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Galileo’s Moons

March 22, 2013

To me, moons are fascinating.  Many planets in our solar system have them, with the majority belonging to the gas giants in the outer solar system.  For me, the most interesting of all the moons is not Earth’s, but rather the Galilean ones, so named because Galileo Galilei was the first to discover them orbiting around Jupiter with his new high-powered telescopes.  We now know that Jupiter’s gigantic gravity field  has captured many moons, actually about 60, but the Galilean ones are the four most important and interesting: Io, Europa, Ganymede, Callisto.  These are their stories.




Io is the most geologically active world in our solar system, with over 400 active volcanoes.  In fact, every one of the dots in the picture above is a volcano.  Because of this rampant volcanism, Io’s surface is also one of the youngest looking in the solar system, since almost all of its impact craters have been filled in by lava.  Surprisingly enough, the cause of  all this constant activity is actually Jupiter.  Io orbits Jupiter in synchronous rotation, and the force required to keep that rotation causes tidal bulges on Io.  This constant change in size and orientation causes friction that creates the internal heat required for volcanic eruptions.  Finally, even with all that volcanic activity causing outgassing, Io still only has an extremely thin atmosphere, made up mostly of sulfur dioxide.  Most expelled gases are nabbed by Jupiter’s gravity and put into orbit in the Io torus, which is a doughnut-shaped ring of gas surrounding Jupiter.




Europa shares many characteristics with Io.  Just like Io, it has a metallic core and rocky mantle.  Also like Io, Europa has no impact craters, meaning it is still tectonically active and that its surface may only be a few million years old.  The differences between the two however, are great.  Rather than volcanoes, Europa’s surface (and crust) is covered by a layer of ice approximately 100 km thick.  What is most exciting about Europa, though, is that it is a prime candidate for the finding of extraterrestrial life.  It is theorized that liquid water exists under a thin shell of ice, because tidal friction caused by Jupiter should provide enough heat to make sure the ocean remains liquid and also drive geological activity (tidal flexing is also what causes the cracks on Europa’s surface).  Scientists hope that primitive life may exist then underwater, near hydrothermal vents, just as on Earth.  One last interesting note about Europa is that its atmosphere, though tenuous, is composed mostly of oxygen, a gas important at least to life on Earth.




Ganymede is the largest moon in our solar system, at 5262 km across.  It also has a geography similar to Europa’s, in that its surface is composed of water ice, and a saltwater ocean is thought to exist 200 km below its surface, sandwiched between layers of ice.  Unlike Europa, however, its surface shows signs of age, along with visible craters on the surface, meaning it is no longer as geologically active as in the past.  What’s also interesting is that a strong magnetic field (perhaps even a magnetosphere) has been detected around the moon, leading to speculation that it still contains a liquid iron core, heated by radioactive decay and tidal heating.  Finally, Ganymede has a tenuous atmosphere, composed mostly of oxygen and ozone.




Callisto is the second largest of the Galilean satellites, and has a geography similar to Ganymede’s (mostly rock and ice), but with even more cratering, indicatingan even cooler interior.  Callisto is considered “boring” by some, since it seems to be a dead world, but it does still have some interesting features.  First of all, it has a very thin atmosphere composed mostly of carbon dioxide.  Second, it has an induced magnetic field, which may be evidence of a salty liquid ocean beneath its surface.  Lastly, it has a 3000 km wide impact crater in the shape of a bulls-eye, named the Valhalla Basin, which was produced about 4 billion years ago.  Upon impact, the foreign object exploded, heating the subsurface ice to above the melting point and producing shock waves in a ripple pattern away from the impact site, giving it its distinctive look.  What’s cool too about Callisto is that it is considered the most suitable place for a base for future human exploration of the Jupiter system, since it is furthest from the intense radiation of Jupiter.

There you have it, the four Galilean moons.  With such a diversity of worlds, I think it’s pretty clear why I find these moons to be so interesting.  Sorry, Earth’s Moon, but in the end, you just can’t compete with these guys.

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