A little over a decade ago, a robotic rover on Mars finally answered a burning question. It’s now clear that the red planet’s ancient lakes have organic matter buried in their sediments. Since then, we’ve continued to find organic molecules scattered across Mars in ways that suggest carbon chemistry was common on our rusty little neighbor.

That doesn’t mean we’ve found signs of alien life. No way; there are many non-biological processes that can create organic molecules. But where this stuff comes from has been a bit of a mystery.

Now, a research team led by planetary scientist Yuichiro Ueno of the Tokyo Institute of Technology has found evidence that its origins lie in the atmosphere, where carbon dioxide bathed in ultraviolet sunlight reacts to form a fog of carbon molecules that falls to the planet’s surface.

While not as exciting as Mars biology, this discovery could help us understand how the ingredients of life arrived on our home planet, Earth, billions of years ago.

“These kinds of complex carbon-based molecules are the prerequisites for life, the building blocks of life,” says chemist Matthew Johnson of the University of Copenhagen.

“So it’s a bit like the old chicken or the egg debate. We show that the organic matter on Mars was formed as a result of photochemical reactions in the atmosphere, meaning that there was no life. It is not yet known whether this ‘egg’, this organic matter, which is a necessary condition for life, gave birth to life on the red planet.”

Photolysis, the process by which molecules are broken down by light, has been thought for some time to play a role in organic chemistry on the Martian surface. Johnson and two colleagues published a paper on the hypothesis in 2013 based on simulations, and others have since followed suit.

But we need strong evidence from Mars that matches the simulation results.

During photolysis of CO₂ carbon monoxide and oxygen atoms. However, there are two isotopes or masses of stable carbon. The most common is carbon-12, which has six protons and six neutrons. Next in order by weight is carbon-13, which has six protons and seven neutrons.

Photolysis works faster for a lighter isotope, so when UV light photolytically splits the mixture of C-12 and C-13 carbon dioxide in the atmosphere, the molecules containing C-12 are depleted more quickly, leaving behind a noticeable “excess” of C-13 carbon dioxide.

This enrichment of carbon-13 in the atmosphere was already detected a few years ago. Researchers analyzed a meteorite that flew from Mars and landed in Antarctica, which contained carbonate minerals formed from CO2 in the Martian atmosphere.

“The main reason for that was that the ratio of carbon isotopes in it matched exactly the ratios we predicted in quantum chemical simulations, but there was a missing piece of the puzzle,” Johnson explains. “We didn’t have another product of this chemical process to confirm the theory, and this is what we have now.”

This missing piece of the puzzle was found in data from the Curiosity rover at Gale Crater. Samples of carbonate minerals found in Martian soil are deficient in carbon-13, which perfectly mirrors the carbon-13 enrichment found in Martian meteorites.

“There is no other way to explain both the depletion of carbon-13 in organic material and the enrichment in the Martian meteorite, both relative to the composition of volcanic CO._2, “On Mars, which has a stable composition similar to volcanoes on Earth, it is released and acts as a foundation,” Johnson says.

This is strong evidence that the carbonaceous organic material found by Curiosity is made up of carbon monoxide produced through photolysis, the researchers said. This gives us a clue about the origin of organic matter on Earth. Billions of years ago, when the solar system was still a baby, Earth, Venus and Mars had very similar atmospheres, suggesting that the same process was happening here on our home planet.

Since then, the three planets have evolved in very different ways, and Mars and Venus appear to be quite inhospitable to life as we know it in their own unique ways. But Mars’ rusty desert environment now gives us a clue about our own origins.

“We haven’t found this ‘smoking gun’ material on Earth yet that would prove the process is happening. Perhaps because the Earth’s surface is much more geologically and literally alive, and therefore constantly changing,” Johnson says.

“But finding it on Mars at a time when the two planets are very similar is a big step forward.” The team’s findings were published on: Nature Geology.