CO2-dominated Atmosphere Research Articles

Potential climates and habitability on Gl 514 b: a super-Earth exoplanet with high eccentricity

ABSTRACT The recently discovered super-Earth Gl 514 b, orbiting a nearby M0.5-1.0 star at 7.6 pc, is one of the best benchmark exoplanets for understanding the potential climate states of eccentric planets. The elongated (e = 0.45$^{+0.15}_{-0.14}$) orbit of Gl 514 b, which only partially lies in the Conservative Habitable Zone, suggests a dynamically young system, where the spin-orbit tidal synchronization may not have yet occurred up to the present time. In the present work, we use a seasonal-latitudinal energy balance model, EOS-ESTM, to explore the potential impact of both constrained and unconstrained planetary, orbital, and atmospheric parameters on the Gl 514 b habitability, mapped in terms of surface temperature. We test three distinct CO2-dominated atmospheres by varying the CH4 concentration values (0 per cent, 0.1 per cent, and 1 per cent) and the total surface pressure. As a general trend, we find that habitable conditions are favoured by high-CH4 and high-pressure regimes. Habitability also increases for high-axis obliquities (at least until the appearance of an icebelt), long-rotation periods, and high-ocean fractional coverage. If the ocean fraction is low, then also the argument of periastron becomes relevant. Our results are robust against changes of the continental distribution. Thus, we conclude that Gl 514 b can potentially maintain temperate surface conditions with modest seasonal temperature variations under a wide variety of planetary, orbital, and atmospheric conditions. Despite no transit have been detected yet, the results found in this work should motivate the community to invest time in future observations.

Atmospheric formaldehyde production on early Mars leading to a potential formation of bio-important molecules

Formaldehyde (H2CO) is a critical precursor for the abiotic formation of biomolecules, including amino acids and sugars, which are the building blocks of proteins and RNA. Geomorphological and geochemical evidence on Mars indicates a temperate environment compatible with the existence of surface liquid water during its early history at 3.8–3.6 billion years ago (Ga), which was maintained by the warming effect of reducing gases, such as H2. However, it remains uncertain whether such a temperate and weakly reducing surface environment on early Mars was suitable for producing H2CO. In this study, we investigated the atmospheric production of H2CO on early Mars using a 1-D photochemical model assuming a thick CO2-dominated atmosphere with H2 and CO. Our results show that a continuous supply of atmospheric H2CO can be used to form various organic compounds, including amino acids and sugars. This could be a possible origin for the organic matter observed on the Martian surface. Given the previously reported conversion rate from H2CO into ribose, the calculated H2CO deposition flux suggests a continuous supply of bio-important sugars on early Mars, particularly during the Noachian and early Hesperian periods.

The Carbon-deficient Evolution of TRAPPIST-1c

Transiting planets orbiting M dwarfs provide the best opportunity to study the atmospheres of rocky planets with current facilities. As JWST enters its second year of science operations, an important initial endeavor is to determine whether these rocky planets have atmospheres at all. M dwarfs are thought to pose a major threat to planetary atmospheres due to their high magnetic activity over timescales of several billion years, and might completely strip atmospheres. Several Cycle 1 and 2 General Observers and Guaranteed Time Observations programs are testing this hypothesis, observing a series of rocky planets to determine whether they retained their atmospheres. A key case study is TRAPPIST-1c, which receives almost the same bolometric flux as Venus. We might therefore expect TRAPPIST-1c to possess a thick, CO2-dominated atmosphere. Instead, Zieba et al. show that it has little to no CO2 in its atmosphere. To interpret these results, we run coupled time-dependent simulations of planetary outgassing and atmospheric escape to model the evolution of TRAPPIST-1c's atmosphere. We find that the stellar wind stripping that is expected to occur on TRAPPIST-1c over its lifetime can only remove up to ∼16 bar of CO2, less than the modern CO2 inventory of either Earth or Venus. Therefore, we infer that TRAPPIST-1c either formed volatile-poor, as compared to Earth and Venus, or lost a substantial amount of CO2 during an early phase of hydrodynamic hydrogen escape. Finally, we scale our results for the other TRAPPIST-1 planets, finding that the more distant TRAPPIST-1 planets may readily retain atmospheres.

