Mysterious dark regions on the planet Mars are made of glass, likely formed in ancient volcanic eruptions, U.S. researchers said.

The dark regions occupy almost 4 million square miles of the Martian northern lowlands. Their composition wasn't clear, but past measurements indicated they were unlike dark regions found elsewhere on the Red Planet, which consist mainly of basalt.

Researchers at Arizona State University analyzed spectrographic data of the regions captured by the Mars Express orbiter and found characteristics of volcanic glass, a shiny substance similar to obsidian that forms when magma cools too fast for its minerals to crystallize, NewScientist.com reported Monday.

The glass is probably in the form of sand-sized grains, similar to that fund in glass-rich volcanic fields in Iceland, researchers Briony Horgan and Jim Bell said.

Such glassy grains are often formed when volcanic magma interacts with water ice and snow.

If that is true of the Martian glass grains, researchers said, it would make the regions potential hot spots for alien life because they would have held chemical-rich water, a key ingredient for life.