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While not outside the realm of possibility—we do know there’s ice buried beneath the Martian surface—such scenarios aren’t as likely as the one scientists favor: The water comes from the atmosphere. If that's true, it’ll be a much tougher resource to tap into.

But how could water from the atmosphere form these dark streaks? On Mars, as on Earth, salts on the surface can absorb atmospheric water vapor and trap it in their crystal structures. Then, when the soggy crystals warm up, they dissolve. The whole liquidy mix surrenders to the tug of gravity, and off it goes, tumbling downhill.

In Chile’s super-dry Atacama desert, this exact type of system—called deliquescence—is the key to supporting some rather extreme life, says NASA astrobiologist Chris McKay.

But there’s no guarantee this is happening on Mars. McKay notes that the type of salts near the Martian streaks, called perchlorates, form different watery mixtures than the salts we’re most used to on Earth. In fact, it’s possible the perchlorate streaks could behave similarly to Antarctica’s Don Juan Pond, which is the saltiest liquid water body on Earth—and totally dead.

“Such a brine is not suitable for life and is of no interest biologically,” McKay says. “Nothing can live in the brine of Don Juan Pond.”

So, seeps fueled by atmospheric humidity might not make the most convenient water well for human colonists, and they might not even be ideal habitats for Martian microbes—but wouldn’t it be worth finding out?

Of course. What we know so far, based on the single example of Earth, is that life tends to show up wherever there’s water. That’s why NASA’s search for life beyond Earth has been driven by the mantra, “Follow the water.”

The frustrating irony here is that NASA can’t follow this particular water. Not yet.