Europa's surface. Galileo spacecraft image of the Tyre impact crater (centre) on the icy surface of Europa, a moon of Jupiter. The crater, surrounded by a 140-kilometre (km) wide system of rings, was formed by the impact of a mountain-sized comet or asteroid. Red areas consist of dirty ice which formed when water contaminated with rocky material came up through cracks in the ice and froze. Blue- green lines are ridges formed after the impact, probably due to tectonic motion of ice sheets over underlying water. Image taken from a distance of 29,000 km on 4 April 1997 by Galileo's solid state imaging (CCD) camera. The image is 214 km wide and the smallest visible objects are 595 metres across.