The Top 10 Biggest Moons (Ours Isn’t Number One)

When we think of moons, we usually think of our Moon. It’s bright and familiar, can be seen with the naked eye, and the only one humans have ever set foot on. But in the grand scheme of the solar system, Earth’s Moon isn’t as big as you might think.

10

Oberon (Uranus) – 1,523 km

• Diameter: 1,529 km
• Size compared to Earth’s Moon: 44%
• Year discovered: 1672

Rhea is Saturn’s second-largest moon, discovered way back in 1672 by astronomer Giovanni Cassini. It’s mostly made of ice and rock and, like a lot of Saturn’s moons, it’s covered in craters. Some of these craters have bright streaks running through them, possibly from ice exposed by past impacts.

NASA’s Cassini spacecraft took a closer look at Rhea and confirmed that it doesn’t have much of an atmosphere, though there’s a chance it could have a thin layer of oxygen or even a hidden ocean beneath its surface.

One of the more interesting (and weird) discoveries was the possibility of a ring system around Rhea. If true, it would be the only moon we know of with its own rings.

8

Titania (Uranus) – 1,578 km

• Diameter: 2,707 km
• Size compared to Earth’s Moon: 77.9%
• Year discovered: 1846

Triton is Neptune’s biggest moon, discovered in 1846 by William Lassell. What makes it fascinating is that it orbits backward (called a retrograde orbit), opposite to Neptune’s rotation. That’s a big clue that Triton wasn’t originally part of Neptune’s system but was probably captured from elsewhere in the solar system.

Another wild thing about Triton? It has ice volcanoes. Instead of lava, these volcanoes erupt nitrogen gas and frozen material. When Voyager 2 flew by in 1989, it even caught geysers shooting nitrogen straight into space. That kind of activity is rare on moons.

Triton’s surface is covered in ice, and there’s a chance that liquid water exists deep below. If so, it could be one of the few places in the solar system where life might be possible. It also has a super-thin atmosphere made of nitrogen, but don’t expect to breathe there.

6

Europa (Jupiter) – 3,122 km

• Diameter: 3,475 km
• Size compared to Earth’s Moon: 100%
• Year discovered: Prehistory

Earth’s Moon is the only one we’ve got, but it’s still pretty special. It’s the fifth-largest moon in the solar system and likely formed billions of years ago when a Mars-sized object slammed into Earth. This theory is known as the Giant Impact Hypothesis.

Its surface is covered in craters, mountains, and dark plains called maria, which were created by ancient volcanic activity. Unlike any other moon, humans have actually set foot on it, thanks to the Apollo missions between 1969 and 1972.

Besides lighting up the night sky (isn’t it beautiful?), it also controls Earth’s tides and helps keep the planet’s rotation stable. Without it, life on Earth would be very different.

4

Io (Jupiter) – 3,643 km

• Diameter: 4,280 km
• Size compared to Earth’s Moon: 138.7%
• Year discovered: 1610

Callisto is Jupiter’s second-largest moon, and if there’s one thing that stands out about it, it’s the craters. In fact, it’s considered the most heavily cratered object in the solar system, meaning its surface has been unchanged for billions of years. It’s basically a time capsule from the early solar system.

Unlike some of Jupiter’s other big moons, Callisto is pretty geologically dead. No volcanoes, no shifting ice, no signs of a subsurface ocean. Just a cold, battered landscape that’s been collecting impact scars for eons.

That being said, Callisto has one thing going for it. It’s far enough from Jupiter to avoid the worst of the planet’s radiation. Some scientists think this makes it a good candidate for a future human base, since it would be one of the safest places to set up camp in the Jupiter system. I probably wouldn’t live there, though.

2

Titan (Saturn) – 5,150 km