A city-sized impact crater has been found on the frozen surface of Jupiter's moon Europa.
Detected by the Galileo spacecraft, the impact zone may shed new light on the enigmatic icy surface of this strange ice-crusted satellite.
The scar from the past impact, made by a comet or small asteroid, is revealed in false-colour images.
The bright, circular feature at the centre right of this image has a diameter of about 80 kilometres (50 miles), making it comparable in size to the largest cities on Earth.
The brightest parts of the image are regions with a high proportion of relatively pure water-ice. Blue indicates non-ice materials are also present.
Astronomers are unsure about the composition of the darker materials. They may consist of minerals formed by evaporation of salty brines, or they may be rich in sulphuric acid.
The ring seen around the impact crater is the ejecta, consisting of icy subsurface material blasted out of the crater by the impact. The darker area in the centre may be debris from the impacting body.
Life question
Europa is a focus of attention for astronomers because an ocean of liquid water may exist beneath the icy crust, possibly providing an environment suitable for life. The Galileo spacecraft has made many close passes of the moon, and in the future a new spacecraft mission, the Europa Orbiter, will make a more detailed study of this fascinating world.
Large impact craters are extremely rare on Europa; only eight are known. The small number of craters suggests that Europa's surface may be quite young in geological terms.
This discovery may provide insights about the age and level of geological activity of Europa's surface.
To the east of the crater are two, or perhaps three, similar but less well-defined quasi-circular features. They raise the possibility that this crater is one member of a catena, or chain of craters.
This would lend still greater interest to this area as a potential target for focused investigations by later spacecraft missions.
Galileo has been orbiting Jupiter and its moons since December 1995. Its primary mission ended in December 1997, and after that Galileo successfully completed a two-year extended mission.
The spacecraft is in the midst of yet another extended journey called the Galileo Millennium Mission.