The stunning Korolev crater in the northern lowlands of Mars is filled with ice all year round owing to a trapped layer of cold Martian air that keeps the water frozen.

The 50-mile-wide crater contains 530 cubic miles of water ice, as much as Great Bear Lake in northern Canada, and in the centre of the crater the ice is more than a mile thick.

Images beamed back from the red planet show that the lip around the impact crater rises high above the surrounding plain. When thin Martian air then passes over the crater, it becomes trapped and cools to form an insulating layer that prevents the ice from melting.

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