Moon and Jupiter 
The largest moon in the solar system has a two-toned appearance, as though someone started painting its surface but left some big splotches of the original color. And in a way, that’s just what happened. The motions of its icy crust have repaved most of the surface, but left large patches of the original crust in view.
Ganymede orbits Jupiter, which is in good view tonight, to the lower left of our own Moon at nightfall. It looks like a brilliant star.
Ganymede is about half again as big as the Moon — bigger even than the planet Mercury. And it’s put together a lot like a planet, with a series of layers: an iron core, surrounded by lightweight rock, and topped by ice. The ice may contain a large ocean of liquid water.
Ganymede’s crust shows two tones of brown.
The darker tone most likely consists of regions of Ganymede’s original crust, which are pockmarked by lots of impact craters.
The lighter tone shows a slightly younger crust, which formed as Ganymede’s interior was heated by Jupiter’s gravity. These regions are marked by deep canyons and tall cliffs that can run for hundreds of miles. They may have been created by the motions of large blocks of ice at the top of the crust — motions that gave Ganymede a bit of an unfinished appearance.
Again, look for Jupiter near the Moon tonight. Through binoculars, Ganymede and three other moons look like tiny stars arrayed near the bright planet.
More about Jupiter and our Moon tomorrow.
Script by Damond Benningfield, Copyright 2011