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.

In 2011 Mr Ojha, then an undergraduate student at the University of Arizona, spotted dark streaks on the walls of certain Martian craters that had been photographed from orbit by a satellite called Mars Reconnaissance Orbiter. These, he noticed, seemed to wax and wane with the Martian seasons—darkening in summer and fading in winter. Though he cautiously dubbed the streaks “recurrent slope lineae”, or RSLs, rather than giving them a name that included the “w” word or anything connected with it, he nevertheless suspected that water was their cause.

The surface temperature on Mars only rarely rises above pure water’s freezing point, but researchers know that chemicals which have been detected in the regolith (the crushed rock that passes for soil on Mars) can act as antifreeze. That might depress the freezing point of brine containing them to one which, at least in summer, permitted such brine to stay liquid. The summer darkening of the RSLs, he thought, was caused by brine of this nature flowing. The winter lightening, conversely, was caused by it freezing again. Rough-and-ready modelling suggested the streaks were growing and fading at approximately the right rate for this hypothesis to be correct.

Mr Ojha’s latest paper examines the question in detail. He and his colleagues used spectroscopy to examine four RSLs from various parts of Mars. This technique lets researchers work out a substance’s composition from the light it reflects. The results suggest the streaks contain compounds (specifically, magnesium perchlorate, magnesium chlorate and sodium perchlorate) that are excellent antifreezes, which would be able to lower water’s melting point by more than 70°C. That would be enough for ice containing them to melt in the Martian summer. Moreover, all of these substances form by the action of water on other minerals, bolstering the case that the RSLs are intermittent streams.

Mr Ojha is careful to point out that he has not detected water directly (a more cautious approach than the one taken at a press conference called by NASA to publicise the story), and that even what he has detected is at the limits of what Mars Reconnaissance Orbiter can accomplish. But the presence of compounds that need water to form, as well as acting as antifreezes, is powerful indirect evidence. The case for there being liquid water—or, at least, brine—on modern Mars has thus become almost unequivocal.

Details remain to be worked out, including where the water in question comes from. Subsurface ice is one possibility. Another is that it might condense out of Mars’s atmosphere, though this is both thin and dry. Whatever its origin, though, the amounts in question are modest in the extreme. But even small amounts of water are intriguing to biologists. If Martians did manage to evolve during their planet’s earlier, wetter phase, the continued presence of water means it is just about possible that a few especially hardy types have survived until the present day—clinging on in dwindling pockets of dampness in the way that some “extremophile” bacteria on Earth are able to live in cold, salty and arid environments.

Looking for Martians will be harder than looking for water. Spotting bacteria from orbit is beyond the capabilities of any probe that engineers know how to build, and even analysing samples on the surface is fraught with difficulty. NASA has two rovers trundling around the Martian surface, but sending them to investigate is forbidden by NASA’s “planetary protection” rules. The worry is that the machines may play host to Earthly extremophiles that have survived the trip to Mars. If so, investigating the water risks contaminating the sample. At best, that could interfere with the readings. At worst it could damage the first alien ecosystem to be discovered.

Other rovers are coming. NASA plans to send one to arrive in 2020. At the moment that rover too would be forbidden from going anywhere near liquid water. Designing and launching an uncontaminated probe would be tricky. But finding a way to look at some RSLs up close is now top of every Mars scientist’s to-do list.