here, we complete a comprehensive survey of the warming potential of all known greenhouse gases and perform detailed calculations for 15 different minor gas species under early Martian conditions. We find that of these 15 species, , , , , and cause significant greenhouse warming at concentrations of 0.1 ppmv or greater. However, the most highly effective greenhouse gas species also tend to be more condensable, soluble and vulnerable to photolytic destruction. To provide a reference for future atmospheric evolution and photochemical studies, we have made our warming potential database freely available online.

Abstract Recent findings of NO near Gale Crater on Mars have been explained by two pathways: formation of nitric acid (HNO 3 ) in a warm climate or formation of peroxynitric acid (HO 2 NO 2 ) in a cool climate. Here, we put forth two hitherto unexplored pathways: (a) deposition of nitric/peroxynitric acid onto ice particles in a cold atmosphere, which settle quickly onto Mars' surface and (b) solar energetic particle‐induced production of nitric/peroxynitric acid. The deposition rates are enhanced and NO production is more efficient under the higher atmospheric pressures typical of Mars' ancient atmosphere. Depending on the unknown rate at which nitric/peroxynitric acid is lost from the surface, the new pathways could result in larger NO‐levels than those detected by the Mars Science Laboratory. We predict a 2:1 ratio of nitrite:nitrate would have deposited in cool surface climates with an icy atmosphere, whereas orders of magnitude more nitrate than nitrite is expected from warm surface climates.

Abstract Atmospheric retrievals are widely used to constrain exoplanet properties from observed spectra. We investigate how the common nonphysical retrieval assumptions of vertically constant molecule abundances and cloud-free atmospheres affect our characterization of an exo-Earth (an Earth-twin orbiting a Sun-like star). Specifically, we use a state-of-the-art retrieval framework to explore how assumptions for the H 2 O profile and clouds affect retrievals. In the first step, we validate different retrieval models on a low-noise simulated 1D mid-infrared (MIR) spectrum of Earth. Thereafter, we study how these assumptions affect the characterization of Earth with the Large Interferometer For Exoplanets (LIFE). We run retrievals on LIFE mock observations based on real disk-integrated MIR Earth spectra. The performance of different retrieval models is benchmarked against ground truths derived from remote sensing data. We show that assumptions for the H 2 O abundance and clouds directly affect our characterization. Overall, retrievals that use physically motivated models for the H 2 O profile and clouds perform better on the empirical Earth data. For observations of Earth with LIFE, they yield accurate estimates for the radius, pressure–temperature structure, and the abundances of CO 2 , H 2 O, and O 3 . Further, at R = 100, a reliable and bias-free detection of the biosignature CH 4 becomes feasible. We conclude that the community must use a diverse range of models for temperate exoplanet atmospheres to build an understanding of how different retrieval assumptions can affect the interpretation of exoplanet spectra. This will enable the characterization of distant habitable worlds and the search for life with future space-based instruments.

The increased brightness temperature of young rocky protoplanets during their magma ocean epoch makes them potentially amenable to atmospheric characterization to distances from the solar system far greater than thermally equilibrated terrestrial exoplanets, offering observational opportunities for unique insights into the origin of secondary atmospheres and the near surface conditions of prebiotic environments. The Large Interferometer For Exoplanets (LIFE) mission will employ a space-based mid-infrared nulling interferometer to directly measure the thermal emission of terrestrial exoplanets. Here, we seek to assess the capabilities of various instrumental design choices of the LIFE mission concept for the detection of cooling protoplanets with transient high-temperature magma ocean atmospheres, in young stellar associations in particular. Using the LIFE mission instrument simulator (LIFEsim) we assess how specific instrumental parameters and design choices, such as wavelength coverage, aperture diameter, and photon throughput, facilitate or disadvantage the detection of protoplanets. We focus on the observational sensitivities of distance to the observed planetary system, protoplanet brightness temperature using a blackbody assumption, and orbital distance of the potential protoplanets around both G- and M-dwarf stars. Our simulations suggest that LIFE will be able to detect (S/N $\geq$ 7) hot protoplanets in young stellar associations up to distances of $\approx$100 pc from the solar system for reasonable integration times (up to $\sim$hours). Detection of an Earth-sized protoplanet orbiting a solar-sized host star at 1 AU requires less than 30 minutes of integration time. M-dwarfs generally need shorter integration times. The contribution from wavelength regions $<$6 $μ$m is important for decreasing the detection threshold and discriminating emission temperatures.

