As far as we know, carbon is critical to life. It could indicate biological activity if we detect a strong carbon signature.

Is a strong carbon signal in Martian rocks indicative of biological processes?

When you are hunting for life, any strong carbon signal is intriguing. It is a part of all the forms of life we know of. Carbon can become concentrated in the environment for a variety of reasons. It doesn't mean life is involved in carbon signatures.

The carbon atoms have six protons. Carbon atoms have different numbers of neutrons. C12 and C13 are stable, and C14 is a radionuclide. C12 has six neutrons, C13 has seven, and C14 has eight.

Life prefers C12 when it comes to carbon isotopes. They use it to make food. The reason is simple. When C12 bonds with other atoms, it makes fewer connections than C13 does. Life will always seek the easiest way to do things. It's easier to use C12 because it forms less bonds than C13. Life never takes the hard way when an easier way is available.

The rover is looking for signs of life in the crater. It drills into rock, extracts a sample, and puts it into a chemistry laboratory. SAM stands for Sample Analysis at Mars. Inside SAM, the rover bakes the sample and converts the carbon in the rock into methane. The process is done in a way that doesn't cause any problems. The Tunable Laser Spectrometer is used to find out what carbon isotopes are in methane.

The sample analysis tool is called SAM. SAM is made up of three different instruments and they search for and measure organic chemicals and light elements that are essential to life. The image is courtesy of NASA/JPL-Caltech.

The SAM team recently discovered something noteworthy after looking at 24 rock samples. The elevated ratios of C12 to C13 were found in six of the samples. The samples from these six sites contained more than 70 parts per thousand more C12 than the Earth-based standard. 98% of the carbon is C12 Earth, and the remaining 1% is C13.

The findings were presented in a new study. The title is "Depleted carbon isotope compositions observed at Gale crater, Mars." Christopher House is a scientist at Penn State University.

If these results were obtained on Earth, they would show that a biological process produced the abundance of C12

Methane was produced on ancient Earth. They are called methanogens and prokaryotes from the Archaea domain. In anoxic wetlands, in the gut of ruminants, and in the hot springs, methanogens are still present.

The methane enters the atmosphere when it interacts with ultraviolet light. More complex molecule rained down on the Earth from those interactions. Their carbon signatures are preserved in Earth rocks. If that happened on Mars, it could account for the findings of the rover.

This is Mars. If the history of searching for life on Mars tells us anything, it is not to get ahead of ourselves.

Paul Mahaffy was the principal investigator for the sample analysis at the Mars lab. If not life, we are looking at what else could have caused the carbon signature.

The panorama was taken on Vera Rubin Ridge. The image is from NASA/JPL-Caltech.

There are multiple plausible explanations for the 13C observed in evolved methane, but no single explanation can be accepted without further research.

Our Earth bias makes it difficult to understand carbon signatures like this one. The majority of what scientists know about atmospheric chemistry is based on Earth. It is difficult for scientists to keep their minds open to new possibilities when they discover a carbon signature on Mars. The history of the search for life on Mars shows this.

The hardest thing to do is let go of Earth and let go of the bias that we have in regards to chemistry, physics and the environment on Mars. The international team of scientists led by Eigenbrode were able to detect many organic molecules on the Martian surface.

"We need to think outside the box, and that's what this paper does."

There are two non-biological explanations for the carbon signature. One involves clouds.

Our Solar System passed through a cloud hundreds of millions of years ago according to the hypothesis. It happens about once every 100 million years, so scientists can't discount it. The type of lighter carbon detected by the rover in the crater may have been in a cloud that was rich in hydrogen. The cloud would have caused Mars to get cold. The elevated C12 would have been prevented by the cooling and glaciation. The paper states that the dust particles from the glaciers should be left on the glacier's surface.

The hypothesis fits since the C12 levels were found at the top of ridges and high points in the crater. The paper states that the samples were gathered from a variety of lithologies and are temporally spread throughout the mission operations to date. The chain of events is not likely to be the molecular cloud hypothesis.

The drill pointed skyward was used by the rover to explore Vera Rubin Ridge at the base of Mount Sharp. raw images were taken on Oct. 2, 2017, and the mosaic was colourized. NASA/JPL/Ken Kremer/ Di Lorenzo.

The ultraviolet light hypothesis is a non-biological one. In this scenario, UV light would interact with carbon dioxide gas in the atmosphere of Mars to create new molecule. The molecule would have rained down on Mars and become part of the rock there. The hypothesis is similar to how methanogens produce C12 on Earth.

Christopher House said all three explanations fit the data. We need more data to rule them out.

There are three hypotheses that could explain the carbon signature. The deposition of 13C-depleted organic material was created by biologically produced methane from the Martian interior. The orange shows photochemical reactions that can result in various atmospheric products, some of which would be deposited as organic material with easily-broken chemical bonds. The hypothesis of the cloud is shown in the grey. The image is from House et al.

The carbon signal we are detecting on Mars is biological. We need to understand if the same explanation works for Mars or if there are other explanations.

Almost half of the samples had elevated levels. They are higher than scientists have found in meteorites and the Martian atmosphere. All of the locations have ancient, well-preserved surfaces, and that's one of the reasons the samples came from them.

The findings aretantalizingly interesting. Scientists are still learning about the carbon cycle on Mars. It is tempting to make assumptions about the carbon cycle of Mars. Carbon may cycle through Mars in ways we don't know yet. It is still valuable knowledge when it comes to understanding Mars' carbon signature, whether or not it is a signal for life or not.

Andrew Steele, a scientist at the Carnegie Institute for Science in Washington, D.C., said that defining the carbon cycle on Mars is key to trying to understand how life could fit into that cycle. We have done that successfully on Earth, but we are just beginning to define that cycle for Mars.
It is difficult to draw conclusions about Mars based on Earth's carbon cycle. Steele said that there is a huge chunk of the carbon cycle on Earth that involves life, and because of life, there is a chunk of the carbon cycle on Earth we can't understand.

There is still work being done on Mars. The meaning of these samples, along with a better understanding of Mars' carbon cycle, lies ahead. More rock will be sample to measure carbon isotope concentrations. It will sample rock from other well-preserved ancient surfaces to see if results are similar to these. There is no way to prepare for one of the unpredictable methane events that it would encounter.

The Perseverance rover is looking for signs of ancient life on Mars. Perseverance's sampling activities can be influenced by results from Curiosity. The image is from NASA/JPL-Caltech.

The results will help inform Perseverance's sample collection at Jezero Crater. Perseverance can determine if carbon signals are biological or not.

Perseverance is gathering samples for return to Earth. Who knows what we will learn from those samples, because scientists will study them more effectively than the rover can.

There is a chance of ancient life on Mars, but it is not certain.