Moon and Jupiter

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The planet Jupiter rises below the Moon early tomorrow. It looks like a brilliant star. It's in the east about 45 minutes before sunrise, but it's so low that you need a clear horizon to see it.

Jupiter has more than 60 moons of its own. The largest is Ganymede, the biggest moon in the solar system -- bigger even than the planet Mercury. And the second-largest is Callisto.

The two moons resemble each other. They're both made of rock and ice, with icy surfaces scarred by impacts. But the resemblance doesn't go very deep. Ganymede appears to be made up of layers: a solid iron core, a layer of rock around the core, and a thick layer of frozen water on top. Callisto is more jumbled, so it doesn't have the same distinct layering.

A recent study by Amy Barr and Robin Canup at the Southwest Research Institute says the moons are different because one of them took a more severe beating.

About 3.8 billion years ago, all the planets and moons in the solar system were bombarded by comets. Jupiter's gravity pulled in many of these big balls of ice and rock. Since Ganymede is closer to Jupiter, it got hit by more of them than Callisto did, and the ones that hit it were moving much faster.

The energy from the impacts melted Ganymede most of the way through. That allowed the metal and rock to sink to the middle. Callisto didn't melt as thoroughly, so it remained jumbled -- a mixed-up ball of rock and ice orbiting the solar system's largest planet.

Script by Damond Benningfield, Copyright 2010

For more skywatching tips, astronomy news, and much more, read StarDate magazine.