Influx of Water and Oxygen into Terrestrial Atmospheres and the Impact on False-positive Biosignatures

Saturn’s moon Titan receives volatiles into the top of its atmosphere — including atomic oxygen — sourced from cryovolcanoes on Enceladus. Similar types of material exchange amongst exoplanets could cause the abiotic formation of biogenic gases such as oxygen and ozone that might otherwise be considered potential biosignatures. We simulate this process on TRAPPIST-1 e, using a 1-D photochemical model (Atmos) by treating it as an abiotic, Archean Earth-like planet that receives water and oxygen from space while methane outgasses from the surface. In these simulations, we predict atmospheric composition as a function of different rates of infalling material and methane surface fluxes with both cloudy and clear sky conditions. We also produce synthetic spectral observations using the planetary spectrum generator (PSG) for JWST, Origins, HabEx and LUVOIR to test the detectability of abiotic-generated oxygen and ozone.

One of the most promising biosignatures is the simultaneous detection of methane and oxygen (and/or ozone) which we looked for in all of the synthetic observations. For the sake of brevity, we share only the oxygen flux results in the abstract while both oxygen and water flux results will be presented in person. Our results in Fig. 1 show the column density of methane and ozone as a function of oxygen flux into the top of the atmosphere. The labeled maximum limit for the top of the atmosphere flux is based on studies of ionized oxygen escape on Proxima Cen-b being modeled as an Earth-twin. This result shows that the modeled atmospheres are generally insensitive to the oxygen flux until after the plausible limit has been passed.

An example of our spectral results are shown in Fig. 2 for all telescopes used in the clear sky conditions. Even though multiple methane signals appeared, no detectable ozone or oxygen signals were seen at the same time. It is only after the oxygen flux limit is exceed (Fig. 3) that a potential ozone feature appears. We determine that the incoming flux of material needed to trigger a false-positive reading by any of these observatories is at least an order of magnitude above what is physically plausible given atmospheric escape considerations.