The Curiosity rover, led by NASA’s Jet Propulsion Laboratory in Southern California, has spent the past nine years exploring an area of the Red Planet’s Gale Crater. At this place, erosion reveals layers of very interesting ancient rocks. By taking samples from these sediments, we can obtain valuable information about the history of Mars. This planet was formed at the same time as the Earth 4.5 billion years ago, but it has undergone a very different evolution. Formerly wide rivers flowed over its surface; today the planet has become arid and desert.
The recent analysis of samples collected by Curiosity in this crater raises new questions about the past Martian environment.
An unexpected carbon cycle
Carbon is very present in living things, because it can easily bond with other atoms, and therefore form complex molecules. On Mars, researchers are particularly interested in its isotopes. The isotopy of an atom is the fact that it exists in a lighter or heavier form. Carbon 12, for example, has a nucleus with 6 protons and 6 neutrons. It is lighter than carbon 13 (6 protons and 7 neutrons), which itself is lighter than carbon 14 (6 protons and 8 neutrons).
To observe carbon isotopes, the Curiosity rover heated its samples up to 850°C and analyzed what was released. Molecules of methane (1 carbon atom bonded to 4 hydrogen atoms) dissipated when the rock extracts were heated. The researchers found a strangeness in the released methane: it was abnormally rich in carbon 12. How is this done?
“We have put forward three hypotheses that could explain this dominance of carbon 12”, indicates Caroline Freissinet, CNRS researcher at LATMOS (Laboratory Atmospheres, Environments, Spatial Observations). They are presented here in random order.
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1. The living hypothesis
The depletion of carbon 13 in carbon 12 (loss of a neutron) could have a biological origin. It is indeed the isotope favored by life because it is the lightest. “But this hypothesis is the least probable”, emphasizes Caroline Freissinet.
For it to be valid, this would imply that the methane was produced by life itself (by microbial mats for example), then that the methane produced is then used by these living organisms. All of this, of course, before Mars’ atmosphere escaped into space. If this hypothesis should not be totally ruled out, it remains implausible because it is not compatible with the observations made on the planet.
>> Read also: What form of life could we find on Mars?
2. An interstellar dust deposit
We know that every 100 to 200 million years Mars passes through a molecular cloud. When the planet passes through this cluster, dust containing carbon settles on its surface. These grains have been shown to contain molecules rich in carbon 12.
Thus, the dominance of carbon 12 in the samples taken by Curiosity could be explained by this deposit of interstellar dust.
“But here again, the hypothesis has its limits,” raises the specialist. Indeed: in the heated samples, the researchers observed that the sulfur was also depleted in its heavy isotope, sulfur 34. In other words, there seems to be a correlation between carbon and sulfur depletion. However, passing through a cloud of molecules does not explain it.
3. Reactions to the Sun’s Rays
Third theory: photons from solar radiation would have triggered reactions in the Martian atmosphere (photochemistry). Result: complex molecules with many carbons could have been created. By a phenomenon of “isotopic fractionation”, these complex molecules could have been enriched in carbon 12.
The latter would have settled on the ground, would have been covered and protected by several layers of sediments for 3 to 4 billion years. Then with erosion, they would have found themselves again on the surface, where the Curiosity rover draws its samples today.
This theory could explain why the methane released by heating the rock extracts has the signature of complex molecules with carbon depleted in the heavy isotope.
Problem: this configuration also does not explain why the sulfur found in the rover samples is also lighter.
“This study raises more questions than it answers,” notes the researcher.
Because this strange carbon cycle is not part of what we usually observe on Mars or on Earth. It may be a phenomenon specific to the red planet, which we have yet to discover!
>>Also discover: New organic molecules found by Curiosity on Mars!
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