If life ever existed on ancient Mars, it would have lived up to several miles below the surface of the Red Planet, likely due to subsurface melting of thick ice sheets fuelled by geothermal heat, according to a study.
The research, published in the journal Science Advances, examined various Mars datasets to see if heating via geothermal or underground heat would have been possible 4.1 billion to 3.7 billion years ago or in the Noachian era.
They showed that the conditions needed for subsurface melting would have been ubiquitous on ancient Mars.
Even if Mars had a warm and wet climate 4 billion years ago, with the loss of the magnetic field, atmospheric thinning and subsequent drop in global temperatures over time, liquid water may have been stable only at great depths, the researchers found.
Therefore, life, if it ever originated on Mars, may have followed liquid water to progressively greater depths, they said.
"At such depths, life could have been sustained by hydrothermal (heating) activity and rock-water reactions. So, the subsurface may represent the longest-lived habitable environment on Mars," said lead author Lujendra Ojha, an assistant professor at Rutgers University-New Brunswick in the US.
The study may help resolve what is known as the faint young sun paradox -- a lingering key question in Mars science.
"Even if greenhouse gases like carbon dioxide and water vapour are pumped into the early Martian atmosphere in computer simulations, climate models still struggle to support a long-term warm and wet Mars," said Ojha.
"I and my co-authors propose that the faint young sun paradox may be reconciled, at least partly, if Mars had high geothermal heat in its past," he said.
The researchers noted that our sun is a massive nuclear fusion reactor that generates energy by fusing hydrogen into helium.
Over time, they explained, the Sun has gradually brightened and warmed the surface of planets in our solar system.
According to the researchers, about 4 billion years ago, the Sun was much fainter so the climate of early Mars should have been freezing.
However, the surface of Mars has many geological indicators, such as ancient riverbeds, and chemical indicators, such as water-related minerals.
These suggest the Red Planet had abundant liquid water in the Noachian era, the researchers said.
This apparent contradiction between the geological record and climate models is the faint young sun paradox, they said.
On rocky planets like Mars, Earth, Venus and Mercury, heat-producing elements like uranium, thorium and potassium generate heat via radioactive decay.
In such a scenario, liquid water can be generated through melting at the bottom of thick ice sheets, even if the Sun was fainter than now.
On Earth, for example, geothermal heat forms subglacial lakes in areas of the West Antarctic ice sheet, Greenland and the Canadian Arctic.
The researchers noted that it is likely that similar melting may help explain the presence of liquid water on cold, freezing Mars 4 billion years ago.