Boiling water may be responsible for some enigmatic geological features on the surface of Mars, according to a new study.
Scientists have long eyed slopes on the Red Planet that seem to be changing seasonally. These slopes appear to have sediment flowing down them, sometimes carving out gullies and other crevices. Researchers have suggested that a fluid might be driving these processes, but until recently there was no direct evidence of any such fluid.
In September, a team of scientists announced that they had spotted evidence of hydrated salts, suggesting salty water flows down mountain, canyon and crater slopes. This has been taken as strong evidence that water is present on Mars, which is a key ingredient for life as we know it on Earth. But Mars is very dry and there's very little water in the planet's atmosphere. So how did a big enough flow form to carve out these features?
"Is it possible to explain the features we're seeing with smaller amounts of water?" Alfred McEwen, a planetary geologist at the University of Arizona's Lunar and Planetary Laboratory in Tuscan, says. That question drove a laboratory experiment investigating how water might have formed such features, the results of which were published Monday in the journal Nature Geoscience.
"A small amount of water can trigger larger-scale change on the surface than we would expect," Dr. McEwen, a co-author on the new paper, tells The Christian Science Monitor. And that's because the conditions on the Red Planet are different from our own.
On Mars, the atmospheric pressure is much lower than on Earth because the atmosphere is thinner. So "the way water behaves on Mars is different from the way that it behaves on Earth," McEwen says. Instead of being brought to a boil by high temperatures, it's that different pressure that changes the state of the water.
A similar change in conditions alters the boiling point on Earth, too. At sea level, water boils at 212 degrees F. But on the top of Mount Everest, where the atmosphere is thinner, water boils at 140 degrees F.
The research team, led by Marion Massé at the Université de Nantes and the National Center for Scientific Research (CNRS) in France, set up slopes of sediment in the lab under Earth's conditions and Mars'. They placed a chunk of ice on top of the slopes at a temperature of 293 Kelvin (67.7 degrees F.) and under Mars barometric pressure, 6.5 mbar or 9 mbar (Earth's standard sea level pressure is 1000 mbar).
As the ice melted, it slowly released liquid water into the sediments.
In the experiment, the slopes under Mars conditions were altered significantly more than those under Earth conditions. This was because the water boiled, displacing the sediment with its bubbles.
"One thing that's important to note is that these experiments, as clever as they are, are not perfect scale models of everything that's happening in the flows on Mars or on Earth," Taylor Perron, a planetary geologist at Massachusetts Institute of Technology in Cambridge, who was not part of the research team, tells the Monitor.
On both planets, the sediments would be various sizes and shapes. These different physical properties would change the way the water acted on them, he says. "What they're doing is creating what we call a qualitative analog, in which the physical phenomenon might be similar even if the forces and the exact characteristics of the materials involved are not."
Where is the water coming from?
"That's a big question," McEwen says.
Dr. Perron agrees, "that's an especially puzzling problem because there are places on Mars where these flows appear to happen repeatedly in the same place, which means you must be recharging it somehow."
Subsurface ice melt could explain it, McEwen says. But these flows are forming close to Mars' equator, and the ice would disappear quickly.
Rain isn't falling on Mars. In fact, there is very little atmospheric water.
That's where the hydrated salts spotted last year might come in, says McEwen. Maybe over time these salts extract water from the atmosphere, locking it away in their molecular structure until temperatures rise enough for it to liquify. That would fit with the seasonal nature of the flows. This is just an idea at this point, McEwen says, "but maybe that could work."
Salty or fresh?
But Perron points out that "there's quite a menagerie of different features" that appear to have some sort of flow. And this diversity of features suggests there could be a diversity of forces acting on the surface of Mars.
In fact, there may be freshwater creating some of these flows as well. The research team found that pure water is less stable in Martian surface conditions, so it boils more readily. And that could create some of the more dramatic features.
But there has been no evidence of freshwater spotted on the Red Planet.
Freshwater could go undetected, as spacecraft orbiting Mars wouldn't pick up a spectral signal if there was only a little bit among grains of sand, suggests Perron. The hydrated salts were spotted by an imaging spectrometer on the Mars Reconnaissance Orbiter (MRO).
"It's possible that there are some features on Mars that were created by fresher flows and some were created by saltier flows," Perron says. "But the saltier ones are the only ones that leave behind the clear calling cards."
A step forward in the hunt for life on Mars
Water, especially freshwater, may be a key piece of the puzzle in making a place livable, but the proposal that it could be driving these geological processes is just a step forward in the hunt for extraterrestrial life.
Rovers likely won't check out these sites just yet either, as scientists worry about contaminating any life or conditions that might exist. So for now, these flows will continue to be monitored and studied from the skies.
"It's definitely very exciting that they've raised the possibility that freshwater might play a more important role in these features than we appreciated," Perron says. "But it doesn't mean we've discovered life on Mars."