The Moon probably formed after Earth got a big whack. A body as big as Mars slammed into our planet, blasting molten rock and other debris into space. Much of this material then came together to form the Moon.

Many other bodies in the solar system have suffered big whacks of their own. Three likely examples are found among the more than 80 known moons of Saturn, which is close to the upper left of the Moon this evening and looks like a bright star.

Tethys is about 660 miles across. Its major feature is Odysseus Crater, which is more than a third of the diameter of the moon itself. The impact that gouged it was so powerful that it would have shattered a solid body. That suggests that Tethys was still partially molten when it got hit. That provided enough “cushion” to keep Tethys from flying apart.

Mimas is smaller than Tethys, but it has a crater of about the same proportions. Known as Herschel, it’s fresher and deeper than Odysseus. It makes the little moon look like a Star Wars death star.

Finally, Dione appears to have been spun halfway around its axis. It has more craters on its trailing hemisphere than on its leading hemisphere — just the opposite of what you’d expect. A possible explanation is that a good-sized impact spun it around. But it seems odd that Dione would have been turned precisely 180 degrees. So scientists are still looking into the history of this little moon that might have gotten a big whack.

Script by Damond Benningfield

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