Mars is the only currently accessible geologic record that can provide an independent test of Earth-derived models of planetary habitability. Mars is the only planet whose surface is known to have become uninhabitable. We are studying this environmental disaster as a benchmark for rocky exoplanets.
The most important climate-regulating greenhouse gas on Earth and Mars today is CO2. To constrain past pCO2 on Mars (on Mars, pCO2 ≈ total atmospheric pressure), we used Mars’ geologic record of the changing distribution of paleochannels and paleolakes, in combination with Global Climate Models. It is usually assumed that CO2 warming was near-maximum at times in Mars history when rivers flowed. To the contrary, we found evidence that even after average CO2 had dropped, rivers still flowed. Specifically, we found a decline over time in the preferred elevation for rivers, and a shift from early‑stage elevation control to late-stage latitude control. These changes are simply explained, based on comparison to Global Climate Models (GCMs), by a reduction in the total strength of the atmospheric greenhouse effect, with a decline in average pCO2 from ≫102 mbar for early river-forming climates, to ≲102 mbar for later river-forming climates. To explain late-stage rivers, strong non-CO2 greenhouse warming must have continued intermittently through ~3 Ga. River‑forming climates at low average pCO2 on Mars constrain Mars warming mechanisms and atmospheric evolution, challenge Habitable Zone theory, and raise the likelihood of false negatives in the search for habitable exoplanets.
Zooming in to Mars' atmospheric evolution, the figure shows how our result changes our view of the Red Planet's atmospheric decay. Estimates from the shifting spatial distribution of water flow on Mars on average pCO_2 (thick green arrows) are shown in the context of independent estimates on paleo-atmospheric pressure on Mars (symbols) and expectations from CO2 + H2O Habitable Zone theory (green dashed line). Wet-to-dry transition age from Martin et al. 2017 (jarosite). Purple symbols: constraints from analysis of meteorite data (not all consistent with one another). Downward-pointing gold triangles: upper limits from the embedded-crater method. Dark gray symbols: constraints from analysis involving rover data. Sky-blue lines: extrapolation of present-day CO2 escape-to-space rate from MAVEN and ASPERA-3. Modern-era value (blue triangle) includes both the present-day atmosphere (blue circle) and also CO2 known to be currently stored in polar ice deposits, which would be released at high obliquity. The gray band is drawn to guide the eye (omitting possible ancient periods of atmospheric collapse), taking account of the possibility of additional present-day polar CO2 ice reserves.