A little over a decade ago, a robotic rover on Mars finally found the answer to a pressing question: the Red Planet does indeed have Contains organic matter It is buried in sediments from an ancient lake bottom.
Since then, we continue to find organic molecules distributed on Mars in ways that suggest carbon chemistry is widespread throughout our tiny rusty neighbor.
This doesn’t mean we’ve found any traces of extraterrestrial life — on the contrary, there are plenty of non-biological processes that can produce organic molecules — but where exactly that material comes from is a bit of a mystery.
Now, a research team led by planetary scientist Yuichiro Ueno of the Tokyo Institute of Technology has found evidence of that origin: carbon dioxide exposed to the sun’s ultraviolet rays in the atmosphere reacted to form a mist of carbon molecules that then rained down on the planet’s surface.
While not as thrilling as Martian biology, the discovery could help shed light on how the ingredients for life got to our home planet billions of years ago.
“These complex carbon-based molecules are prerequisites for life, and we could say they are the building blocks of life.” chemist Matthew Johnson says of the University of Copenhagen.
“So it’s a bit like the old chicken-and-egg debate. We show that the organic matter found on Mars was formed by photochemical reactions in the atmosphere, which means that there was no life there. This is the ‘egg’, the prerequisite for life. Whether this organic matter led to life on the Red Planet remains to be seen.”
The idea that photolysis (the process by which molecules are broken down by light) plays a role in the organic chemical reactions seen on the Martian surface has been discussed for some time. Johnson and two colleagues In 2013, he published a paper on this hypothesis.Based on simulations, and others Further investigation.
But what we need is hard evidence from Mars that matches the simulation results.
Photolysis of CO2 It produces carbon monoxide and oxygen atoms.However, there are two stable isotopes, or masses, of carbon. Most common The heaviest is carbon-12, which contains 6 protons and 6 neutrons, and the next heaviest is carbon-13, which contains 6 protons and 7 neutrons.
Photolysis works faster on lighter isotopes, so when ultraviolet light photolyzes a mixture of C-12 and C-13 carbon dioxide in the atmosphere, the C-12-containing molecules are reduced more quickly, leaving a visible “excess” of C-13 carbon dioxide.
This concentration of carbon-13 in the atmosphere was identified several years ago, and the researchers analyzed it. Meteorite from Mars It landed on the Antarctic and contained carbonate minerals formed from carbon dioxide in the Martian atmosphere.
“The smoking gun is that the ratios of carbon isotopes match exactly what quantum chemical simulations predict, but that there was a missing piece of the puzzle.” Johnson explains.
“What was missing to support the theory was another product of this chemical reaction, and now we have it.”
The missing piece of the puzzle was found in data collected by the Mars rover Curiosity at Gale Crater: carbonate mineral samples found on the Martian surface showed a depletion of carbon-13 that perfectly mirrored the enrichment of carbon-13 found in Martian meteorites.
“The depletion of carbon-13 in organic matter and the enrichment of carbon-13 in Martian meteorites are related to the composition of volcanic CO, but cannot be explained in any other way.2 The radiation emitted by Mars has a consistent composition, similar to that of volcanoes on Earth, and serves as a baseline.” Johnson says.
This is strong evidence that the carbon-based organic matter Curiosity found formed from carbon monoxide produced by photolysis, the researchers say, and it could provide clues about the origin of organic matter on Earth.
Billions of years ago, when the solar system was in its infancy, the atmospheres of Earth, Venus and Mars were very similar, and a similar process probably occurred on our home planet.
The three planets have since evolved on very different paths – Mars and Venus, each in their own unique ways, seem entirely inhospitable to life as we know it – but Mars’ rusty desert environment now offers clues about our own origins.
“On Earth, we have yet to find the ‘smoking gun’ that proves this process occurred, probably because the Earth’s surface is much more active and constantly changing, both geologically and literally.” Johnson says.
“But it’s a big step to find something on Mars from a time when the two planets were so similar.”
The team’s findings were: Nature Chemistry.
