The southern pole of Mars is hidden beneath a "deep and wide" layer of ice - enough that if it melted*, it would cover the whole planet in a sea 36 feet deep. Shallow for a sea, but still a fair quantity of aqua.

The findings are published in the 15 March online edition of the journal Science. Lead author Jeffrey Plaut said: "The south polar layered deposits of Mars cover an area bigger than Texas. The amount of water they contain has been estimated before, but never with the level of confidence this radar makes possible."

The precise measurements were taken with MARSIS, a joint NASA Italian Space Agency instrument on the European Space Agency's Mars Express orbiter. The radar instrument can see through the ice to the ground below. At its deepest, this is 2.3km below the surface of the ice.

Giovanni Picardi, professor at the University of Rome, and principal investigator for the instrument commented: "MARSIS is showing itself to be a very powerful tool to probe underneath the Martian surface...the details we are seeing are truly amazing."

He added that the instrument is still to be fine tuned, and should soon be capable of producing even more detailed information on the composition of the surface and subsurface.

So far, there is one signal that has puzzled the scientists: at the base of one layer of ice, there is a particularly light reflection. The characteristics are such that if it were warmer, the team would confidently identify it as liquid water. But it so cold that liquid water is extremely unlikely, the researchers say.

These polar layered deposits hold most of the water on Mars, and extend beyond a bright white polar cap of frozen carbon dioxide. The radar pings suggest that 90 per cent of the material below the polar cap is frozen water. ®

*Not on our list of likely events, either.

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