Living microbes could be hanging out beneath the Martian surface, according to a new study of Mars rocks from Brown University. The research shows that Mars could provide a stable, nourishing environment there for billions of years.
All life needs energy to survive. The life on Earth’s surface mostly gets that energy from the sun, but microbes can survive without light if they get their energy elsewhere.
“To have sufficient chemical energy for life, you need both reducing compounds and oxidizing compounds,” says Jesse Tarnas, a planetary scientist at NASA’s Jet Propulsion Lab. He led the study while completing his Ph.D. at Brown University. Reduction is just a chemical process that gives a molecule more electrons, and oxidation is one that takes them away. Microbes would need these basic chemical fuel types, along with liquid water, to survive.
The surface of Mars is barren, heavily radiated, and cold. But underground, deep enough down, scientists think the warmth of the planet’s core keeps water liquid.
Where this underground water touches Martian rocks, certain chemical reactions can take place, producing the reduction and oxidation chemicals essential for life. The chemicals form because Mars rocks, like those on Earth, typically have small amounts of radionuclides—atoms that are unstable and eventually release radiation—trapped inside them. But this radiation isn’t as overpowering as that on the surface, which kills pretty much everything.
When these nuclides emit radiation, it breaks up nearby water molecules into hydrogen gas and oxides, both highly reactive chemicals which go on to create other chemicals that can sustain life.
Testing the ingredients for life on Mars
The team used data from Martian meteorites gathered across the globe—including the famous Allan Hills 84001 meteorite—to figure out how much of these crucial chemicals could form on the red planet. They looked at the composition of the Mars rocks and calculated the amount of reducing and oxidizing chemicals those rocks could produce over time, then compared this to the rates that Earth microbes would chomp them up.
They found that some types of Mars rocks could support life’s requirements long-term. The researchers then estimated how many microbes could survive in different rocky areas under Mars, assuming that these microbes would be similar to those deep underground or on the seafloor on Earth, which feed off of sulfates instead of oxygen.
[Related: Was there ever life on Mars? Perseverance’s SHERLOC laser sniffs for microscopic clues]
This study addresses two really important factors in the habitability of another planet, says Allan Treiman, a planetary geologist at the Lunar and Planetary Institute of the Universities Space Research Association, who was not involved in the study. First, he says, “the source of energy, and second, the consistency of environments,” or how stable they are.
Though we haven’t found liquid water on Mars directly, he says, “it’s a pretty good guess that there is a lot of liquid water down there.”
This water most likely exists in pockets, Tarnas says. “Fresh” water would be several kilometers down—quite a long way to drill. But salt water pockets, like road salts here on Earth, freeze at lower temperatures, and could exist just a few hundred meters down.
This water, along with the small amount of radiation coming off Mars rocks, could supply a steady stream of the chemical energy life needs for billions of years, the researchers found. “If life did ever arise on Mars, and if groundwater is still present there,” Tarnas says—which he admits are two big ifs—“then it’s possible that habitable environment could have been a refuge for [life] from billions of years ago, all the way up to today.”
‘Nature’s drill’ on Mars could be another route to finding life
It may still be a while before rovers or astronauts dig that deep, but with technologies like transient electromagnetic sounding, Tarnas says, researchers can get an idea of where and how much water is hiding beneath the surface.
And there’s another way to glimpse the deep down of Mars.
“Nature gives us a pretty good drill, which are impact craters,” Treiman says. Large impacts on the surface of Mars have ejected rocks from deep down under the surface. “The bigger the crater, the further down you can get.”
Mars rovers like Perseverance could find and study these impact-ejected rocks to learn what Mars is like down below. Exposed on the surface, the rocks wouldn’t still have living microbes on them. But they could have microfossils, organic molecules, or signs of contact with water.
Treiman says his gut feeling all along has been that there isn’t life on Mars. “But I may have to reevaluate that,” he says. “Based on this paper, it was pretty convincing.”