Seasonal Thaws under Mid- to Low-pressure Atmospheres on Early Mars

Abstract Despite decades of scientific research on the subject, the climate of the first 1.5 Gyr of Mars' history has not been fully understood yet. Especially challenging is the need to reconcile the presence of liquid water for extended periods of time on the Martian surface with the comparatively low insolation received by the planet, a problem which is known as the Faint Young Sun paradox. In this paper, we use the Earth-like planet surface-temperature model (or ESTM), a latitudinal energy-balance model with enhanced prescriptions for meridional heat diffusion, and the radiative-transfer code EOS to investigate how seasonal variations of temperature can give rise to local conditions which are conducive to liquid-water runoffs. We include the effects of the Martian dichotomy, a northern ocean with either 150 or 550 m of global equivalent layer, and simplified CO2 or H2O clouds. We find that 1.3–2.0 bar CO2-dominated atmospheres can produce seasonal thaws due to inefficient heat redistribution, provided that the eccentricity and the obliquity of the planet are sufficiently different from zero. We also studied the impact of different values for the argument of perihelion. When local favorable conditions exist, they nearly always persist for >15% of the Martian year. These results are obtained without the need for additional greenhouse gases (e.g., H2, CH4) or transient heat-injecting phenomena (e.g., asteroid impacts, volcanic eruptions). A moderate amount (0.1%–1%) of CH4 significantly widens the parameter space region in which seasonal thaws are possible.

Measurements of sound propagation in Mars' lower atmosphere

Acoustics has become extraterrestrial and Mars provides a new natural laboratory for testing sound propagation models compared to those ones on Earth. Owing to the unique combination of a microphone and two sound sources, the Ingenuity helicopter and the SuperCam laser-induced sparks, the Mars 2020 Perseverance rover payload enables the in situ characterization of unique sound propagation properties of the low-pressure CO2-dominated Mars atmosphere. In this study, we show that atmospheric turbulence is responsible for a large variability in the sound amplitudes from laser-induced sparks. This variability follows the diurnal pattern of turbulence. In addition, acoustic measurements acquired over one Martian year reveal a variation of the sound intensity by a factor of 1.8 from a constant source due to the seasonal cycle of pressure and temperature that significantly modifies the acoustic impedance and shock-wave formation. Finally, we show that the evolution of the Ingenuity tones and laser spark amplitudes with distance is consistent with one of the existing sound absorption models, which is a key parameter for numerical simulations applied to geophysical experiments on CO2-rich atmospheres. Overall, these results demonstrate the potential of sound propagation to interrogate the Mars environment and will therefore help in the design of future acoustic-based experiments for Mars or other planetary atmospheres such as Venus and Titan.

N-waves propagating through the Mars atmosphere

One of the particular interest of the SuperCam microphone onboard the NASA’s Perseverance rover is that it is coupled with its own reproducible sound source, the blast waves generated by the SuperCam laser-induced plasma expansion. This unique emission-reception pair on Mars is used to investigate the propagation of spark-induced N-waves through the turbulent CO2-dominated Mars atmosphere. The analysis of waveforms shows large distortions over a burst of 30 subsequent shots, which is consistent with previous laboratory recordings of N-waves through thermal turbulence. Especially, this distortion leads to a scattering of the peak pressure amplitude, whose distribution can be fitted with a generalized gamma function. For bursts recorded farther than 4 m, the distribution gets asymmetric with a shift of the maximum toward the left of the median due to peaks amplified more than twice the median amplitude, likely due to caustics that randomly focus the acoustic wave. On the other hand, over a Martian year, the absolute peak pressure varies by ∼50% due to changes in the acoustic impedance and Rankine-Hugoniot conditions, due to the seasonal cycles of pressure and temperature at the surface of Mars.

