Scientists used a radar instrument on the European Space Agency's Mars Express spacecraft to determine the thickness and volume of ice deposits covering an area larger than Texas.

"This is the first time that a ground-penetrating system has ever been used on Mars," said Jeffrey Plaut of NASA's Jet Propulsion Laboratory. "All the other instruments used to study the surface of Mars in the past really have only been sensitive to what occurs at the very surface."

Researchers are eager to learn about the history of water on Mars because water is fundamental to whether the planet has ever harboured life.

Channels on the Martian surface suggest the planet was once very wet, in contrast to its present desertified state.

Plaut believes that the amount of water in the past may have been the equivalent of a global layer hundreds of meters deep, while the polar deposits represent a layer of perhaps tens of meters.

"We have this continuing question facing us in studies of Mars, which is: where did all the water go?" Plaut said.

"Even if you took the water in these two ice caps and added it all up, it's still not nearly enough to do all of the work that we've seen that the water has done across the surface of Mars in its history."