They look dark, but mysterious expanses on Mars are mainly made of glass forged in past volcanoes.
The dark regions make up more than 10 million square kilometres of the Martian northern lowlands, but their composition wasn't clear. Past spectral measurements indicated that they are unlike dark regions found elsewhere on the Red Planet, which consist mainly of basalt.
Briony Horgan and Jim Bell of Arizona State University in Tempe analysed near-infrared spectra of the regions, gathered by the Mars Express orbiter. They found absorption bands characteristic of the iron in volcanic glass, a shiny substance similar to obsidian that forms when magma cools too fast for its minerals to crystallise (Geology, DOI: 10.1130/G32755.1).
The glass likely takes the form of sand-sized grains, as it does in glass-rich fields in Iceland. The spectra suggest the grains are coated with silica-rich "rinds".
On Earth, such rinds coat volcanic glass weathered by water. How the glassy grains formed on Mars is unknown, but Horgan says magma from Martian volcanoes interacting with water ice and snow is a possibility. That would make these regions (pictured right) potential hotspots for alien life because they would have held chemical-rich water - a key ingredient for life.