A new method for finding nearby white dwarfs exoplanets and detecting biosignatures

ABSTRACT We demonstrate that the James Webb Space Telescope (JWST) can detect infrared (IR) excess from the blended light spectral energy distribution of spatially unresolved terrestrial exoplanets orbiting nearby white dwarfs. We find that JWST is capable of detecting warm (habitable-zone; Teq = 287 K) Earths or super-Earths and hot (400–1000 K) Mercury analogues in the blended light spectrum around the nearest 15 isolated white dwarfs with 10 h of integration per target using MIRI’s medium-resolution spectrograph (MRS). Further, these observations constrain the presence of a CO2-dominated atmosphere on these planets. The technique is nearly insensitive to system inclination, and thus observation of even a small sample of white dwarfs could place strong limits on the occurrence rates of warm terrestrial exoplanets around white dwarfs in the solar neighbourhood. We find that JWST can also detect exceptionally cold (100–150 K) Jupiter-sized exoplanets via MIRI broad-band imaging at $\lambda = 21\, \mathrm{\mu m}$ for the 34 nearest (<13 pc) solitary white dwarfs with 2 h of integration time per target. Using IR excess to detect thermal variations with orbital phase or spectral absorption features within the atmosphere, both of which are possible with long-baseline MRS observations, would confirm candidates as actual exoplanets. Assuming an Earth-like atmospheric composition, we find that the detection of the biosignature pair O3+CH4 is possible for all habitable-zone Earths (within 6.5 pc; six white dwarf systems) or super-Earths (within 10 pc; 17 systems) orbiting white dwarfs with only 5–36 h of integration using MIRI’s low-resolution spectrometer.

The TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI). I. Dry Cases—The Fellowship of the GCMs

With the commissioning of powerful, new-generation telescopes such as the James Webb Space Telescope (JWST) and the ground-based Extremely Large Telescopes, the first characterization of a high molecular weight atmosphere around a temperate rocky exoplanet is imminent. Atmospheric simulations and synthetic observables of target exoplanets are essential to prepare and interpret these observations. Here we report the results of the first part of the TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) project, which compares 3D numerical simulations performed with four state-of-the-art global climate models (ExoCAM, LMD-Generic, ROCKE-3D, Unified Model) for the potentially habitable target TRAPPIST-1e. In this first part, we present the results of dry atmospheric simulations. These simulations serve as a benchmark to test how radiative transfer, subgrid-scale mixing (dry turbulence and convection), and large-scale dynamics impact the climate of TRAPPIST-1e and consequently the transit spectroscopy signature as seen by JWST. To first order, the four models give results in good agreement. The intermodel spread in the global mean surface temperature amounts to 7 K (6 K) for the N2-dominated (CO2-dominated) atmosphere. The radiative fluxes are also remarkably similar (intermodel variations less than 5%), from the surface (1 bar) up to atmospheric pressures ∼5 mbar. Moderate differences between the models appear in the atmospheric circulation pattern (winds) and the (stratospheric) thermal structure. These differences arise between the models from (1) large-scale dynamics, because TRAPPIST-1e lies at the tipping point between two different circulation regimes (fast and Rhines rotators) in which the models can be alternatively trapped, and (2) parameterizations used in the upper atmosphere such as numerical damping.

