Mars. Mosaic of images taken by the Viking I and II probes centred at a latitude of 30 degrees north and a longitude of 270 degrees. The north polar cap (white) is clearly visible at top centre. It is formed by water ice and frozen carbon dioxide and extends outward about to the 80 degrees latitude circle. The north polar region is surrounded by a vast circumpolar plain (dark), known as Vastitas Borealis, having a diameter of about 10,000 km. The equatorial band and the southern hemisphere of the planet are marked by heavily cratered areas which contrast with the sparsely cratered plains of the northern hemisphere.