The CO2 inventory on Mars

We use spacecraft measurements to determine the inventory of CO2 gas on Mars. We estimate the amounts of gas from an early, thicker Martian atmosphere that have been lost to space by impact ejection and by solar and solar-wind stripping, and that have been removed to non-atmospheric reservoirs of CO2 ice, clathrate hydrate, adsorbed gas, or carbonate minerals. Loss to space has removed 1–2 bars of CO2. Deeply buried carbonates may contain up to the equivalent of a bar of CO2, with the other sinks likely containing no more than ∼90 mbar of CO2. These sinks can readily account for loss of the bulk of an early CO2 atmosphere thought to be necessary to provide early greenhouse warming. Loss of CO2 from the atmosphere to these sinks is the likely explanation for the transition in climate from an early, warmer atmosphere to the cold, dry atmosphere that has been present since early in the Hesperian epoch.