Reflected Spectroscopy of Small Exoplanets II: Characterization of Terrestrial Exoplanets

A space telescope capable of high-contrast imaging has been recognized as the avenue toward finding terrestrial planets around nearby Sun-like stars and characterizing their potential habitability. It is thus essential to quantify the capability of reflected light spectroscopy obtained through direct imaging for terrestrial exoplanets, and existing work focused on planetary analogs of modern Earth. Here we go beyond Earth analogs and use a Bayesian retrieval algorithm, ExoReL , to determine what we could learn about terrestrial exoplanets from their reflected light spectra. Recognizing the potential diversity of terrestrial exoplanets, our focus is to distinguish atmospheric scenarios without any a priori knowledge of the dominant gas. We find that, while a moderate-resolution spectrum in the optical band (0.4−1.0 μm) may sufficiently characterize a modern Earth analog, it would likely result in incorrect interpretation for planets similar to Archean Earth or having CO2-dominated atmospheres. Including observations in the near-infrared bands (1.0−1.8 μm) can prevent this error, determine the main component (N2, O2, or CO2), and quantify trace gases (H2O, O3, and CH4) of the atmosphere. These results are useful to define the science requirements and design the wavelength bandwidth and observation plans of exoplanet direct imaging missions in the future.

Absorption of methane broadened by carbon dioxide in the 3.3 μm spectral region: From line centers to the far wings

This work studied the infrared absorption of methane broadened by carbon dioxide, which can contribute to the radiative budget of CO2-dominated atmospheres. Fourier transform absorption spectra of CH4 perturbed by CO2 were recorded in the 3.3 μm spectral region, at room temperature and total pressures ranging from 3 to 25 bars. These experimental data were modeled using a theoretical approach taking collision-induced line mixing into account. Comparisons between measured and calculated spectra demonstrate that the proposed model is capable of accurately representing the absorption of methane broadened by collisions with CO2, from line centers to the far wings. For practical applications, this rigorous spectral modeling was used to derive a simple χ-factor model to represent the spectral shape of CO2-broadened CH4 line wings. Comparisons with experimental values show that, at room temperature, the proposed χ-factor reproduces the measured spectra to within 10% in the band wings where the absorption is mainly due to the far line wings, while the usual Lorentzian profile leads to relative differences several orders of magnitude larger. This line-shape correction was successfully validated through comparisons with heavily saturated spectra of the ν3 band of methane recorded at sub-atmospheric pressures.

Mars soundscape: Review of the first sounds recorded by the Perseverance microphones

On February 18, 2021, NASA’s Perseverance rover landed in Jezero Crater carrying the two first microphones operating on the surface of Mars: the SuperCam microphone, positioned on top of the rotating rover’s mast and the EDL microphone fixed on the body of the rover. Working flawlessly since then, they provide the first characterization of Mars’ acoustic environment in the audible range and beyond, from 20 Hz to 50 kHz. Recorded sounds originate from three main sources: the atmosphere (turbulence, wind), the shock-waves generated by the Supercam pulsed laser ablating rocks, and hardware-induced artificial sounds such as the signal generated by the high-speed rotating blades of the Ingenuity helicopter. After one year, the Perseverance playlist features more than 5 hours of martian sounds. In addition to providing an unprecedented short timescale characterization of the wind, temperature fluctuations, and the turbulence dissipative regime, this dataset highlights the unique sound propagation properties of the low-pressure CO2-dominated Mars atmosphere: acoustic impedance varying with the season, large intrinsic attenuation of the high frequencies, and the dispersion of the sound speed in the audible range. This presentation will review these results to date.

In situ recording of Mars soundscape

Before the Perseverance rover landing, the acoustic environment of Mars was unknown. Models predicted that: (1) atmospheric turbulence changes at centimetre scales or smaller at the point where molecular viscosity converts kinetic energy into heat1, (2) the speed of sound varies at the surface with frequency2,3 and (3) high-frequency waves are strongly attenuated with distance in CO2 (refs. 2–4). However, theoretical models were uncertain because of a lack of experimental data at low pressure and the difficulty to characterize turbulence or attenuation in a closed environment. Here, using Perseverance microphone recordings, we present the first characterization of the acoustic environment on Mars and pressure fluctuations in the audible range and beyond, from 20 Hz to 50 kHz. We find that atmospheric sounds extend measurements of pressure variations down to 1,000 times smaller scales than ever observed before, showing a dissipative regime extending over five orders of magnitude in energy. Using point sources of sound (Ingenuity rotorcraft, laser-induced sparks), we highlight two distinct values for the speed of sound that are about 10 m s−1 apart below and above 240 Hz, a unique characteristic of low-pressure CO2-dominated atmosphere. We also provide the acoustic attenuation with distance above 2 kHz, allowing us to explain the large contribution of the CO2 vibrational relaxation in the audible range. These results establish a ground truth for the modelling of acoustic processes, which is critical for studies in atmospheres such as those of Mars and Venus.

