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