Since NASA's Curiosity rover's descent on Mars in 2012, it has rolled the planet's dusty surface searching for signs of ancient small life forms like microbes. The rover uses its seven-foot-long arm to drill into rocks and analyze the dust's makeup for chemical fingerprints of the Red Planet's history. A new analysis of sediment samples collected by Curiosity revealed an unusual amount of carbon isotopes, reports Science's Paul Voosen.
Chemical signatures like these are considered strong—but heavily debated—evidence for prehistoric, microbial life here on Earth, but the two planets are ultimately too different to make any definitive claims based on direct comparisons alone. Alternatively, scientists suggest the strange isotopes could have been caused by space dust or the degradation of carbon dioxide from ultraviolet light, reports Andrew Griffin for the Independent. Researchers published details of the carbon signature this week in the Proceedings of the National Academy of Sciences.
Carbon is considered a building block of all life on planet Earth. In the carbon cycle, carbon atoms move from the atmosphere to the ground and then back to the atmosphere through processes like photosynthesis, decomposition, and human and animal emissions on land and sea. The process of carbon cycling can be used to trace biological activity on Earth, and researchers applied this idea to the possibility of life on Mars, reports Ashley Strickland for CNN.
Researchers reviewed 24 Martian sediment samples collected from six exposed locations in the Gale Crater that contained "mudstones of an ancient lake," Science reports. Before the samples were analyzed, the rover baked them at 1,562 degrees Fahrenheit to separate elements in the soil, per CNN. Baking the samples released methane, which was analyzed by another tool inside the rover that revealed it contained carbon isotopes.
Carbon has two stable isotopes, carbon-12 and carbon-13, the Independent reports. Most organic molecules are made up of carbon-12, and carbon-13 has an extra neutron, which allows it to make stronger bonds, Science reports. When analyzing the samples, the team found that the soils were highly depleted of the isotope carbon-13.
"The samples extremely depleted in carbon-13 are a little like samples from Australia taken from sediment that was 2.7 billion years old," says Christopher H. House, a geoscientist at Pennsylvania State University and the study's first author, in a statement. "Those samples were caused by biological activity when methane was consumed by ancient microbial mats, but we can't necessarily say that on Mars because it's a planet that may have formed out of different materials and processes than Earth."
If a depleted carbon-13 signature were measured on Earth, it would result from microbes consuming methane, CNN reports. The rover on Mars has detected methane, but it is too low to measure carbon isotopes in it, Science reports.
Life isn't the only possible explanation for these carbon isotopes. A large cloud of galactic dust that the solar system passes through every hundred million years could have lowered temperatures on ancient Mars, and froze water that the planet may have had. When glaciers formed, a layer of dust may have been left on top of the ice. The sediment containing the carbon isotopes would have remained long after the ice melted, per CNN. However, the researchers have stated that the Gale Crater has little evidence of past glaciation, and more evidence is needed, the Independent reports.
Another explanation could be carbon dioxide on the Red Planet reacted with ultraviolet radiation and converted the CO2 into other organic compounds like formaldehyde. Mars' atmosphere is 96 percent carbon dioxide. Other studies—including unpublished lab results from Yuichiro Ueno, a planetary scientist from the Tokyo Institute of Technology—have found that UV rays can generate a carbon signal in this way, per Science. "“The reported carbon isotope ratios are exactly what I have expected,” Ueno, who was not involved in the study, tells Science.
"All three possibilities point to an unusual carbon cycle unlike anything on Earth today," House says in a statement. "But we need more data to figure out which of these is the correct explanation. It would be nice if the rover would detect a large methane plume and measure the carbon isotopes from that, but while there are methane plumes, most are small, and no rover has sampled one large enough for the isotopes to be measured.”
Curiosity did come across such a plume in 2019, "but there’s no way to predict whether that will happen again," per NASA.
Editor's Note, January 20, 2022: A previous version of this article incorrectly stated that carbon-13 has an extra electron, when, in fact carbon-13 has an extra neutron. The story has been edited to correct that fact.