Thursday, July 16, 1998 Published at 11:11 GMT 12:11 UK
Lost ocean of Jupiter moon
A crater chain formed by a fragmented comet
The latest photos released of Jupiter's latest moon, Ganymede, suggest it may have once had an ocean beneath a layer of ice. This would have made it very similar to one of Jupiter's other moons, Europa, which has been suggested could harbour life. Our science editor Dr David Whitehouse reports
Although the moon is now a dead world, etched across its surface are indications of a more active past.
A dramatic feature is a chain of craters named Enki Catena. This row of 13 craters probably formed when a comet was pulled to pieces by Jupiter's gravity after it has passed too close to the planet.
Soon afterwards it crashed onto Ganymede. A similar event occurred in 1994 when fragmented comet Shoemaker-Levy 9 plunged into the cloud tops of Jupiter.
Such bright and dark groves cover about half of Ganymede's icy surface. the height difference between the highest and lowest points in the groves is about 200 metres.
Based on the surface markings, astronomers believe that 3 billion years ago Ganymede may have had an ocean, like the one that may be below Europa's frozen crust.
"We don't know and that's why we're out there looking," said James Head, a Brown University planetary scientist.
"You have heat, liquid water, organic material coming in from impacts from meteorites. The ingredients are there."
If Ganymede were orbiting the sun instead of Jupiter it would be considered a major planet. It is bigger than Mercury and not a lot smaller than Mars.