Perseverance has successfully landed on the 3.5 to 4 billion year old rocks around Jezero Crater on Mars. With multiple international missions actively exploring our red planet neighbor, scientists eagerly await findings that will challenge and advance our understanding of Mars.
What is found may also shed light on processes dominating the earliest era on our home planet, Earth.
While the public has seized on the astrobiology objectives and the search for signs of ancient life from a time when Mars is thought to have been warmer, wetter and more habitable, NASA scientist and Deputy Project Scientist for the Mars 2020 Rover, Katie Stack Morgan has emphasized the importance of exploration of prebiotic environments on Mars.
Mars’ multi-billion year old rocks provide abundant real estate for prebiotic exploration. Such opportunities are much rarer on Earth, where the destructive forces of plate tectonics have recycled much of the planet’s oldest rocks. Ironically, the success of life itself adds to the challenge for prebiotic science on Earth, making it difficult to find remnants of the “abiotic” world that preceded the origin of life.
In a paper published this month in Geochimica cosmochimica acta, a team of researchers from University of Toronto and the MARUM Centre at Bremen shine a light on an otherworldly niche, deep beneath the Earth’s surface, where processes that may have dominated prebiotic Earth and Mars, can still be identified today.
In the billion-year old fracture waters flowing from rock buried 2.4 km below the surface of the Canadian Shield, this new study have identified carbon compounds – specifically formate and acetate at some of the highest concentrations found in natural groundwaters. These one- and two-carbon bearing compounds, respectively, provide possible substrates (food sources) for deep subsurface microorganisms discovered in subsurface rocks and groundwaters.
What surprised researchers here were not just the high concentrations of these simple organic compounds, but the fact that they were largely the only carbon compounds present – together accounting for almost all of the dissolved carbon found in these deep ancient waters. Further investigation based on their carbon isotope signatures (naturally occurring ratios of the isotopes od 12-carbon to 13-carbon) pointed to a source for the compounds outside the conventional biogeochemical reactions known to dominate the cycling of these two molecules at the Earth’s surface.
Previous studies had shown that radiolysis, naturally occurring radiochemistry deep in the Earth’s crust can produce both hydrogen and sulfate, by cleaving water molecules and oxidizing sulfide minerals in the ancient rocks, respectively. This new study provides the first evidence, that such radiolytic chemistry may also be a key part of the deep carbon cycle – producing carbon compounds through abiotic organosynthesis in the deep dark places of the planet where the Sun’s photosynthetic energy cannot reach.
Most life on Earth gets its energy – directly or indirectly – from the sun. But there are other options. If radiolytic reactions sustain an abiotic carbon cycle similar to what might have prevailed on Earth before life arose, studying these processes provides us with a bridge between the abiotic-biological worlds.
“Microbial subsurface communities are often chemosynthetic, not photosynthetic,” Barbara Sherwood Lollar says. “In chemosynthesis, a molecule like hydrogen ‘donates’ electrons, and sulfate ‘accepts’ them. Basically all metabolism works through this kind of exchange of electrons. That’s how life works. But until now, the role of chemical reactions supplying deep carbon compounds in this setting had not been known.”
This study suggests that a radiolytically driven H, S and C deep cycle provides a mechanism for sustaining deep subsurface microorganisms in the absence of interaction with the surface photosphere, and suggests a model for planetary habitability capable of sustaining chemolithotrophic life on planets or moons where photosynthesis may never have arisen.
This specialized niche, deep beneath the rocks of northern Ontario in Canada, is providing clues into the organic-producing reactions that dominated the carbon cycle on Earth before the rise of life, and that may persist today on planets and moons such as Mars, Enceladus, Europa and Titan. Such natural analog environments for the investigation of abiotic organic chemistry outside the laboratory are a rare find.
Sherwood Lollar says “While we watch and await the fascinating new discoveries to come from Perseverance, science continues to explore the boundaries and links between life, and what came before, through exploration of the unknown right beneath our feet”.
- Sherwood Lollar, B., Heuer, V.B., McDermott, J., Tille, S., Warr, O., Moran, J.J., Telling, J., and Hinrichs K.-U. (2021). A window into the abiotic carbon cycle – acetate and formate in fracture waters in 2.7 billion year old host-rocks of the Canadian Shield. Geochimica Cosmochimica acta 294:295-314.
Contact for press and public outreach on this story:
- University of Toronto
University Professor Barbara Sherwood Lollar
barbara.sherwoodlollar@utoronto.ca