Oxygen False Positives on Habitable Zone Planets Around Sun‐Like Stars

Abstract

The search for life beyond Earth is a key motivator for exoplanet astronomy. Among the various life detection approaches that have been proposed, atmospheric oxygen is arguably the most promising biosignature. This is because any organism that adapts to exploit free energy from starlight will have a competitive advantage over organisms that are limited by geochemical sources of free energy. The earliest incontrovertible evidence for life on Earth around 3.5 Ga coincides with fossilized photosynthetic stromatolites (Buick, 2008). While it is unknown whether the evolution of oxygenic photosynthesis is contingent-the metabolism emerged only once in Earth's history (Mulkidjanian et al., 2006)-oxygenic photosynthesis confers a unique evolutionary advantage over other forms of photosynthesis since the required substrates, carbon dioxide

Was Venus Ever Habitable? Constraints from a Coupled Interior–Atmosphere–Redox Evolution Model

“…The advective heat term depends on melt production, MP (m 3 s −1 ), and the average melt density ρ melt = 3000 kg m −3 , which is assumed to be slightly less than that of the bulk mantle. Equations (2), (4), and (5) govern thermal evolution except in rare cases where transition to runaway greenhouse causes surface temperature to increase above mantle potential temperature, in which case a conduction regime is adopted (see Krissansen-Totton et al 2021).…”

“…The sizes of these oxygen sinks are selfconsistently calculated from the planetary interior evolution and mantle volatile content: outgassing fluxes are calculated using the melt-gas equilibrium outgassing model of Wogan et al (2020). Sulfur species are ignored in our nominal model because, for Earth-like bulk abundances, outgassing of S-bearing gases only modestly increases total oxygen sinks (Krissansen-Totton et al 2021). Outgassing fluxes depend on mantle oxygen fugacity, degassing overburden pressure, the volatile content of the mantle, specifically H 2 O and CO 2 content, the rate at which melt is produced, and the tectonic regime (see below).…”

Waterworlds Probably Do Not Experience Magmatic Outgassing

“…To illustrate the implications of a solubility limit to outgassing for waterworld atmospheric evolution, we incorporated solubility-limited outgassing into a coupled model of planetary geochemical, thermal, and climate evolution. The model, which is described in full in Krissansen-Totton et al (2021), considers terrestrial planet atmospheric evolution from a post-accretion magma ocean onward. C-O-H-bearing volatiles are partitioned between the magma ocean and the atmosphere assuming chemical equilibrium, and then following magma ocean solidification volatile exchange between the atmosphere and interior may occur via magmatic outgassing, serpentinization, carbonatization, and hydration reactions.…”

“…The coupled atmosphere-interior model used to generate Figure 4 in the main text is described in full in Krissansen-Totton et al (2021), and the code is available at https://github.com/ joshuakt/Oxygen-False-Positives. This section describes the modifications made to this model to simulate Trappist-1 waterworlds.…”

“…The rate at which cations are dissolved from the seafloor, F kinetic , is controlled by the temperature (T deep ) and pH (pH) of hydrothermal fluids, as well as plate velocity, ν plate : Here, W coef = 1.3 × 10 11 kg yr −1 is a constant scaled to fit Earth fluxes, n plate Earth = 0.03 m yr −1 is the average plate speed on the modern Earth, and E SF = 90 kJ mol −1 is an effective activation energy. Plate speed, ν plate , is calculated assuming the following relationship between melt production, MP (m 3 yr −1 ), This reduces to an overall dependence of plate velocity on interior heatflow of n µ heatflow plate (Krissansen-Totton et al 2021). Here, we are assuming that only CO 2 availability limits seafloor weathering, and that the supply of weatherable oceanic, represented by plate velocity, scales freely with heatflow.…”