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The ice that covers the Martian south pole stands nearly four kilometres high and is bigger than Texas, an international team of scientists announced Thursday.

If it were to melt, it would cover the planet in 11 metres of water.

The scientists were surprised at the purity of the ice, which has much less dust in it than expected, said Dr. Jeffrey Plaut lead author of a paper published Thursday in the journal Science. He works at the Jet Propulsion Laboratory at the California Institute of Technology

He and his colleagues were able to make the most precise measurements yet of the frozen layers that blanket the underside of Mars. They used sounding radar carried by the European Space Agency's Mars Express orbiter.

Their findings mark another step in the quest to learn more about water on Mars, which many scientists believe is essential for life on the planet.

Water wouldn't stay liquid for long in the frigid temperatures and thin air on Mars. It either freezes or turns into water vapour.

But there is plenty of evidence the planet was not always the frozen desert it is today. Previous missions have sent back images of dried up river beds. Earlier this month, scientists reported that deposits that appeared to be signs of an ancient ocean were more likely produced by a network of springs.

The team is now analyzing data collected from the Martian north pole by the Mars Advanced Radar for Subsurface and Ionospheric Sounding. The instrument was jointly funded by National Aeronautics and Space Administration and the Italian Space Agency.

It transmits radio waves, which pass through the surface of the planet, and bounce off features below. A similar technique is used to study the interior of ice sheets and glaciers on Earth.

The polar regions on Earth and on Mars have equivalent amounts of ice, says Dr. Plaut.