A nitrogen-rich atmosphere on ancient Mars consistent with isotopic evolution models

The ratio of nitrogen isotopes in the Martian atmosphere is a key constraint on the planet's atmospheric evolution. However, enrichment of the heavy isotope expected due to atmospheric loss from sputtering and photochemical processes is greater than measurements. A massive, multi-bar early CO2-dominated atmosphere and recent volcanic outgassing have been proposed to explain this discrepancy, and many previous models have assumed atmospheric nitrogen rapidly reached a steady state where loss to space balanced volcanic outgassing. Here we show using time-dependent models that the abundance and isotopic composition of nitrogen in the Martian atmosphere can be explained by a family of evolutionary scenarios in which the initial partial pressure of nitrogen is sufficiently high that a steady state is not reached and nitrogen levels gradually decline to present-day values over 4 billion years. Our solutions do not require a multi-bar early CO2 atmosphere and are consistent with volcanic outgassing indicated by both geologic mapping and the atmospheric 36Ar/38Ar ratio. Monte Carlo simulations that include these scenarios estimate that the partial pressure of N2 was 60 - 740 mbar (90% confidence, with a median value of 310 mbar) at 3.8 billion years ago when the valley networks formed. We suggest that such a high nitrogen partial pressure could have contributed substantially to warming on early Mars.

Circumpolar ocean stability on Mars 3 Gy ago

What was the nature of the Late Hesperian climate, warm and wet or cold and dry? Formulated this way the question leads to an apparent paradox since both options seem implausible. A warm and wet climate would have produced extensive fluvial erosion but few valley networks have been observed at the age of the Late Hesperian. A too cold climate would have kept any northern ocean frozen most of the time. A moderate cold climate would have transferred the water from the ocean to the land in the form of snow and ice. But this would prevent tsunami formation, for which there is some evidence. Here, we provide insights from numerical climate simulations in agreement with surface geological features to demonstrate that the Martian climate could have been both cold and wet. Using an advanced general circulation model (GCM), we demonstrate that an ocean can be stable, even if the Martian mean surface temperature is lower than 0°C. Rainfall is moderate near the shorelines and in the ocean. The southern plateau is mostly covered by ice with a mean temperature below 0°C and a glacier return flow back to the ocean. This climate is achieved with a 1-bar CO2-dominated atmosphere with 10% H2 Under this scenario of 3 Ga, the geologic evidence of a shoreline and tsunami deposits along the ocean/land dichotomy are compatible with ice sheets and glacial valleys in the southern highlands.

Exosphere Modeling of Proxima b: A Case Study of Photochemical Escape with a Venus-like Atmosphere

Abstract Exoplanets orbiting M dwarfs within habitable zones are exposed to stellar environments more extreme than that terrestrial planets experience in our solar system, which can significantly impact the atmospheres of the exoplanets and affect their habitability and sustainability. This study provides the first prediction of hot oxygen corona structure and the associated photochemical loss from a 1 bar CO2-dominated atmosphere of a Venus-like rocky exoplanet, where dissociative recombination of O2 + ions is assumed to be the major source reaction for the escape of neutral O atoms and formation of the hot O corona (or exospheres) as on Mars and Venus. We employ a 3D Monte Carlo code to simulate the exosphere of Proxima Centauri b (PCb) based on the ionosphere simulated by a 3D magnetohydrodynamic model. Our simulation results show that variability of the stellar wind dynamic pressure over one orbital period of PCb does not affect the overall spatial structure of the hot O corona but contributes to the change in the global hot O escape rate that varies by an order of magnitude. The escape increases dramatically when the planet possesses its intrinsic magnetic fields as the ionosphere becomes more extended with the presence of a global magnetic field. The extended hot O corona may lead to a more extended H exosphere through collisions between thermal H and hot O, which exemplifies the importance of considering nonthermal populations in exospheres to interpret future observations.

