Context. Current observation techniques are able to probe the atmosphere of some giant exoplanets and get some clues about their atmospheric composition. However, the chemical compositions derived ...from observations are not fully understood. For instance, the CH4/CO abundance ratio is often inferred to be different from the value that has been predicted by chemical models. Recently, the warm Neptune GJ 3470b has been discovered, and because of its close distance from us and high transit depth, it is a very promising candidate for follow-up characterisation of its atmosphere. Aims. We study the atmospheric composition of GJ 3470b to compare to the current observations of this planet and to prepare for future ones but also to understand the chemical composition of warm (sub-)Neptunes as a typical case study. The metallicity of such atmospheres is totally uncertain and are likely to vary to values up to 100× solar. We explore the space of unknown parameters to predict the range of possible atmospheric compositions. Methods. We use a one-dimensional chemical code to compute a grid of models with various thermal profiles, metallicities, eddy diffusion coefficient profiles, and stellar UV incident fluxes. Thanks to a radiative transfer code, we then compute the corresponding emission and transmission spectra of the planet and compare them with the observational data already published. Results. Within the parameter space explored we find that methane is the major carbon-bearing species in most cases. We, however, find that for high metallicities with a sufficiently high temperature, the CH4/CO abundance ratio can become lower than unity, as suggested by some multiwavelength photometric observations of other warm (sub-)Neptunes, such as GJ 1214b and GJ 436b. As for the emission spectrum of GJ 3470b, brightness temperatures at infrared wavelengths may vary between 400 and 800 K depending on the thermal profile and metallicity. Conclusions. Combined with a hot temperature profile, a substantial enrichment in heavy elements by a factor of ≥100 with respect to the solar composition can shift the carbon balance in favour of carbon monoxide at the expense of methane. Nevertheless, current observations of this planet do not allow us yet to determine which model is more accurate.
Abstract
Ongoing observing projects like the James Webb Space Telescope and future missions offer the chance to characterize Earth-like exoplanetary atmospheres. Thereby, M dwarfs are preferred ...targets for transit observations, for example, due to their favorable planet–star contrast ratio. However, the radiation and particle environment of these cool stars could be far more extreme than what we know from the Sun. Thus, knowing the stellar radiation and particle environment and its possible influence on detectable biosignatures—in particular, signs of life like ozone and methane—is crucial to understanding upcoming transit spectra. In this study, with the help of our unique model suite INCREASE, we investigate the impact of a strong stellar energetic particle event on the atmospheric ionization, neutral and ion chemistry, and atmospheric biosignatures of TRAPPIST-1e. Therefore, transit spectra for six scenarios are simulated. We find that a Carrington-like event drastically increases atmospheric ionization and induces substantial changes in ion chemistry and spectral transmission features: all scenarios show high event-induced amounts of nitrogen dioxide (i.e., at 6.2
μ
m), a reduction of the atmospheric transit depth in all water bands (i.e., at 5.5–7.0
μ
m), a decrease of the methane bands (i.e., at 3.0–3.5
μ
m), and depletion of ozone (i.e., at ∼9.6
μ
m). Therefore, it is essential to include high-energy particle effects to correctly assign biosignature signals from, e.g., ozone and methane. We further show that the nitric acid feature at 11.0–12.0
μ
m, discussed as a proxy for stellar particle contamination, is absent in wet-dead atmospheres.
Abstract
Carbon monoxide (CO) is predicted to be the dominant carbon-bearing molecule in giant planet atmospheres and, along with water, is important for discerning the oxygen and therefore ...carbon-to-oxygen ratio of these planets. The fundamental absorption mode of CO has a broad, double-branched structure composed of many individual absorption lines from 4.3 to 5.1
μ
m, which can now be spectroscopically measured with JWST. Here we present a technique for detecting the rotational sub-band structure of CO at medium resolution with the NIRSpec G395H instrument. We use a single transit observation of the hot Jupiter WASP-39b from the JWST Transiting Exoplanet Community Early Release Science (JTEC ERS) program at the native resolution of the instrument (
R
∼ 2700) to resolve the CO absorption structure. We robustly detect absorption by CO, with an increase in transit depth of 264 ± 68 ppm, in agreement with the predicted CO contribution from the best-fit model at low resolution. This detection confirms our theoretical expectations that CO is the dominant carbon-bearing molecule in WASP-39b’s atmosphere and further supports the conclusions of low C/O and supersolar metallicities presented in the JTEC ERS papers for WASP-39b.
