Scientists are now pretty positive that Mars used to have liquid water – and that it probably still has some today. Some of the best evidence for this liquid comes from the analysis of streaky, dark marks that appear periodically on the planet's surface. According to a study published Monday in Nature Geoscience, the streaks may form when boiling water causes grains of sand to gurgle and pop off of the surface.

Researchers led by Marion Massé of the University of Nantes used a special chamber to mimic the conditions of a Martian summer day. We're all familiar with the way a block of ice placed in a sandy slope would behave on Earth, of course. But because of Mars's incredibly thin atmosphere (a symptom of the planet's negligible magnetic field, which has allowed solar radiation to strip it nearly bare), pressure is very low. So low, in fact, that water can boil at 32 degrees Fahrenheit, the same temperature that would make it freeze on Earth. Mars can drop to more than 100 degrees below zero, but at noon parts of the planet can reach a balmy 70 degrees Fahrenheit – so there's plenty of opportunity for water to boil away as soon as it appears.

When Massé and her colleagues replicated those conditions, their block of ice melted into the sand – but then immediately started boiling up from below the surface. The boiling caused grains of sand to blast into the air, and those sandy bubbles tumbled down the hill to form ridges. When the remaining water seeped down the ridges, it formed streaks strikingly similar to the ones seen on Mars.

On the one hand, these findings could be good news for the prospect of past or present life on Mars: They provide yet another explanation for how water might create the planet's bold streaks. And unlike last year's big water-streaks-on-Mars study, which suggested that seasonal brines might saturate the sand to color the terrain, these findings favor fresh water. But the researchers note that their experiment showed that very little water was required for the process – so their results could hint that Mars is drier than it looks.

But the results are far from definitive: The researchers couldn't replicate Martian gravity, so the faux-Mars chamber wasn't a perfect facsimile. Plus, National Geographic reports, the experiment only accounted for the way water would behave under perfect summer temperatures – and the streaks in question appear even when the planet is a bit cooler than that.

With a pair of super successful rovers on Mars, you might wonder why we've never gone and poked around in these mysterious streaks. But that's a big no-no. Since the dark streaks could be water, they're some of the most likely locations of microbial life – whether those buggies are still teeming or have left behind trace remains of biological processes long past. NASA engineers do their best to make sure every spacecraft that leaves Earth is as sterile as possible, but they know that the tenacious microbes of Earth still manage to hitch a ride into space. If Curiosity or Opportunity were to roll into a wet patch, the terrain would be compromised before their instruments even warmed up. Sure, we'd know pretty quickly whether the streaks were water – but we'd never be able to trust that the microbes we found there were actually Martians.

So for now we'll have to stick with orbital observations – and sandbox science experiments.

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