Mars was once rich in carbon dioxide, suggesting life on the red planet.
Widespread deposits of carbonate rock are buried a few miles beneath the surface, according to new research.
If they are abundant it means the greenhouse gas could have helped make it a much wetter and warmer place hundreds of millions of years ago.
This revealed extensive deposits of the mineral almost four miles below the crust that were exposed by a massive meteorite impact, reports the journal Nature Geoscience.
The team believes these represent ancient sediments that were subsequently buried by volcanic material ejected during eruptions from Syrtis Major. The mineral probably came from carbonate-rich water, which interacted with the rock — pointing to early Mars being hotter than previously thought.
Joseph Michalski of the Planetary Science Institute, Arizona, and Paul Niles of the NASA Johnson Space Centre, Houston, said the surface of Mars is now cold, dry, acidic and inhospitable to life.
Similar conditions may have persisted for billions of years, suggesting the best place to search for habitable environments is the subsurface.
The researchers said: “One hint of habitable conditions at depth is the presence of atmospheric methane, which may have formed through hydrothermal processes in the crust, in the presence of carbon dioxide. A dense CO2 atmosphere means a greenhouse effect.