BALTIMORE — Scientists have found enough water ice at Mars’ south pole to blanket the entire planet in more than 11 meters of water if thawed.

Using an instrument aboard the European Space Agency’s Mars Express orbiter , astronomers used radar to penetrate for the first time about 4 kilometers beneath the south pole’s frozen surface, and found that nearly pure water ice lies beneath.

Discovered in the early 1970s, layered deposits of ice and dust cap the north and south poles of Mars. Until now, the deposits have been difficult to study closely with existing telescopes and satellites.

“This is the first time that a ground-penetrating system has ever been used on Mars,” said Jeffrey Plaut, the new radar study’s lead author and a scientist at NASA’s Jet Propulsion Laboratory of Pasadena, Calif .

While NASA’s Mars Odyssey spacecraft also carries instruments designed, among other things, to probe beneath icy polar surfaces, Plaut said “all the other instruments used to study the surface of Mars in the past really have only been sensitive to what occurs at the surface.”

Plaut and his colleagues probed the deposits with radar echo sounding, which typically is used on Earth to study the interior of glaciers. The instrument, called the Mars Advanced Radar for Subsurface and Ionospheric Sounding , beams radio waves that penetrate the planet’s surface and bounce off features with different electrical properties.

The reflected beams revealed that 90 percent or more of the frozen polar material is pure water ice, sprinkled with dust particles. The scientists calculated that the water would form a nearly 11-meter -deep ocean if spread across the planet.

“It’s the best evidence that’s been obtained to date for that thickness,” said Ken Herkenhoff, a planetary geologist at the U.S. Geological Survey in Flagstaff, Ariz., who studies the martian polar regions. Herkenhoff was not involved in the current study.

Scientists have long known that Mars’ north polar cap is a massive storehouse of water ice, and the research team says they will use their radar technique to refine past estimates of its thickness and composition.

“These polar ice deposits are by far the largest reservoir of water or water ice that we know of on Mars,” Plaut said.

However, the deposits still do not contain enough water to account for the flowing streams thought to meander along Mars’ surface in the past, he said.

“There’s evidence that about 10 times or maybe even 100 times that much water has flowed across the surface of Mars to carve the various channels, the outflow valleys and other features we see in the images and topography data,” Plaut said.

Some scientists theorize that a subterranean plumbing system once ferried loads of water beneath the martian surface. Plaut said his team also will search for underground pools with the radar technique.