What is #40: Iapetus?

Yep, another moon for this edition of "What Is?". :-) The name of this moon is Iapetus, a moon orbiting Saturn.

NASA/JPL/Space Science Institute / Public domain

Iapetus is a big moon, although not one that is as well known as the other big moons. It is the 3rd largest moon orbiting Saturn and the 11th largest moon in all of the Solar System. In the image above you can see it pot-marked with craters, meaning it has been impacted many times, including a very large crater in the bottom half of the image.

An interesting feature on this moon is that one hemisphere is bright and the other is dark. This was known when the moon was first discovered in by Cassini in the late 1600s, but at the time it was not known what caused this difference. The Cassini spacecraft discovered the reason in 2008. This moon is tidally locked, so its same side always faces Saturn. Therefore the same side of the moon is always leading the way as it orbits Saturn. This side has swept up dark dust over many, many, years, depositing it on that side of the surface. Therefore one side is darker due to collected dust, and the other side is brighter. Pretty cool!

Guess what? You can probably expect another moon in the next issue of this series! :-)