Moons Visited and Revisited
A volcanic eruption on the surface of Io
taken by the Voyager spacecraft. Credit: NASA/VoyagerPlanets hog a lot of press, inside and outside the Solar System, but there's a lot to be said for those "second class" worlds that are the satellites of the planets–some of which would be true planets (fascinating ones, too) if it weren't for the fact that they orbit another planet instead of the Sun.
I'd like to point out a few things about these little worlds of which you may not be aware.
I have to start with Earth's own satellite–the Moon (or Luna, by the Latin name). Whether or not large moons are common around Earth-sized planets, they really are not common in our Solar System. Earth has one very sizable moon, a quarter the diameter of Earth itself, whereas none of the other terrestrial planets really do. Mars' two moons are pint-sized–roughly comparable to Mount Diablo in size. And that's it. (I can avoid talking about Pluto's moon, Charon, since Pluto isn't an official planet… please don't throw the rotten fruit!)
Mars' satellites–14-mile Phobos and 6-mile Deimos–are possibly asteroids captured from the Asteroid Belt, which neighbors Mars. Many of Jupiter's current count of 63 moons are also probably absconded asteroids.
It's Jupiter where the satellite worlds truly begin, Luna notwithstanding. As I mentioned, some moons would be planets if only they orbited the Sun directly. Jupiter's largest moon (the Solar System's largest, for that matter) is Ganymede, with a diameter that exceeds that of Mercury.
Another of Jupiter's four large "Galilean" moons, Io, is the most volcanically active object known in the Solar System. Europa has probable oceans of water hidden under its fractured icy crust –oceans, I said, perhaps as deep as 30 miles! Think of the possibilities with that much liquid water! The heating that powers Io's volcanoes and keeps Europa's oceans thawed comes from tidal stressing by Jupiter's powerful gravity–another interesting planet-moon interaction.
One oddity about many of the moons of the Solar System–including our own–is that they are gravitationally locked to their parent planet. That is, like our Moon, they always keep the same side facing their planet. What this also means is that, as the moon orbits its planet, the same face always leads "forward," in its direction of travel, the other side always trailing aft–in some cases producing amusing results. For example, Saturn's Iapetus is half bright white and half dark, its face that leads the way in its orbit around Saturn collecting ice particles in much the way that the wind-ward side of a house collects snow, which piles up on that side.
Saturn's Titan is definitely a world unto itself, with a cold nitrogen atmosphere much thicker than even Earth's, clouds and haze of methane and ethane, as well as precipitation, runoff, lakes, and even seas of these compounds.
And don't forget the one thing moons have that their planets don't: location! Imagine the view you'd have from the moon of a gas giant planet: a view of the gas giant– magnificent clouded Jupiter, stunning ringed Saturn, blue-green Uranus and Neptune. You don't find views like these from a planet… at least, not in our Solar System.
Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.
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