The biggest moons
Published on May 2, 2006 at 6:44 pm.
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So, just what is the largest moon in the Solar System? Well, that sort of depends upon how you define “largest.â€Â Most any textbook will tell you that Ganymede is the largest moon. This was not always the case. That is, I mean that the books used to say differently, not that the moons were different sizes! When I was first learning about the Solar System, most books claimed that Titan was largest. So, what changed?
Well, let’s give a bit of background. Ganymede is one of the four largest moons of Jupiter. In fact, it is the largest of Jupiter’s moons. Ganymede is, in fact, quite large. At 5262km diameter, Ganymede is even larger than the planet Mercury, which is only 4879km in diameter.  Made of a mixture of rock and ice, Ganymede would be considered a planet were it not in orbit around Jupiter.
Titan is Saturn’s largest moon. Most textbooks list Titan as having a diameter of 5150km. Incidentally, that is still larger than the planet Mercury. But, why would the older textbooks say that Titan is larger than Ganymede? Well, this is because Titan has an atmosphere with a cloud layer. Titan’s even has a pretty decent nitrogen rich atmosphere, with slightly higher density than Earth’s atmosphere. Titan’s clouds are quite thick, forming a cloud back 200km above Titan’s surface. So, measuring from cloud top to cloud top, you get a diameter of 5550km. Even before 1980, when Voyager I gave the first close-up data, we knew that Titan had an appreciable atmosphere. However, no one really thought that the clouds would be so high. After all, just about all of Earth’s clouds lie below 10km altitude. Even if Titan’s clouds were two or three times higher, that would still make the diameter of the moon somewhat larger than Ganymede. So, why are Titan’s clouds so high? Well, the answer comes from the fact that Titan has a lower gravity than Earth. So, the atmosphere balloons out much higher. A similar sort of effect is at work with Saturn and Jupiter. Saturn has nearly 1/3 the mass of Jupiter, but it is only somewhat smaller in physical size (about 84% the diameter).
So, this makes which one bigger? As I said, most books simply list Ganymede as largest. But Titan has an atmosphere and clouds. Where a moon or planet end? When we talk about Earth or Mars, we usually give the size of the solid body as the size of the planet. However, when we talk bout Jupiter or Saturn, we give the size as the diameter of the planet from cloud top to cloud top. Of course, Saturn and Jupiter have fairly small solid cores, and it would seem silly to give a diameter that is only 15% of what you see, wouldn’t it? So, which method do we use to give the sizes of the moons? Hmm. That is why you sometimes, even today, see some lists that have Titan on top, and Ganymede as second largest. For sure, if you are trying to land a spacecraft on one or the other, you’d worry more about the edge of Titan’s atmosphere, which is even farther out than the cloud tops.
However, just to complicate the matter, there is another way to measure moon size, and that is relative to the size of the planet. By this measure, neither Ganymede nor Titan rate at the top. The winner is Charon, one of Pluto’s moons. At 1190km, it isn’t very big, but compared to Pluto’s 2390km it is positively huge:  nearly 50% the planet’s size. This makes Pluto and Charon nearly a double planet.
For that matter, the Earth’s moon ranks very high on this way of looking at things. Our Moon, at 3475km, the Moon is 27% of Earth’s diameter. Ganymede is only a bit under 4% of Jupiter’s diameter, and Titan is less than 5% of Saturn’s diameter, even using the cloud top to cloud top diameter. Why such a discrepancy between Titan and Ganymede to their planets compared with the Moon and Charon to their planets? Well, we think that this is because of how they formed. Ganymede, Titan, and many other of the moons likely formed as a consequence of their parent planets’ forming, likely from material coming together to form those planets. However, our Moon and Charon likely formed from giant impacts on their parent bodies. These impacts carved out large amounts of material that coalesced to form Charon and our Moon.Â
Anyway, as long time readers of this blog will see, we have once again come to a point where a simple question doesn’t really have a simple answer that everyone agrees upon. Back long ago, we started these labels of things, and we started our lists long before we knew about what we were talking about. Unfortunately, this means that we didn’t always categorize things best, or list them right. Oh, well. At least, it gives me a chance to have something to blog about.
-Astroprof
