Just over a decade ago, a robotic rover on Mars finally revealed an answer to a pressing question. It is now clear that the red planet does, indeed, have organic material buried in the sediments of its ancient lake beds.
Since then, we’ve continued to find organic molecules on Mars distributed in a way that suggests carbon chemistry is widespread in our rusty little neighbor.
This does not mean that we have found signs of alien life. Far from it; there are many non-biological processes that can produce organic molecules. But exactly where the material came from has created a bit of a puzzle.
Now, a team of researchers led by planetary scientist Yuichiro Ueno of the Tokyo Institute of Technology has discovered evidence of its origin in the atmosphere, where carbon dioxide bathed in the sun’s ultraviolet rays reacted to form a cloud of carbon molecules that rained down. on the surface of the planet.
While not as exciting as Martian biology, the discovery could help us understand how the ingredients for life ended up right here on our planet Earth billions of years ago.
“Such complex carbon-based molecules are the prerequisite of life, the building blocks of life, one might say,” says chemist Matthew Johnson of the University of Copenhagen.
“So this is a bit like the old debate about which came first, the chicken or the egg. We show that the organic material found on Mars was formed through atmospheric photochemical reactions – without life, that is. This is the ‘egg.’ a prerequisite for life Whether this organic material resulted in life on the red planet remains to be seen.
The notion that photolysis—the process where molecules are broken apart by light—plays a role in the organic chemistry found on the surface of Mars has been around for a while. Johnson and two colleagues published a paper on the hypothesis in 2013, based on simulations, and others have since investigated further.
What we need, however, is hard evidence from Mars that is consistent with the simulation results.
CO photolysis2 produces carbon monoxide and oxygen atoms. But there are two isotopes, or masses, of stable carbon. By far the most common is carbon-12, which contains six protons and six neutrons. The next heaviest is carbon-13, which contains six protons and seven neutrons.
Photolysis works faster on the lighter isotope. So when UV light photolytically splits the mixture of C-12 and C-13 carbon dioxide in the atmosphere, the C-12-containing molecules are depleted more quickly, leaving behind an apparent ‘excess’ of C-13 carbon dioxide .
This atmospheric enrichment with carbon-13 was already identified several years ago. The researchers analyzed a meteorite that came from Mars and landed in Antarctica, which contained carbonate minerals that formed from CO2 in the Martian atmosphere.
“The smoking gun here is that the ratio of carbon isotopes in it exactly matches our predictions in quantum chemical simulations, but there was a piece of the puzzle missing,” Johnson explains.
“We lacked the other product of this chemical process to confirm the theory, and this is what we have now.”
That missing piece of the puzzle was found in data taken by the Curiosity rover in Gale Crater. In samples of carbonate minerals found in the soil on Mars there is a depletion of carbon-13 that perfectly mirrors the enrichment of carbon-13 found in the Martian meteorite.
“There is no other way to explain the depletion of carbon-13 in the organic material and the enrichment in the Martian meteorite, both relative to the composition of volcanic CO.2 emitted on Mars, which has a constant composition, similar to Earth’s volcanoes, and serves as a base,” says Johnson.
This is strong evidence that the organic carbon material found by Curiosity was formed from carbon monoxide produced by photolysis, the researchers say. And this gives us a clue about the origin of organic material on Earth.
Billions of years ago, when the Solar System was just an infant, Earth, Venus and Mars all had very similar atmospheres, suggesting that the same process likely occurred here on our planet.
Since then, the three planets have evolved along very different paths, and Mars and Venus seem quite inhospitable to life as we know it in their own particular ways. But the rusty Martian desert environment has now given us a clue to our origins.
“We have not yet found this ‘smoking gun’ material here on Earth to prove that the process has occurred. Perhaps because the Earth’s surface is much more alive, geologically and literally, and therefore constantly changing,” says Johnson.
“But it’s a big step that we’ve now found on Mars, from a time when the two planets were very similar.”
The team’s findings are published in Nature Geoscience.