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 O 2 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 O 2 -and CO 2 -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 CO 2 -dominated atmospheres. For Venus-like D/H ratios 100 times that of Earth, TRAPPIST-1 b transit signals of up to 79 ppm are possible by observing HDO. Similarly, 18 O/ 16 O ratios 100 times that of Earth produce signals at up to 94 ppm. Detection at S/N=5 may be attained on these bands with as few as four to eleven transits, with optimal use of JWST NIRSpec Prism. Consequently, H 2 O and CO 2 isotopologues could be considered as indicators of past ocean loss and atmospheric escape for JWST observations of terrestrial planets around M dwarfs.

A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System

“…While there are multiple theoretical uncertainties in the above dependencies that have yet to be fully worked out, the conclusion of these works is that O 2 -dominated atmospheres are one of the most serious candidates for the composition of TRAPPIST-1 planetary atmospheres (Lincowski et al. 2019 ).…”

“…CO 2 is the most widespread molecule in the atmospheres of the terrestrial planets of the solar system, and is one of the most likely gases to accumulate in the atmospheres of TRAPPIST-1 planets (Lincowski et al. 2019 ).…”

“…Lincowski et al. ( 2019 ) proposed that JWST-NIRSpec 8 (Near-InfraRed Spectrograph) in particular in prism mode (Bagnasco et al. 2007 ; Ferruit et al.…”

“…Isotopologues such as HDO or 18 OCO (at mixing ratios compatible with solar system values) could be detected through their near-infrared absorption bands with as few as 4 to 11 transits at 5 on JWST-NIRSpec Prism (Lincowski et al. 2019 ). However, the presence of clouds and/or hazes would likely preclude detection of these isotopologues with JWST (Fauchez et al.…”

The Fundamental Connections between the Solar System and Exoplanetary Science

“…These combined constraints suggest that these terrestrial worlds may have high-molecular-weight atmospheres like the terrestrial planets in our own system, although the data are also consistent with no atmospheres at all. With the launch of the James Webb Space Telescope, observations of the TRAPPIST-1 system and other nearby terrestrial worlds will have the capability to detect the presence and composition of atmospheres (Lincowski et al, 2018;Lustig-Yaeger et al, 2019b;Morley et al, 2017;Wunderlich et al, 2019), potentially revealing past processes like atmosphere and ocean loss (Lincowski et al, 2018(Lincowski et al, , 2019Lustig-Yaeger et al, 2019a). These observations may also provide our first opportunity to search for signs of life, such as CH 4 in combination with other biosignatures, in the atmosphere of a terrestrial exoplanet (Krissansen-Totton, Garland, et al, 2018;Wunderlich et al, 2019), and complement observations from the ground with extremely large telescopes that will search for O 2 using high resolution spectroscopy (López-Morales et al, 2019;Lovis et al, 2017).…”