The holy grail of space exploration is extraterrestrial life. An important precursor to finding that is finding liquid water - and that is exactly what Nasa announced yesterday: scientists have discovered water on Mars. The findings, described in the journal Nature Geoscience, offer an explanation for peculiar long streaks that sometimes appear on sloping Martian terrain.The dark features, which can be as wide as 5m and more than 100m long, were first noticed in 2010. By analysing their reflected light characteristics, eight scientists concluded that the streaks consisted of mineral salts that easily absorb moisture - and that they were most probably deposited by flowing water.The Red Planet has been known for years to have water ice and its surface bears the topographical scars of ancient water flows. But this latest research is the first to provide compelling evidence of the continued existence of flowing water on Mars.The findings date back to 2010, when Lujendra Ojha, then a University of Arizona undergraduate and now the lead author of the new research, started sifting through images from a powerful Nasa camera orbiting the Red Planet.His professor, Alfred McEwen, is a co-author of the article in Nature Geoscience and the lead scientist for Nasa on its Mars camera project."There were these stark, linear, narrow features forming" all over the planet, Ojha said, "and they were forming only when the temperature was ideal for liquid water."In the Martian winter the features disappeared.The answer to the ultimate question, Can Mars host life? must come from astrobiology, the study of life's origins.Towards the end of their paper the authors say the conditions on Mars - arid and salt-rich - are similar in some ways to those in the Atacama Desert, in western Latin America. There, water-retaining salt beds are oases for microbes.Unfortunately for ET-hunters, on Mars "the water activity in perchlorate solutions might be too low to support known terrestrial life", the authors wrote.There's an irony in searching for life-friendly conditions on Mars, or elsewhere. The more promising the discovery, the more thoughtful humans (or our robots) should be in approaching it. Areas in which life might propagate are declared "special regions" and treated with caution by scientists. That's because virtually everything on Earth - your smartphone, bananas, yesterday's socks, spacecraft - is slathered with microbes."Maybe in the future we can have a mission to go there," Ojha said, "but I think we have to be really careful about transferring life from Earth to Mars. We might be the ones creating the second Genesis."..