Blue streaks on a red planet

ASTRONOMERS know that Mars was once wet. There is plenty of evidence for that, from dried-up river valleys to the presence of chemicals that need water to form. Modern Mars, though, is a freezing desert. In the 4.5 billion years since the planet came into existence, a sizeable chunk of its water has boiled away into space, another chunk is thought to have seeped deep into its interior, and what little remains on or near the surface has frozen solid. That is depressing for those who seek Martians, even bacterial ones, for most biologists agree that liquid water is a sine qua non of life.

But Mars may not be entirely dry. Over the past few years evidence has accumulated that water still trickles over its surface from time to time. And a paper just published in Nature Geoscience by Lujendra Ojha of the Georgia Institute of Technology, in Atlanta, and his colleagues offers the most convincing proof yet.