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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.

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 CO2 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, H2O2, HNO3, NH3, SO2, and C2H4 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., Comment: 14 pages, 12 figures, published in JGR Planets

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 $\mu$m is important for decreasing the detection threshold and discriminating emission temperatures., Comment: 18 pages, 19 figures; accepted for publication in A&A

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., Comment: Final published version, in press at Astrobiology

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., Comment: Accepted to ApJL, 12 pages, 9 figures

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., Comment: Published in ApJ; 17 pages (main text incl. 9 figures and 5 tables) + appendix; comments are welcome

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., Comment: Accepted by ApJ

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 CO2 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 $\nu_1$ and bending mode $\nu_2$. 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 CO2 radiative forcing on Earth, our results may have implications for understanding radiation and climate on other planets., Comment: 15 pages, 7 figures, accepted to The Planetary Science Journal

The Large Interferometer For Exoplanets (LIFE) initiative aims to develop a space based mid-infrared (MIR) nulling interferometer to measure the thermal emission spectra of temperate terrestrial exoplanets. We investigate how well LIFE could characterize a cloudy Venus-twin exoplanet to: (1) test our retrieval routine on a realistic non-Earth-like MIR spectrum of a known planet, (2) investigate how clouds impact retrievals, (3) refine the LIFE requirements derived in previous Earth-centered studies. We run retrievals for simulated LIFE observations of a Venus-twin exoplanet orbiting a Sun-like star located 10 pc from the observer. By assuming different models (cloudy and cloud-free) we analyze the performance as a function of the quality of the LIFE observation. This allows us to determine how well atmosphere and clouds are characterizable depending on the quality of the spectrum. Our study shows that the current minimal resolution ($R=50$) and signal-to-noise ($S/N=10$ at $11.2\mu$m) requirements for LIFE suffice to characterize the structure and composition of a Venus-like atmosphere above the cloud deck if an adequate model is chosen. However, we cannot infer cloud properties. The accuracy of the retrieved planet radius ($R_{pl}$), equilibrium temperature ($T_{eq}$), and Bond albedo ($A_B$) depend on the choice of model. Generally, a cloud-free model performs best and thus the presence of clouds cannot be inferred. This model dependence of retrieval results emphasizes the importance of developing a community-wide best-practice for atmospheric retrieval studies. If we consider higher quality spectra (especially $S/N=20$), we can infer the presence of clouds and pose first constraints on their structure., Comment: Accepted for publication in A&A; Updates: LIFE-series number changed from 10 to 9, additional references added to the discussion, language editing; 15 pages (main text incl. 8 figures and 6 tables) + appendix; comments are welcome. This paper is part of a series on the LIFE telescope. Related series papers: arXiv:2101.07500, arXiv:2203.00471, arXiv:2112.02054, arXiv:2204.10041

In Earth's current climate, moist convective updraft speeds increase with surface warming. This trend suggests that very vigorous convection might be the norm in extremely hot and humid atmospheres, such as those undergoing a runaway greenhouse transition. However, theoretical and numerical evidence suggests that convection is actually gentle in water vapor-dominated atmospheres, implying that convective vigor may peak at some intermediate humidity level. Here, we perform small-domain convection-resolving simulations of an Earth-like atmosphere over a wide range of surface temperatures and confirm that there is indeed a peak in convective vigor, which we show occurs near T_s ~ 330 K. We show that a similar peak in convective vigor exists when the relative abundance of water vapor is changed by varying the amount of background (non-condensing) gas at fixed T_s, which may have implications for Earth's climate and atmospheric chemistry during the Hadean and Archean. We also show that Titan-like thermodynamics (i.e., a thick nitrogen atmosphere with condensing methane and low gravity) produce a peak in convective vigor at T_s ~ 95 K, which is curiously close to the current surface temperature of Titan. Plotted as functions of the saturation specific humidity at cloud base, metrics of convective vigor from both Earth-like and Titan-like experiments all peak when cloud-base air contains roughly 10% of the condensible gas by mass. Our results point to a potentially common phenomenon in terrestrial atmospheres: that moist convection is most vigorous when the condensible component is between dilute and non-dilute abundance.

