The more we learn about Mars, it seems, the icier the Red Planet appears to be. The recently departed Phoenix lander dug up water ice and even spotted falling snow from its position in the northern polar plains. And now data from the Mars Reconnaissance Orbiter point to vast glaciers buried beneath thin layers of crustal debris, much closer to the equator.

The findings, published today in Science, come from the spacecraft's shallow radar, or SHARAD, which is able to penetrate the surface and examine what lies beneath. In this case, SHARAD indicated that two long-visible mid-latitude features, one of which is roughly three times the size of Los Angeles, are almost completely composed of water ice. (The suspect glaciers are covered by debris that obscures them but also insulates the ice from sublimating into water vapor, much as street grit forms an opaque, protective blanket over roadside snowbanks.)

Some researchers had believed the features, which are plentiful at Martian mid-latitudes, were primarily rock, lubricated by a relatively small amount of ice, says study co-author Roger Phillips, a planetary scientist at the Southwest Research Institute in Boulder, Colo. But the SHARAD results suggest a generally homogeneous glacier largely lacking in internal rock fragments. "I was surprised that the ice appears so clean in the radar data and that the surface layer is so thin," says lead author Jack Holt, a geophysicist at the University of Texas at Austin. Victor Baker, a planetary scientist at the University of Arizona in Tucson who did not contribute to the study, says the evidence for water ice glaciers is compelling. "It's still an indirect measurement, but I'm totally convinced that it's ice," he says. "It's a physical measurement that you can't really interpret in any way other than the presence of ice."

Baker says that studies of Mars's surface indicate a history of glacial features in the planet's distant past. "Now what's a little bit surprising in some ways," he adds, "is that the ice is still there." Indeed, Mars's climate is not hospitable to ice formation so close to the planet's equator. (The researchers focused on the latitude band stretching from 30 to 60 degrees south latitude—on Earth, Porto Alegre, Brazil, is roughly 30 degrees south of the equator.)

"Ice should not accumulate, nor can it exist, at [the] surface at these latitudes under the current climate of Mars," Holt says. "So in addition to a protective layer, it requires regional glaciations in the past." Without a sizable moon to stabilize it, the authors point out, Mars's axial tilt is much less stable than that of Earth, causing fluctuations in its climate over time.

It's unclear how much ice is in these formations, but one of the features surveyed appears to be roughly a half mile (0.8 kilometer) thick. Phillips notes that SHARAD is not finished peering into glaciers. "We haven't surveyed the entire mid-latitudes region yet," he says. (The current study focuses on the southern hemisphere alone but notes that similar features exist north of the equator, as well.)

With more time and more sweeps of the Mars Reconnaissance Orbiter subsurface radar, the researchers say, they will be able to better estimate the ice's volume. But taken together, the glaciers could constitute the largest stores of water on Mars outside its polar regions.

Those stores, Baker says, would be key to potential human exploration of Mars. "Ice is the critical resource," he says, noting that solar energy—which is more abundant at lower latitudes—can be harnessed to yield hydrogen and oxygen. "Mars doesn't have much free oxygen, and people need that to breathe. And hydrogen is a great fuel for getting back from Mars."