We propose an interpretation of the composition of volatiles observed in comets based on their trapping in the form of clathrate hydrates in the solar nebula. The formation of clathrates is ...calculated from the statistical thermodynamics of
Lunine and Stevenson (1985, Astrophys. J. Suppl. 58, 493–531), and occurs in an evolutionary turbulent solar nebula described by the model of
Hersant et al. (2001, Astrophys. J. 554, 391–407). It is assumed that clathrate hydrates were incorporated into the icy grains that formed cometesimals. The strong depletion of the N
2 molecule with respect to CO observed in some comets is explained by the fact that CO forms clathrate hydrates much more easily than does N
2. The efficiency of this depletion, as well as the amount of trapped CO, depends upon the amount of water ice available in the region where the clathration took place. This might explain the diversity of CO abundances observed in comets. The same theory, applied to the trapping of volatiles around 5 AU, explains the enrichments in Ar, Kr, Xe, C, and N with respect to the solar abundance measured in the deep troposphere of Jupiter
(Gautier et al., 2001a,b).
Context. The atmosphere of hot Jupiters can be probed by primary transit and secondary eclipse spectroscopy. Owing to the intense UV irradiation, mixing, and circulation, their chemical composition ...is maintained out of equilibrium and must be modeled with kinetic models. Aims. Our purpose is to release a chemical network and the associated rate coefficients, developed for the temperature and pressure range relevant to hot Jupiters atmospheres. Using this network, we study the vertical atmospheric composition of the two hot Jupiters (HD 209458b and HD 189733b) with a model that includes photolyses and vertical mixing, and we produce synthetic spectra. Methods. The chemical scheme has been derived from applied combustion models that were methodically validated over a range of temperatures and pressures typical of the atmospheric layers influencing the observations of hot Jupiters. We compared the predictions obtained from this scheme with equilibrium calculations, with different schemes available in the literature that contain N-bearing species, and with previously published photochemical models. Results. Compared to other chemical schemes that were not subjected to the same systematic validation, we find significant differences whenever nonequilibrium processes take place (photodissociations or vertical mixing). The deviations from the equilibrium, hence the sensitivity to the network, are larger for HD 189733b, since we assume a cooler atmosphere than for HD 209458b. We found that the abundances of NH3 and HCN can vary by two orders of magnitude depending on the network, demonstrating the importance of comprehensive experimental validation. A spectral feature of NH3 at 10.5 μm is sensitive to these abundance variations and thus to the chemical scheme. Conclusions. Due to the influence of the kinetics, we recommend using a validated scheme to model the chemistry of exoplanet atmospheres. The network we release is robust for temperatures within 300–2500 K and pressures from 10 mbar up to a few hundred bars, for species made of C, H, O, and N. It is validated for species up to 2 carbon atoms and for the main nitrogen species (NH3, HCN, N2, NOx). Although the influence of the kinetic scheme on the hot Jupiters spectra remains within the current observational error bars (with the exception of NH3), it will become more important for atmospheres that are cooler or subjected to higher UV fluxes, because they depart more from equilibrium.
The dissipation of the tidal energy deposited on eccentric planets may induce a heating of the planet that affects its atmospheric thermal structure. Here we study the influence of tidal heating on ...the atmospheric composition of the eccentric (e = 0.16) "hot Neptune" GJ 436b, for which inconclusive chemical abundances are retrieved from multiwavelength photometric observations carried out during primary transit and secondary eclipse. We build up a one-dimensional model of GJ 436b's atmosphere in the vertical direction and compute the pressure-temperature and molecular abundances profiles for various plausible internal temperatures of the planet (up to 560 K) and metallicities (from solar to 100 times solar), using a radiative-convective model and a chemical model which includes thermochemical kinetics, vertical mixing, and photochemistry. We find that the CO/CH sub(4) abundance ratio increases with metallicity and tidal heating, and ranges from 1/20 to 1000 within the ranges of metallicity and internal temperature explored. Water vapor locks most of the oxygen and reaches a very high abundance, whatever the metallicity and internal temperature of the planet. The CO sub(2)/H sub(2)O abundance ratio increases dramatically with metallicity, and takes values between 10 super(-5)-10 super(-4) with solar elemental abundances and ~0.1 for a metallicity 100 times solar. None of the atmospheric models based on solid physical and chemical grounds provide a fully satisfactory agreement with available observational data, although the comparison of calculated spectra and observations seems to point to models with a high metallicity and efficient tidal heating, in which high CO/CH sub(4) abundance ratios and warm temperatures in the dayside atmosphere are favored.
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