That's because the materials were spotted at the red planet's equator—but they appear to contain a large amount of water like that previously seen only at the Martian poles.

The finding is based on new high-resolution radar data from the Martian subsurface, which show similarities between the properties of deposits on a hilly equatorial formation called Medusae Fossae and the sediments at the ice-rich poles.

Lead researcher Thomas Watters, of the Smithsonian Institution's National Air and Space Museum, said that the new data suggest two possible scenarios.

"We can't exclude the possibility that these deposits are dry, low-density materials," Watters said.

But the observed properties of the materials could also mean that the Martian equator is rich in ice.

Kenneth Tanaka, an astrogeologist with the U.S. Geological Survey (USGS) in Flagstaff, Arizona, said that the idea of ice on Mars's equator is somewhat shocking.

"It would be like finding evidence of ice caps on Earth at the Equator," Tanaka said. "It's kind of very strange."

Watters and colleagues describe the find in this week's issue of the online advance journal Science Express.

Icy Middle?

Watters' team examined the Martian surface using a radar instrument called MARSIS aboard the European Space Agency's Mars Express orbiter.

One of the team's areas of study was the Medusae Fossae Formation—a series of large, oddly textured plateaus at the equator that are covered with materials easily eroded by wind.

"This is the first direct measurement of the thickness of these Deposits," Watters said. "We didn't know whether they were just a thin veneer or much thicker."

The radar observations found the Medusae Fossae Formation to have deposits more than 1.4 miles (2.5 kilometers) thick in places.

The data also revealed that the deposits are highly porous, meaning they could be pure ice protected by a meters-thick blanket of sediments or that they are made of an airy, pumicelike substance.

"The most surprising aspect of the research was that the electrical properties of the Medusae Fossae Formation deposits were so similar to [those of] polar layered deposits that are known to be ice rich," Watters said.

"I really didn't expect it, because Medusae Fossae Formation deposits don't look much like the polar layered deposits morphologically."

If the equatorial plateaus do contain lots of ice, they could hold as much water as the red planet's south pole.

What's more, Watters added, an icy equator would mean that "it is not impossible that [the plateaus] could harbor some form of life."

(Read full coverage of the search for water—and life—on Mars.)

Volcanic Ash

But Watters and his co-authors also say that volcanic ash or wind-blown deposits could be sources of materials porous enough to look like ice in the radar images.

Tanaka, of the USGS, has focused his research on the theory that volcanic ash has contributed to the low-density material in the Medusae Fossae area.

"The problem was we couldn't find any volcanoes" nearby, he said. "There's a lack of smoking guns."

Although he calls the new radar research exciting, Tanaka said it might be too early to draw conclusions.

For example, he said, images from the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter reveal places where the Medusae Fossae Formation is not covered up by sediment.

(See some of the most recent color images of Mars from the HiRISE camera.)

In these exposed places, any ice once hidden below the sediment blanket would likely evaporate.

Tanaka said he is most interested in the geologic processes that would be involved in depositing and eroding porous material on the Martian equator.

"What I think really needs to be studied is, can this kind of porosity exist on Mars?" he said.

On Earth volcanic pumice can be up to 90 percent porous when it's first deposited, but gravity soon forces it to collapse on itself, decreasing its porosity.

Since gravity is lower on Mars, highly porous deposits might retain their properties for longer.

"The question is how thick of a deposit could be sustained and have such low porosity," Tanaka said. "What needs to be followed up … is a plausible alternative to ice."