There is still a lot of mystery about the carbon samples collected by the Mars rover

Since August 6, 2012, NASA’s Mars Curiosity rover has been orbiting Gale Crater on Mars. In doing so, he examined several soil samples and transmitted the results back to Earth. One of the carbon isotopes in these soil samples shows three possible explanations for the origin of the carbon in it. But all three are uncertain and completely unfamiliar.

Carbon has two stable isotopes: C12 and C13. Both isotopes are found everywhere, but because carbon-12 reacts faster than carbon-13, examining the relative amounts of both isotopes in samples can provide definitive information about the carbon cycle that occurred, even a very long time ago.

Curiosity has spent the past nine years exploring an area in Gale Crater where layers of ancient rock have been discovered. The Mars rover drilled into the surface of these layers, heated up and analyzed with a spectrometer in the absence of oxygen. From this it can be concluded that some samples contain a very small amount of carbon-13, while others contain an excess.

A team of researchers, led by Frederic Schmidt of the University of Paris-Saclay, has three possible explanations for the exceptionally ‘depleted’ samples: a cosmic dust cloud, ultraviolet radiation that breaks down carbon dioxide, or the ultraviolet breakdown of biologically produced methane.

molecular cloud

Every few hundred million years, our solar system passes through a galactic molecular cloud, but such a cloud does not deposit much dust. To form a layer like Curiosity found, the cloud had to cool the Martian climate when the planet still had water, allowing glaciers to form. The dust then collects on top of the ice and is bound to stay in place as the glaciers melt. For example, there may have been a layer of carbon with a little C13 in it, but the problem is that there is no or no evidence of glaciers at Gale Crater.

Another possibility is that in the atmosphere of Mars, under the influence of ultraviolet (solar) rays, carbon dioxide was converted into organic compounds such as formaldehyde. There are studies that suggest this is possible, but to confirm or rule this out, more empirical research needs to be done.

The third possible method for producing carbon-13 depleted samples is biological in nature. On Earth, a signature of highly depleted C13 in ancient rocks indicates that microorganisms may have been at work. However, according to the researchers, there is currently no evidence that microorganisms ever lived on the (former) surface of Mars.

Based on the available data, researchers do not dare to give a definitive answer about the origin of C13-poor soil samples. But the situation was not yet hopeless: Curiosity is still collecting and analyzing soil samples.

Image: Artist’s impression of Kasei Valles on Mars, where glaciers flowed into the planet’s former northern ocean. (NASA / USGS / ESA / DLR / FU Berlin (G. Neukum))