Carbon in Martian rock samples reveals the secrets of life

Since 2012, NASA's Curiosity Mars rover has traveled across the Martian surface, drilled holes in rocks to sample and analyzed them in the chemistry lab on board, trying to tease out evidence of life on Mars.

On Jan. 17, scientists from the Mars rover project published an interesting phenomenon about carbon in the Proceedings of the National Academy of Sciences, which is not necessarily evidence that life once existed on Mars, but it is very strange. They found that Curiosity detected that some rocks contained large amounts of light carbon isotopes in their carbon. On Earth, this is strong evidence of the existence of ancient microorganisms.

However, given that this was discovered on Mars, the researchers were reluctant to jump to conclusions easily, so they have been looking for non-biological explanations for the phenomenon, including ultraviolet light and interstellar dust.

Christopher House, a biogeochemist at Pennsylvania State University at Park, said the study did increase the likelihood that microbes once existed and still exist on Mars today.

Carbon exists in two stable isotopic forms: "light" carbon-12 and "heavier" carbon-13. Since carbon-13 has one more neutron, the molecular bonds formed will be slightly "stronger". Therefore, life evolved to choose carbon-12, which is more easily decomposed. Most organic molecules produced by life activities are also rich in carbon-12, for example, methane from rice paddies contains more light carbon than non-biological methane emitted by deep-sea hydrothermal vents.

The team observed 24 different rock samples drilled along the way through Gaelic Crater, which houses mudstone from ancient lakes. The analysis results of different samples vary greatly, with the carbon-12 and carbon-13 levels drilled at 6 points more than 70% higher than the Earth-based reference standard.

"It's a dramatic discovery." House said the phenomenon was most pronounced in samples from the top of the ridge and other topographical heights of impact craters. As a result, the team believes that the enriched carbon was deposited from the atmosphere billions of years ago, not left by lake sediments.

Concentrating light carbon to such a high level may require multiple steps, a process that the researchers speculated about.

One hypothesis is that microbes deep in Mars feed on lighter carbon in magma, releasing methane gas. Other microbes on the surface then feed on the emitted methane, further increasing the light carbon content and sequestering the light carbon after its death.

However, the Mars rover has yet to find physical traces of ancient microbes. Therefore, the researchers believe that it is also possible that microbes deep in Mars are concentrated in light carbon driven by ultraviolet light. Ultraviolet light may break down methane produced by microorganisms, further enriching light carbon while producing sub-products such as formaldehyde, which are eventually deposited on the surface of Mars.

In addition, it is also possible that the young solar system, including early Mars, traveled through an interstellar cloud containing gas and dust. Scientists believe this phenomenon occurs every 100 million years or so. This interstellar cloud contains light carbon, comparable to the carbon observed by Curiosity. Dust clouds may have blocked sunlight, causing Mars to freeze deeply, forming glaciers, and preventing the light carbon in cosmic dust from being diluted by other sources of carbon. House said the incident was a great coincidence, and there was no evidence of glaciers in Gaelic Crater, but that possibility could not be ruled out.

Mark Harrison, a planetary scientist at the University of California who was not involved in the study, also believes that carbon enrichment is indeed a hint of the existence of ancient life, but judging the existence of life through such signs is also controversial on Earth, so non-biological explanations cannot be excluded. To solve the above mystery, more exploration and research are needed. (Xu Rui)

Source: China Science Daily