Low, wide glaciers half a mile thick adorn the middle latitudes of Mars, say scientists who used radar probes to peer into debris-covered formations.
The rounded slopes of material skirting steep ridges have cropped up in numerous satellite images over the years, generating controversy over whether they are mostly made of rock or ice.
Using NASA's Mars Reconnaissance Orbiter, John Holt of the University of Texas at Austin and colleagues found that the landforms appear to be water ice covered by rocky rubble.
The ice exists at much lower latitudes than any other known deposits on the Martian surface, and some experts say the trapped water has the potential to support humans in future missions to Mars.
"Altogether, these glaciers almost certainly represent the largest reservoir of water ice on Mars that's not in the polar caps," Holt said in a statement.
"Just one of the features we examined is three times larger than the city of Los Angeles … and there are many more."
As for the rock-or-ice debate, "I do think it puts to rest the controversy regarding these deposits," Holt added in an email. "But I suppose it's really up to the science community to decide that."
Radar Echoes
Most known concentrations of Martian ice exist at the poles, where temperatures are low enough to sustain large swaths of water ice.
The mysterious aprons of material had therefore puzzled scientists ever since NASA's Viking orbiters revealed them at mid-latitude in both hemispheres in the 1970s.
Although they resemble fan-shaped deposits of debris seen elsewhere on Mars, the formations are larger, steeper, and have features that suggest they were formed by an ancient flow of viscous material.
One theory contended that the landforms are flows of rocky debris lubricated by a little ice, while others suggested they are lumps of ice covered by just enough debris to prevent sublimation (turning directly from a solid to a gas).
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