Abstract We investigate the impact of nightside cloud formation on the observable day/night contrast of tidally locked terrestrial planet atmospheres. We demonstrate that, in the case where the planetary dayside is only 10 s of Kelvin hotter than the planetary nightside, the presence of optically thick nightside clouds can lead to observations that mimic a planet without an atmosphere, despite the planet actually hosting a significant (10 bar) atmosphere. The scenario presented in this work requires a level of intrinsic atmospheric day/night temperature contrast such that the nightside can form clouds while the dayside is too hot for cloud formation to occur. This scenario is most likely for hotter terrestrials and terrestrials with low volatile inventories. We thus note that a substantial dayside/nightside temperature difference alone does not robustly indicate that a planet does not host an atmosphere, and additional observations and modeling are essential for characterization. We further discuss several avenues for future study to improve our understanding of the terrestrial planets and how best to characterize them with JWST.

Abstract The equilibrium rotation rate of a planet is determined by the sum of torques acting on its solid body. For planets with atmospheres, the dominant torques are usually the gravitational tide, which acts to slow the planet’s rotation rate, and the atmospheric thermal tide, which acts to spin up the planet. Previous work demonstrated that rocky planets with thick atmospheres may produce strong enough thermal tides to avoid tidal locking, but a study of how the strength of the thermal tide depends on atmospheric properties has not been done. In this work, we use a combination of simulations from a global climate model and analytic theory to explore how the thermal tide depends on the shortwave and longwave optical depth of the atmosphere, the surface pressure, and the absorbed stellar radiation. We find that for planets in the habitable zones of M stars only high-pressure but low-opacity atmospheres permit asynchronous rotation owing to the weakening of the thermal tide at high longwave and shortwave optical depths. We conclude that asynchronous rotation may be very unlikely around low-mass stars, which may limit the potential habitability of planets around M stars.

Standard definitions of habitability assume that life requires the presence of planetary gravity wells to stabilize liquid water and regulate surface temperature. Here the consequences of relaxing this assumption are evaluated. Temperature, pressure, volatile loss, radiation levels and nutrient availability all appear to be surmountable obstacles to the survival of photosynthetic life in space or on celestial bodies with thin atmospheres. Biologically generated barriers capable of transmitting visible radiation, blocking ultraviolet, and sustaining temperature gradients of 25-100 K and pressure differences of 10 kPa against the vacuum of space can allow habitable conditions between 1 and 5 astronomical units in the solar system. Hence ecosystems capable of generating conditions for their own survival are physically plausible, given the known capabilities of biological materials on Earth. Biogenic habitats for photosynthetic life in extraterrestrial environments would have major benefits for human life support and sustainability in space. Because the evolution of life elsewhere may have followed very different pathways from on Earth, living habitats could also exist outside traditional habitable environments around other stars, where they would have unusual but potentially detectable biosignatures.

We investigate the impact of nightside cloud formation on the observable night-day contrast of tidally-locked terrestrial planet atmospheres. We demonstrate that, in the case where the planetary dayside is only 10s of Kelvin hotter than the planetary nightside, the presence of optically thick nightside clouds can lead to observations that mimic a planet without an atmosphere, despite the planet actually hosting a significant (10 bar) atmosphere. The scenario presented in this work requires a level of intrinsic atmospheric day/night temperature contrast such that the nightside can form clouds while the dayside is too hot for cloud formation to occur. This scenario is most likely for hotter terrestrials and terrestrials with low volatile inventories. We thus note that a substantial dayside/nightside temperature difference alone does not robustly indicate that a planet does not host an atmosphere and additional observations and modeling are essential for characterization. We further discuss several avenues for future study to improve our understanding of the terrestrial planets and how best to characterize them with JWST.

Atmospheric retrievals are widely used to constrain exoplanet properties from observed spectra. We investigate how the common nonphysical retrieval assumptions of vertically constant molecule abundances and cloud-free atmospheres affect our characterization of an exo-Earth (an Earth-twin orbiting a Sun-like star). Specifically, we use a state-of-the-art retrieval framework to explore how assumptions for the $\mathrm{H_2O}$ profile and clouds affect retrievals. In a first step, we validate different retrieval models on a low-noise simulated 1D mid-infrared (MIR) spectrum of Earth. Thereafter, we study how these assumptions affect the characterization of Earth with the Large Interferometer For Exoplanets (LIFE). We run retrievals on LIFE mock observations based on real disk-integrated MIR Earth spectra. The performance of different retrieval models is benchmarked against ground truths derived from remote sensing data. We show that assumptions for the $\mathrm{H_2O}$ abundance and clouds directly affect our characterization. Overall, retrievals that use physically motivated models for the $\mathrm{H_2O}$ profile and clouds perform better on the empirical Earth data. For observations of Earth with LIFE, they yield accurate estimates for the radius, pressure-temperature structure, and the abundances of $\mathrm{CO_2}$, $\mathrm{H_2O}$, and $\mathrm{O_3}$. Further, at $R=100$, a reliable and bias-free detection of the biosignature $\mathrm{CH_4}$ becomes feasible. We conclude that the community must use a diverse range of models for temperate exoplanet atmospheres to build an understanding of how different retrieval assumptions can affect the interpretation of exoplanet spectra. This will enable the characterization of distant habitable worlds and the search for life with future space-based instruments.

