U.S. space researchers say they have found more evidence that the planet Mars could have once been able to support life.

美国太空研究人员说，他们发现更多证据显示火星可能曾经存在能够维持生命的环境。

NASA scientists said Tuesday the Curiosity Mars rover found nitrates in Martian rocks. Nitrates are compounds that contain nitrogen, an element essential for life.

美国航空航天局的科学家们星期二说，火星探测器“好奇号”在火星岩石中发现了硝酸盐。硝酸盐中含有氮，而氮是维持生命的必要元素。

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.

但科学家们说，没有证据显示火星上的氮来自生物，并强调火星表面不能维持任何形式的生命。

They believe the nitrates are ancient and probably came from meteorites¹ or lightning strikes.

科学家们相信，火星上的硝酸盐存在已久，很可能来自流星或闪电。

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.

目前还没有迹象显示火星可能曾经适于居住。过去发现，火星表面曾被淡水覆盖。