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 H2, 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 H2, the climatic effects of H2 remain poorly understood. Previous work showed that H2 induces strong greenhouse warming at the outer edge of the habitable zone. Here we use a 1D radiative-convective model to show that H2 also leads to strong warming near the inner edge of the habitable zone. Unlike H2's greenhouse warming at the outer edge, however, its effect near the inner edge is driven by thermodynamics: H2's large thermal scale height allows the atmosphere to store more water vapor than either a pure-H2O atmosphere or an atmosphere with a heavy background gas, such as N2 or CO2, thereby amplifying the greenhouse effect of H2O. Using idealized grey calculations, we then present a general argument for how different background gases affect the inner edge of the habitable zone. H2 stands out for its ability to induce novel "Souffle" 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 H2-rich, they were likely also hot: 1 bar of H2 is sufficient to raise surface temperatures above 340 K, and 50 bar of H2 are sufficient to raise surface temperatures above 450 K.