Detection of water vapor in the atmosphere of temperate rocky exoplanets would be a major milestone on the path towards characterization of exoplanet habitability. Past modeling work has shown that cloud formation may prevent the detection of water vapor on Earth-like planets with surface oceans using the James Webb Space Telescope (JWST). Here we analyze the potential for atmospheric detection of H2O on a different class of targets: arid planets. Using transit spectrum simulations, we show that atmospheric H2O may be easier to be detected on arid planets with cold-trapped ice deposits on the surface, because such planets will not possess thick H2O cloud decks that limit the transit depth of spectral features. However, additional factors such as band overlap with CO2 and other gases, extinction by mineral dust, overlap of stellar and planetary H2O lines, and the ultimate noise floor obtainable by JWST still pose important challenges. For this reason, combination of space- and ground-based spectroscopic observations will be essential for reliable detection of H2O on rocky exoplanets in the future., Comment: 14 pages, 4 figures, accepted for publication in ApJL

Rocky planets are common around other stars, but their atmospheric properties remain largely unconstrained. Thanks to a wealth of recent planet discoveries and upcoming advances in observing capability, we are poised to characterize the atmospheres of dozens of rocky exoplanets in this decade. Theoretical understanding of rocky exoplanet atmospheres has advanced considerably in the last few years, yielding testable predictions of their evolution, chemistry, dynamics and even possible biosignatures. Here we review key progress in this field to date and discuss future objectives. Our major conclusions are as follows: 1) Many rocky planets may form with initial H$_2$-He envelopes that are later lost to space, likely due to a combination of stellar UV/X-ray irradiation and internal heating. 2) After the early stages of evolution, a wide diversity of atmospheric compositions is expected, due to variations in host star flux, atmospheric escape rates, interior exchange and other factors. 3) Observations have ruled out the presence of hydrogen-dominated atmospheres on several nearby rocky exoplanets, and the presence of any thick atmosphere on one target. More detailed atmospheric characterization of these planets and others will become possible in the near future. 4) Exoplanet biosphere searches are an exciting future goal. However, reliable detections for a representative sample of planets will require further advances in observing capability and improvements in our understanding of abiotic planetary processes., Comment: Pre-print of 2022 Annual Reviews article, with minor updates from previously submitted version. 48 pages including references, 11 figures

Earth's distant past and potentially its future include extremely warm "hothouse" climate states, but little is known about how the atmosphere behaves in such states. One distinguishing characteristic of hothouse climates is that they feature lower-tropospheric radiative heating, rather than cooling, due to the closing of the water vapor infrared window regions. Previous work has suggested that this could lead to temperature inversions and significant changes in cloud cover, but no previous modeling of the hothouse regime has resolved convective-scale turbulent air motions and cloud cover directly, thus leaving many questions about hothouse radiative heating unanswered. Here, we conduct simulations that explicitly resolve convection and find that lower-tropospheric radiative heating in hothouse climates causes the hydrologic cycle to shift from a quasi-steady regime to a "relaxation oscillator" regime, in which precipitation occurs in short and intense outbursts separated by multi-day dry spells. The transition to the oscillatory regime is accompanied by strongly enhanced local precipitation fluxes, a significant increase in cloud cover, and a transiently positive (unstable) climate feedback parameter. Our results indicate that hothouse climates may feature a novel form of "temporal" convective self-organization, with implications for both cloud coverage and erosion processes.

Terrestrial-type exoplanets orbiting nearby red dwarf stars (M dwarfs) are the first potentially habitable exoplanets suitable for atmospheric characterization in the near future. Understanding the stability of water in cold-trap regions on such planets is critical because it directly impacts transmission spectroscopy observations, the global energy budget, and long-term surface water evolution. Here we diagnose the humidity distribution in idealized general circulation model (GCM) simulations of terrestrial-type exoplanets. We use the `tracer of last saturation' technique to study the saturation statistics of air parcels. We find that on synchronously rotating planets, the water vapor abundance in the nightside upper troposphere depends weakly on planetary rotation, while more water vapor builds up in the nightside lower troposphere on fast-rotating planets. We then discuss how last-saturation statistics can elucidate the multiple moist climate equilibrium states on synchronously and asynchronously rotating arid planets. We show that the multiple moist climate states arise from the cold-trapping competition between the substellar upper atmosphere and cold surface regions. We find that fast synchronously rotating planets tend to trap surface water on the nightside as a result of their weak atmospheric and strong surface cold traps compared to the slow rotating case. These results elucidate the nature of the water cycle on arid rocky exoplanets and will aid interpretation of atmospheric observations in the future., Comment: 28 pages, 10 figures, 2 tables, 1 movie, accepted for publication in The Planetary Science Journal

