Learn what to look for in the night sky with today’s Nerdy Curio from Stardate. One of the moons of Saturn is the Jekyll and Hyde of the solar system, presenting two very different faces to the universe.

Iapetus is about 900 miles in diameter — less than half the size of our own moon. And it’s locked so that one side always faces Saturn, just as the same side of the Moon always faces Earth. That means that the same portion of Iapetus always faces forward as it orbits Saturn. And that may be responsible for its two-faced appearance: The leading hemisphere is almost as dark as charcoal, while the trailing hemisphere is quite bright.

The dark hemisphere is coated with compounds that are rich in carbon, while the bright hemisphere is mostly ice.

The dark material may have been blasted off the surface of another of Saturn’s moons, and swept up by Iapetus. This dark material forms a layer that’s only about a foot thick. But it absorbs energy from the Sun, warming the surface. That caused ices below the coating to vaporize, darkening the surface even more. Some of the vapor then migrated to the opposite side of Iapetus, coating that hemisphere with fresh ice, making it brighter — giving the odd moon a two-faced appearance.

Look for Saturn close to the lower left of our moon as they rise late this evening. Saturn looks like a bright star. The true star Antares, which is about half as bright as Saturn, is a bit farther to the lower right of the Moon. You need a telescope to see Iapetus and Saturn’s other moons.

Script by Damond Benningfield

Aired May 12, 2017

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