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

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 theKband. A nondetection of CO could suggest the existence of an ocean, whereas significant CH(4)hints at the presence of a biosphere. © 2020. The American Astronomical Society. All rights reserved.This research was supported by DFG projects RA-714/7-1, GO 2610/1-1, SCHR 1125/3-1, and RA 714/9-1. We acknowledge the support of the DFG priority program SPP 1992 "Exploring the Diversity of Extrasolar Planets (GO 2610/2-1)." M.L.-P. acknowledges financial support from the State Agency for Research of the Spanish MCIU through project ESP2017-87143-R, the "Center of Excellence Severo Ochoa" award to the IAA-CSIC (SEV-2017-0709), and EC FEDER funds. P.C.S. gratefully acknowledges support by the German Aerospace Center under DLR 50.OR.1901. We thank Michael Gillon for providing the SPECULOOS target list, Franklin Mills for sending cross-section data of several sulfur species, and Vladimir Krasnopolsky for providing chemical profiles of Mars. The SED used in this study is based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program No. 15071. Support for program No. 15071 was provided by NASA through a grant from the Space Telescope Science Institute. We thank the anonymous referee for the helpful and constructive comments.Peer reviewe