Object: Spiral patterns in Martian valleys
Origin: Solidified lava lake
Some say Mars's northern valley formed in fire, some say in ice: now curious spirals on the floor of the valley have been glimpsed – and hold with those who favour fire.
The Athabasca Valles region, a channelled and scabbed valley just north of the red planet's equator, was clearly carved by floods of fluid coursing through it tens of millions of years ago. But, in a beautiful echo of the apocalyptic poem "Fire and Ice" by American poet Robert Frost, no one had been able to work out whether that fluid was water or molten lava.
Graduate student Andrew Ryan of Arizona State University and colleagues seem to have settled the debate with their discovery of subtle spirals on the valley floor that could only be formed from lava. "These coils can only be formed by volcanic processes," says Ryan.
Athabasca Valles is famous for its large dark plates, which seem to be floating in a network of channels that were probably carved in a flood of liquid water. The plates look a lot like the broken floes of pack ice found in the Arctic Ocean on Earth. The land between them is criss-crossed by a network of polygons that look like frozen glacial terrain found on Earth and near Mars's poles.