Last updated at 09:42 29 July 2005

A beautiful circle of pale blue, it looks like an exotic pool where travellers could take a dip.

But don't even think about packing your swimming gear and heading there.

For this is a patch of ice sitting on the floor of a crater near the north pole of Mars.

The picture was taken by the high-resolution stereo camera on board the European Space Agency probe Mars Express.

Water

Scientists obtained definitive proof of water on Mars more than 18 months ago and since then have produced pictures of varying quality. This is the most spectacular image to date.

The impact crater is located on Vastitas Borealis, a broad plain that covers much of the far northern Martian latitudes.

It is 21.7 miles wide and has a maximum depth of about 1.2 miles. The circular patch of frozen water is present all year round. Faint traces of ice are also visible along the rim of the crater and the crater walls.

A large dune field is thought to lie beneath the ice layer, some of which is exposed at the edge of the bright patch.

The existence of large bodies of water on Mars does not necessarily mean there was, or is, life there. But it makes it much more likely, although any such life is likely to be confined to primitive micro-organisms.