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A giant patch of frozen water has been pictured nestled within an unnamed impact crater on Mars.

The photographs were taken by the High Resolution Stereo Camera on board Mars Express, the European Space Agency probe which is exploring the planet.

The ice disc is located on Vastitas Borealis, a broad plain that covers much of Mars' far northern latitudes.

The existence of the water-ice patch on Mars raises the prospect that past or present life will one day be detected.

It also boosts the chances that manned missions could eventually be sent to the Red Planet - because they would probably need accessible water to survive.

Highly visible

The highly visible ice lake is sitting in a crater which is 35 km (23 miles) wide, with a maximum depth of about two km (1.2 miles).

Scientists believe the water-ice is present all year round because the temperature and pressure are not sufficient to allow it to change states.

Researchers studying the images are sure it is not frozen carbon dioxide (CO2), because CO2 ice had already disappeared from the north polar cap at the time the image was taken.

The team has also been able to detect faint traces of water-ice along the rim of the crater and on the crater walls.

Mars is covered with deep gorges, apparently carved out by rivers and glaciers, although most of the water vanished millions of years ago.

Earlier this year, the European Space Agency detected what they called a huge "frozen sea", but it is located below a crust of surface deposits.

Large reserves of water-ice are also known to be held at the poles on Mars, and probably at great depth at many locations around the planet.