For the first time, scientists say they have clear evidence that the chemical building blocks of life exist on Mars.
What they can't say yet is whether there is, or ever was, life on the Red Planet.
The new evidence comes from a pair of rocks. NASA's six-wheeled Curiosity rover drilled into the planet in late 2014 and early 2015. The two rock samples, from sites named Confidence Hills and Mojave, are at the bottom of Gale Crater.
Powder from the rocks went into an analyzer on the rover called SAM that can determine what they were made of.
But the SAM results were hard to interpret — there were a lot of extraneous signals that didn't make any sense. So NASA astrobiologist Jennifer Eigenbrode at the Goddard Space Flight Center spent the intervening years figuring out which signals were clearly junk and removing them.
Then she looked at the analyzer results again. "There were signals there that were telling us that we had detected certain types of organic molecules," she says. Organic molecules contain carbon, the chemical element central to life.
That raises the obvious question: Where did the carbon come from?
"We don't know," Eigenbrode says.