On Mars, a little water can go a long way. Those mysterious dark lines that grow and fade on some of the Red Planet's slopes might not have all been caused by water flowing on the surface, but violently boiling off of it, a new laboratory study finds.
The findings, published in the journal Nature Geoscience, reveal the potential pitfalls of judging Martian geology by Earthly standards — and could alter our understanding of water's role in shaping the surface of our rusty, dusty neighbor.
Planetary scientists have long been entranced by dark streaks that appear on some Martian slopes known as "recurring slope lineae," so named because of their tendency to lengthen in the spring and summer and shrink during fall and winter. Researchers believed that these RSL might be caused by water flowing briefly on the Martian surface, but it was hard to find proof.
That's because the Red Planet's atmosphere is extremely thin — so thin that water at room temperature would boil off of the surface near the equator, unable to stay stable as a liquid. (And near the frigid poles, it would likely freeze solid.)
But with enough salts in it to raise the boiling temperature, briny water just might last long enough to flow down a slope and leave those characteristic dark stripes. And last year, scientists using NASA's Mars Reconnaissance Orbiter said they'd found the smoking gun — signs of hydrated salts in the streaks. Where there's watery salt, the thinking went, there must have once been salty water.
The discovery excited scientists at the time, including John Grunsfeld, who recently retired as associate administrator of NASA's Science Mission Directorate.
"It suggests that it would be possible for there to be life today on Mars," he told the audience at a press briefing at the time.
But this model required pulling water molecules out of the atmosphere, and that's in short supply, said study coauthor Alfred McEwen, a planetary scientist at the University of Arizona and leader for the HiRISE camera on the Mars Reconnaissance Orbiter. How could there possibly be enough of it in the air to leave such a dramatic mark on Martian slopes?
Using a satellite orbiting far above the surface, it's hard to get an up-close, continuous picture of the transient phenomena that cause these streaks. So the scientists turned to the next best option, which is to recreate a Martian environment on Earth and watch what happens.
It turns out that there might be another factor complicating this picture, said study lead author Marion Masse, a geologist at the University of Nantes in France. She and her colleagues set up a laboratory experiment in special chambers that allowed them to bring the atmospheric pressure and temperature down to Martian standards. Then they placed blocks of ice at the top of miniature slopes to see how the meltwater interacted with the surface. They tested both freshwater and briny water, to see if there were any differences in behavior between the two.
They found that the freshwater that made it to the surface evaporated so quickly and so violently that it flung sand grains into the air, creating little piles that developed into ridges ahead of the flow. As these ridges grew larger, they eventually collapsed, releasing miniature avalanches of dry sand. The lower the pressure, the more violent this process was.
"Sometimes when we lower the pressure, it's like popcorn on the surface," she said of the flying sand grains.
The situation with salty water was somewhat different, as it was more viscous and a little less prone to immediate boiling. But it could also travel farther and form channels that could occasionally become explosive at low pressures. In both fresh and briny cases, even with their differences, the resulting patterns looked remarkably similar to the recurring slope lineae on the Red Planet.
When examining Martian geology, scientists often look to parallels on Earth to explain or understand the sculpting of the rocky planet's terrain. But this explosive boiling process, which has no known analogue on our own planet, is a lesson in the perils of looking only to terrestrial explanations, Masse said.
"With our experiments we see that it's not really possible to compare water flow on Earth and on Mars ... [the] mechanism is completely different," Masse said.
McEwen agreed, also pointing out that it may actually help to bolster the earlier findings that brines may cause the recurring slope lineae. After all, if fresh and even briny water have the potential to move so much soil, it means that the tiny amount of water in the atmosphere might not be such an inefficient source.
"This helps us to think differently about how water would behave on Mars," he said.
There's a strange irony at work here, the scientists pointed out.
"Paradoxically, instead of requiring the stability of substantial water or brines, it is the instability of water on Mars that may explain the morphological activity needed to form the observed features," Wouter Marra of the University of Utrecht in the Netherlands, who was not involved in the paper, wrote in a commentary on the study.
As for the exceedingly slim possibility for life on Mars, it means that water on the surface does not need to be stable to sculpt many of the features we see today — and it's hard to see how microbes might survive in such explosive situations. (It was hard to see how microbes might survive even stable water on the planet's surface; to be stable, it would have to be so salty as to be practically unusable.)
But it doesn't seem to change too much for the water beneath the surface, McEwen pointed out – and that's where any microbes, if they exist, would probably be living, sheltered from the thin atmosphere and any harmful radiation.
"It is not easy to conduct fieldwork on Mars, so physical experiments are an important tool to explore how processes operate under Martian surface conditions and the geomorphic impact of these processes," Marra wrote in the commentary. "Such experimental insights are essential to interpret what we see in satellite imagery and to provide information for numerical models attempting to reconstruct climate conditions on Mars."