Monday, March 8, 1999 Published at 13:57 GMT
Best ever view of Ganymede
If it were on its own it would be a planet
By BBC News Online Science Editor Dr David Whitehouse
If it orbited the Sun on its own then it would undoubtedly be a planet. The ice world of Ganymede is the largest moon in the solar system. In fact, it is larger than some planets.
With a diameter of 5,260 kilometres, it is larger than the planets Mercury and Pluto and just over three quarters the size of Mars.
The most-detailed ever view of Ganymede as a whole body has just been compiled from images obtained by the Galileo spacecraft that has been orbiting Jupiter since December 1995.
The image shows the trailing hemisphere of the moon. It is speckled with bright young craters.
Ganymede's craters are remarkably smooth. They seem to have become flattened as the moon's icy surface slowly relaxes.
Its surface shows a mixture of old, dark, cratered terrain and lighter, younger, regions streaked with grooves and ridges.
In reality, Ganymede's colours are subtle browns and greys, but the colours in the image on this page have been enhanced to increase surface contrasts.
The violet shades extending from the top and bottom are likely to be due to frost particles in Ganymede's polar regions.
Another moon of Jupiter, Europa, has captured most of the attention in recent years. This is because of the possibility of an ocean beneath its frozen surface.
New observations of Ganymede suggest that Europa may not be alone in having an under-ice ocean - a possible abode for life.