Ganymede is quite different from the inner worlds of Io and Europa. It is the
largest of the Galilean moons at 5140 km. Ganymede is about 1 million km from Jupiter with a period of 7.1 days. Ganymede has a much lighter density than Io and Europa, only 1.9 g/cm3. This indicates that Ganymede is half rocky material and half ice.

     Ganymede is a heavily cratered world with a surface also crossed with grooves which appear to be fractures in the icy surface. Since the grooves do not contain many craters it is believed to have formed more recently. The typical size of the grooves are 3 to 4 km across and about 300 meters deep with gentle sloping sides.

When Ganymede was formed, it cooled much slower than Io and Europa. The grooves were probably formed early in the moon's history, but after most of the cratering had taken place. When the icy crust fractured, the rocky slush oozed up from beneath giving the grooves their coloring and covering up any craters that lay in its path.

     Ganymede has changed little since its early history. Intrestingly, Ganymede has the same period of rotation as it does revolution (just like our moon.) This means that the same side of Ganymede always faces Jupiter. The side facing away from Jupiter is marked by a dark cratered region called Galileo Regio.

This image is an area called Urus Sulkus, the grooves are clearly visible. In the lower middle area a large impact crater has blasted up fresher and cleaner icey material from beneath the surface. Younger grooves can be seen crossing over older grooves. The image covers an area about 35 km by 55 km (34 by 25 miles.) It was taken by the Galileo spacecraft June 1996.

     There is no evidence of activity on Ganymede now. It is believed that there has been no activity since Ganymede's first billion years.