Conditions beneath the surface on ancient Mars were ripe for life, according to a new study.
Scientists now believe that there was enough chemical energy beneath the surface on the planet for microbes to thrive.
"We showed, based on basic physics and chemistry calculations, that the ancient Martian subsurface likely had enough dissolved hydrogen to power a global subsurface biosphere," said Jesse Tarnas, lead author of the study.
Mr Tarnas, a graduate student at Brown University, added: "Conditions in this habitable zone would have been similar to places on Earth where underground life exists."
The work, published in the journal Earth and Planetary Science Letters, refers to subsurface lithotrophic microbial ecosystems – also known as SliMEs.
These ecosystems allow microbes beneath the surface to receive their energy without sunlight, but by peeling electrons from molecules in their environments.
One of the easiest ways to do this is by peeling electrons from dissolved hydrogen, and the new study has found that there would have been an abundance of this on ancient Mars.
"One of the most interesting options for exploration is looking at megabreccia blocks – chunks of rock that were excavated from underground via meteorite impacts," said Mr Tarnas.
"Many of them would have come from the depth of this habitable zone, and now they're just sitting, often relatively unaltered, on the surface."
Professor John Mustard, who is an active part of selecting a landing site for NASA's Mars 2020 rover, said that these chunks of rock are present in two of the current potential landing sites.
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He said: "The mission of the 2020 rover is to look for the signs of past life.
"Areas where you may have remnants of this underground habitable zone – which may have been the largest habitable zone on the planet – seem like a good place to target."