Ancient Mars Had Prime Conditions for Life to Thrive Below Its Surface

Ancient Mars probably had prime conditions to support life below its surface, according to a study published in the journal Earth and Planetary Science Letters.

A team of researchers, led by Jesse Tarnas at Brown University, found that there was likely enough chemical energy in subterranean regions of the Red Planet for tiny microbes to thrive around billions of years ago. Conditions in this subsurface habitable zone would have been similar to places on Earth where underground life exists, according to Tarnas.

"This study was undertaken because of new work showing that hydrogen production could be much higher in Earth's ancient greenstone belts than previously thought," he said. "Hydrogen production has been shown to support abundant microbial communities in these regions of the Earth's crust."

These underground areas on our planet are known as subsurface lithotrophic microbial ecosystems—or SliMEs for short. Microbes in these ecosystems cannot harness energy to power themselves from the Sun so instead they steal electrons from molecules in their environment—particularly from dissolved hydrogen molecules.

In their research, the team calculated how much of this hydrogen fuel Mars could produce via radiolysis—the process through which radiation breaks down water into its constituent hydrogen and oxygen molecules. They found that radiolysis was taking place in sufficient quantities to sustain a global subsurface habitable zone, rich in hydrogen, around 4 billion years ago.

"Radiolysis supports abundant life underground on Earth," Tarnas said. "We found that radiolysis also likely produced enough chemical energy to support an underground biosphere on ancient Mars."

"Using climate and geothermal heat flux models, we constrained where a habitable zone could have existed within the crust in different climate and groundwater composition scenarios," he said. "We conclude that this habitable zone likely existed for hundreds of millions of years regardless of the background climate. This tells us that if life did evolve on Mars, it would have had a place to inhabit for long periods of time."

Scientists have long been fascinated by the possibility that Mars could have once hosted life—an idea that has gained more traction with the mounting evidence that liquid water may have flowed on the surface at some point. However, ancient Martian climate models suggest that the temperature rarely reached above freezing, suggesting that wet periods on the surface may have been brief.

This likely would have hurt the chances of life developing in the long-term, leading some scientists to turn their interest to searching for signs of life under the surface.

While the latest results don't provide proof that life once existed on Mars, they do indicate that if microbes did evolve, they would have had plentiful energy resources to draw on.

Interestingly, the latest findings hold up, even under a variety of different climate scenarios. In fact, the amount of subsurface hydrogen available as fuel actually increases under extremely cold climate scenarios. In these situations, a thick layer of ice on the surface would help prevent hydrogen escaping from below the ground.

"People have a conception that a cold early Mars climate is bad for life, but what we show is that there's actually more chemical energy for life underground in a cold climate," Tarnas said. "That's something we think could change people's perception of the relationship between climate and past life on Mars."

According to the researchers, the study has important implications for future exploration of the Red Planet. For example, areas where the ancient subsurface is exposed may be excellent targets in the search for signs of life.

Furthermore, it's possible that the hydrogen produced by radiolysis could be converted to methane and stored by human explorers.

"Methane will likely be important in future human exploration of Mars since SpaceX plans on using it, along with oxygen, as rocket propellant," Tarnas said. "Both of these compounds can be stored in clathrate hydrates—where gas molecules are trapped in crystalline ice-cages—like those currently being released on Earth by melting in the Arctic."

"They currently plan on building a propellant production plant to make methane and oxygen out of water-ice and atmospheric CO2, but tapping these methane clathrate hydrate deposits, if they exist, could provide a more efficient way of getting some of that energy.

Dirk Schulze-Makuch, an expert in astrobiology from Washington State University, who was not involved in the latest research, agreed with the findings presented by the scientists.

"This is a nice study to provide some quantification of the habitability of the subsurface of early Mars, which will be very helpful for follow-up studies," he told Newsweek. "For me, that habitability was never in question, and the next step would be to investigate how long that habitability could be maintained, or—more pointedly—is the Martian subsurface still habitable today?"

Other groups of scientists have also provided evidence recently that boost the chances Mars may have once hosted life. One study published in the journal Nature, for example, showed that Mars's crust formed no later than 4,547 million years ago, much earlier than previously thought, and at least 100 million years sooner than the Earth's.

This means that liquid water could have existed at the surface of the planet by that time and, hence, this environment may have been amiable for life to develop. This is much earlier than Earth, by about 100 million years, meaning that life may have first originated on Mars, although this is still a speculative idea.

This article has been updated to include additional comments from Jesse Tarnas and Dirk Schulze-Makuch.