Pretty much everyone can rattle off the names of our solar system's eight (formerly nine) planets, but ask the average person to list some moons and you'll be lucky if they can tell you more than two or three.
Now, you obviously can't expect people to remember the name of every single satellite in the solar system (after all, they outnumber the planets by around 20 to 1), but if you have even the slightest interest in astronomy, it wouldn't kill you to be familiar with at least an even ten. So with that in mind, we've assembled this reference guide to ten of the solar system's most noteworthy moons.
Moon: Europa
Parent Planet: Jupiter
Why You Should Know it: Despite being covered by distinctive, criss-crossing cracks and ridges, Europa's water-ice surface is largely free of craters, making it perhaps the smoothest solid body in the entire solar system. More interesting than Europa's frozen exterior, however, is what lies hidden beneath it.
A growing body of evidence suggests that there is a vast liquid ocean lurking underneath Europa's solid surface, and a lot of scientists are eager to tap this watery interior in search of life. As Michael Shara — curator of the astrophysics division at the American Museum of Natural History — told us: "If we can figure out a way of putting a probe through the ice… who knows what we could find [in the subsurface oceans of Europa]. It would be fascinating to go look, and I think we have no choice but to go look; we must do it."
Moon: Io
Parent Planet: Jupiter
Why You Should Know it: Io is very close in size to our own moon, but it couldn't be more different. Despite having a mean surface temperature of less than -250 degrees Fahrenheit, Io is home to over 400 raging volcanos, making it the single most geologically active object in the solar system.
Moon: Mimas
Parent Planet: Saturn
Why you should know it: This list is an important reference for any self-respecting science geek, but Mimas is especially relevant for fans of science fiction for what should be obvious reasons. In brief: Mimas is no space station. It's a moon. Like many of Saturn's orbiting bodies, Mimas is small and icy, but it's also home to "Herschel" — the name astronomers have given the massive crater situated on the moon's leading hemisphere.
At 139-kilometers wide, Herschel is almost one-third the diameter of Mimas itself, causing the moon to bear a striking resemblance to the Death Star. Incidentally, Herschel crater was discovered three years after the release of A New Hope (Star Wars has a way of predicting astronomical discoveries). Bonus geek points: Temperature maps of Mimas reveal hot regions that look exactly like Pac-Man eating a dot.
Moon: Enceladus
Parent Planet: Saturn
Why You Should Know it: Enceladus is one wacky little moon. Like Europa, its surface is covered in water ice, but it's also home to some of the most impressive geysers in the solar system. Scientists had suspected for years that Enceladus was venting water vapor from its surface, but it wasn't until 2005 that they had direct visual confirmation that the moon was doing so by spewing jets of the stuff from geysers on its surface.
More recent images have revealed that this water vapor is expelled so violently that it makes it all the way to Saturn itself, making Enceladus the only known moon to affect its planet's climate directly. And, of course, water vapor could also indicate the presence of a water ocean, making Enceladus yet another attractive destination in the search for alien life.
Moon: Triton
Parent Planet: Neptune
Why You Should Know it: Of all the biggest, "major" moons in the solar system, Triton is the only one that orbits in a direction opposite that of its parent planet's rotation. Astrophysicists call this a "retrograde orbit," and it's typical of moons that have been "captured" by their parent planet.
But Triton not only orbits Neptune more closely than most captured moons, it's also orders of magnitude larger (as a point of reference, Triton is even bigger than ex-planet Pluto). Understanding how Neptune managed to wrangle Triton into its close, nearly perfect circular orbit could teach us a lot about how cosmic bodies interact, and also about the Kuiper belt — the mysterious ring of icy bodies encircling our solar system from which Triton is thought to have been apprehended.
Moon: Iapetus
Parent Planet: Saturn
Why You Should Know it: Iapetus may be one of the most mysterious moons we've ever discovered. For one thing, it is two completely different colors; the moon's trailing hemisphere is as bright and reflective as snow, but its leading hemisphere is as dark as freshly poured asphalt — a characteristic that has led many astronomers to refer to it as the "painted" or "yin-yang" moon.
How the yin-yang moon acquired this bizarre coloration is a mystery; nobody is even sure what the dark material is made of. Equally puzzling, however, is the prominent ridge that runs almost perfectly along Iapetus' equator (a feature that has earned it yet another nickname: "the walnut moon"). Like the moon's two-toned coloration, nobody is sure how the ridge, pictured here, came to be.
Moon: Phobos
Parent Planet: Mars
Why You Should Know it: Mars' moon Phobos may not be the biggest moon on this list. It may not have the most interesting geology, or the most peculiar orbit, or the most promising environment when it comes to harboring extraterrestrial life. Be that as it may, there is a very, very good chance that it will become the second moon — and the third cosmic body — to host human travelers on mankind's journey out into the Universe.
Mars will almost certainly be the first planet that we'll put put astronauts on, but landing people on Phobos first would be easier, less expensive, and more strategically sensible than landing them on the Red Planet right away, making Phobos what NASA scientists have called the "critical link between Moon and Mars exploration."
Moon: Titan
Parent Planet: Saturn
Why You Should Know it: If moons could be considered for reclassification under planetary status, Titan would be the first to come under review. It is the only moon in the entire solar system with a dense atmosphere (which can be clearly seen in the form of an enveloping haze in many recent Cassini images, including the one featured here); it experiences rain and snow; and it's even home to geological features like lakes, valleys, plains and deserts. In fact, according to NASA's Dr. Rosaly Lopes, "Titan looks more like the Earth than any other body in the Solar System."
Plus, as an added bonus: you can shatter Ewoks (or, alternatively, Wookies) on Titan by tossing them into lakes of liquid farts.
Moon: Hyperion
Parent Planet: Saturn
Why You Should Know it: Phil Plait — astronomer extraordinair and master of ceremonies over at Bad Astronomy — once called Hyperion "the solar system's weirdest moon" — and that's saying something. For one thing, Saturn is home to some pretty wonky moons (just look at how many of the natural satellites on this list orbit the ringed planet); secondly, Phil Plait has written about some weird moons in his day — so what makes Hyperion the weirdest? Well, a lot of things, but for starters: the loofah-like moon happens to be weirdly foamy. Writes Plait:
As you can see, it's saturated with craters. But they look funny! The overwhelming impression I get is that Hyperion is made of resilient foam, like a packing peanut. I'm also fascinated by the ginormous crater that dominates this face of the moon. If Hyperion were made of stiff rock, an impact that size would've shattered it like a bullet hitting a pebble. But if the composition of the moon is able to compress and compact - like foam, or something with lots of pockets of empty space inside it - the impact would do pretty much what we see here.
Moon: Moon
Parent Planet: Earth
Why You Should Know it: It's hard to go wrong with the original. Sure, it's the first moon humans ever observed, but it wasn't until the 1950s that we finally managed to get a glimpse of its far side; and just last week we learned that the Moon may have been partly responsible for sinking the Titanic, demonstrating that our Moon has been — and will always be — a source of wonder and mystery. [Photo, and top photo, by Rick Baldridge via NASA]
Want to know which moon is the solar system's awesomest? Check out our Satellite Smackdown.
All images via Wikimedia Commons unless otherwise indicated