Jupiter blazes just to the left of the almost-full Moon this evening. The giant planet outshines all the other planets and stars in the sky at that hour, so it makes a brilliant counterpoint to the Moon.
Jupiter has a whole entourage of moons of its own, the largest of which puts our moon to shame. In fact, Ganymede is the largest moon in the entire solar system — half again the diameter of our moon, and bigger even than the planet Mercury.
Unlike Mercury and the Moon, though, Ganymede contains a lot of frozen water. The ice forms an outer layer that’s hundreds of miles thick. An ocean of liquid water may slosh about deep inside the ice.
The surface of Ganymede consists of two completely different types of terrain, which give it a streaky, blotchy appearance. Dark regions appear to be parts of the original crust, and they’re covered with lots of impact craters.
Light-colored regions appear to be younger. The main features in these areas are grooves that can be a couple of thousand feet tall and hundreds of miles long. They might have formed when the gravity of some of Jupiter’s other moons tugged at Ganymede. That cracked and folded the ice, wiping out any evidence of the original crust of this big, icy moon.
Ganymede and three other Jovian moons are visible through binoculars. They look like tiny stars arrayed near Jupiter. Around midnight tonight, they’re all on the left side of Jupiter, with Ganymede closest to the giant planet.
Script by Damond Benningfield, Copyright 2013
For more skywatching tips, astronomy news, and much more, read StarDate magazine.