NASA’s Curiosity rover, which has been scouring Gale Crater and the slopes of Mount Sharp (Aeolis Mons) since 2012, has detected an important carbon signature in samples collected using its assortment of research instruments. The samples feature a rich carbon signature that is associated with biological processes on Earth.

Scientists were able to detect the signature from samples analyzed by the TLS (Tunable Laser Spectrometer) and SAM (Sample Analysis at Mars) instruments.

While the detection of this carbon signature is intriguing, it doesn’t directly point to ancient life on the Red Planet. Curiosity scientists have not yet found conclusive evidence supporting ancient or present-day life on Mars — though that is an element for Curiosity’s cousin rover, Perseverance.

“We’re finding things on Mars that are tantalizingly interesting, but we would really need more evidence to say we’ve identified life,” said Paul Mahaffy, former principal investigator of the SAM instrument at NASA’s Goddard Space Flight Center.

“So we’re looking at what else could have caused the carbon signature we’re seeing if not life.”

Detecting the signatures

Curiosity’s SAM instrument lab is comprised of five primary components: a gas chromatograph, a mass spectrometer, a Tunable Laser Spectrometer, a Sample Manipulation System, and ovens.

For this particular study, Christopher House of Pennsylvania State University led a team that used 24 different samples collected by Curiosity during its time in Gale Crater. Each sample was taken from five geologically diverse regions in the area that featured well-preserved ancient surfaces.

The Highfield drill hole, drilled by Curiosity on Dec. 10, 2021. This particular drill hole and sample was rich in carbon. (Credit: NASA/JPL-Caltech/MSSS)

Each sample was placed into SAM’s ovens, where they were heated to approximately 850° Celcius — allowing gases inside the samples to release into SAM’s instruments. Once these gases reached the instruments, the Tunable Laser Spectrometer then measured the isotopes of some carbon released from the samples.

Isotopes like those measured in the House et al. research are vital to understanding the biological and chemical processes that both did — and still are — taking place on Mars. Curiosity is the first rover capable of interpreting and studying carbon isotopes directly on the Martian surface.

But why is carbon so important?

Carbon is commonly considered one of the most — if not the most — important elements in life as it is currently understood and known to have evolved. Carbon is also continuously around us in the air, water, and ground.

Understanding carbon and its characteristics on Mars could be crucial to the search for life on the Red Planet and in other locations throughout our solar system — like some of the moons of Jupiter and Saturn.

Carbon signatures are comprised of several different types of atoms that vary in size and weight. Living creatures on Earth use a carbon-12 atom to metabolize nourishment and/or photosynthesize instead of the larger and heavier carbon-13 atom.

If astrobiologists and researchers can determine the types of carbon atoms present in samples that feature carbon signatures, they can gain a better understanding of what conditions the sample was exposed to before being sampled by a rover. For example, if large amounts of carbon-12 atoms were found, it could mean the signatures are connected to life-related chemistry — as long as other environmental evidence supports.

Additionally, investigating and understanding the ratio between the types of carbon atoms present in a carbon signature provides insight into the type of life that could have existed and the environment it lived in.

Curiouser & curiouser! I found samples with unusual carbon isotopes, which are key in understanding the evolution of planets. On Earth, this is linked to life but it may still be created by geology. What does it mean on Mars? Ah, that’s the great puzzle!

— Curiosity Rover (@MarsCuriosity) January 18, 2022

For the House et al. study, researchers found roughly half of the 24 samples contained large amounts of carbon-12 atoms — a potentially stunning discovery given carbon-12’s relationship to life on Earth. The amount of carbon-12 measured is quite a bit more than what has been previously measured in other Martian atmosphere and meteorite samples.

But life might not be the answer to this large amount of carbon-12 identified using Curiosity.

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“On Earth, processes that would produce the carbon signal we’re detecting on Mars are biological. We have to understand whether the same explanation works for Mars, or if there are other explanations because Mars is very different,” said House.

Curiosity scientist Andrew Steele of the Carnegie Institution for Science added: “There’s 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 because everywhere we look there is life.”

Mars likely formed with a different mix of carbon isotopes than Earth, and carbon could be cycling through the planet without any life interfering. But, researchers and planetary scientists are still trying to understand how carbon and other elements cycle on Mars, and this involves trying to understand the exact isotopic ratios and what exactly led to certain atoms and elements forming on Mars in the past.

“Defining the carbon cycle on Mars is absolutely key to trying to understand how life could fit into that cycle,” said Steele. “We have done that really successfully on Earth, but we are just beginning to define that cycle for Mars.”

The House et al. paper provides details of the SAM and Tunable Laser Spectrometer results while also providing hypotheses on what could have caused the carbon signature and what it means.

The biological hypothesis is inspired by life on Earth, with researchers hypothesizing that ancient bacteria on the Martian surface could have produced a unique carbon signature when they released methane into the atmosphere. From there, ultraviolet light could convert the methane gas into larger complex molecules that could have then rained  onto the surface where they could still be preserved with the unique carbon signature released by the bacteria.

The first of the two nonbiological hypotheses suggests that the carbon signature in the samples could be a result of an interaction between ultraviolet light and carbon dioxide gas in the Martian atmosphere — producing new, carbon-containing molecules. These new molecules would have later settled onto the Martian surface.

The second nonbiological hypothesis suggests that the carbon signature is the result of leftover carbon from a time when the solar system could have passed through a giant molecular cloud that was rich in the type of carbon detected in the Curiosity samples. This pass through the carbon-rich molecular cloud is thought to have occurred hundreds of millions of years ago — still fairly recent in terms of the cosmic time scale.

“All three explanations fit the data. We simply need more data to rule them in or out,” House added.

Some of the near 35 holes Curiosity has drilled in Gale Crater. Many of these drill holes and samples were used in the House et al. study. (Credit: NASA/JPL-Caltech/MSSS)

Data from the House et al. study is expected to help teams working with Curiosity’s cousin rover, Perseverance, in Jezero Crater as data from the carbon signature study could provide insight on where to collect samples that can be returned to labs on Earth and analyzed in a way to determine whether or not the carbon signature was produced by life.

The House et al. research was published in the January 2022 issue of the Proceedings of the National Academy of Sciences journal.

(Lead image: Curiosity takes a selfie in front of surface outcrop Mont Mercou while exploring Mount Sharp. Credit: NASA/JPL-Caltech/MSSS)

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