Life on Mars may once have thrived miles beneath the surface of the Red Planet where geothermal heat could have melted the thick ice sheets, scientists claim.
Researchers from Rutgers University used computer simulations to calculate the likelihood of life emerging on the Red Planet and where it may have first formed.
They say that even if you pump greenhouse gases like carbon dioxide and water vapour into the early Martian atmosphere in simulations - you still can't get a long-term warm and wet surface that would be capable of sustaining life.
However, they say that four billion years ago the Red Planet had high geothermal activity underground that melted ice sheets and may have allowed life to thrive.
Geothermal activity on Mars could be a solution to the long-standing 'faint young Sun paradox', according to the US research team.
A vertically exaggerated, false-colour view of a large, water-carved channel on Mars called Dao Vallis that is evidence liquid water once flowed on the Red Planet four billion years ago
Four billion years ago the Sun was 25 per cent dimmer than it is today, meaning the climate of early Mars should have been freezing - but signs of minerals, chemicals and ancient riverbeds suggest that during this period Mars had flowing water.
Lead author Dr Lujendra Ojha said, 'the faint young sun paradox may be reconciled, at least partly, if Mars had high geothermal heat in its past.'
It also has implications for NASA's current Perseverance mission. The robot has instruments for collecting rock samples up to 30 feet deep after it lands in February.
If Dr Ojha and colleagues are right, they will 'need a bigger drill' to find signs of microbial life as it likely exists miles below the surface.
It is possible to go much deeper - the largest hole ever dug on Earth is the Kola Superdeep Borehole deep in the Arctic Circle on the Kola Peninsula.
The hole goes down about seven and a half miles - but took nearly 20 years to get that far - so it could be some time before we get samples from deep inside Mars.
The sun is a massive nuclear reactor that generates energy by fusing hydrogen into helium and has gradually brightened and warmed the surface of planets.
About four billion years ago, the Sun was much fainter than it is today - back to a time when it was only about 600 million years old.
The 'top soil' or regolith on the Martian surface has evidence of ancient riverbeds, chemicals and minerals that show it was a waterworld 4.1 to 3.7 billion years ago.
On rocky planets like Mars, Earth, Venus and Mercury elements like uranium, thorium and potassium generate heat via radioactive decay.
In such a scenario, liquid water occurs through melting at the bottom of thick ice sheets - even if the Sun was weaker than now.
Four billion years ago the Sun was 25 per cent dimmer than it is today, meaning the climate of early Mars should have been freezing - but signs of minerals, chemicals and ancient riverbeds suggest that during this period Mars had flowing water
Schematics of two possible end-member aqueous environments on Mars during the Noachian
On Earth, for example, geothermal heat forms subglacial lakes in areas of the West Antarctic ice sheet, Greenland and the Canadian Arctic.
Similar melting probably explains rivers and seas that have been found, dried up, on cold, freezing Mars four billion years ago.
Analysis of various datasets showed the conditions needed for subsurface melting were widespread - so could happen more easily than realized.
Even a warm, wet climate without a magnetic field, atmospheric thinning and a drop in global temperatures, would only have enabled flowing water at great depths.
This is where life would have been found - if it ever originated, said the researchers.
NASA's Mars Perseverance rover is being sent to Mars to take samples from beneath the surface to get a better picture of the ancient barren world
Dr Ojha added: 'At such depths, life could have been sustained by hydrothermal activity and rock-water reactions.
'So, the subsurface may represent the longest-lived habitable environment on Mars.'
NASA's Mars InSight spacecraft which landed in 2018 is the first to study the deep interior of the planet - although it will only drill 16ft inside the planet.
However, the mission will be looking at the crust, mantle and core of the Red Planet using a series of cutting edge instruments.
These will use seismology, heat flows and tracking to see deep into the rocky world.
'It may allow scientists to better assess the role of geothermal heat in the habitability of Mars during the Noachian era,' said Dr Ojha.
The findings have been published in the journal Science Advances.