Understanding when global glaciations occur on Earth-like planets is a major challenge in climate evolution research. Most models of how greenhouse gases like CO2 evolve with time on terrestrial planets are deterministic, but the complex, nonlinear nature of Earth's climate history motivates study of non-deterministic climate models. Here a maximally simple stochastic model of CO2 evolution and climate on an Earth-like planet with an imperfect CO2 thermostat is investigated. It is shown that as stellar luminosity is increased in this model, the decrease in the average atmospheric CO concentration renders the climate increasingly unstable, with excursions to a low-temperature state common once the received stellar flux approaches that of present-day Earth. Unless climate feedbacks always force the variance in CO2 concentration to decline rapidly with received stellar flux, this means that terrestrial planets near the inner edge of the habitable zone may enter Snowball states quite frequently. Observations of the albedos and color variation of terrestrial-type exoplanets should allow this prediction to be tested directly in the future., Comment: 8 pages, 3 figures, accepted April 14th to ApJ Letters

Reconciling the geology of Mars with models of atmospheric evolution remains a major challenge. Martian geology is characterized by past evidence for episodic surface liquid water, and geochemistry indicating a slow and intermittent transition from wetter to drier and more oxidizing surface conditions. Here we present a new model incorporating randomized injection of reducing greenhouse gases and oxidation due to hydrogen escape, to investigate the conditions responsible for these diverse observations. We find that Mars could have transitioned repeatedly from reducing (H2-rich) to oxidizing (O2-rich) atmospheric conditions in its early history. Our model predicts a generally cold early Mars, with mean annual temperatures below 240 K. If peak reducing gas release rates and background CO2 levels are high enough, it nonetheless exhibits episodic warm intervals sufficient to degrade crater walls, form valley networks and create other fluvial/lacustrine features. Our model also predicts transient buildup of atmospheric O2, which can help explain the occurrence of oxidized mineral species such as manganese oxides at Gale Crater. We suggest that the apparent Noachian--Hesperian transition from phyllosilicate deposition to sulfate deposition around 3.5 billion years ago can be explained as a combined outcome of increasing planetary oxidation, decreasing groundwater availability and a waning bolide impactor flux, which dramatically slowed the remobilization and thermochemical destruction of surface sulfates. Ultimately, rapid and repeated variations in Mars' early climate and surface chemistry would have presented both challenges and opportunities for any emergent microbial life., Comment: Published March 8 2021 in Nature Geoscience

The evolution of a single raindrop falling below a cloud is governed by fluid dynamics and thermodynamics fundamentally transferable to planetary atmospheres beyond modern Earth's. Here, we show how three properties that characterize falling raindrops -- raindrop shape, terminal velocity, and evaporation rate -- can be calculated as a function of raindrop size in any planetary atmosphere. We demonstrate that these simple, interrelated characteristics tightly bound the possible size range of raindrops in a given atmosphere, independently of poorly understood growth mechanisms. Starting from the equations governing raindrop falling and evaporation, we demonstrate that raindrop ability to vertically transport latent heat and condensible mass can be well captured by a new dimensionless number. Our results have implications for precipitation efficiency, convective storm dynamics, and rainfall rates, which are properties of interest for understanding planetary radiative balance and (in the case of terrestrial planets) rainfall-driven surface erosion., Comment: submitted to JGR: Planets; 40 pages, 8 figures, 3 tables, 4 appendices; supporting information with 9 pages, 8 figures, 1 table; associated code at https://github.com/kaitlyn-loftus/rainprops

