For the last five years, scientists have stared agog at Mars’s surface, bewitched by gullies etched into the landscape, seemingly by flows of salty water.
But a new study demonstrates that these formations and others like them might be carved by a process unlike anything seen on Earth: boiling meltwater that flings dirt around like popping popcorn.
The unexpected geological process, described on May 2 in Nature Geoscience, was re-created in the lab during a simulated Martian day. The finding could help explain Mars’s recurring slope lineae, enigmatic gullies that have had planetary scientists scratching their heads since their discovery in 2011.
The gullies’ dark colors, incremental growth, and floors filled with salt deposits suggest that briny water flows on modern Mars, potentially providing habitats for hardscrabble Martian life.
But how exactly the gullies form remains a mystery, in large part because the planet’s surface is extraordinarily hostile to liquid water. Mars’s atmosphere is bone dry and more than 50 times thinner than the air at Mount Everest’s summit.
In such rarified conditions, water will boil at temperatures as low as zero degrees Celsius, says study leader Marion Massé of France’s University of Nantes.
“What we don’t have on Mars are video cameras looking at these features—it’d be awesome if we could,” adds study co-author Susan Conway of the University of Nantes. “We don’t know what’s going on on small timescales. How do they form? How do they grow?”
(Not) Under Pressure
To test how gullies might flow from water dripping down a Martian hillside, Massé, Conway, and their colleagues set up simulated hills in the Large Mars Chamber at England’s Open University. The facility is a steel decompression chamber like those used by scuba divers.
At Earthlike air pressures and temperatures around 68°F (20°C), meltwater from an ice cube dribbled down a ramp coated with a thin layer of sandy soil, darkening the sand without moving it.
But once the team replicated the setup at Mars-like air pressures, something unexpected happened: The water melted and seeped into the soil, then began to boil, flinging sand down the hill into a pile in front of the water’s leading edge.
As the boiling continued, the pile became unstable, ultimately spilling down the hill and forming a small ridge. Water continued to flow downhill past the ridge, creating additional piles and ridges. This process eventually formed elaborately ridged channels about a foot (30 centimeters) long that look remarkably similar to slope lineae.
Though the researchers knew that water would boil at Mars-like air pressures, its effect on sand movement took them completely by surprise. “We weren’t expecting it,” says Conway. “We all crowded around the chambers, going, ‘Aw, that’s so cool! Let’s hope it’s not a one-off.’”
Water We Waiting For?
Massé and Conway maintain that the newly discovered process, which also works with salty brines, suggests that even a small amount of water can move large amounts of sand—strengthening the case that liquid water carves the recurring slope lineae, although perhaps not in the way originally imagined.
Outside experts, however, are more cautious, largely because the experiment was simplified by design. Jennifer Hanley of Lowell Observatory, an expert on how ice behaves on other worlds, points out that the experiment was conducted only at a best-case temperature for the Martian summer. But recurring slope lineae also form during Mars’s spring, when surface temperatures are cooler.
A big hurdle to ultimately solving the mystery is that we don’t have a safe way of seeing the slope lineae up close. Even NASA’s Mars Reconnaissance Orbiter, which has the highest resolution camera currently orbiting the red planet, wouldn't have a sharp enough view.
And don’t even think about using a rover to get a closer look. NASA is steering its robots clear of recurring slope lineae on the off chance that the machines would contaminate the sites with Earth life—throwing off tests for Martian life or even abetting a microbiological alien invasion.
“We know of organisms that actually could live in these highly saline environments,” says Hanley. “It’s really tempting—but how do we go and look for these things without introducing our own biases, or potentially introducing life?”
Still, “I thought it was a good use of lab data to understand something that we’re unsure of,” she adds. “It’s definitely something we need to take into account [when] trying to understand these liquid flows.”