When the words "intriguing," "Mars," and "ancient life" appear in the same NASA statement, my ears get fresh. On Sunday, NASA spoke of a new study looking at "unusual carbon signals" measured by the Curiosity rover in the Gale crater of the red planet.
Curiosity has not found evidence of ancient microbial life on Mars, but scientists do not rule it out as a possible explanation for the rover's findings. Stone samples in powder form examined by the rover show the kind of carbon signatures associated with biological life on Earth. But Mars may be telling a completely different story.
The study is to be published this week in the journal Proceedings of the National Academy of Sciences.
Carbon is a key element in life on our own planet, so it's important to study what it looks like on Mars. "For example, living beings on Earth use the smaller, lighter carbon-12 atom to metabolize food or for photosynthesis compared to the heavier carbon-13 atom," NASA said. "Thus, significantly more carbon 12 than carbon 13 in ancient rocks, along with other evidence, suggests to scientists that they look at signatures of life-related chemistry."
Curiosity heated rock samples in a laboratory on board and used its Tunable Laser Spectrometer instrument to measure the gases released by the samples. Some of the rock samples had "surprisingly large amounts of carbon 12" compared to what has been found in the atmosphere on Mars and in Mars meteorites.
According to a statement from Penn State, the researchers suggested several explanations: "a cosmic dust cloud, ultraviolet radiation that decomposes carbon dioxide or ultraviolet decomposition of biologically created methane."
The cloud idea connects back to an event where the solar system passed through a galactic cloud of dust hundreds of millions of years ago, which could have left carbon-rich deposits on Mars. The second idea suggests that ultraviolet light could have interacted with carbon dioxide gas in the Martian atmosphere, leaving molecules with the characteristic carbon signature on the surface.
An idea of biological origin could have involved bacteria releasing methane into the atmosphere, which was then converted into molecules that settled on Mars, leaving the carbon signature that Curiosity found.
Mars and Earth have experienced very different lives, so scientists are wary of applying Earth's expectations to Mars data. "All three possibilities point to an unusual carbon cycle that bears no resemblance to Earth today," said Penn State geoscientist Christopher House, who led the study. "But we need more data to figure out which of these is the correct explanation."
Curiosity has been on Mars since 2012 and continues to explore rocks and sediments as it travels around the crater. Its study of carbon isotopes may not yet be able to answer the question of whether the red planet once hosted life, but the investigation continues.