Hot Hydrogen Climates Near the Inner Edge of the Habitable Zone

Abstract

Young terrestrial planets can capture or outgas hydrogen-rich atmospheres with tens to hundreds of bars of H 2 , which persist for 100 Myrs or longer. Although the earliest habitable conditions on Earth and terrestrial exoplanets could thus arise while the atmosphere is still dominated by H 2 , the climatic effects of H 2 remain poorly understood. Previous work showed that H 2 induces strong greenhouse warming at the outer edge of the habitable zone. Here we use a 1D radiative-convective model to show that H 2 also leads to strong warming near the inner edge of the habitable zone. Unlike H 2 's greenhouse warming at the outer edge, however, its effect near the inner edge is driven by thermodynamics: H 2 's large thermal scale height allows the atmosphere to store more water vapor than either a pure-H 2 O atmosphere or an atmosphere with a heavy background gas, such as N 2 or CO 2 , thereby amplifying the greenhouse effect of H 2 O. Using idealized grey calculations, we then present a general argument for how different background gases affect the inner edge of the habitable zone. H 2 stands out for its ability to induce novel 'Soufflé' climates, which further support its warming effect. Our results show that if the earliest conditions on a planet near the inner edge of the habitable zone were H 2 -rich, they were likely also hot: 1 bar of H 2 is sufficient to raise surface temperatures above 340 K, and 50 bar of H 2 are sufficient to raise surface temperatures above 450 K.

Formation and dynamics of water clouds on temperate sub-Neptunes: the example of K2-18b

“…the inner edge of the habitable zone for planets endowed with a thick H 2 -dominated envelope) has never been directly evaluated for a large range of planetary masses, gravities and types of host star. We acknowledge some recent progress in this regard (Koll & Cronin 2019). Finally, the cloud feedback proposed by Yang et al (2013) may be less efficient for H 2 -dominated atmospheres, for which moist convection is inhibited (Leconte et al 2017).…”

Unveiling Shrouded Oceans on Temperate sub-Neptunes via Transit Signatures of Solubility Equilibria versus Gas Thermochemistry

“…The exoplanet community already has ways to detect an H 2 atmosphere by transmission spectroscopy via its pressure scale height 1 order of magnitude larger than that of an N 2 or CO 2 atmosphere (Miller-Ricci et al 2008). However, the mass of the H 2 atmosphere-the parameter that controls the temperature at the bottom of the atmosphere and thus the possibility for liquid water (Pierrehumbert & Gaidos 2011;Ramirez & Kaltenegger 2017;Koll & Cronin 2019)-is not directly measurable from the transmission spectrum. Also, a planet's mass and radius typically allow multiple models of the interior structure (e.g., Rogers & Seager 2010;Valencia et al 2013).…”

Vertically Resolved Magma Ocean–Protoatmosphere Evolution: H2, H2O, CO2, CH4, CO, O2, and N2as Primary Absorbers

“…We explore single‐species atmospheres composed of H 2 , H 2 O, CO 2 , CH 4 , CO, O 2 , and N 2 , since these volatiles can become the dominant absorbers under plausible scenarios of planetary accretion and early evolution. For instance, H 2 can affect the long‐term climate due to collision‐induced absorption (Koll & Cronin, 2019; Pierrehumbert & Gaidos, 2011) and its interaction with N 2 has been proposed as a contributor to greenhouse warming on early Earth (Wordsworth & Pierrehumbert, 2013a). Furthermore, H 2 is abundant during planet formation and can create transient reducing climates on young planets.…”