Ultra-hot disintegrating exoplanets have been detected with tails trailing behind and/or shooting ahead of them. These tails are believed to be made of dusts that are carried upward by the supersonic flow escaping the planet's gravity field from the fiercely heated permanent day-side. Conserving angular momentum, this day-side escape flux would lead the planet in orbit. In order to explain the trailing tails in observation, radiation pressure, a repulsive force pushing the escape flow away from the host star is considered to be necessary. We here investigate whether escape could occur on the night-side as the escape flow is deflected by the pressure gradient force. We demonstrate in an idealized framework that escape flux from the night-side could dominate that from the day-side; and the former may naturally explain the commonly-observed trailing tails based on angular momentum conservation, without the need to invoke radiation pressure, which has previously been thought to be the key. We also find analytical approximations for both dayside and nightside escape fluxes, which may be applied to study planetary evolution of disintegrating planets and to infer planetary sizes from observations of the properties of their dusty tails.

We present gray gas general circulation model (GCM) simulations of the tidally locked mini-Neptune GJ 1214b. On timescales of 1,000-10,000 Earth days, our results are comparable to previous studies of the same planet, in the sense that they all exhibit two off-equatorial eastward jets. Over much longer integration times (50,000-250,000 Earth days) we find a significantly different circulation and observational features. The zonal-mean flow transitions from two off-equatorial jets to a single wide equatorial jet that has higher velocity and extends deeper. The hot spot location also shifts eastward over the integration time. Our results imply a convergence time far longer than the typical integration time used in previous studies. We demonstrate that this long convergence time is related to the long radiative timescale of the deep atmosphere and can be understood through a series of simple arguments. Our results indicate that particular attention must be paid to model convergence time in exoplanet GCM simulations, and that other results on the circulation of tidally locked exoplanets with thick atmospheres may need to be revisited., Comment: 36 pages, 17 figures, 2 tables

A key factor in determining the potential habitability of synchronously rotating planets is the strength of the atmospheric boundary layer inversion between the dark side surface and the free atmosphere. Here we analyse data obtained from polar night measurements at the South Pole and Alert Canada, which are the closest analogues on Earth to conditions on the dark sides of synchronously rotating exoplanets without and with a maritime influence, respectively. On Earth, such inversions rarely exceed 30 K in strength, because of the effect of turbulent mixing induced by phenomena such as so-called mesoscale slope winds, which have horizontal scales of 10s to 100s of km, suggesting a similar constraint to near-surface dark side inversions. We discuss the sensitivity of inversion strength to factors such as orography and the global-scale circulation, and compare them to a simulation of the planet Proxima Centauri b. Our results demonstrate the importance of comparisons with Earth data in exoplanet research, and highlight the need for further studies of the exoplanet atmospheric collapse problem using mesoscale and eddy-resolving models., Comment: Accepted by Astrophysical Journal Letters; 14 pages, 3 figures

Terrestrial-type exoplanets orbiting nearby red dwarf stars (M-dwarfs) are among the best targets for atmospheric characterization and biosignature searches in the near future. Recent evolutionary studies have suggested that terrestrial planets in the habitable zone of M-dwarfs are probably tidally locked and have limited surface water inventories as a result of their host stars' high early luminosities. Several previous climate simulations of such planets have indicated that their remaining water would be transported to the planet's permanent nightside and become trapped as surface ice, leaving the dayside devoid of water. Here we use a three-dimensional general circulation model with a water cycle and accurate radiative transfer scheme to investigate the surface water evolution on slowly rotating tidally locked terrestrial planets with limited surface water inventories. We show that there is a competition for water trapping between the nightside surface and the substellar tropopause in this type of climate system. Although under some conditions the surface water remains trapped on the nightside as an ice sheet, in other cases liquid water stabilizes in a circular area in the substellar region as a wetland. Planets with 1 bar N$_2$ and atmospheric CO$_2$ levels greater than 0.1 bar retain stable dayside liquid water, even with very small surface water inventories. Our results reveal the diversity of possible climate states on terrestrial-type exoplanets and highlight the importance of surface liquid water detection techniques for future characterization efforts., Comment: accepted for publication in ApJL

