It’s one of the biggest questions in our solar system: was there ever life on Mars ? And, if so, will we ever find evidence of it? We don’t have the answers yet, but new research has revealed a promising place to look for clues.
A new study shows evidence that ancient Mars likely had an ample supply of chemical energy to fuel microbes under the surface . Lacking energy from sunlight, subterranean microbes here on Earth, known as subsurface lithotrophic microbial ecosystems (SliMEs), often derive their electron energy from molecules in their surrounding environments.
Because they are far from the warmth and light of the Sun, which triggers photosynthesis, the process on which most life on Earth depends, SLiME depend on a different energy mechanism. It’s called chemosynthesis ; and chemolithotrophic life forms, as they are known, use the energy stored in the chemical bonds of inorganic compounds such as hydrogen sulfide or hydrogen to produce carbohydrates from carbon dioxide.
Dissolved molecular hydrogen is a great source of electrons and is known to power SLiMEs on Earth. “ Based on basic calculations of physics and chemistry, we were able to show that the ancient Martian subsurface probably had enough dissolved hydrogen to feed a global subsurface biosphere ,” said Jesse Tarnas, from Brown University (USA), co-author of the paper that publishes the journal Earth and Planetary Science Letters.
On Mars, conditions are less hospitable. But the research team determined that radioactive elements in the crust of Mars could have driven radiolysis, the process by which radiation breaks down water into hydrogen and oxygen. This could have produced enough hydrogen to sustain a horde of hungry Martian SLiME.
“We know that radiolysis helps provide energy for microbes underground on Earth,” said planetary scientist Jack Mustard, “so what Jesse did was look up the history of radiolysis on Mars.”
Because the decay rates of these elements are known constants, the team was able to calculate how many of them could have been in the crust 4 billion years ago . This radioactive decay would also have caused the radiolytic decay of the water. Next, they had to calculate how much water would have been available based on the geothermal evidence. They found that groundwater would have been abundant in the porous rocks of Mars.
And finally, climate models allowed them to locate the ideal place for life: not so cold as to be frozen, but not so close to the hot core of the planet that everything alive was boiled.
“Conditions in this habitable zone would have been similar to places on Earth where life exists underground,” Tarnas said. This discovery doesn’t exactly mean that there was microbial life on our neighboring red planet , but it does suggest that if there really was life, the Martian subsurface at least held the key ingredients to support life for hundreds of millions of years, the researchers noted.
One of the positive implications of this work is that these areas where ancient subsurface is exposed could be excellent places to search for evidence of past lives for future Martian exploration.
“One of the most interesting options for exploration is looking at blocks or chunks of rock that were excavated underground by meteorite impacts,” said Tarnas. “Many of them would have come from deep within this habitable zone, and are now situated, relatively unchanged, on the surface.”
Other places where signs of Martian life can be found include what appears to be an underground reservoir of liquid water, where subglacial microbes could be living just as they do on Earth or in iron-rich rocks next to dry lakes. where the fossils could have been preserved.
Reference: Radiolytic H2 production on Noachian Mars: Implications for habitability and atmospheric warming. JDTarnasa JFMustarda B.Sherwood LollarbM.S.Bramblea KMCannonc AMPalumboa A.-C. Plesad Earth and Planetary Science Letters D OI: https://doi.org/10.1016/j.epsl.2018.09.001
