A little over a decade ago, a robotic rover on Mars finally uncovered an answer to a pressing question. It's now clear that the red planet does, indeed, have organic material buried in the sediment of its ancient lakebeds.
Since then, we've continued to find organic molecules on Mars distributed in a way that suggests carbon chemistry is widespread across our small rusty neighbor.
This doesn't mean we've 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 posed 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 origins in the atmosphere, where carbon dioxide bathed in ultraviolet sunlight reacted to form a mist of carbon molecules that rained onto the planet's surface.
While it's not quite as thrilling as Martian biology, the dicovery could help us figure out how the ingredients for life ended up right here on our home planet of Earth, billions of years ago.
"Such carbon-based complex 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 has been formed through atmospheric photochemical reactions – without life that is. This is the 'egg', a prerequisite of life. It still remains to be shown whether or not this organic material resulted in life on the red planet."
The notion that photolysis – the process whereby molecules are broken apart by light – plays a role in the organic chemistry found on the surface of Mars has been kicking around for a while. Johnson and two colleagues published a paper on the hypothesis in 2013, based on simulations, and others have subsequently investigated further.
What we need, though, is hard evidence from Mars that's 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. 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 mix of C-12 and C-13 carbon dioxide in the atmosphere, molecules containing C-12 are depleted faster, leaving a noticeable 'excess' of C-13 carbon dioxide behind.
This atmospheric carbon-13 enrichment had already been identified a few years ago. The researchers analyzed a meteorite that came from Mars and landed in Antarctica, containing carbonate minerals that formed from the CO2 in the Martian atmosphere.
"The smoking gun here is that the ratio of carbon isotopes in it exactly matches our predictions in the quantum chemical simulations, but there was a missing piece in the puzzle," Johnson explains.
"We were missing the other product of this chemical process to confirm the theory, and that's what we've now obtained."
That missing puzzle piece was found in data obtained by the Curiosity rover in the Gale crater. In the samples of carbonate minerals found on the ground on Mars is a carbon-13 depletion that perfectly mirrors the carbon-13 enrichment found in the Martian meteorite.
"There is no other way to explain both the carbon-13 depletion in the organic material and the enrichment in the Martian meteorite, both relative to the composition of volcanic CO2 emitted on Mars, which has a constant composition, similar as for Earth's volcanoes, and serves as a baseline," Johnson says.
This is strong evidence that the carbon organic material found by Curiosity formed from the 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 but a babe, Earth, Venus, and Mars all had very similar atmospheres, suggesting the same process likely occurred here on our home planet.
The three planets have since 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 rusted desert environment of Mars has now given us a clue about our own origins.
"We have not yet found this 'smoking gun' material here on Earth to prove that the process took place. Perhaps because Earth's surface is much more alive, geologically and literally, and therefore constantly changing," Johnson says.
"But it is a big step that we have now found it on Mars, from a time when the two planets were very similar."
The team's findings have been published in Nature Geoscience.