That’s No Moon

NASA’s Mars Opportunity Rover has finally gone dark, after over 14 years of sending data about the Red Planet, and it’s reminded me of some of my favourite missions to the various planets in the solar system.  I’ll have to do some more in-depth posts about these in the future, but I thought I’d visit a couple of the moons of Saturn today, because there’s some weird sh*t in the Solar System.


Cassini photo of Mimas

Mimas was one of the first moons of Saturn, discovered by William Herschel in 1789.  It’s probably a big ball of ice, with a surface area around the size of Spain.  One thing Herschel couldn’t see when he discovered it was that Mimas has a big impact crater that makes it look like the Death Star. (The Death Star predates the discovery of the crater, however).  It’s named after Herschel. The crater is nearly a third the diameter of the entire moon, and there are fractures on the opposite side that may have been made when Mimas was hit by whatever created the Herschel crater.  Other than looking like something from Star Wars, Mimas is pretty dull.


Cassini photo of Iapetus

Iapetus has some big-ass craters, too.  But most interestingly, Iapetus is half dark-coloured and half light.  You can see this in the NASA photo (from the Cassini spacecraft) above.  It was discovered by Cassini in 1671, who kept seeing it on one side of Saturn but not the other, and who posited that the moon must be both tidally locked to Saturn (presenting the same face at all times) and have significantly different colouring on its two “sides.”  It also, as it turns out, orbits in a heavily inclined orbit and retrograde to most of Saturn’s other satellites.

So why is it apparently covered in dark gunk on one side?  There’s another one of Saturn’s moons, Phoebe, that has more or less created its own faint ring of debris quite far out from Saturn. Phoebe is thought to be a captured comet. Iapetus’s eccentric orbit runs into this ring, and since it orbits in the other direction than the debris in the ring, it gains a coating of “gunk” every time it passes through.

Iapetus has other weird things going on as well. One of them is a prominent ridge right along the equator, rising up some 20 km from the rest of the planet.


The ridge makes the moon look a little like a walnut.  No one’s really sure how it formed, although there are all kinds of theories, including one that Iapetus had a moon of its own that was torn apart and deposited its material neatly along the equator, or another positing that the ridge is ice deposited during a period when the moon was rapidly expanding.  No other object in the Solar System has such a ridge.  The presence of the ridge and the lack of obvious explanation for why it’s there has led to some interesting conspiracy theories, however, that Iapetus is a constructed object and the ridge is the weld line.  The theory goes that the creators of Star Wars were in on the secret via communications with the aliens when they made their movie.

Artist’s conception of what Iapetus’ ridge looks like close up

So apparently, Saturn has not one, but two Death Stars.  Saturn’s largest moon, Titan, has a dense atmosphere.  Maybe it’s Dagobah.