The response of the general circulation in a dry atmosphere to various atmospheric shortwave absorptivities is investigated in a three-dimensional general circulation model with grey radiation. Shortwave absorption in the atmosphere reduces the incoming radiation reaching the surface but warms the upper atmosphere, significantly shifting the habitable zone toward the star. The strong stratification under high shortwave absorptivity suppresses the Hadley cell in a manner that matches previous Hadley cell scalings. General circulation changes may be observable through cloud coverage and superrotation. The equatorial superrotation in the upper atmosphere strengthens with the shortwave opacity, as predicted based on the gradient wind of the radiative-convective equilibrium profile. There is a sudden drop of equatorial superrotation at very low shortwave opacity. This is because the Hadley cell in those cases are strong enough to fill the entire troposphere with zero momentum air from the surface. A diurnal cycle (westward motion of substellar point relative to the planet) leads to acceleration of the equatorial westerlies in general, through the enhancement of the equatorward eddy momentum transport, but the response is not completely monotonic, perhaps due to the resonance of tropical waves and the diurnal forcing.

Jupiter's atmosphere is enriched in C, N, S, P, Ar, Kr and Xe with respect to solar abundances by a factor of ~3. Gas Giant envelopes are mainly enriched through the dissolution of solids in the atmosphere, and this constant enrichment factor is puzzling since several of the above elements are not expected to have been in the solid phase in Jupiter's feeding zone; most seriously, Ar and the main carrier of N, N2, only condense at the very low temperatures, 21-26 K, associated with the outer solar nebula. We propose that a plausible solution to the enigma of Jupiter's uniform enrichment pattern is that Jupiter's core formed exterior to the N2 and Ar snowlines, beyond 30 au, resulting in a Solar composition core in all volatiles heavier than Ne. During envelope accretion and planetesimal bombardment, some of the core mixed in with the envelope causing the observed enrichment pattern. We show that this scenario naturally produces the observed atmosphere composition, even with substantial pollution from N-poor pebble and planetesimal accretion in Jupiter's final feeding zone. We note that giant core formation at large nebular radii is consistent with recent models of gas giant core formation through pebble accretion, which requires the core to form exterior to Jupiter's current location to counter rapid inward migration during the core and envelope formation process. If this scenario is common, gas giant core formation may account for many of the gaps observed in protoplanetary disks between 10s and 100 au., Comment: Accepted for publication by ApJ

Despite surface liquid water's importance to habitability, observationally diagnosing its presence or absence on exoplanets is still an open problem. Inspired within the Solar System by the differing sulfur cycles on Venus and Earth, we investigate thick sulfate (H2SO4-H2O) aerosol haze and high trace mixing ratios of SO2 gas as observable atmospheric features whose sustained existence is linked to the near absence of surface liquid water. We examine the fundamentals of the sulfur cycle on a rocky planet with an ocean and an atmosphere in which the dominant forms of sulfur are SO2 gas and H2SO4-H2O aerosols (as on Earth and Venus). We build a simple but robust model of the wet, oxidized sulfur cycle to determine the critical amounts of sulfur in the atmosphere-ocean system required for detectable levels of SO2 and a detectable haze layer. We demonstrate that for physically realistic ocean pH values (pH > 6) and conservative assumptions on volcanic outgassing, chemistry, and aerosol microphysics, surface liquid water reservoirs with greater than 1e-3 Earth oceans are incompatible with a sustained observable H2SO4-H2O haze layer and sustained observable levels of SO2. Thus, we propose the observational detection of a H2SO4-H2O haze layer and of SO2 gas as two new remote indicators that a planet does not host significant surface liquid water., Comment: published in ApJ; associated code at https://github.com/kaitlyn-loftus/alien-sulfur-cycles

