Scientists are excited when they discover carbon on other planets as it's the foundation for all life on Earth, and the discovery of an unusual mix of the chemical element on Mars could point to the existence of alien life.
It's a possibility, but it's not certain. It's one of three different scenarios that experts think might have produced the carbon found in the Gale crater, collected across nine years from August 2012 to July 2021.
A wide variation in the mix of carbon 12 and carbon 13 is revealed by the heated powder samples, which were used to separate individual chemicals.
The landscape of the planet Mars. NASA/Caltech-JPL/MSSS
Some samples enriched with carbon 13, and some extremely deplete, are examples of unconventional processes created by the carbon cycle in the modern era.
The amount of carbon 12 and carbon 13 in the Solar System are the amounts that existed at the beginning, says Christopher House from Pennsylvania State University.
Carbon 12 reacts more quickly than carbon 13 so it's possible to see the carbon cycle by looking at the relative amounts of each in samples.
A giant cloud of dust is one explanation for the carbon signatures. The Solar System passes through one of these every couple of hundred million years or so, and the cooling effect it creates leaves carbon deposits in its wake. The team says that this is a plausible scenario, but that more investigation is needed.
The conversion of CO2 to organic compounds through abiotic processes could explain what was found. It's something scientists have speculated about before, but further study is needed to confirm whether or not this is actually what's happening.
The third explanation is that either ultraviolet light or microbes once upon a time converted methane produced by biological processes into carbon, which is what we're looking at today. We need more evidence to know for sure, but there are some parallels on Earth.
"The samples that are very low in carbon 13 are very similar to samples from Australia that were over 2 billion years old," says House.
The samples were caused by biological activity when methane was consumed by ancient mats, but we can't say that on Mars because it's a planet that may have formed out of different materials and processes than Earth.
The mission continues, of course. The discovery of the remains of mats, methane or glaciers in the future would help the scientists figure out which is the most likely explanation.
We don't know enough about Mars and its history to come to any conclusions about how these carbon signatures came about. The spot where many of the samples were collected is where further drilling is planned.
The Perseverance rover is planning to return Martian rocks to Earth rather than experiment on them in situ. The two robotic explorers are going to reveal a lot over the next few years.
"All three possibilities point to an unusual carbon cycle unlike anything on Earth today," says House. We need more data to figure out which of these is the correct explanation.
The best course of study when studying another world is to be cautious with your interpretation.
The research has been published.
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