Lava flows coiled like snail shells have been found for the first time on Mars - and they're bigger than anything similar on Earth.
Arizona State University graduate student Andrew Ryan spotted the spirals while studying possible interactions of lava flows and floods of water in the Elysium volcanic province of Mars.
"I was interested in Martian outflow channels and was particularly intrigued by Athabasca Valles and Cerberus Palus, both part of Elysium," says Ryan.
"There's an extensive literature on the area, as well as an intriguing combination of seemingly fluvial and volcanic features."
These features include large slabs, rather like the broken floes of pack ice in the Arctic Ocean. Using the High Resolution Imaging Science Experiment camera on the Mars Reconnaissance Orbiter, Ryan was examining the possibility that the plates could be resting on water ice.
"One evening, I was making a second pass over the HiRISE images when I first noticed puzzling spiral patterns in an image near the southern margin of Cerberus Palus," he says.
"The coils become noticeable in the full-resolution HiRISE image only when you really zoom in. I don't find it surprising that these were overlooked in the past. I nearly missed them too."
On Earth, lava coils can be found on the Big Island of Hawaii, mainly on the surface of ropey pahoehoe lava flows. They've also been seen in submarine lava flows near the Galapagos Rift on the Pacific Ocean floor.
"The coils form on flows where there's a shear stress - where flows move past each other at different speeds or in different directions," says Ryan.
"Pieces of rubbery and plastic lava crust can either be peeled away and physically coiled up, or wrinkles in the lava's thin crust can be twisted around."
The size of the Martian lava coils came as a surprise. The largest is 100 feet across - bigger than any known lava coils on Earth.
"Lava coils may be present in other Martian volcanic provinces or in outflow channels mantled by volcanic features," says Ryan. I expect that we'll find quite a few more in Elysium as the HiRISE image coverage grows over time.