The Planets That Aren't

When we study the planets, there is an entire family of little worlds in our solar system that people often forget about: the planet-size moons. These moons are small planets in their own right—entire worlds with underground oceans, towering mountains, and strange landscapes. Some moons even have atmospheres.

What a Moon Should Look Like

For ages, the Earth's moon was the only moon we could see up close. Its surface is a rocky, cratered desert. Scientists speak of moons as being “dead” or “active” and “alive.” A dead moon is one that has not changed significantly for billions of years. Dead moons are covered in craters, while the surfaces of active ones have been changed. There are no river valleys on our moon, no signs of oceans, and no volcanoes to change the cratered landscape. Our moon looks as if it has been dead for a long time. Most scientists expected to find more of the same on the larger moons in our planetary system. They were in for a few big surprises.

The laws of nature do some seemingly strange things out in space. While the smaller moons are covered with craters gouged out by meteors and comets, the surfaces of the larger moons have been altered by a variety of interesting and ongoing processes. The resulting landscapes are sometimes "strange" beyond anything we can imagine, and the detailed causes even yet are not fully understood. The largest moons of our solar system circle the “gas giant” planets: Jupiter, Saturn, Uranus, and Neptune. These moons are shaped by the gravity of the giant planets they orbit. They are also sculpted and changed by other nearby large moons and by the cold temperatures far from the Sun.

It's Alive! The Active Moons

Until the Voyager 1 spacecraft flew by Jupiter in 1979, Jupiter's moons were thought of as silent balls of rock and ice. Then came an amazing discovery, in an overexposed photo of the pizza-faced moon Io. The photo had been taken to show the stars behind the moon, but the picture also showed a faint cloud hovering above a dark spot on Io's surface. It was the 200-km-high plume of an erupting volcano! In fact, Io is covered by volcanoes. It's a small world, about the size of Earth's moon.

What gives it so many violent volcanoes? The answer is tidal heating. When Io travels around Jupiter, it does so in an orbit that departs slightly from a circle. So, in some parts of its orbit Io is slightly closer to Jupiter than in other parts. During each trip around Jupiter, Io is flexed back and forth, like a balloon in the hands of a child. This continual flexing generates the heat to drive volcanism. The heat comes out as volcanic eruptions. Many of Io's volcanoes are like geysers, spouting sulfur dioxide high into the airless sky. Other eruptions are more like volcanoes on Earth, where magma (melted rock) pours out in fountains and lava rivers.

Tidal heating is also responsible for the weird and wacky ice moon Europa, just next door to Io. Europa's surface is cracked and ridged. Huge plates of ice have broken, floated around, and then frozen into place again. Europa may have soft ice or even an ocean under its skin. This cosmic ice cube is warmed on the inside by the same push and pull that heats up Io. The question is, If Io has volcanoes, might Europa have undersea volcanoes? Humps have been found on Europa's surface that might be pushed up from such volcanoes on Europa's ocean floor. On Earth, undersea volcanoes are surrounded by rich life. Could the same be true in the alien oceans of Jupiter's strange moon?

Another planet-size moon is Saturn's Titan. Of all the large moons, Titan is the most like a planet. Radar studies show areas that might be liquid, along with rougher regions that may be continents. Titan has a thick nitrogen atmosphere filled with some sort of orange fog. Its atmosphere interacts with sunlight to make complex chemistry, and is much like that of the early Earth before there was oxygen. Titan's surface may have ice mountains covered by gooey chemicals that rain from the sky. In January 2005, the European Space Agency's Huygens probe landed on Titan, giving earthlings their first look at the surface of this mysterious world.

The next stop on our tour of the not-quite-planets is Neptune's moon Triton. Almost as large as our own moon, Triton is a ball of ice covered by frozen nitrogen. Triton is so cold that nitrogen, which makes up most of the air you are breathing right now, is frozen into pink ice. Some scientists believe that sunbeams penetrating deep into the frozen nitrogen warm up the ice beneath the surface in a sort of icy greenhouse effect. When the nitrogen inside melts, it turns to gas. Eventually, it explodes as a geyser. Some geysers on Triton tower 8 km above the ground.

The “Zombie” Moons: Dead or Alive?

There are large moons that are not dead like Earth's moon, but they aren't really alive either. They are somewhere in between, like zombies in a scary movie. These moons have craters, but they also show evidence of flooding, volcanoes, or other geologic “life.” The largest moon in our solar system is one of these.

Ganymede is a planet-size moon of Jupiter. Its rock-and-ice surface has craters, but the craters don't last as long as those on Earth's moon. Instead, they slowly sink into the frozen surface. All that is left of some craters is a ghostly circle. Ganymede also has exotic chains of mountains twisting across its surface. These mountains show scientists that Ganymede was actively building mountains in the recent past. Perhaps it still is. Ganymede also may have an ocean of liquid water far beneath its surface. It has magnetic fields just like ones that would come from an ocean.

Just next door to Ganymede is the massive but slightly smaller moon Callisto. Its surface, however, is covered in dust and small craters. From far away, Callisto looks nearly as dead as Earth's moon, but something is in the process of covering up the small craters. Whatever is changing Callisto's surface is still a mystery.

There are other, smaller zombie moons farther out. At Saturn, the moon Enceladus has craters that have been partially covered by some kind of titanic floods. Its moons Tethys and Dione have winding canyons. Two moons of Uranus, called Ariel and Titania, also have deep chasms across their cratered faces. The weirdest of Uranus' moons is undoubtedly Miranda. It's a mixed-up worldlet with strange, oblong features that scientists playfully call “racetracks.” What caused them? Researchers believe that when a moon or planet forms, heavy material sinks to the center and the lighter stuff floats to the surface, where it finally hardens. This process of settling is called differentiation. Some scientists believe that Miranda was nearly differentiated when it was hit by an asteroid. The collision mixed up the heavy center stuff with the lighter outer elements, making Miranda the jumbled moon we see today.

The moons of the outer solar system are large and diverse. It is because of these massive moons that the lines between planets and moons is so blurry—and so confusing. What makes a planet? A moon? Where do we draw the line? Even though a moon is an object that orbits a planet, we can see that some moons are as complex as the planets themselves.


  1. What is the difference between a dead Moon and an active moon?
  2. Is Io a dead or an active moon? Why?
  3. What is a zombie moon?
  4. What is happening on Callisto?
  5. What do you think could be causing the changes on Callisto?