Was Venus Ever Habitable? Constraints from a Coupled Interior–Atmosphere–Redox Evolution Model

Abstract Venus’s past climate evolution is uncertain. General circulation model simulations permit a habitable climate as late as ∼0.7 Ga, and there is suggestive—albeit inconclusive—evidence for previous liquid water from surface geomorphology and mineralogy. However, it is unclear whether a habitable past can be reconciled with Venus’s inferred atmospheric evolution. In particular, the lack of leftover atmospheric oxygen argues against recent water loss. Here, we apply a fully coupled model of Venus’s atmospheric–interior–climate evolution from post-accretion magma ocean to present. The model self-consistently tracks C-, H-, and O-bearing volatiles and surface climate through the entirety of Venus’s history. Atmospheric escape, mantle convection, melt production, outgassing, deep water cycling, and carbon cycling are explicitly coupled to climate and redox evolution. Plate tectonic and stagnant lid histories are considered. Using this coupled model, we conclude that both a habitable Venusian past and one where Venus never possessed liquid surface water can be reconciled with known constraints. Specifically, either scenario can reproduce bulk atmospheric composition, inferred surface heat flow, and observed 40Ar and 4He. Moreover, the model suggests that Venus could have been habitable with a ∼100 m global ocean as late as 1 Ga, without violating any known constraints. In fact, if diffusion-limited water loss is throttled by a cool, CO2-dominated upper atmosphere, then a habitable past is tentatively favored by our model. This escape throttling makes it difficult to simultaneously recover negligible water vapor and ∼90 bar CO2 in the modern atmosphere without temporarily sequestering carbon in the interior via silicate weathering to enhance H escape.

Atmospheric Escape From TOI-700 d: Venus versus Earth Analogs

Abstract The recent discovery of an Earth-sized planet (TOI-700 d) in the habitable zone of an early-type M-dwarf by the Transiting Exoplanet Survey Satellite constitutes an important advance. In this Letter, we assess the feasibility of this planet to retain an atmosphere—one of the chief ingredients for surface habitability—over long timescales by employing state-of-the-art magnetohydrodynamic models to simulate the stellar wind and the associated rates of atmospheric escape. We take two major factors into consideration, namely, the planetary atmospheric composition and magnetic field. In all cases, we determine that the atmospheric ion escape rates are potentially a few orders of magnitude higher than the inner solar system planets, but TOI-700 d is nevertheless capable of retaining a 1 bar atmosphere over gigayear timescales for certain regions of the parameter space. The simulations show that the unmagnetized TOI-700 d with a 1 bar Earth-like atmosphere could be stripped away rather quickly (<1 gigayear), while the unmagnetized TOI-700 d with a 1 bar CO2-dominated atmosphere could persist for many billions of years; we find that the magnetized Earth-like case falls in between these two scenarios. We also discuss the prospects for detecting radio emission of the planet (thereby constraining its magnetic field) and discerning the presence of an atmosphere.

