A valley-like region on Mars hosted plenty of mild-temperature surface water and underground geothermal activity nearly 4 billion years ago, making it one of the best places to look for signs of ancient life on the planet, according to new computer modeling based on minerals found there.
In a paper published March 29 in JGR Planets, planetary scientists at the University of Arkansas ran simulations to determine the formation of a unique mix of rock types at the site of Nili Fossae. It also provides some insights into what kind of atmosphere Mars had in the past that was heavy enough to support liquid water on its surface.
"That kind of location would be a good spot for future exploration to look for potential traces of the biological activity or biochemical activity," said lead author Vincent Chevrier, an associate professor of planetary science. "There may be some traces of early biochemical activity that could be self-forming that region."
Nili Fossae is a field of curved faults and troughs, known as a graben, and is located less than 200 miles north of the recently landed Perseverance rover. It is of particular interest to scientists studying Mars because it contains some of the planet's largest deposits of phyllosilicates — such as clay and serpentine — and carbonates, both of which form in the presence of water and are rare on the red planet.
Minerals form only in certain conditions specific to each type, so several minerals in a single location can help narrow down the conditions that created them all, Chevrier said.
How Mars' atmosphere sustained large amounts of liquid water still contains mysteries: It needed greenhouse gases to warm the planet, but the lack of widespread carbonate deposits means carbon dioxide was present in only small concentrations. Hydrogen gas and methane are viable candidates, but their geological function on Mars is not well understood, according to Chevrier.
In the newly published study, he and Arkansas colleague Marietta Morisson ran computer simulations of the evolution of rocks at Nili Fossae. Beginning with a simulated water-based soup of ions, they tested two main models: evaporation at 25 degrees Celsius to model weathering on the Martian surface, and a range of temperatures between 0 and 300 degrees Celsius to model above-ground and underground hydrothermal activity.
In both scenarios, they tested low and high concentrations of carbon dioxide, past- and present-equivalent atmospheric pressures as well as oxidizing and reducing conditions.
"You start from the liquid of composition whatever, and you evaporate it and you see what precipitates over time," Chevrier said. "You start also from the solution, from liquid, and you evolve the temperature and you see what precipitates over … a whole range of temperatures."
The simulations showed that several processes were at play at Nili Fossae. An iron-based phyllosilicate called nontronite forms with plenty of water and temperatures below water's boiling point, alongside the mineral ferrihydrite. This mixture leads to the creation of carbonates and iron-rich serpentine phyllosilicates during evaporation in reducing conditions in low temperatures, while many other phyllosilicates and carbonates are created in high temperatures, depending on the carbon dioxide concentration.
The study found that the conditions would have additionally created greenhouse gases hydrogen, carbon monoxide and methane, though not enough to fill the atmosphere unless the process was widespread on Mars.
The findings confirmed that Mars' surface had a low concentration of carbon dioxide, Chevrier said, though it may have been higher underground.
The authors also said a contested observation of a methane plume at Nili Fossae could have feasibly occurred given the geothermal history in the region. Although the 2009 discovery has been dismissed by some scientists as measurement artifacts from Earth's atmosphere, the authors suggested that methane could have been emitted by recent or ongoing underground hydrothermal reactions, or methane trapped below the surface may have been disturbed and emitted.
Because of the conditions once present in Nili Fossae and methane's important role in biochemistry, Chevrier said it is a prime location to search for signs of Martian life. And even if it does not turn up signs of ancient living beings, he said, it may inform which conditions and compounds are conducive to abiogenesis, when life springs from non-life.
Chevrier plans to conduct similar simulations of the rocks at Mawrth Vallis, one of Mars' oldest valleys and the other major source of phyllosilicates on the planet. He hopes that some of the minerals found there, such as iron compounds, will provide more answers about the ancient weather on Mars and whether hydrogen and methane were abundant in its atmosphere and warmed the planet as greenhouse gases.
Although Nili Fossae is not expected to be visited by a Mars rover in the near future, the recently arrived Perseverance rover is expected to make carbonate measurements in the Jezero Crater in which it landed. The rover analysis would reveal new details on the composition of these rocks, which Chevrier said could be used to further refine his findings.
"With all the instruments on the rover, you have way, way more constraints on the meteorology, the chemical composition," he said, "so you can really refine those kinds of models with way, way more precise conditions."
The study, "Carbonate-phyllosilicate parageneses and environments of aqueous 6 alteration in Nili Fossae and Mars," published March 29 in JGR Planets, was authored by Vincent Chevrier and Marietta Morisson, University of Arkansas.