A little over a decade ago, a robotic rover on Mars finally discovered an answer to a pressing question. It is now clear that the red planet does have organic material buried in the sediment 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 little rusty neighbor.
This does not mean that we have found signs of extraterrestrial life. Far from it; there are many non-biological processes that can produce organic molecules. But where exactly the material came 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 found evidence of its origin in the atmosphere, where carbon dioxide bathed in ultraviolet sunlight reacts to form a fog of carbon molecules that rain down on the planet’s surface .
Although not as exciting as Martian biology, the discovery could help us understand how the ingredients for life ended up right here on our home planet Earth billions of years ago.
“Such carbon-based complex 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 formed through atmospheric photochemical reactions—without life, that is. it is the “egg”, a prerequisite for life. Whether this organic material gave rise to life on the red planet remains to be seen.”
The idea that photolysis—the process by which 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 subsequently investigated further.
What we need, however, is hard evidence from Mars that is consistent with the simulation results.
The photolysis of CO2 produces carbon monoxide and oxygen atoms. But there are two isotopes or masses of stable carbon. 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 a noticeable “surplus” of C-13 carbon dioxide behind.
This enrichment of atmospheric carbon-13 was already identified several years ago. The researchers analyzed a meteorite that came from Mars and landed in Antarctica, containing carbonate minerals formed by 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 were missing the other product of this chemical process to confirm the theory, and this is what we now have.”
This missing piece of the puzzle was found in the data obtained by the Curiosity rover in Gale Crater. Samples of carbonate minerals found on the Martian soil are depleted of carbon-13, which perfectly mirrors the enrichment of carbon-13 found in the Martian meteorite.
“There is no other way to explain both the depletion of carbon-13 in the organic material and the enrichment of the Martian meteorite, both in terms of volcanic CO composition2 emitted on Mars, which has a consistent composition similar to Earth’s volcanoes, and serves as a baseline,” Johnson says.
This is strong evidence that the carbonaceous organic material found by Curiosity formed from carbon monoxide produced by photolysis, the researchers said. And this gives us insight into the origin of organic material on Earth.
Billions of years ago, when the solar system was just a baby, Earth, Venus and Mars had very similar atmospheres, suggesting that the same process probably happened here on our home 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 idiosyncratic ways. But the rusty desert environment of Mars is already giving us a glimpse of our own origins.
“We have not yet found this ‘smoky’ material here on Earth to prove that the process took place. Maybe 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 it on Mars, from a time when the two planets were very similar.”
The team’s findings are published in Nature Geoscience.
