Greetings! Today I want you to think about Jupiter. It’s such a large planet; we can barely comprehend its size. It is the second largest known object in our solar system–the only thing bigger in size and mass is the sun! But what I really want to talk about are the moons of Jupiter. Think about how alien, how different the landscape is compared to that of our single moon. Now Jupiter, he’s got 67 different moons! Granted, a majority of them are just asteroids captured in its orbit. But the main point I want to discuss, are the four big moons of Jupiter. They are referred to as the “Galilean” moons: Io, Europa, Ganymede, Callisto.
Now, to start off, I want to try and drill into your heads the sheer size of Jupiter. I found this neat little image that compares North America to Jupiter. I know, even just thinking about the size of a continent can be a little difficult, but this may help put some things in perspective.
There, now that we have that settled, lets talk about the moons. Although Jupiter has tons of moons, these four moons make up 99% of the mass of objects orbiting Jupiter. That is how big they are; even then, they aren’t very big compared to the planet.
Now that we have some approximate estimations of sizes, lets dig in to the interesting characteristics of them. One interesting thing is that the three inner ones (Io, Europa, Ganymede) rotate in a 1:2:4 period. Meaning, when Ganymede completes one orbit, Europa will have completed two orbits and Io will have done four. This sort of synchronization is kind of beautiful right?
Io is the innermost of these four moons and is the fourth largest moon in the solar system. This moon also happens to be the most geologically active object observed in the solar system too. There are over 400 active volcanoes. In the picture, the black spots are mostly large volcanoes, and some are even higher than our Mount Everest. The extreme heating is generated because of all the gravitational activity it experiences from Jupiter and the other large moons. The yellow color is primarily caused from silicates, sulfur, and sulfur dioxide.
Io actually has an atmosphere as well; it is very thin but is there. It is primarily composed of sulfur dioxide. So if a planned Ionian landing were to occur, the ship will not actually need heat shielding. But due to its proximity to Jupiter, radiation levels are very high.
Europa is probably the moon that gets the most attention. This is because that the entire surface of the moon is composed of water ice and it is believed that beneath the surface lies an ocean of liquid water. They believe this because the surface features tend to rotate, suggesting that the outer layer moves at a different pace from the core. Basically, it makes it appear that liquid is beneath the surface. In the end, this moon is particularly popular because of the whole search-for-life quest. Many believe that liquid water is the best sign for life, and if the estimates are real, this moon has lots of water.
It too has a thin atmosphere. The atmosphere is actually composed of O2, molecular oxygen. But it is very thin, almost nonexistent. There have even been water vapor plumes detected. All of these clues make studying this moon a very hot topic in popular science. So you can bet that the near future will have some missions dedicated for this moon.
Ganymede is the largest moon in the solar system, in fact it is actually larger than Mercury! It also is the only known moon to have a magnetosphere, similar to Earth’s, which makes scientists believe this has an active iron-rich liquid core. This moon also contains a thin atmosphere of O, O2, and O3.
The surface composition is about half rocky material and half water ice. This moon is also believed to have several layers of subsurface oceans. This moon is also up for possible future explorations, though not as much as Europa.
Callisto is the second largest Jovian moon and the third largest moon in the solar system. It is the only one of the Galilean moons that is not in an orbital resonance, thus it does not experience appreciable tidal heating. It is the only one that does not show evidence of subsurface processes (volcanism, plate tectonics). It is also believed to have evolved primarily by impacts, as you can see from the numerous craters on its surface.
This moon has low radiation levels because of its distance from Jupiter. Due to that, it has largely been the focus for any possible future base involving humans. The moon is composed of rocky material and water ice. It is believed to have a subsurface ocean as well. Like the others, it has a very thin atmosphere, but is mostly carbon dioxide.
That is just four moons! There are many, many more around just Jupiter. The variety of moons is just crazy; there are different ones around other planets with unique characteristics, but that is a discussion for another day. What did you think? Learn anything interesting?