It's one of the biggest questions in the Solar System: was there ever life on Mars? And, if yes, will we ever find evidence for it? We still don't have the answers, but new research has revealed a promising place to look for clues.
Instead of scouring the dusty plains and mountains, maybe we should look under the ground, below the Martian surface.
According to planetary scientists at Brown University, the breakdown of water molecules in rocks on Mars likely produced enough chemical energy to sustain a population of chemosynthetic microbes for hundreds of millions of years.
"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 planetary scientist Jesse Tarnas.
"Conditions in this habitable zone would have been similar to places on Earth where underground life exists."
There's evidence in the rocks on Mars that the dry and dusty planet held abundant water long ago - although there's some debate about whether it flowed on the surface or underground.
In fact, the same is true today. What seemed to be evidence of water flows on Mars is not conclusive at all; yet other evidence suggests that there's a lot of water hanging around under the surface.
And if there was - or is - water on the Red Planet, computer simulations run by the researchers suggest that could have made hospitable conditions for a life-form similar to one found right here on Earth.
They're called subsurface lithoautotrophic microbial ecosystems, or SLiMEs, and they consist of communities of microorganisms that live deep under the ground, in the dark.
Because they're far from the warmth and light of the Sun, which triggers photosynthesis, the process on which most life on Earth depends, SLiMEs rely on a different mechanism for energy.
It's called chemosynthesis; and chemolithotrophic lifeforms, as they are known, use energy stored in the chemical bonds of inorganic compounds such as hydrogen sulfide or hydrogen gas to produce carbohydrates from carbon dioxide.
On Mars, conditions are less hospitable. But the research team determined that radioactive elements in Mars' crust could have driven radiolysis - the process whereby radiation breaks water down into hydrogen and oxygen. This could have produced enough hydrogen to sustain a horde of hungry Martian SLiMEs.
"We know that radiolysis helps to provide energy for underground microbes on Earth," said planetary scientist Jack Mustard, "so what Jesse did here was to pursue the radiolysis story on Mars."
The team started with data from the gamma ray spectrometer aboard NASA's Mars Odyssey spacecraft to map radioactive elements thorium and potassium in the Martian crust. This data was then used to calculate the abundance of uranium.
Because the decay rates of these elements are known constants, the team could then calculate how much of them were in the crust 4 billion years ago. This radioactive decay would also have driven the radiolytic breakdown of water.
Next, they had to estimate how much water would have been available based on the geothermal evidence. They found that groundwater would have been abundant in Mars's porous rocks.
And finally, climate modelling allowed them to locate the sweet spot for life - not so cold that it was frozen, but likewise not so close to the planet's hot core that anything alive would boil.
They determined that the planet hosted a global habitable zone several kilometres thick, wherein radiolysis would have generated enough hydrogen to support a community of microbes for hundreds of millions of years, through a range of climates.
It doesn't, of course, mean that life is there right now. But it does help figure out where to send planned rover Mars 2020 when it goes in search of signs of any Martian life, even if it's long expired.
"One of the most interesting options for exploration is looking at megabreccia blocks — chunks of rock that were excavated from underground via meteorite impacts," Tarnas said. "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."
Other places signs of Martian life might be found include what seems to be an underground reservoir of liquid water, where subglacial microbes might be living just as they do on Earth, and iron rich rocks next to dried-up lakes, where fossils might have been preserved.
The team's research has been published in the journal Earth and Planetary Science Letters.