Mars ~3 Ga had river-forming climates at low average pCO2, raising the likelihood of false negatives in the search for habitable exoplanets

Abstract:

Mars is the only planet that is known to retain a geologic record that can be used to test Earth-derived models of long-term planetary habitability. Mars’ river valleys are dry today, and the planet has lost most of its initial atmospheric volatiles; however it remains unclear whether river-forming climates required high pCO₂ (≈atmospheric pressure). Atmospheric pressure affects the spatial distribution of climate-sensitive landforms on Mars. We find evidence in paleochannel distributions, corrected for the first time for detectability biases, combined with a new aridity database, for a decline in average pCO₂ from >10² mbar for early river-forming climates, to ≲10² mbar for later river-forming climates. Strong non-CO₂ greenhouse warming occurred intermittently through ~3 Ga. The end of rivers on Mars could have been caused by further loss of pCO₂, or alternatively by a reduction in non-CO₂ greenhouse warming. These new results challenge Habitable Zone theory. Searches for habitable exoplanets are guided by the Habitable Zone concept (HZ). This concept predicts that habitable worlds near the cold edge of the HZ should have high pCO₂. However, we find that Mars (at the cold edge of the HZ) had river-forming climates at low average pCO₂. The persistence of river-forming climates at low average pCO₂ at the cold edge of the HZ raises the likelihood of false negatives in the search for habitable exoplanets. So far, rocky exoplanet atmosphere detection has used methods that do not reliably detect ~0.1 bar atmospheres. Mars had a habitable surface climate at ~0.1 bar average atmospheric pressure. Therefore nondetection of an atmosphere on an exoplanet does not preclude its habitability.