Hundreds of large spirals have been discovered on Mars, and scientists think the coiled features are evidence of a type of lava flow never before seen on the red planet.
If so, the spirals would suggest that volcanoes—not ice floes, as other experts believe—shaped an unusual area near the red planet's equator.
Athabasca Valles is a region of flow channels and valleys covered with terrain plates, structures that show clear evidence of something fracturing and drifting across the planet's surface millions of years ago.
Scientists have been divided, however, as to whether the plates were made by the hardening of a massive lava flow or by icy "rafts"—much like Arctic pack ice—from an ancient inland sea.
(Related: "Lava, Not Water, Made Mars 'Riverbed.'")
Now, high-resolution pictures of Athabasca Valles from NASA's Mars Reconnaissance Orbiter have revealed 269 coils up to a hundred feet (30 meters) wide.
"There are no known mechanisms to naturally produce spiral patterns in ice-rich environments on the scale and frequency observed in our study area," study authors Andrew Ryan and Philip Christensen write in a new paper in this week's issue of the journal Science.
Instead Ryan and Christensen, of Arizona State University, think the coils are most similar to features seen in smooth, slow-moving Hawaiian lava flows.
"Everything that we have observed in Athabasca Valles can be formed by lava. Although you could attribute certain features to ice, the lava coils indicate that this is not the case," Ryan told National Geographic News in an email.
Valley Shaped by Ice or Fire?
On Earth, so-called pahoehoe lava flows can move past each other in different directions or at different speeds. The resulting shear produces a twisting motion, which coils the rubbery crust atop the flows into a distinctive shape—one that closely resembles the newfound Martian spirals.
(Also see "Mars Volcanoes May Re-Erupt, Hawaii Comparison Shows.")
On Earth similar spirals are found underwater on a mid-ocean ridge, where two "conveyor belts" of spreading crust move outward in opposite directions, causing the upwelling lava between them to swirl.
As with many things Martian, however, the new data remain open to interpretation.
John Murray, with the Department of Earth Sciences at the Open University in the U.K., said Ryan's paper is interesting and that the spiral features are quite curious. But Murray still believes that the surface near Athabasca Valles shows the lingering effects of an ancient frozen sea.
"I think there are so many features here that it's difficult to explain them other than [the theory] that this was essentially water that froze and has since sublimated away," he said. Sublimation is when a solid turns directly into a gas.
"There is no lava that behaves in so many different ways."
(Related: "Weird Mars Deposits Could Be Vast 'Ice Cap' at Equator.")
The appearance of the plates themselves, not the spirals in between, remains a main sticking point for Murray.
"You do get plates in lava, but on the scale of a few meters," he said. "Here you're talking about things which are kilometers long, and the only way you can do that really is to have a liquid that's extremely mobile and fluid—water or something like water.
"If you freeze the top of that, as in the Arctic, you do get ice floes that are several kilometers or more, which is what you get on Mars in this region. You never see anything like that in a lava flow."
A Place to Look for Life?
The debate pitting lava against ice is more than purely geological. If Athabasca Valles does show clear evidence of water ice, the region could become an important target for future Mars astrobiology missions.
"If this really is the remains of a large inland sea, that's where you're going to find life, if there is any there," Murray said.
"Even if the sea was five million years old, there may still be subsurface remains of the ice, and ... microfossil traces could perhaps be found there."
Study author Ryan, meanwhile, is confident the odd features of Athabasca Valles can be explained by volcanism alone.
"I don't believe it would be a very good place to search for life," he said. "However, it is a very interesting place to study the volcanic and geologic history of Mars."