Dark patches visible across much of the northern Martian hemisphere aren't canals or vegetation, as once thought, but volcanic glass according to a new study.
The discovery by Dr Briony Horgan and Professor James Bell from Arizona State University, provides evidence of the same sort of processes seen on Earth, also happening on the red planet.
Using near infrared spectroscopic data collected by the European Space Agency's Mars Express orbiter, Horgan and Bell found widespread weathered volcanic glass covering the surface of the Martian lowlands.
"The volcanic glass (probably basalt) was created when hot magma reacted explosively with ice or water," says Horgan.
"The same sort of thing happens in Iceland, where volcanoes erupt under glaciers. The interaction with the ice and the water causes the magma to become super-explosive, creating tonnes of sand sized black particles."
"These form vast sand dunes covering about a quarter of Iceland's surface, which is exactly what we're seeing on Mars," says Horgan.
"We see these dark plains and enormous glassy sand dune fields up in the northern Martian polar regions."
Horgan and Bell also found evidence of dips in the spectrum consistant with weathering caused by the glass being exposed to acidic water.
Their finding, which appear in the journal Geology, represents the first detection of widespread surface weathering during the Amazonian epoch - the most recent of the three Martian geologic periods.
"The deposits are young by Martian standards, just a few billion years old, but that's still well after the planet's warm and wet period ended," says Horgan.
According to Horgan, the evidence of weathering also supports the explosive magma hypothesis.
"It works really well because we know there's lots of ice on Mars today," says Horgan.
"In the past we think Mars went through multiple ice ages with huge icesheets, glaciers and snow packs covering the surface."
Horgan says the melt water would have included dissolved chemicals that weathered the surface of the glass deposits even under the arid conditions of Mars.
According to Horgan the creation of explosive volcanic glass also creates interesting habitats for study by astrobiologists.
"If these things were created by magma ice interactions, they would have caused huge outflows of hot, chemically rich liquid water, which would have created a habitable environment which is one of the big drivers for the Mars program today."