Last year, the discovery of dark streaks in the sands of Mars during warmer months suggested NASA had found evidence of flowing liquid water on the barren world.
But now, a new study from researchers at Northern Arizona University and the Jet Propulsion Laboratory in La Cañada Flintridge is causing that theory to dry up.
Scientists Christopher Edwards and Sylvain Piqueux used thermal imaging instruments aboard the Mars Odyssey orbiter to compare the temperatures of the streaks against other areas where the phenomenon, dubbed “recurring slope lineae,” does not appear. The temperatures should change during the seasonal flows, but researchers found no difference even during seasons when the RSLs were active.
“We used a very sensitive technique to quantify the amount of water associated with these features,” Edwards said in a statement. “The results are consistent with no moisture at all and set an upper limit at three percent water.”
That means for two pounds of soil, there’s no more than an ounce of water at best. The study puts that water concentration on par with Atacama Desert and the Antarctic’s Dry Valleys, the driest places on Earth.
NASA believed those dark streaks were very salty water seeping down the hillside during the summer and spring months, then disappearing in the fall. The hydrated salt allowed the water to stay liquid in spite of Mars’ harsh environment, where liquid boils and freezes in a very narrow temperature range. NASA used the Mars Reconnaissance Orbiter, Odyssey’s partner in orbit, for the initial inspection of the RSLs.
Why quick change?
Emily Lakdawalla, the senior editor and resident Mars expert at the Pasadena-based Planetary Society, said this is just the standard scientific process in action. You try to prove or disprove theories by looking at it from all possible angles.
The difference is that today every exciting scientific detail is immediately released and broadcast to the public, making discoveries like the RSLs feel more final than they actually are.
“The answer is that scientists don’t actually know what is going on, they have different hypotheses that are supported by different evidence,” she said. “You sometimes get this seesaw back and forth of conclusions about exciting things.”
A meta-analysis that looks at the different theories and tries to piece together the facts is needed to better understand the processes, but that type of analysis isn’t terribly common in the space exploration community, she said. Part of the excitement came from the idea that present-day liquid water could be used as a resource for astronauts or even allow life to exist on Mars, but the RSLs high salt content made it a poor source for either. Even without water, the RSLs show an important and complex geological behavior that we don’t yet understand.
“I think it’s just as interesting as it was before. We don’t understand what is driving this behavior,” she said. “They’re variable and different, and most of the places that we study in space, at least most on the terrestrial planets, don’t change much from time to time. The RSLs are a feature on Mars that does change, that we can watch.”
No conclusive answers
The new study doesn’t eliminate the possibility of the hydrated salt theorized last year, but it is evidence that the streaks may be caused by a drier process, like an avalanche instead of a large amount of water seeping to the surface.
But Richard Zurek, the chief scientist for the Mars Program Office at JPL, stresses there hasn’t been any answer yet that fits perfectly, whether it includes water or not. An avalanche for example doesn’t explain why it occurs during the warmer seasons.
“These things are so mysterious because anything we’ve been able to come up with, there’s at least one bit of information that doesn’t seem to fit with it,” Zurek said. “It’s very difficult to think of what else could be involved, if it’s not water.”
The thermal imaging covers football fields’ length and has some margin of error, according to NASA. The best way to understand the mystery would be to study it on the ground instead of from orbit.
But currently, NASA’s robotic explorers don’t meet the sterilization standards necessary to risk possibly touching water. They could introduce life into the environment inadvertently, Zurek said.
One idea may be to use a drone-like device in a future mission to drop sterilized probes into the soil, or to use a rover to observe chemical changes without touching the soil. There’s a lot of science to perform on Mars still, so the RSLs aren’t high on the priority list.
“Definitely, (the study) is a strike against the hypothesis, but we still don’t have an answer for this, so we’re going to keep exploring,” Zurek said.