Abstract We simulate atmospheric fractionation in escaping planetary atmospheres using IsoFATE , a new open-source numerical model. We expand the parameter space studied previously to planets with tenuous atmospheres that exhibit the greatest helium and deuterium enhancement. We simulate the effects of extreme-ultraviolet-driven photoevaporation and core-powered mass loss on deuterium–hydrogen and helium–hydrogen fractionation of sub-Neptune atmospheres around G, K, and M stars. Our simulations predict prominent populations of deuterium- and helium-enhanced planets along the upper edge of the radius valley with mean equilibrium temperatures of ≈370 K and as low as 150 K across stellar types. We find that fractionation is mechanism dependent, so constraining He/H and D/H abundances in sub-Neptune atmospheres offers a unique strategy to investigate the origin of the radius valley around low-mass stars. Fractionation is also strongly dependent on retained atmospheric mass, offering a proxy for planetary surface pressure as well as a way to distinguish between desiccated enveloped terrestrials and water worlds. Deuterium-enhanced planets tend to be helium dominated and CH 4 depleted, providing a promising strategy to observe HDO in the 3.7 μ m window. We present a list of promising targets for observational follow-up.

Recent findings by the Mars Science Laboratory (MSL) have confirmed the presence of nitrates near Gale Crater on Mars. In this work, we consider the formation and deposition of HNOx species in cold early Mars climates. We find that solar energetic particles could facilitate nitrogen fixation by photochemically generating pernitric and nitric acid, which then deposit onto icy particles that settle onto Mars’ surface. This study demonstrates that such deposition would be more efficient under higher atmospheric pressures, consistent with Mars’ ancient atmosphere, and could account for the nitrate levels detected by the MSL. We find a more rapid deposition rate for pernitric acid over nitric acid (in agreement with Smith et al., 2014), and a significant enhancement of deposition rates through consideration of deposition onto icy particles. This distinction could be crucial for interpreting the MSL data.

Abstract Although the scientific principles of anthropogenic climate change are well-established, existing calculations of the warming effect of carbon dioxide rely on spectral absorption databases, which obscures the physical foundations of the climate problem. Here, we show how CO 2 radiative forcing can be expressed via a first-principles description of the molecule’s key vibrational-rotational transitions. Our analysis elucidates the dependence of carbon dioxide’s effectiveness as a greenhouse gas on the Fermi resonance between the symmetric stretch mode ν 1 and bending mode ν 2 . It is remarkable that an apparently accidental quantum resonance in an otherwise ordinary three-atom molecule has had such a large impact on our planet’s climate over geologic time, and will also help determine its future warming due to human activity. In addition to providing a simple explanation of CO 2 radiative forcing on Earth, our results may have implications for understanding radiation and climate on other planets.

We simulate atmospheric fractionation in escaping planetary atmospheres using IsoFATE, a new open-source numerical model. We expand the parameter space studied previously to planets with tenuous atmospheres that exhibit the greatest helium and deuterium enhancement. We simulate the effects of EUV-driven photoevaporation and core-powered mass loss on deuterium-hydrogen and helium-hydrogen fractionation of sub-Neptune atmospheres around G, K, and M stars. Our simulations predict prominent populations of deuterium- and helium-enhanced planets along the upper edge of the radius valley with mean equilibrium temperatures of 370 K and as low as 150 K across stellar types. We find that fractionation is mechanism-dependent, so constraining He/H and D/H abundances in sub-Neptune atmospheres offers a unique strategy to investigate the origin of the radius valley around low-mass stars. Fractionation is also strongly dependent on retained atmospheric mass, offering a proxy for planetary surface pressure as well as a way to distinguish between desiccated enveloped terrestrials and water worlds. Deuterium-enhanced planets tend to be helium-dominated and CH4-depleted, providing a promising strategy to observe HDO in the 3.7 um window. We present a list of promising targets for observational follow-up.