As much as three percent of Mars could sustain life, say scientists from the Australian National University - but most habitable areas are underground.
A greater area of the planet than expected could provide the water and comfortable temperatures for terrestrial – and potentially Martian – microbes to survive, they say.
"Our models tell us that if there is water present in the Martian sub-surface then it could be habitable – as an extensive region of the subsurface is at temperatures and pressures comfortable for terrestrial life," says lead author of the study PhD student Eriita Jones.
On Earth, there's a hot, deep biosphere that extends to around five kilometers. And on Mars, says the team, that biosphere could extend to around 30 kilometers.
The team's already modelled the Earth and identified water that's inhabited and water that isn't. Applying the same technique to Mars, they conclud that a large fraction of the Martian sub-surface could be harbouring habitable water.
"We found that about three per cent of the volume of present-day Mars has the potential to be habitable to terrestrial-like life. This is compared to only about one per cent of the volume of the Earth being inhabited," says co-author Dr Charley Lineweaver.
"Our conclusion is that the best way to find water – or potentially microbes – on Mars is to dig. Sadly, NASA’s Curiosity Rover, which is scheduled to land on Mars in August, has a limited capacity to scratch the surface to 10 or 20 centimetres."