Everyone memorizes the names of the eight planets. But how many of us have memorized the seven large moons of the solar system?
We’ll start with our moon. Even today, the moon is a powerful symbol in our culture. We find the moon in the sky to be a comforting familiarity. There’s romance in moonlight, and it evokes dreams of faraway places. The moon is very important for astronomy because it fuels our imagination of the universe.
Our Own, Quite Respectable, Moon
The moon is about one-fourth the size of Earth; Earth is about 13,000 kilometers in diameter, and the moon is about 3,500 kilometers in diameter. Now, you may think that Earth and the moon are pretty close together, but they’re not. Actually, the real distance between Earth and the moon is about 30 Earth diameters away from the moon—that’s about 380,000 kilometers. It looks really far away, but we have traversed that gap. The moon is the only place in the solar system that humans have visited, besides our planet.
When we look at the moon, we can see the basics of it. Its surface is dominated by craters, and from those craters we understand the history of impacts throughout the solar system. When you look at the full moon, you can see that there are some dark regions. These are the marae, which were originally believed to be lunar seas but we now know to be lava-filled plains. A lot of them have circular boundaries around them, indicating that they are lava-filled plains due to very large impacts that punched through the crust of the moon, and the lava then flowed up and filled the circular basin.
The seven large moons are, in decreasing order of size: Ganymede, Titan, Callisto, Io, our moon, Europa, and Triton.
Compared to the other large moons of the solar system, our moon is actually quite respectable. The seven large moons are, in decreasing order of size: Ganymede, Titan, Callisto, Io, our moon, Europa, and Triton. Ganymede, Callisto, Io, and Europa are the four Galilean moons that Galileo saw in 1610. Titan is the large moon of Saturn, and Triton is the large moon of Neptune.
These large moons are comparable to the rocky planets. In fact, Ganymede and Titan are even larger than Mercury, and all of these are larger than Pluto. These seven are the major moons, the ones that you really want to know about if you are studying the solar system.
Ganymede: Regions of Light and Dark
The largest moon in our solar system is Ganymede. Ganymede is ice-covered and has impact spots where the impact has broken up the ice and extruded fresh snow on top of the ice.
Images of this moon present a very strong contrast between the lighter regions of Ganymede and the dark regions. The dark regions are heavily cratered and have much more ancient terrain than the lighter regions. The lighter regions have been reprocessed. The color in the dark regions comes from some rock material. Basically, if some of the ice diffuses away, then what is left is more rocky material, and it has a darker appearance.
In images of Ganymede, these light regions show long parallel grooves. This indicates the surface of Ganymede has extended and flexed. Basically, this is ice tectonics, the sort that is present on Venus, but this is happening on the ice moon of Ganymede.
There is also some evidence of what we call cryovolcanism. These are icy flows from the interior of the moon. Ice from underneath the surface wells up and spreads out, sort of like volcanism, but in this case the lava would be slush. The ice that we see on Ganymede is a key to understanding a lot of the moons because most of the outer solar system is ice, and most of the moons are in the outer solar system.
Callisto’s Vanishing Craters
Another moon out there is Callisto. Callisto is ice-covered like Ganymede, and it has a very dark surface, so you would expect it to be an older surface like the dark regions of Ganymede, and you would be right. It has an abundance of craters, so when the Galileo spacecraft went to visit Callisto, scientists expected to see lots and lots of small craters, sort of like we see on our moon. They did not find them.
There is some kind of erosion process on Callisto that is erasing the small craters. It is possibly due to the sublimation of the ice. The ice sublimates and diffuses away from the moon. The surface then is a little more porous and is able to collapse and erase these small craters. The sublimation, as on Ganymede, would expose the dark material and give Callisto its darker appearance. There are only a few signs of ice geologic activity on Callisto, similar to what we see on Ganymede.
The Only Other Moon We’ve Landed On
A fourth large moon is Titan, Saturn’s largest moon. It’s the only moon with an atmosphere, but what makes it really special is it’s the only other moon besides our moon that we have landed on. A movie of the Huygens probe, released from the Cassini satellite, depicts the probe landing on Titan. The movie takes about 4.5 hours before the Huygens probe lands on the surface.
In the video, you can see the probe passing through the smog layer of Titan, and then slowly you are able to see the terrain underneath. As you get through the clouds, you can see the dark regions that scientists originally thought to be methane oceans on Titan, but now they know are just ice. As the Huygens probe descends, the icy terrain is revealed. It’s just hills and plains on Titan. Slowly the probe descends, and as it hits the ground, the video switches to the side view.
Icy rocks lie all around. These are basically ice balls. In the video, the surface of Titan looks to be kind of bright, but this is basically just due to the very long exposure and the lamp used to light it up. One scientist described the process as trying to take pictures of an asphalt parking lot at dusk.
Millions of Smaller Moons
The major moons are vastly outnumbered by the smaller moons. While there are seven large moons, there are about 160 smaller moons. There are about 10 medium-sized moons greater than about 1,000 kilometers in diameter, and there are about 20 small moons greater than about 100 kilometers in diameter, and then there are about 50 tiny moons, only about 10 kilometers to 100 kilometers. And that leaves about 70 moons that are even smaller that are just a few kilometers or even fewer across.