When Mars was still a young world more than 3.7 billion years ago, it was regularly pummeled by asteroids that would have often splashed into its abundant surface water. Now some scientists say simple life forms may have been plentiful in the planet's crust at the time. And they have suggestions on where to look for evidence.
A team of researchers based in France and at the University of Arizona fed data into computer models about the early climate on Mars and the chemical composition of its crust. They wanted to see if the so-called Noachian time period could have produced the right conditions to support a particular kind of life.
Specifically the team wanted to see if methanogenic hydrogenotrophs, a type of microorganism that feeds on hydrogen and produces methane, could have thrived on ancient Mars. On Earth, we find similar microbes everywhere from the human gut to soils and underwater hydrothermal vents.
The computer simulations revealed that, so long as the surface of Mars wasn't fully iced over at the time, that microscopic life forms could have been abundant in the planet's moist crust, which scientists often refer to as regolith, on a scale comparable to the amount of life in the early oceans on Earth.
"The porous brine-saturated regolith would have created a physical space sheltered from ultraviolet and cosmic radiation," reads a paper published by the team Monday in the latest issue of the journal Nature Astronomy.
The crust may have also provided some needed warmth for the basic life forms to spend their days chowing down on hydrogen and carbon dioxide for energy and excreting methane as a waste product.
"On Earth, hydrogenotrophic methanogenesis was among the earliest metabolisms, but its viability on early Mars has never been quantitatively evaluated," the authors said.
The models also show that the ecosystem created by the organisms may not have been sustainable for the long term. The resulting changes to the Martian atmosphere may have triggered changes to the planet's climate, cooling it and forcing the microbes to move progressively deeper in the planet's crust.
If this is what happened on early Mars, it also supports one hypothesis of why life seems to be so rare in the universe: The so-called "Gaian Bottleneck" idea states that early forms of life on often volatile young planets may struggle to evolve fast enough to survive.
For now, all of this is in the realm of theory until fossil evidence of early life is actually found on Mars. Fortunately, the researchers have suggestions for the best places to look. The paper identifies three sites as best bets to search for indications of ancient methanogens near the surface on Mars: Hellas Planitia, Isidis Planitia and Jezero Crater.
Conveniently, that last one just happens to be the landing site of NASA's Perseverance rover. Keep digging, Percy!
