Oh, Those Moons!

Hey, sky fans. Thanks for stopping by and sharing some of your morning coffee here.

The other night, while I was standing outside with my family looking at the sky and taking a couple of sunset photos, I got thinking about Jupiter, which was poking through the clouds. A couple of weeks ago, the Juno probe made its way into orbit there, and will start sending back all kinds of juicy and salacious news before long. The photos, which aren’t its main goal at all, will be incredible. I can’t wait.

When you think about Jupiter, there are a lot of superlatives that come to mind: biggest planet; biggest storms; most radioactive; most moons; biggest moons; shiniest car (I’d imagine). It goes on. I’ve been thinking a lot about those moons. What most people know about Jupiter’s moons is there’s a lot of them, 67 in what are believed to be fairly stable orbits, with new ones discovered from time to time.

The four most famous, Ganymede, Callisto, Io, and Europa, are big and planet-sized. In fact, Ganymede is one of the two moons in the solar system bigger than the planet Mercury. For 1100 points, what’s the other? These four were first seen in 1610 by Galileo Galilei, and, so, they’ve come to be called the Galilean moons. These moons are incredible places, Io orbits so close and quickly than Jupiter’s gravity stretches and squeezes it like a rubber stress ball to the point that it’s noticeably not a sphere. It has sulfur volcanoes erupting on its surface. Europa is covered in a sheet of water ice and has a giant ocean underneath that lots of really smart people think could, could, have life in it. Ganymede has its own magnetic field! Amazing.

But what about the other moons? No one ever talks about them. Interestingly enough, that’s because they’re all really small, and there’s not a lot to talk about. Most of the rest of the moons fall off quickly into small-town sized rocks. Many of them are only around 2 km (a bit less than a 1.5 miles) across, or smaller.

One of the things I find most interesting about these other ones is that there’s a whole other set of them orbiting even closer in than Io does! They quietly spend their time in and among Jupiter’s rings. These are actually surprisingly interesting moons, themselves. Two of them orbit faster than Jupiter spins!

Thebe is the furthest of these inner moons, and the second biggest. It wasn’t even discovered until 1979 when the Voyagers sped through.

After Europa, which is the smallest of the Galileans at about 3100 km across (just under 2000 miles), the next biggest moon is Amalthea. Amalthea, like the others, is oblong and lumpy, far from a sphere, and made of water ice and all kinds of other stuff; enough to make it look red. It’s the biggest of those inner four moons, but is still only about 165 km across on average. It’s not a sphere, so there’s all kind of exciting math involved with figuring that.

Amalthea Voyager-1.png
Amalthea by NASA – NASA (see page with photos, description of this image), Public Domain, https://commons.wikimedia.org/w/index.php?curid=34742364

Adrastea is the smallest of this group, only about 16 km across on average (again, it’s not a sphere), and was also only discovered when the Voyagers drove by. It’s so close to Jupiter’s clouds it finishes an orbit around Jupiter in only about eight Earth hours, faster than a Jovian day.

The closest in of the bunch is Metis, which is sort of lumpy and potato-shaped. It manages to zip all the way around Jupiter in just under seven hours. Like Adrastea, this is shorter than a day on Jupiter, so if you were sitting on Jupiter’s cloud tops with a beer and a book, you’d see the two of them rise, cut across the sky, set, and then rise again a few hours later. The only other moon in the solar system that does this is Mars’s Phobos, which also orbits in about seven hours. Mars is positively tiny compared to Jupiter, though. So, Phobos can orbit much closer and still make it all the way around that fast. It has a much shorter distance to go to get back to the starting line. Imagine getting all the way around a planet the size of Jupiter so quickly. It’s remarkable. For comparison, it takes our Moon nearly an entire month to make its way around.

I agree, there isn’t a whole lot to these tiny moons, especially when you compare them to the giants, but if you look closer, you never know what you might find.

Clear skies, everyone!





2 thoughts on “Oh, Those Moons!

    1. Yes! We have a winner! Yeah, it’s Titan. I don’t know about you, but there’s something that I have a hard time wrapping my head around about a moon bigger than a planet. The word planet has nothing to do with size, but there’s a context there. It’s just really amazing to think how big those giant planets are. Thanks for stopping by!


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