Number 3: Io

Hey, Sky Fans! Two more days and 2016 will be behind us. This means, for tonight’s sixth night of Hanukkah, we’re up to the number 3 moon on our list of the best, most amazing moons of the year. I’d have been with you sooner today, but my daughter got a Rubik’s Cube as a gift, which means I’ve spent more time than I’d like telling her about the tough times of the early ’80s when the roads were littered with Cubes, thrown from the windows by frustrated drivers. I can still only get one side. For now, that’s good enough for me. You know how it is.

There are so many great places to talk about, it’s hard to pick ones to fill out the last three spots. You know, I’d like to think of number 3, today, and tomorrow’s second best as more or less a tie. One thing’s for sure, as I’ve researched this list, I’ve learned more than I was hoping to learn by doing it in the first place.

On we go.

#3: Io

One thing I’ve learned (again) is how all these moon-hogging planets seem to have collected their satellites into groups of a few big ones and then a mess of smaller ones scattered around them. We talked about the underrated Uranus team, for instance, the other day.

Without a doubt, the most famous of them all are Jupiter’s bunch, the Galilean moons, which were discovered in 1609 or 1610 by Galileo Galilei, and were the first things discovered that orbited another planet. This set in motion a whole bunch of other things that eventually dissolved the geocentric model of the universe, and sent Galileo to prison.

Those four moons, Io, Europa, Ganymede and Callisto are positively huge, and they’re all incredibly fascinating. Ganymede is the biggest in the solar system and is one of two moons that are bigger than the planet Mercury. Do you know the other?

Poor Mercury, yesterday, we talked about it having a closer orbit than a moon, and today it’s smaller than a moon. It’s taken quite a beating over the last two days. No offense, old friend.

Even, Europa, the smallest of the bunch, is the 14th biggest object in the solar system other than the Sun. This means it’s bigger than Pluto. These moons don’t kid around. If you’re looking for superlatives, these are the places. Biggest, oldest, most cratered, you name it. I could talk for days about these places, and maybe I will next year.

Three of them are thought to have water, and lots of it. Europa, which is already seen of as one of the most likely places for us to find life in the solar system (even more likely than on Earth, huzzah!), has an enormous ocean of water, presumably free of spent gasoline, under a giant sheet of ice. Earlier this year, NASA announced they’ve spotted it shooting water off into space. Maybe they can do some science on those plumes and see if there’s life there. Ganymede, and Callisto, too, have oceans sloshing around. Wouldn’t it be amazing if it turned out to be the moons, not the planets, that have life? In a lot of ways we owe Jupiter a debt of gratitude. Its gravity keeps us safe from giant rocks, and it keeps its moons warm enough to have liquid water far outside the Sun’s habitable zone.

Io as seen by the Galileo Probe (from NASA)
Io as seen by the Galileo Probe (from NASA)

But, why Io?

Let’s put water aside for a minute. Maybe Io’s the most interesting one for exactly that reason: it has very little water to speak of or even none. What it does have, though, is sulfur! If there were air there, too, we’d have a hard time getting past the smell of rotten eggs. When the Voyagers flew through the neighborhood in 1979, volcanoes were spotted shooting sulfur off into space. As it turns out, Io is the most volcanically active place in the solar system, with hundreds and hundreds of volcanoes crammed into a space a quarter the size of the Earth. It’s so close to Jupiter, about 400,000 miles from those incredible cloud tops, that it’s able to get all the way around in less than two Earth days. This tight orbit causes Io to be stretched and twisted. It’s more machine, now, than moon (I’m a funny guy). It’s squished, and nonspherical. This tidal squashing helps make these sulfur volcanoes happen.

Here’s to the number 3 moon on our list, Io! Even though it was mostly out of the news this year, and maybe it’s getting a bit of a participation trophy, or maybe it’s carrying the flag for the rest of them, it’s really an amazing place and deserves to be thought about more than it is, thanks to its water-hog cousins.

Clear skies, everyone!



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