The low temperatures and high ultraviolet (UV) radiation levels at the surface of Mars today currently preclude the survival of life anywhere except perhaps in limited subsurface niches. Several ideas for making the martian surface more habitable have been put forward previously, but they all involve massive environmental modification that will be well beyond human capability for the foreseeable future. Here we present a new approach to this problem. We show that widespread regions of the surface of Mars could be made habitable to photosynthetic life in the future via a solid-state analogue to Earth's atmospheric greenhouse effect. Specifically, we demonstrate via experiments and modelling that under martian environmental conditions, a 2 to 3-cm thick layer of silica (SiO2) aerogel will simultaneously transmit sufficient visible light for photosynthesis, block hazardous ultraviolet radiation, and raise temperatures underneath permanently to above the melting point of water, without the need for any internal heat source. Placing silica aerogel shields over sufficiently ice-rich regions of the martian surface could therefore allow photosynthetic life to survive there with minimal subsequent intervention. This regional approach to making Mars habitable is much more achievable than global atmospheric modification. In addition, it can be developed systematically starting from minimal resources, and can be further tested in extreme environments on Earth today., Comment: 15 pages, 5 figures, published in Nature Astronomy July 15, 2019

Low-gravity waterworlds ($M\lesssim 0.1 M_{\oplus}$) are of interest for their potential habitability. The weakly bound atmospheres of such worlds have proportionally larger radiative surfaces and are more susceptible to escape. We conduct a unified investigation into these phenomena, combining analytical energy balance and hydrodynamic escape with line-by-line radiative transfer calculations. Because outgoing radiation is forced to increase with surface temperature by the expansion of the radiative surface, we find that these worlds do not experience a runaway greenhouse. Furthermore, we show that a long-lived liquid water habitable zone is possible for low-gravity waterworlds of sufficient mass. Its inner edge is set by the rate of atmospheric escape, because a short-lived atmosphere limits the time available for life to evolve. In describing the physics of the parameter space transition from "planet-like" to "comet-like", our model produces a lower bound for habitability in terms of gravity. These results provide valuable insights in the context of the ongoing hunt for habitable exoplanets and exomoons., Comment: published in ApJ; text updated to match published version

The radii and orbital periods of 4000+ confirmed/candidate exoplanets have been precisely measured by the Kepler mission. The radii show a bimodal distribution, with two peaks corresponding to smaller planets (likely rocky) and larger intermediate-size planets, respectively. While only the masses of the planets orbiting the brightest stars can be determined by ground-based spectroscopic observations, these observations allow calculation of their average densities placing constraints on the bulk compositions and internal structures. Yet an important question about the composition of planets ranging from 2 to 4 Earth radii still remains. They may either have a rocky core enveloped in a H2-He gaseous envelope (gas dwarfs) or contain a significant amount of multi-component, H2O-dominated ices/fluids (water worlds). Planets in the mass range of 10-15 Earth masses, if half-ice and half-rock by mass, have radii of 2.5 Earth radii, which exactly match the second peak of the exoplanet radius bimodal distribution. Any planet in the 2-4 Earth radii range requires a gas envelope of at most a few mass percentage points, regardless of the core composition. To resolve the ambiguity of internal compositions, we use a growth model and conduct Monte Carlo simulations to demonstrate that many intermediate-size planets are water worlds., Comment: PNAS link: https://www.pnas.org/content/116/20/9723 Complete data and mass-radius tables are available at: https://www.cfa.harvard.edu/~lzeng/planetmodels.html

We are now on a clear trajectory for improvements in exoplanet observations that will revolutionize our ability to characterize their atmospheric structure, composition, and circulation, from gas giants to rocky planets. However, exoplanet atmospheric models capable of interpreting the upcoming observations are often limited by insufficiencies in the laboratory and theoretical data that serve as critical inputs to atmospheric physical and chemical tools. Here we provide an up-to-date and condensed description of areas where laboratory and/or ab initio investigations could fill critical gaps in our ability to model exoplanet atmospheric opacities, clouds, and chemistry, building off a larger 2016 white paper, and endorsed by the NAS Exoplanet Science Strategy report. Now is the ideal time for progress in these areas, but this progress requires better access to, understanding of, and training in the production of spectroscopic data as well as a better insight into chemical reaction kinetics both thermal and radiation-induced at a broad range of temperatures. Given that most published efforts have emphasized relatively Earth-like conditions, we can expect significant and enlightening discoveries as emphasis moves to the exotic atmospheres of exoplanets., Comment: Submitted as an Astro2020 Science White Paper

