New research suggests that ancient Mars likely had enough chemical energy for microbes to thrive underground. “Based on fundamental physical and chemical calculations, we have shown that the subsurface layer of ancient Mars probably had enough dissolved hydrogen to power the global subsurface biosphere,” says Jesse Tarnas, a graduate student at Brown University and lead author of the study, published in Earth and Planetary Science Letters.
"The conditions in this potentially habitable area could be similar to those on earth where underground life exists."
Where is life hiding on Mars?
The earth is home to the so-called subsurface lithotrophic microbial systems. In the absence of sunlight, these underground microbes often take their energy by ripping electrons from molecules in their environment. Dissolved molecular hydrogen is an excellent electron donor. It feeds such microbes on Earth.
New research shows that radiolysis, a process by which radiation breaks water molecules into their constituent hydrogen and oxygen, could create a lot of hydrogen in the ancient Martian subsurface. Scientists estimate that the concentration of hydrogen in the crust 4 billion years ago should have been roughly comparable to that on Earth, which feeds many microbes today.
These findings do not mean that life definitely existed on ancient Mars, but they do suggest that if life did exist, the Martian subsurface would have the necessary ingredients to sustain it for hundreds of millions of years. This work also has implications for future exploration of Mars, as the areas where the ancient subsurface comes out could be a great place to look for old life.
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Going underground
Ever since it was revealed that rivers and lakes once flowed on Mars, scientists have been obsessed with the possibility that the Red Planet may once hold life. But while the evidence for the existence of water in the past is incontrovertible, it is unclear for how much of Martian history the water actually flowed. The best climate models for early Mars produce temperatures that barely exceed freezing point, which means the planet's wet periods could be very short-lived. This is not the best scenario for sustaining life on the surface for a long time, and therefore some scientists believe that past Martian life under the surface may have felt better.
Scientists studied data from a gamma-ray spectrometer that flies aboard the Mars Odyssey spacecraft. They mapped the abundance of radioactive elements thorium and potassium in the Martian crust. Starting from the map, they managed to find a third radioactive element, uranium. The decay of these three elements provides radiation that leads to the radiolytic decay of water. And because these elements decay at a certain rate, the abundance model can be used to calculate the presence of the elements 4 billion years ago. So the team came up with the idea of a radioactive outbreak that actively pushed radiolysis.
The next step was to estimate how much water was available for this radiation. Geological evidence suggests that the porous rocks of the ancient Martian crust had a lot of groundwater breaking through the pores. The scientists used measurements of the density of the Martian crust to roughly estimate how many pores were available to fill with water.
Finally, the team used geothermal and climate models to determine where ancient life might have been. It should have been not so cold that not all the water froze, but not very warm either.
Combining these analyzes, the scientists concluded that Mars likely had a global subsurface potentially habitable zone several kilometers thick. In this zone, the production of hydrogen through radiolysis has generated more than enough chemical energy to support microbial life, based on what we know on Earth. And this zone had to persist for hundreds of millions of years.
These findings persisted even when scientists simulated different climate scenarios - some warmer, some colder. Remarkably, Tarnas said, the amount of underground hydrogen available as a source of energy increases in extremely cold climatic scenarios. Because a thicker layer of ice above the habitable zone serves as a cover that prevents hydrogen from escaping from the subsurface.
“People have the idea that the cold climate of early Mars is bad for life, but as we can see, there is more chemical energy in cold climates for life underground,” Tarnas says. "We think it could change people's attitudes towards climate and past life on Earth."
Research implications
Tarnas and Mustard say these findings will help in understanding where to send the next spacecraft to look for signs of life on Mars.
“One of the most interesting exploration options is finding blocks of megabreccia - chunks of rock that were ripped out of the ground by a meteorite impact,” Tarnas says. "Many of them came from the depths of the habitable zone, and now they are, often intact, on the surface."
Mustard, who was heavily involved in the selection process for the Mars 2020 rover, says that these kinds of breccia blocks are present in at least two locations reviewed by NASA: Northeast Syrtis Major and Midway.
Ilya Khel