Ancient Mars had right conditions for underground life, new research suggests

Ancient Mars had right conditions for underground life, new research suggests
New research shows that ancient Mars likely had ample chemical energy to support the kinds of underground microbial colonies that exist on Earth. Credit: NASA / JPL

A new study shows evidence that ancient Mars probably had an ample supply of chemical energy for microbes to thrive underground.

"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 Jesse Tarnas, a graduate student at Brown University and lead author of a study published in Earth and Planetary Science Letters. "Conditions in this habitable zone would have been similar to places on Earth where underground exists."

Earth is home to what are known as subsurface lithotrophic microbial ecosystems—SliMEs for short. Lacking energy from sunlight, these subterranean microbes often get their energy by peeling electrons off of molecules in their surrounding environments. Dissolved molecular hydrogen is a great electron donor and is known to fuel SLiMEs on Earth.

This new study shows that radiolysis, a process through which radiation breaks water molecules into their constituent hydrogen and oxygen parts, would have created plenty of hydrogen in the ancient Martian subsurface. The researchers estimate that hydrogen concentrations in the crust around 4 billion years ago would have been in the range of concentrations that sustain plentiful microbes on Earth today.

The findings don't mean that life definitely existed on ancient Mars, but they do suggest that if life did indeed get started, the Martian subsurface had the key ingredients to support it for hundreds of millions of years. The work also has implications for future Mars exploration, suggesting that areas where the ancient subsurface is exposed might be good places to look for evidence of past life.

Going underground

Since the discovery decades ago of ancient river channels and lake beds on Mars, scientists have been tantalized by the possibility that the Red Planet may once have hosted life. But while evidence of past water activity is unmistakable, it's not clear for how much of Martian history water actually flowed. State-of-the-art climate models for early Mars produce temperatures that rarely peak above freezing, which suggests that the planet's early wet periods may have been fleeting events. That's not the best scenario for sustaining life at the surface over the long term, and it has some scientists thinking that the subsurface might be a better bet for past Martian life.

"The question then becomes: What was the nature of that subsurface life, if it existed, and where did it get its energy?" said Jack Mustard, a professor in Brown's Department of Earth, Environmental and Planetary Sciences and a study coauthor. "We know that radiolysis helps to provide energy for underground microbes on Earth, so what Jesse did here was to pursue the radiolysis story on Mars."

The researchers looked at data from the gamma ray spectrometer that flies aboard NASA's Mars Odyssey spacecraft. They mapped out abundances of the radioactive elements thorium and potassium in the Martian crust. Based on those abundances, they could infer the abundance of a third radioactive element, uranium. The decay of those three elements provides the radiation that drives the radiolytic breakdown of water. And because the elements decay at constant rates, the researchers could use the modern abundances to calculate the abundances 4 billion years ago. That gave the team an idea of the radiation flux that would have been active to drive radiolysis.

The next step was to estimate how much water would have been available for that radiation to zap. Geological evidence suggests there would have been plenty of groundwater bubbling about in the porous rocks of the ancient Martian crust. The researchers used measurements of the density of the Martian crust to estimate roughly how much pore space would have been available for water to fill.

Finally, the team used geothermal and climate models to determine where the sweet spot for potential life would have been. It can't be so cold that all water is frozen, but it also can't be overcooked by heat from the planet's molten core.

Combining those analyses, the researchers conclude that Mars likely had a global subsurface habitable zone several kilometers in thickness. In that zone, hydrogen production via radiolysis would have generated more than enough to support microbial life, based on what's known about such communities on Earth. And that zone would have persisted for hundreds of millions of years, the researchers conclude.

The findings held up even when the researchers modeled a variety of different climate scenarios—some on the warmer side, others on the colder side. Interestingly, Tarnas says, the amount of subsurface hydrogen available for energy actually goes up under the extremely cold climate scenarios. That's because a thicker layer of ice above the habitable zone serves as a lid that helps to keep hydrogen from escaping the subsurface.

"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."

Exploration implications

Tarnas and Mustard say the findings could be useful in thinking about where to send spacecraft looking for signs of past Martian life.

"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."

Mustard, who has been active in the process of selecting a landing site for NASA's Mars 2020 rover, says that these kinds of breccia blocks are present in at least two of the sites NASA is considering: Northeast Syrtis Major and Midway.

"The mission of the 2020 rover is to look for the signs of past life," Mustard said. "Areas where you may have remnants of this underground habitable zone—which may have been the largest on the planet—seem like a good place to target."

Explore further

NASA finds ancient organic material, mysterious methane on Mars

More information: J.D. Tarnas et al, Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming, Earth and Planetary Science Letters (2018). DOI: 10.1016/j.epsl.2018.09.001
Provided by Brown University
Citation: Ancient Mars had right conditions for underground life, new research suggests (2018, September 24) retrieved 21 October 2019 from
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Sep 24, 2018
the ancient Martian subsurface likely had enough dissolved hydrogen to power a global subsurface biosphere

No matter what we find or don't find on Mars, the implications for our understanding of the rest of universe are huge. Regardless of anything else we do in space, we need to thoroughly explore Mars and learn as much as we can if we want to keep moving forward.

I hope someday the majority of people would agree I am simply stating what is now obvious, because I am.

Sep 25, 2018
Very true. If only to get away from the nitwits who are populating this once magnificent planet Earth where such soulless creatures who resemble humans (or used to) decided to decimate the things of beauty and joy in their pursuit of decaying cultures and the rats' nests cities of which they are holding in a complete stranglehold.
Where is the justice? Will it be a bigger improvement on planet Mars?

Sep 25, 2018
I doubt if those too incompetent to protect and sustain the Biosphere of their home planet, will prove to be any less incompetent attempting to rehabilitate another world.

It is much easier to repeat one's errors than to make the effort to correct them.

Sep 25, 2018
In all likelihood, it will be Mars that will rehabilitate humans; especially those who have had a relatively easy existence on planet Earth and have thus lost their sense of value for the only home (so far) they have ever had.

Most humans are stupid and, the ones who go to Mars will probably bring their stupidity with them. And yet, they and those who remain here must be protected.
There was one future astronaut who said publicly in the media that when he got to Mars, he would light up and smoke a joint from his stash of cannabis that he will bring with him.
Such may be the level of qualified spacefarers who would travel to Mars just to smoke and get their fill of THC.
After reading that, I decided that a colony on Mars would be doomed to failure, even if it was only one maroon who decides to remove all doubt of stupidity.
And there are others who will not take seriously their role as colonists. Hopefully, they will not be able to return to Earth.

Sep 25, 2018
We need to settle Mars in a secure location that is sustainable. That means sending our best and brightest... problem solvers. People that can think 'out of the box and realize endless possibilities in simple situations. In a place with extensive archaeovolcanism there could be caves of live in many forms, some motile and maybe even evolve. We may find that Martian life actually presently existts, and if likely HUNGRY. Maybe even for US.

Sep 27, 2018
Modern day cave men or is that cave women

Marvellous, send are best and brightest to live in caves as Martian cave men or is that cave women!
Job vacancy:- graduate cave persons wanted, job prospects include personal cave, no socialising, no Taco Bells, no cinema's no television, Oh and by the way no going outside as your cave as its in a airless environment and no return flight, numeration not required as there in nothing to buy, no shops, no Mercedes Benz cars, no flights to the Florida everglades, no trips to Disney world.

The high light of the years to come are dust devils and worldwide dust storms
A forgettable trip of your life time – Good Look on your new career

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