Exploring diverse planetary atmospheres requires modeling tools that are both accurate and flexible. Here, we develop a three-dimensional general circulation model (3D GCM) that for the first time uses a line-by-line approach to describe the radiative transfer. We validate our GCM by comparing with published results done by different 1D and 3D models. To demonstrate the versatility of the model, we apply the GCM to the hot Earth-sized exoplanet GJ 1132b and study its climate and circulation assuming an atmosphere dominated by abiotic oxygen (O$_2$). Our simulations show that a minor CO$_2$ composition can change the circulation pattern substantially, intensifying the equatorial superrotation in particular. Computation of the phase-resolved spectroscopy indicates that the vertical profile of the superrotating jet could be inferred in future spectro-photometric observations by the phase shift of the hotspot in the CO$_2$ principle absorption band centered at 667 cm$^{-1}$. We also show that atmospheric mass could potentially be constrained by the phase amplitude in the O$_2$ vibrational fundamental band for planets with O$_2$-rich atmospheres, although further experimental and/or theoretical O$_2$-O$_2$ collision-induced absorption data at high temperatures is needed to confirm this. More physical schemes such as moist dynamics will be implemented in the GCM in the future so that it can be used to tackle a wide variety of planetary climate problems., Comment: accepted for publication in ApJ

While recently discovered exotic new planet-types have both challenged our imaginations and broadened our knowledge of planetary system workings, perhaps the most compelling objective of exoplanet science is to detect and characterize habitable and possibly inhabited worlds orbiting in other star systems. For the foreseeable future, characterizations of extrasolar planets will be made via remote sensing of planetary spectroscopic and temporal signals, along with careful fitting of this data to advanced models of planets and their atmospheres. Terrestrial planets are small and significantly more challenging to observe compared to their larger gaseous brethren; however observatories coming on-line in the coming decade will begin to allow their characterization. Still, it is not enough to invest only in observational endeavors. Comprehensive modeling of planetary atmospheres is required in order to fully understand what it is that our grand telescopes see in the night-sky. In our quest to characterize habitable, and possibly inhabited worlds, 3D general circulation models (GCMs) should be used to evaluate potential climate states and their associated temporal and spatial dependent observable signals. 3D models allow for coupled, self-consistent, multi-dimensional simulations, which can realistically simulate the climates of terrestrial extrasolar planets. A complete theoretical understanding of terrestrial exoplanetary atmospheres, gained through comprehensive 3D modeling, is critical for interpreting spectra of exoplanets taken from current and planned instruments, and is critical for designing future missions that aim to measure spectra of potentially habitable exoplanets as one of their key science goals. We recommend continued institutional support for 3D GCM modeling teams that focus on planetary and exoplanetary applications., Comment: NAS ASTRO 2020, White Paper! 2 figures, and 10 pages

The habitable zone (HZ) is the region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. The classical HZ definition makes a number of assumptions common to the Earth, including assuming that the most important greenhouse gases for habitable planets are CO2 and H2O, habitable planets orbit main-sequence stars, and that the carbonate-silicate cycle is a universal process on potentially habitable planets. Here, we discuss these and other predictions for the habitable zone and the observations that are needed to test them. We also, for the first time, argue why A-stars may be interesting HZ prospects. Instead of relying on unverified extrapolations from our Earth, we argue that future habitability studies require first principles approaches where temporal, spatial, physical, chemical, and biological systems are dynamically coupled. We also suggest that next-generation missions are only the beginning of a much more data-filled era in the not-too-distant future, when possibly hundreds to thousands of HZ planets will yield the statistical data we need to go beyond just finding habitable zone planets to actually determining which ones are most likely to exhibit life., Comment: White paper submitted to the NAS Astro 2020 decadal survey(5 pages, 1 figure + cover page, 13 co-authors + 10 co-signers). First author name added to description. In this arxiv version, one more co-signer was added after the NAS version was submitted. Link: http://surveygizmoresponseuploads.s3.amazonaws.com/fileuploads/623127/4458621/64-2dc3fcc201a3075e3da6233f1a91c668_RamirezRamsesM.pdf