Mars had right conditions for subsurface life some 3.7 to 4.1 billion years ago (Noachian period), according to new research from Brown University.
An artist’s impression of habitable Mars. Image credit: Daein Ballard / CC BY-SA 3.0.
“We showed, based on basic physics and chemistry calculations, that the ancient Martian subsurface likely had enough dissolved hydrogen to power a global subsurface biosphere,” said study lead author Jesse Tarnas, a PhD student at Brown University.
“Conditions in this habitable zone would have been similar to places on Earth where underground life exists.”
Earth is home to what are known as subsurface lithotrophic microbial ecosystems.
Lacking energy from sunlight, these microbial communities often get their energy by peeling electrons off of molecules in their surrounding environments. Dissolved molecular hydrogen is a great electron donor and is known to fuel these microbes on our planet.
Tarnas and colleagues shows that radiolysis, a process through which radiation breaks water molecules into their constituent hydrogen and oxygen parts, would have created plenty of hydrogen in the ancient Martian subsurface.
They estimate that hydrogen concentrations in the Martian crust around 4 billion years ago would have been in the range of concentrations that sustain plentiful microbes on Earth today.
The findings don’t mean that life definitely existed on ancient Mars, but they do suggest that if life did indeed get started, the Martian subsurface had the key ingredients to support it for hundreds of millions of years.
“What was the nature of that subsurface life, if it existed, and where did it get its energy? We know that radiolysis helps to provide energy for underground microbes on Earth, so what Jesse did here was to pursue the radiolysis story on Mars,” said Brown University’s Professor Jack Mustard, co-author of the study.
The researchers looked at data from the gamma ray spectrometer that flies aboard NASA’s Mars Odyssey spacecraft.
They mapped out abundances of the radioactive elements thorium and potassium in the Martian crust. Based on those abundances, they could infer the abundance of a third radioactive element, uranium.
The decay of those three elements provides the radiation that drives the radiolytic breakdown of water.
And because the elements decay at constant rates, the scientists could use the modern abundances to calculate the abundances 4 billion years ago. That gave them an idea of the radiation flux that would have been active to drive radiolysis.
The next step was to estimate how much water would have been available for that radiation to zap.
Geological evidence suggests there would have been plenty of groundwater bubbling about in the porous rocks of the ancient Martian crust.
The researchers used measurements of the density of the Martian crust to estimate roughly how much pore space would have been available for water to fill.
Finally, they used geothermal and climate models to determine where the sweet spot for potential life would have been. It can’t be so cold that all water is frozen, but it also can’t be overcooked by heat from the planet’s molten core.
Combining those analyses, the researchers conclude that Mars likely had a global subsurface habitable zone several miles in thickness.
In that zone, hydrogen production via radiolysis would have generated more than enough chemical energy to support microbial life, based on what’s known about such communities on Earth. And that zone would have persisted for hundreds of millions of years.
The study was published recently online in the journal Earth and Planetary Science Letters.
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J.D.Tarnas et al. 2018. Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming. Earth and Planetary Science Letters 502: 133-145; doi: 10.1016/j.epsl.2018.09.001
