Mars clearly had extensive water in the past, and there's still plenty of it locked up as ice in glaciers and the polar ice caps. But the atmosphere is too thin and cold to allow liquid water to exist on the surface, which makes prospects for life on the Red Planet far less likely.

Back in 2011, however, researchers suggested that, contrary to our expectations, there might still be some water seeping out onto Mars' surface. Darkened features were identified on a variety of slopes, and they seemed to appear during warmer seasons and vanish as temperatures plunged again. The Mars Reconnaissance Orbiter appeared to detect water at the site. But other researchers proposed a physical mechanism that didn't involve water that could account for the seasonal changes.

Now, a review of the evidence in Nature Geoscience argues that there are problems with almost all of the potential causes for these seasonal features. And, in the absence of a compelling case for water, it's best to assume that the harsh conditions mean what we typically thought they did: Mars is a dry planet.

Get wet?

The features in question are called recurring slope lineae, or RSL. We spotted them because we've had a superb camera in orbit around the Red Planet for more than a decade. That means lots of sites have been imaged multiple times, allowing us to detect changes over time. RSL appear as a darkening on slopes made of loose, granular material—essentially piles of sand. Their appearance is seasonal, with the darker phase occurring as local conditions warm up. They're also limited to the equatorial regions, suggesting that a certain level of warmth is needed for their formation.

There's typically some exposed bedrock up-slope of the RSL, providing a potential source of groundwater. And, as mentioned above, spectroscopic data showed the presence of water in the region.

None of these, however, are slam dunks for the presence of water, the new review argues. For example, many minerals incorporate water as part of their structure, which may have been what was picked up via spectroscopy. The presence of prominent bedrock features is also not persuasive, as some of these are rather isolated; they can't be part of a network of fissures that water flows through from an uphill source.

The one plausible way to keep water liquid under Martian conditions is to have it saturated with salts, which will lower its evaporation rate. But the review argues that it won't lower it enough. It also points out that the RSL would eventually build up a salt deposit near the bottom of the slope, and nothing of the sort has been detected yet (the review mentions that it should only take a few years to deposit a cubic meter of salt).

Finally, there's the issue of the length of the RSL. All of them tend to stop just above the bottom of the slope. As the authors of the review argue, "this would require the volume of liquid to correspond to the length of slope available, producing more liquid on longer slopes." Instead, they favor an explanation that involves flowing grains of the loose material that make up the slopes. As the grains flow downward, they'd naturally stop as material piles up near the bottom of the slope. And, as noted above, there has been a a physical mechanism proposed that should start things flowing as the area warmed up with the changing seasons.

All wet?

But even the authors admit there are some problems with this idea. To begin with, when a slope destabilizes and some of the material flows downhill, the largest particles should flow the farthest. This should leave the slope in a more stable state and less likely to have RSL appear the Martian year following; instead, they seem to reappear in the same places.

Then there are color issues. Darkening could be ascribed to the uncovering of material that hasn't been lightened by its exposure to harsh Martian conditions. But many of the RSL show a complicated pattern of darker and lighter features. In addition, there's no obvious mechanism to lighten the RSL back up again in less than a single Martian year. The review suggests a coating of dust might help, but there would have to be additional factors involved.

Finally, like the salt left behind by evaporated brines, the downward flow should leave piles of material at the base of the slope and near any features like boulders that protrude from it. But there's no sign of that in most of the images. So, based on these issues, the idea that these are granular flows has nearly as many problems as the watery explanation.

So where does that leave us? The paper argues that we're right back where we started: we don't expect liquid water on the surface of Mars, and the RSL simply aren't conclusive evidence of it. "Flowing liquid water in the current Martian climate has always been an extraordinary claim," the authors write. "The observations and interpretations presented here suggest that RSL are no longer extraordinary evidence." As long as we're not sure what they are, they can't be used as evidence of anything else.

Nature Geoscience, 2017. DOI: 10.1038/s41561-017-0012-5 (About DOIs).