Since the early 1990s, it has been known that Early Mars river-forming climates require slightly more greenhouse forcing than can be explained by the greenhouse effect of CO2 and H2O vapor. This mis-match is a challenge to our understanding of the circumstellar Habitable Zone. I will present new evidence that the mis-match between Mars geologic data and models is more severe than has previously been recognized, report new 3D simulations of a long-term warming mechanism that can match the Mars data, and review lessons from Early Mars that can help us interpret data from habitable-zone exoplanets.
To get a river-forming climate, models so far have assumed near-optimal pCO2 (1-2 bar) plus an extra greenhouse gas (e.g., H2). However, our new analysis of the distribution of ~3 Ga fluvial landforms indicates that they formed at low average atmospheric pressure and therefore at low average pCO2. This result raises the likelihood of false negatives in the search for habitable exoplanets, and weakens (although it does not remove) the correlation between pCO2 and surface habitability.
We do not know what supplied the necessary extra non-CO2 warming. An attractive candidate is warming by high-altitude water ice clouds. We present new MarsWRF simulations of this warming mechanism.
Finally, I will review work by our collaboration and by others on trends, rhythms and aberrations in Early Mars climate, with a focus on the implications for interpreting data from habitable zone exoplanets. For example, there is strong evidence that Early Mars river-forming climates switched on and off on timescales (103–107 yr) that could fool telescopic observers into thinking that a habitable planet was uninhabitable, or vice versa.