The weird, dark streaks that appear and disappear seasonally on the Martian surface may be caused by boiling water, new research suggests. The study, published today in Nature Geoscience, is the first one that tries to explain how exactly water — the key ingredient for microbial life — creates the streaks on the Red Planet.
In the study, scientists melted ice in a so-called "Mars Chamber" — a room that simulates conditions on Mars — to see how it interacted with the planet’s sandy soil. Because of the low atmospheric pressure on the Martian surface, water on Mars stays liquid only for a short period of time; it either quickly freezes or boils. In their experiments, the researchers discovered that the water boiled and created streaks just like the mysterious ones on the Red Planet.
Ever since the weird streaks were detected on Mars’ slopes in 2010 by NASA’s Mars Reconnaissance Orbiter (MRO), scientists have been trying to figure out what could be creating the so-called recurring slope lineae. Last September, NASA announced that the streaks are formed by salty water flowing downhill, growing thicker in the warmer seasons and shrinking when Mars gets colder. Today’s study shows how the process might actually happen.
"It’s a great laboratory study, like a science fair project," says David Stillman, a senior research scientist at Southwest Research Institute’s department of space studies, who did not take part in the study. "It’s really neat how they were able to go into the lab, into these chambers, and simulate what would happen on Mars, because no one had ever predicted anything like this happening before."
Researchers conducted a series of lab experiments to see how water and sediments interact on Mars. They recreated the planet’s thin atmosphere and placed a 7.5-ounce block of ice at the top of a slope covered with thin sand. Then, they watched the melting ice boil and evaporate, ejecting the sand grains into the air and building small piles of sand. As more grains pile up, the piles collapse, forming similar streaks to the ones observed on Mars’ surface.
"Water in Martian atmospheric conditions behaves rather differently than what we’re used to," says Alfred McEwen, a professor of planetary geology at the University of Arizona who co-authored the study. "If people are going to go [to Mars], we need to understand these activities."
No one knows where the water on Mars is coming from. There are three possible sources: an ice reservoir underneath the planet’s surface; a deep, underground aquifer; or the atmosphere, which has a certain level of humidity. McEwen believes the water comes from the atmosphere, although it is fairly dry and is known to have little water in it. The study, which used a very small amount of water to replicate the streaks, is further proof of this, he says. "That’s what this lab experiment shows, that even with a small amount of water boiling, you trigger larger-scale geomorphology changes. That’s why I think this is so promising. It explains it with small amounts."
The study, of course, is limited, since it’s hard to faithfully recreate a Martian environment on a terrestrial lab. The temperature in the Mars Chamber, for example, was warmer than Mars usually is, Stillman says. (The Red Planet has an average of -80 degrees Fahrenheit, but on a warm day near the equator it can get to 70 degrees Fahrenheit.) And the layer of sand used in the experiments was extremely thin — much thinner than it’s likely to be on Mars.
Marion Massé, a researcher at the planetology and geodynamics lab at the Université de Nantes and the study author, says that the experiments were limited by how small the Mars Chamber is, about 3 feet in diameter and 7 feet long. McEwen agrees. "Ideally you would simulate activities at a scale of tens-of-meters types of a lab set-up," he says. "Of course when you have to put it all in a [small] vacuum chamber, that’s extremely difficult."
Unfortunately, it seems that there won’t be a way to confirm today’s findings with real-life data. The images and data sent by spacecrafts like the MRO orbiting the planet don’t have high enough resolution to show what’s happening at such small scale on the surface, so they can't confirm that boiling water is actually creating the streaks. And the Curiosity rover can’t go near the streaks and take measurements without contaminating the water. "It’s a cool experiment, but I think it doesn’t tell us exactly what’s going on right now with these features," Stillman says. "It’s just another hypothesis."
Despite its limitations, today’s study is — so far, anyway — the only game in town, when it comes to trying to figure out how the weird streaks on Mars have formed. However, the study doesn’t answer the million-dollar question that’s raveling scientists. Does the water forming these streaks contain any form of life? "That’s kind of the holy grail that everyone’s going to," Stillman says, "but this paper stops short of really addressing that."