U.S. space researchers said Tuesday that they had found more evidence that the planet Mars could have once been able to support life.
NASA scientists said the Curiosity rover found nitrates in Martian rocks. Nitrates are compounds that contain nitrogen, an element essential for life.
But the scientists said there was no evidence that the nitrogen had come from living things, and they stressed that the Martian surface could not support any forms of life.
They said they thought the nitrates were ancient and probably had come from meteorites or lightning strikes.
This was not the first sign that the planet might have been habitable at one time. Past discoveries have shown signs that fresh water once covered the Martian surface.