Distinguishing between Wet and Dry Atmospheres of TRAPPIST-1 e and f

Abstract

The nearby TRAPPIST-1 planetary system is an exciting target for characterizing the atmospheres of terrestrial planets. The planets e, f, and g lie in the circumstellar habitable zone and could sustain liquid water on their surfaces. During the extended pre-main-sequence phase of TRAPPIST-1, however, the planets may have experienced extreme water loss, leading to a desiccated mantle. The presence or absence of an ocean is challenging to determine with current and next-generation telescopes. Therefore, we investigate whether indirect evidence of an ocean and/or a biosphere can be inferred from observations of the planetary atmosphere. We introduce a newly developed photochemical model for planetary atmospheres, coupled to a radiative-convective model, and validate it against modern Earth, Venus, and Mars. The coupled model is applied to the TRAPPIST-1 planets e and f, assuming different surface conditions and varying amounts of CO 2 in the atmosphere. As input for the model we use a constructed spectrum of TRAPPIST-1, based on near-simultaneous data from X-ray to optical wavelengths. We compute cloud-free transmission spectra of the planetary atmospheres and determine the detectability of molecular features using the Extremely Large Telescope (ELT) and the James Webb Space Telescope (JWST). We find that under certain conditions the existence or nonexistence of a biosphere and/or an ocean can be inferred by combining 30 transit observations with ELT and JWST within the K band. A nondetection of CO could suggest the existence of an ocean, whereas significant CH 4 hints at the presence of a biosphere.

Detectability of biosignatures on LHS 1140 b

“…Grenfell et al (2007); Rauer et al (2011);von Paris et al (2011); Grenfell et al (2013);von Paris et al (2015) and Gebauer et al (2017). Recently a major update of the climate radiative transfer module (called REDFOX; Scheucher et al 2020) and chemistry module (called BLACKWOLF; Wunderlich et al 2020) enabled e.g. CO 2 -and H 2 -dominated atmospheres to be consistently simulated.…”

“…REDFOX includes absorption of 20 molecules 1 using spectroscopic cross sections from the HITRAN 2016 line list (Gordon et al 2017) and 81 molecules using UV and VIS cross sections mainly taken from the MPI Mainz Spectral Atlas (Keller-Rudek et al 2013) as described in Scheucher et al (2020) and Wunderlich et al (2020). Additionally Rayleigh scattering of eight molecules 2 , Mlawer-Tobin-Clough-Kneizys-Davies absorption (MT_CKD; Mlawer et al 2012) Scheucher et al 2020, for details).…”

“…The atmosphere in the climate module is divided into 101 pressure levels and the chemistry module into 100 pressure layers. The eddy diffusion profile can be calculated according to the parameterization shown in Wunderlich et al (2020) or set to a given profile. Unless indicated otherwise, we use a parameterized eddy diffusion profile.…”

“…Unless indicated otherwise, we use a parameterized eddy diffusion profile. The photochemical module accounts for dry and wet deposition, as well as surface emission fluxes and atmospheric escape (see details in Wunderlich et al 2020). For wet deposition we use the parameterization of Giorgi & Chameides (1985) and the tropospheric lightning emissions of nitrogen oxides, NO x (here defined as NO + NO 2 ) are based on the Earth lightning model of Chameides et al (1977).…”

“…In this work we apply the steady-state, cloud-free, radiativeconvective photochemistry model 1D-TERRA (Scheucher et al 2020;Wunderlich et al 2020) together with the theoretical spectral model GARLIC (Schreier et al 2014) to simulate a range of CO 2 , H 2 -He atmospheres (and mixtures thereof) as well as atmospheric spectra for LHS 1140 b. A central aim of our work is to investigate potential atmospheres of this Super-Earth and determine the detectability of key atmospheric features, in particular potential biosignatures, in the context of the forthcoming JWST and Extremely Large Telescope (ELT).…”