NASA’s Curiosity rover has detected high amounts of an unexpected form of carbon on Mars. That might not sound too exciting, but the kicker is that here on Earth, this chemical signature is usually associated with life.
Let’s temper expectations right off the bat – it’s probably not aliens. There are other, non-biological ways that kind of carbon could have gotten there. But NASA also can’t be sure that it’s not not aliens, and that warrants a closer look.
The Curiosity rover has been drilling samples of rock for years, and then analyzing the chemical composition of the resulting powder. One of the things it can detect is ratios of different isotopes – atoms of the same element with different numbers of neutrons in their nuclei. It was expected that the isotope carbon-13 would be most common, but around half of the samples taken during a recent drilling expedition showed surprisingly large amounts of carbon-12.
Crucially, carbon-12 is usually considered a signature of biological chemistry. Earthly organisms use carbon-12 to metabolize their food, while plants use it to perform photosynthesis. That seems to suggest that the rover has detected evidence of ancient life on Mars, however, the team says we just don’t know enough about the Red Planet’s carbon cycle to be sure.
“On Earth, processes that would produce the carbon signal we’re detecting on Mars are biological,” said Christopher House, lead author of the study. “We have to understand whether the same explanation works for Mars, or if there are other explanations, because Mars is very different.”
As for those other explanations, the team gives two non-biological hypotheses about the source of this carbon-12. The first says that ultraviolet light from the Sun could have interacted with carbon dioxide in Mars’ atmosphere, which would have produced carbon-rich molecules that then settle on the surface. And the second story suggests that the solar system may have passed through a huge molecular cloud hundreds of millions of years ago, which could have caused more carbon-12 to rain down to the surface.
And then there’s the third hypothesis – that ancient bacteria living on and just below the surface of Mars would have released methane into the atmosphere. This would then have interacted with UV light and been converted into more complex molecules, creating the carbon signature detected by Curiosity billions of years later.
As tempting as it is to want to believe the third story, the team cautions that non-biological origins are probably the more likely culprit. What works on Earth doesn’t necessarily apply to Mars.
“The hardest thing is letting go of Earth and letting go of that bias that we have and really trying to get into the fundamentals of the chemistry, physics and environmental processes on Mars,” said Jennifer L. Eigenbrode, an author of the study.
Future work, including that done by the Red Planet’s most recent arrival Perseverance, could help uncover more information about the carbon cycle on Mars, and whether or not this intriguing chemical signature is actually evidence of life.
The research is due to be published in the journal PNAS.