Chemically distinct regions of Venus’s atmosphere revealed by measured N2 concentrations

A defining characteristic of the planet Venus is its thick, CO2-dominated atmosphere. Despite over 50 years of robotic exploration of Venus, including thirteen successful atmospheric probes and landers, our knowledge of N2, the second most abundant compound in the atmosphere, is highly uncertain1. We report the first measurement of the nitrogen content of Venus’s atmosphere at altitudes between 60 and 100 km. Our result, 5.0 ± 0.4 v% N2, is 40% higher than the value of 3.5 v% N2 reported for the lower atmosphere (<50 km altitude). Our discovery of altitude-dependent variations in the N2 content of the atmosphere defies early expectations of homogeneous composition below 100 km (for example, see ref. 1), and necessitates complete chemical modelling to investigate chemical versus physical explanations for the enhanced N2 in the upper atmosphere. The existence of chemically distinct lower and upper atmosphere regions complicates the use of remote sensing measurements of the upper atmosphere to infer the properties of the lower atmosphere and surface, an important lesson that also extends to the growing field of exoplanet astronomy. A MESSENGER flyby of Venus has detected a difference in N2 composition between the upper and lower atmosphere. The presence of substantially distinct regions within Venus’s atmosphere has implications on remote sensing techniques for exoplanets.

Observing Isotopologue Bands in Terrestrial Exoplanet Atmospheres with the James Webb Space Telescope: Implications for Identifying Past Atmospheric and Ocean Loss

Abstract Terrestrial planets orbiting M dwarfs may soon be observed with the James Webb Space Telescope (JWST) to characterize their atmospheric composition and search for signs of habitability or life. These planets may undergo significant atmospheric and ocean loss due to the superluminous pre-main-sequence phase of their host stars, which may leave behind abiotically generated oxygen, a false positive for the detection of life. Determining if ocean loss has occurred will help assess potential habitability and whether or not any O2 detected is biogenic. In the solar system, differences in isotopic abundances have been used to infer the history of ocean loss and atmospheric escape (e.g., Venus, Mars). We find that isotopologue measurements using transit transmission spectra of terrestrial planets around late-type M dwarfs like TRAPPIST-1 may be possible with JWST, if the escape mechanisms and resulting isotopic fractionation were similar to Venus. We present analyses of post-ocean-loss O2- and CO2-dominated atmospheres containing a range of trace gas abundances. Isotopologue bands are likely detectable throughout the near-infrared (1–8 μm), especially 3–4 μm, although not in CO2-dominated atmospheres. For Venus-like D/H ratios 100 times that of Earth, TRAPPIST-1b transit signals of up to 79 ppm are possible by observing HDO. Similarly, 18O/16O ratios 100 times that of Earth produce signals at up to 94 ppm. Detection at signal-to-noise ratio = 5 may be attained on these bands with as few as four to 11 transits, with optimal use of JWST’s NIRSpec Prism. Consequently, H2O and CO2 isotopologues could be considered as indicators of past ocean loss and atmospheric escape for JWST observations of terrestrial planets around M dwarfs.

The Role of N2 as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats.

Since the Archean, N2 has been a major atmospheric constituent in Earth's atmosphere. Nitrogen is an essential element in the building blocks of life; therefore, the geobiological nitrogen cycle is a fundamental factor in the long-term evolution of both Earth and Earth-like exoplanets. We discuss the development of Earth's N2 atmosphere since the planet's formation and its relation with the geobiological cycle. Then we suggest atmospheric evolution scenarios and their possible interaction with life-forms: first for a stagnant-lid anoxic world, second for a tectonically active anoxic world, and third for an oxidized tectonically active world. Furthermore, we discuss a possible demise of present Earth's biosphere and its effects on the atmosphere. Since life-forms are the most efficient means for recycling deposited nitrogen back into the atmosphere at present, they sustain its surface partial pressure at high levels. Also, the simultaneous presence of significant N2 and O2 is chemically incompatible in an atmosphere over geological timescales. Thus, we argue that an N2-dominated atmosphere in combination with O2 on Earth-like planets within circumstellar habitable zones can be considered as a geo-biosignature. Terrestrial planets with such atmospheres will have an operating tectonic regime connected with an aerobic biosphere, whereas other scenarios in most cases end up with a CO2-dominated atmosphere. We conclude with implications for the search for life on Earth-like exoplanets inside the habitable zones of M to K stars.