The old Mars had good conditions for underground life, a new research suggests

A new study shows that the old Mars probably had sufficient chemical energy for microbes to grow underground.

"We have shown, on the basis of physical and chemical calculations, that the old Martian subsurface probably had enough dissolved hydrogen to feed a global underground biosphere," said Jesse Tarnas, a student at Brown University and senior author. 'a study published in Scientific letters of the Earth and planets. "The conditions in this habitable zone would have been similar to those of the Earth where the underground life exists".

The Earth is home to what are known as subterranean lithotrophic microbial ecosystems – SliMEs for short. Lacking energy from sunlight, these underground microbes often get their energy by removing the electrons from the molecules in their environment. Molecular dissolved hydrogen is a large electron donor and is known to supply SLiME on Earth.

This new study shows that radiolysis, a process by which radiation breaks water molecules in their constituent parts of hydrogen and oxygen, would have created a lot of hydrogen in the old Martian subsurface. The researchers estimate that hydrogen concentrations in the crust about 4 billion years ago would have been in the range of concentrations supporting many microbes on Earth.

The results do not mean that life definitely existed on the old Mars, but they suggest that if life actually began, the Martian basement had the key ingredients to support it for hundreds of millions of years. years. The work also has implications for the future exploration of Mars, suggesting that areas where the old basement is exposed could be good places to look for evidence of past life.

Go underground

Since the discovery, decades ago, of ancient river canals and lake beds on Mars, scientists have been seduced by the possibility that the red planet once hosted life. But while evidence of past water activity is undeniable, it is unclear for how much water the Martian story has actually sunk. Advanced climate models for early Mars produce temperatures rarely above freezing, suggesting that the first humid periods of the planet may have been ephemeral events. This is not the best scenario for keeping life on the surface in the long run, and some scientists believe that the basement could be a better bet for past Martian life.

"The question then becomes: what was the nature of this underground life, if it existed, and where did it find its energy?" said Jack Mustard, a professor in the Department of Earth Sciences, Environment and Brown Planets and co-author of the study. "We know that radiolysis helps to provide energy to the Earth's underground microbes, so Jesse has attempted to continue the history of radiolysis on Mars."

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

The next step was to estimate the amount of water available for this radiation. Geological evidence suggests that there would have been a lot of bubbling groundwater in the porous rocks of the ancient Martian crust. The researchers used measurements of the density of the Martian crust to estimate approximately how many pores would have been available to fill the water.

Finally, the team used geothermal and climate models to determine where potential life might have been. It can not be so cold that all the water is frozen, but it can not be overcooked by the heat of the melted heart of the planet either.

By combining these analyzes, the researchers conclude that Mars likely had a global underground habitable zone several kilometers thick. In this area, the production of hydrogen by radiolysis would have generated more than enough chemical energy to support microbial life, based on what we know of these communities on Earth. And this area would have persisted for hundreds of millions of years, the researchers conclude.

The results were retained even when researchers modeled a variety of different climate scenarios – some on the warmer side, others on the colder side. It is interesting to note that, according to Tarnas, the amount of hydrogen below the surface available for energy increases in extremely cold climate scenarios. This is because a thicker layer of ice above the habitable area serves as a cover to prevent hydrogen from escaping from the surface.

"People have the impression that a cold climate in early March is bad for life, but what we are showing is that there is actually more chemical energy for life underground in a cold climate "said Tarnas. "We think this could change people's perception of the relationship between climate and life on Mars."

Implications of exploration

Tarnas and Mustard say the results could be helpful in knowing where to send the spaceships in search of signs of past Martian life.

"One of the most interesting options for exploration is to examine megabreccia blocks, pieces of rock that have been extracted from the basement by meteorite impacts," said Tarnas. "Many of them would have come from the depths of this livable area, and now they are just sitting, often relatively unaltered, on the surface."

Mustard, who has been active in the selection process of a landing site for NASA's March 2020 rover, says that these types of gap blocks are present in at least two of the sites envisioned by the NASA. NASA: Northeast Syrtis Major and Midway.

"The mission of the 2020 rover is to look for signs of past life," said Mustard. "The areas where you can have remnants of this underground habitable area – which could be the largest habitable area on the planet – seem to be a good place to target."

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More information:
J.D. Tarnas et al, Production of Radiolytic H2 on Mars Noachian: Implications for habitability and atmospheric warming, Scientific letters of the Earth and planets (2018). DOI: 10.1016 / j.epsl.2018.09.001