Just over a decade ago, a robotic Mars rover most finally found an answer to a pressing question. It is now clear that there are indeed organic materials on the Red Planet buried in the sedimentary earth layers of ancient lake beds, NOVA reported.
Since then, we continue to find organic molecules on Mars distributed in a way that suggests carbon chemistry is widespread on Mars, Science Alert reported.
This does not mean that signs of extraterrestrial life have been found. There are many non-biological processes that can produce organic molecules. But where exactly the material came from is a bit of a mystery.
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 falls on the surface of the planet.
Although not as exciting as Martian biology, the discovery could help us understand how the ingredients of life ended up right here on our home planet Earth billions of years ago.
"Such complex carbon-based molecules are a prerequisite for life, and can be said to be the building blocks of life. So it's a bit like the old argument about which came first - the chicken or the egg. We demonstrate that the organic materials found on Mars were formed by atmospheric photochemical reactions - that is, without life. This is the "egg", which is a prerequisite for life. Whether or not this organic material gave rise to life on the Red Planet remains to be proven," said chemist Matthew Johnson of the University of Copenhagen.
The idea that photolysis -- the process by which molecules break apart under the influence of light -- plays a role in the organic chemistry found on the surface of Mars has been floating around for some time.
In 2013, Johnson and two colleagues published a paper on this hypothesis based on simulations, and others have since continued to investigate it.
What we need, however, is hard evidence from Mars to match the results of the simulations.
The photolysis of CO2 produces carbon monoxide and oxygen atoms. However, there are two isotopes or masses of stable carbon. The most common is carbon-12, which contains six protons and six neutrons. Next in weight is carbon-13, which contains six protons and seven neutrons.
Photolysis occurs faster with the lighter isotope. So when UV light splits the mixture of C-12 and C-13 carbon dioxide in the atmosphere, the molecules containing C-12 are depleted more quickly, leaving a noticeable "excess" of carbon dioxide C-13.
Enrichment of the atmosphere with carbon-13 was established several years ago. The researchers analyzed a meteorite that came from Mars and fell in Antarctica, containing carbonate minerals that formed from CO2 in the Martian atmosphere. "The big discovery here is that the ratio of carbon isotopes in it matches exactly what we predicted in the quantum chemical simulations, but there was one piece missing from the puzzle," Johnson explains.
"We lacked the other product of this chemical process to confirm the theory, and that is exactly what we got now," he added.
This missing piece of the puzzle was found in the data received by the Mars rover "Curiosity" in Gale Crater. Carbon-13 depletion was observed in samples of carbonate minerals found on the Martian soil, which fully mirrored the carbon-13 enrichment 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 in the Martian meteorite, both relative to the composition of the volcanic CO2 emitted on Mars, which has a constant composition similar to that of terrestrial volcanoes, and serves as a starting point," Johnson added.
This is conclusive evidence that the carbonaceous organic material found by Curiosity was formed from carbon monoxide produced by photolysis, the researchers say. This gives us an idea of the origin of organic materials on Earth.
Billions of years ago, when the solar system was very small, Earth, Venus and Mars had very similar atmospheres, suggesting that the same process may have 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 idiosyncratic ways. The rusty desert environment of Mars, however, is already giving us information about our own origins.
"We have not yet found that great discovery - material here on Earth to prove that the process took place. Perhaps because the surface of the Earth is much more alive, geologically and literally, and therefore constantly changing. Still, it's a big step to now find it on Mars, since the two planets were very similar." Johnson also said.