Since it is the basis of all life on Earth, the discovery of carbon on other planets is always exciting for scientists – and the Curiosity rover discovered an unusual mixture of chemical elements that could hypothetically indicate the existence of alien life.
This is by no means certain, but it is possible. This is one of three different scenarios that experts believe could have generated the carbon found in the sediments at Gale Crater collected over the nine years from August 2012 to July 2021.
A total of 24 powder samples were heated by Curiosity to separate individual chemicals, revealing wide diversity in terms of the mixture of carbon-12 and carbon-13 isotopes: two stable carbon isotopes that can show how the carbon cycle may have changed over time.
Part of the Martian landscape where the samples were taken. (NASA/Caltech-JPL/MCSS)
What makes these variations particularly interesting—some samples are enriched in carbon-13 and some extremely depleted—is that they point to unconventional processes that are different from those created by the carbon cycle in Earth’s modern era.
“The amounts of carbon-12 and carbon-13 in our solar system are the amounts that existed when the solar system was formed,” says geologist Christopher House of Pennsylvania State University.
“Both exist in everything, but because carbon 12 reacts faster than carbon 13, looking at the relative amounts of each in samples can reveal the carbon cycle.”
One explanation for the carbon signatures is a giant molecular cloud of dust. The solar system passes through one of them every couple of hundred million years or so, and the cooling effect it creates leaves carbon deposits behind. The team says this is a plausible scenario, but needs further study.
Alternatively, converting CO2 to organic compounds (such as formaldehyde) through abiotic (non-biological) processes could explain what Curiosity found – in this case, ultraviolet light could be the trigger. This is something that scientists have hypothesized about before, but again, further research is needed to confirm if this is actually happening.
A third explanation remains, which is that either ultraviolet radiation or microbes once converted the methane produced by biological processes – we are dealing with carbon created as a result of life. As with the other two possibilities, we’ll need more surrounding evidence to know for sure, but there are some parallels on Earth.
“The extremely low carbon-13 samples are a bit like the 2.7 billion-year-old samples from Australia,” says House.
“These patterns were caused by biological activity when methane was consumed by ancient microbial mats, but we can’t necessarily say Mars because it’s a planet that may have formed from different materials and processes than Earth.”
Curiosity’s mission, of course, continues. The discovery in the future of remnants of a microbial mat, large methane plumes, or traces of long-lost glaciers will help scientists figure out which of these three explanations is most likely.
However, at this point, we simply don’t know enough about Mars and its history to draw any conclusions about how these carbon signatures came about. Further drilling is planned in the place where many such samples were taken in a month.
Curiosity was recently joined by the Perseverance rover, which plans to return Martian rocks to Earth rather than experimenting with them in situ. Expect a lot more to be revealed by these two exploration robots in the coming years.
“All three possibilities point to an unusual carbon cycle unlike anything on Earth today,” says House. “But we need more data to find out which of these explanations is correct.
“We are careful with our interpretation, which is the best course in learning about another world.”
The study was published in PNAS.
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