Abstract:
The geology of ancient Mars suggests that liquid water flowed on its surface nearly 4 billion years ago. The above-freezing temperatures required to explain the fluvial features occurred either transiently or persistently. Climate models that consider only CO2 and H2O vapor have been unable to recreate warm surface conditions given the dimmer young Sun. Here, I improve upon my previous single-column climate model calculations to demonstrate that a persistently warm atmosphere, containing only ~1% H2 and 3 bar CO2 or ~20% H2 and 0.55 bar CO2, could have raised the mean surface temperature of early Mars above the freezing point of water. Unlike transient warming model scenarios, which must contend with an "ice problem", most persistently warm solutions satisfy available geologic and atmospheric constraints. Vigorous volcanic outgassing from a highly reduced early martian mantle can provide sufficient atmospheric H2 and CO2 - the latter from the photochemical oxidation of outgassed CH4 and CO. I then apply this volcanic H2 mechanism to the classical habitable zone, which is the circular region around a star in which liquid water could exist on the surface of a rocky planet. Although the outer edge of the traditional N2-CO2-H2O habitable zone (HZ) extends out to ~1.67 AU in our Solar System, I show that volcanic outgassing of atmospheric H2 can extend this to ~2.4 AU, assuming 50% H2. The corresponding outer edge orbital distances increase by about ~30% to 60% for A to M stars, respectively. The atmospheric scale heights of such volcanic H2 atmospheres near the outer edge of the HZ also increase, facilitating remote detection of atmospheric signatures. Improved interior, atmospheric (including escape), and planetary accretion models are needed for further progress.