The most inviting environments for life on ancient Mars might have been deep underground, suggest findings published this week in the journal Science Advances.

About 4 billion years ago, Mars received much less heat from the Sun than it does today, yet scientists have found plenty of evidence of liquid water early in the Red Planet’s history. “The paradox is basically [caused by] this discrepancy between what we expect the climate of Mars to have looked like 4 billion years ago and what the geological record shows,” says Lujendra Ojha, a planetary scientist at Rutgers University-New Brunswick.

To understand how water could have persisted in this chilly environment, Ojha and his colleagues estimated whether heat from the planet’s interior could have melted the base of ice sheets in the planet’s southern highlands. They found that, whether ancient Mars was warm and wet or cold and arid, conditions could have been at least toasty enough to create a moist habitable environment that reached several miles below the planet’s surface.

Mars is about 48 million miles farther from the Sun than Earth is, and receives only 43 percent as much sunlight. “The energy received on these planets from the Sun is really the thing that makes or breaks these planets from being habitable,” Ojha says. “If you’re too close, you’re getting too much power from the Sun and your temperature goes up; if you’re too far away it’s rather cold.”

During the Red Planet’s Noachian eon, which took place 4.1 to 3.7 billion years ago, the young Sun was about 30 percent dimmer than it is today. This means that early Mars should have been extremely cold, with temperatures hovering below the freezing point of water. However, the Martian surface is marked by river channels, lakebeds, and possible shorelines, as well as clays and other water-containing minerals. Scientists have debated this apparent contradiction (which has also been observed for Earth), known as the faint young Sun paradox, for decades. One possible solution is that greenhouse gases in the ancient atmosphere kept the surface warm enough for liquid water.

Other researchers have suggested that the heat came from within the planet itself. The crust and mantle of rocky planets contain elements that give off heat when they decay, including thorium, uranium, and potassium. On Earth, hot water and steam carry this geothermal energy to the surface, and it’s been harnessed for heating and energy production in Iceland, the United States, and a number of other countries. It also creates lakes and channels beneath massive glaciers in Antarctica, Iceland, Greenland, and the Canadian Arctic.

To investigate whether this could have happened on Mars, Ojha and his colleagues estimated both the thickness of ice deposits in the planet’s southern hemisphere and the planet’s average annual surface temperature. The team also used measurements collected by NASA’s Mars Odyssey spacecraft of thorium, uranium, and potassium in Martian soils to model how heat might have traveled up from the interior.

“[We] were able to calculate how much geothermal heat could you expect on Mars 4 billion years ago, and was that heat sufficient to create some sort of liquid from thick ice?” Ojha says. “We showed that it is possible.”

Ojha suspects that water could have infiltrated roughly 3 miles into the crust, although he hopes to refine this estimate in the future as NASA’s InSight lander captures more information about the structure of the Martian crust. Subglacial lakes could have been an ideal environment for ancient lifeforms similar to the extremophiles found at undersea vents and around the hydrothermal features of Yellowstone National Park, where the heat and chemical reactions between the water and rock create key ingredients for life, Ojha says. Groundwater habitats would also have sheltered any resident organisms from the cold, radiation, and other hazards found on the surface.

Whether Mars started out wet or icy, the researchers concluded, over time these lakes would have become the only place where liquid water could persist as the planet lost its magnetic field, its atmosphere thinned, and temperatures dropped.

The findings also have potential implications for the search for life beyond our solar system. Geothermal energy might make it possible for life to exist on an otherwise cold and supposedly inhospitable planet on the edge of its solar system’s habitable zone.

“Even though the surface temperature is very low,” Ojha says, “because of these heat-producing elements that are undoubtedly present in any rocky planet, it would create enough heat to create a melt and you could potentially have a deep biosphere.”