More than a decade ago, a robotic rover on Mars finally revealed the answer to a pressing question. It is now clear that the red planet does contain organic material buried in the sediment of its ancient lakes.
Since then, we still find organic molecules on Mars arranged in a way that suggests carbon chemistry is widespread in our little rusty 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 comes from is a bit of a puzzle.
Now a team of researchers led by planetary scientist Yuichiro Ueno of the Tokyo Institute of Technology has uncovered evidence of its origin in the atmosphere, where carbon dioxide bathed in ultraviolet sunlight reacted to form a mist of carbon molecules that rained down on the planet’s surface.
While not as exciting as Martian biology, the discovery could help us figure out how the ingredients for life ended up right here on our home planet Earth billions of years ago.
“Such complex carbon-based molecules are the prerequisite for life, the building blocks of life, you could say,” says chemist Matthew Johnson of the University of Copenhagen.
“So it’s 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 by atmospheric photochemical reactions – thus without life. This is the ‘egg,’ a necessary condition for life. It remains to be seen whether this organic material led to life on the red planet.
The idea that photolysis—the process by which molecules are broken apart by light—plays a role in the organic chemistry found on the Martian surface has been advanced for some time. Johnson and two colleagues published a paper on the hypothesis in 2013, based on simulations, and others have since investigated further.
However, we need hard evidence from Mars that is consistent with the simulation results.
Photolysis of CO2 produces carbon monoxide and oxygen atoms. However, 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. Thus, 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 a noticeable “excess” of C-13 carbon dioxide.
This enrichment of the atmosphere with carbon-13 was already identified several years ago. Scientists analyzed a meteorite that came from Mars and landed in Antarctica and contained carbonate minerals that formed from CO2 in the Martian atmosphere.
“The smoking gun here is that the ratio of carbon isotopes in it matches exactly what we predicted in the quantum chemical simulations, but there was a piece missing from the puzzle,” Johnson explains.
“We were missing another product of this chemical process to confirm the theory, and that’s what we’ve got now.”
This missing piece of the puzzle was found in data obtained by the Curiosity rover in Gale Crater. There is a depletion of carbon-13 in the carbonate mineral samples found on the ground on Mars, which 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 the volcanic CO.2 emitted on Mars, which has a constant composition, similar to that of Earth’s volcanoes, and serves as a baseline,” says Johnson.
This is strong evidence that the carbonaceous organic material found by Curiosity arose from carbon monoxide produced by photolysis, the researchers say. And this gives us an idea of ​​the origin of organic material on Earth.
Billions of years ago, when the Solar System was just a baby, Earth, Venus, and Mars all had very similar atmospheres, suggesting that the same process likely occurred here on our home planet.
The three planets have evolved on very different paths since then, and Mars and Venus appear to be quite inhospitable to life as we know it, in their own idiosyncratic ways. But the rusted desert environment of Mars has now given us a clue about our own origins.
“We haven’t yet found this ‘smoking gun’ material here on Earth to prove that this 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 found it on Mars now, from a time when the two planets were very similar.”
The team’s findings were published in Nature Geoscience.