The exact nature of these deposits has been a geologic mystery for decades.
"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."
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."
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