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Terrestrial planets orbiting M dwarfs may soon be observed with the JamesWebb Space Telescope (JWST) to characterize their atmospheric composition andsearch for signs of habitability or life. These planets may undergo significantatmospheric and ocean loss due to the superluminous pre-main-sequence phase oftheir host stars, which may leave behind abiotically-generated oxygen, a falsepositive for the detection of life. Determining if ocean loss has occurred willhelp assess potential habitability and whether or not any O2 detected isbiogenic. In the solar system, differences in isotopic abundances have beenused to infer the history of ocean loss and atmospheric escape (e.g. Venus,Mars). We find that isotopologue measurements using transit transmissionspectra of terrestrial planets around late-type M dwarfs like TRAPPIST-1 may bepossible with JWST, if the escape mechanisms and resulting isotopicfractionation 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 um),especially 3-4 um, although not in CO2-dominated atmospheres. For Venus-likeD/H ratios 100 times that of Earth, TRAPPIST-1 b transit signals of up to 79ppm are possible by observing HDO. Similarly, 18O/16O ratios 100 times that ofEarth produce signals at up to 94 ppm. Detection at S/N=5 may be attained onthese bands with as few as four to eleven transits, with optimal use of JWSTNIRSpec Prism. Consequently, H2O and CO2 isotopologues could be considered asindicators of past ocean loss and atmospheric escape for JWST observations ofterrestrial planets around M dwarfs.

the astronomical journal

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