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The solar system is full of strange things. But these three moons are especially strange, and kind of ... ugly.

Hosted by: Caitlin Hofmeister
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Giant impact: Janes DM and Melosh HJ. Sinker Tectonics: An Approach to the Surface of Miranda. JGR 1988, 93:B4, 3127--3143.
Faulting: Pappalardo RT, Reynolds SJ, and Greely R. Extensional tilt blocks on Miranda: Evidence for an upwelling origin of Arden Corona. JGR 1997, 102:6, 13,369--13,379.
Sublimation: Denk T, Neukum G, Roatsch T, et al. Iapetus: Unique Surface Properties and a Global Color Dichotomy from Cassini Imaging. Science 2010. 373, 435--439.
Ridge leftover from squatter Iapetus: Porco CC, Baker E, Barbara J, et al. Cassini Imaging Science: Initial Results on Phoebe and Iapetus. Science 2005, 307, 1237--1242.
Ring origin of ridge: Ip, W-H. On a ring origin of the equatorial ridge of Iapetus. Geophysical Research Letters 2006. 33:16, L16203.
Weird cratering:
Chaotic orbit: Wisdom J, Peale SJ, Mignard F. The chaotic rotation of Hyperion. Icarus 1984, 58:2, 137--152.
Bunch o’ rock:

Image Sources:
The universe is full of weird and exciting things, a lot of which we still don’t really understand — from giant electrical currents in distant galaxies to stars that look like they’re surrounded by some kind of alien megastructure.

But even just within our own solar system, there are plenty of strange hunks of rock. Because the solar system has some ugly, pockmarked, and sometimes very potato-like moons.

First up: Miranda, Uranus’s largest moon. Miranda has some very distinct features that have left astronomers wondering why it’s so ugly. In addition to the usual craters and pockmarks that cover any rocky object with basically no atmosphere, Miranda has deep, almost parallel gashes running along its southern hemisphere.

It kind of looks like a ball of yarn, or like a giant space monster was playing basketball and fumbled the ball. What’s especially weird about these deep grooves is that they are confined to three regions called coronae, which is just Latin for “crown”. Astronomers do have some ideas about where these features come from.

One possibility is that Miranda was hit by something so large that it actually broke the moon apart, and the patterns formed as the pieces clumped back together. Don’t you love if astronomers aren’t sure how a thing formed, they’ll often suggest that it got hit by something else. More recently, researchers have suggested that the ridges and grooves are actually Uranus’s fault.

As Uranus and Miranda orbit, they pull on each other, deforming each other a little bit. And that deformation would generate some heat inside Miranda — maybe enough heat to lead to geological activity, like icy volcanoes or stretching, which could have formed those ridges There are still some unresolved questions, though. Like, why do the ridges only show up in the southern hemisphere?

And why only in the coronae? We might have to wait until the next mission to Uranus to find out. Then there’s Iapetus, one of Saturn’s dozens of moons.

Iapetus looks like a half-moon cookie, but not because it’s hidden in shadow. Half of it is covered with some unknown, dark substance. It also has this big ridge running along the equator, which was discovered by the Cassini orbiter in 2005.

Scientists have come to a kind of consensus about the dark material: they believe it’s caused by two factors. First, Iapetus sits right inside the Phoebe ring, which is an enormous, diffuse ring that astronomers discovered surrounding Saturn in 2009. Dark material from the Phoebe ring falls into Iapetus, mostly on the leading side.

Second, ice sublimating, or vaporizing, off of Iapetus’s surface leaves behind a residue of darker particles that were suspended in it. The part of the moon that’s already a little darker because of the material from the Phoebe ring is also a little warmer, so more ice sublimates on that side. The equatorial ridge is still a mystery, though!

One option is that Iapetus used to have rings — or “moon moons”, if you will — but the ring material fell into Iapetus, creating a ridge along what was once the ring’s orbital path. It’s also possible that Iapetus just used to be very squat, and the bulge is all that remains from that period of its life. Hyperion is another of Saturn’s really odd moons.

For one thing, it basically looks like a haggard, gnarly potato. It’s about as big as something can be before its gravity forces it into a more spherical shape. Well, the astronomer’s standby answer of “it probably got hit by something” applies here too!

Astronomers think that Hyperion looks like a potato because it’s the leftover fragments of a larger moon that was hit by something enormous. But those fragments didn’t have enough mass into a compact, spherical shape, and instead are more like a big group of different rocks. You may have also noticed that it looks a lot like a sponge.

That’s probably because Hyperion is mostly ice, and not very dense. When it’s hit by meteorites, they just punch down through the ice like a very aggressive knife through butter, giving it that spongy look. But even though it’s mostly ice, Hyperion is a lot less reflective than we’d expect it to be.

So there’s a layer of some substance on there, but astronomers don’t know exactly what it is. It looks a lot like the stuff on Iapetus’s dark side, and Hyperion is very icy, so sublimation might also play a role on Hyperion. And Hyperion doesn’t just look weird — it also acts weird.

Its rotation is super chaotic, meaning that it’s very hard to predict which way Hyperion will be facing at any given time. That probably comes from the way Hyperion’s lopsided orbit lines up with the orbit of Saturn’s largest moon, Titan, which leads to a gravitational pull from Titan that keeps Hyperion’s orbit lopsided, and its rotation from stabilizing. So, these moons are kind of ugly, and fantastically weird.

But studying them has also taught us a lot about the history of our solar system. Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon who make everything we do possible. If you want to help us keep making episodes like this, you can go to, and don’t forget to go to and subscribe!