But a new study in the journal Geophysical Research Letters suggests that we shouldn't pin our hopes of a Martian oasis on the RSLs just yet.
This year, planetary scientists Christopher Edwards and Sylvain Piqueux took a closer look at the feature using a thermal imaging instrument on board Mars Odyssey, another orbiter. They found no temperature differences between the dark RSL streaks and surrounding terrain — which suggests that the streaks aren't really patches of wet sand streaming down a slope. At best, they say, the RSLs could contain no more than 3 percent liquid water — making them more like mildly damp, slightly salty dirt. And that's an optimistic interpretation, Edwards said; it's possible the RSLs contain no water at all.
"Why this process is happening in this area, or what is causing this darkening, I don’t think is exactly obvious at this point," he continued. "But to say it's flowing liquid water, I don’t think it’s the whole story. It's not necessarily even the right story."
Edwards, now a professor at Northern Arizona University, and Piqueux, of NASA's Jet Propulsion Laboratory, used five years of data from the THEMIS instrument, which measures thermal energy coming off the Martian surface. Looking at data from the same region and many of the same time periods as the images used to first identify the RSL, the scientists compared the temperature of the dark spots with light ones. If the dark RSLs really did contain moist soil, they should have been warmer than the "dry" areas, because water fills all the interstitial gaps in the soil and improves its ability to retain heat. Salt and ice are likewise a "pore filling material" — so even if no liquid water was present, those two would have also made the dark spots warmer.
But the RSLs were no different from the nearby soil. Taking into account the instrument's margin for error, the temperatures of the two spots were within one degree Kelvin of each other (that's less than two degrees Fahrenheit). When the scientists plugged that number into a model that predicts how much interstitial material can be present in soil based on its heat retention, they came up with an upper limit of 3 percent by weight.
"We’re able to place strong constraints on the amount of water that is there and the amount of salt that is there," Edwards said.
This doesn't mean NASA was totally wrong about the RSLs, Edwards cautioned. There's no reason to believe that there aren't perchlorate salts in the soil, especially because they have been found in plenty of other places on the planet. But they are probably present in the RSLs in very small amounts, and Edwards said he thinks it's worth considering mechanisms other than flowing liquid water to explain how they got there.
"Maybe there are salts in the soil and a tiny amount of water rehydrates them and then evaporates on an annual basis," he said. "… We should also make sure we're not ruling out dry processes."
In the meantime, he added, everyone should keep their eyes on the RSLs. They may not contain much — if any — water, but they're still some of the most interesting things on the surface of Mars today. Unlike almost every other process on the Red Planet, the RSLs grow and fade quickly, and they're a rare feature that isn't produced by wind. To Edwards, the uncertainty only makes them more exciting.
"They're super enigmatic," he said. "We still need to see what controls these features."
Correction: An earlier version of this post incorrectly stated the conversion from degrees Kelvin to Fahrenheit. One degree Kelvin is 1.8 degrees Fahrenheit.