Hello, Cosmic Kossacks! You might not know this about me, but I was quite the astronomy nerd when I was a kid. After I got over my dinosaur fascination (as most boys eventually do), I turned that same preoccupation to the planets of the solar system.
I probably don't know as much as I knew then, but I'm going to take a shot at doing a science diary on some of the most intriguing and mysterious bodies in our system -- the Galilean moons of Jupiter:
Ganymede... Callisto... Io... Europa
While Jupiter has as many as 62 moons, many of them with catchy names such as "Hapalyke," "Callirrhoe" and "S/2003 J-17," when most people mention the moons of Jupiter, they are referring to the four heavenly bodies known as the Galilean moons, which are Jupiter's four largest moons.
First spotted by Galileo Galilei ("magnifico-o-o-o-ohhh!") in 1610, each one is a distinctive, treasure-filled world with a lot to teach us about our own planet, and maybe even a little to say about our place in the universe. Galileo named his discoveries on Zeus' many one-night stands. In order of orbit, outward from Jupiter, there's the hot-tempered Io; the smooth, icy Europa; the gigantic and varied Ganymede; and the mysterious, scar-covered Callisto.
If you're an amateur star-gazer, the Galilean moons can be seen from Earth by telescope, which is how Galileo found them in the first place, when telescopes were quite new and considered hot shit.
And now, the Galilean moons...
First in orbit, Io, the volcanic moon!
Io: In bad need of a Stridex pad for many, many years.
Probes sent to get a closer look at Jupiter's moons found that Io's deep orange, red, green and brown pockmarked surface changed quite dramatically in just a few years. This is because Io is home to more than 400 active volcanoes, making it one of the most geologically "alive" bodies in the solar system.
This geologic activity has given rise to huge mountains, some with peaks taller than our own Mount Everest. A single volcano on Io's surface can eject more lava than all of our planet's volcanoes combined. So to call the planet's surface "igneous" is an understatement. Many features on the Ionian surface, which can include literal lakes of fire and lava, carry names derived from Dante's Inferno.
Unlike earthly volcanoes, Ionian ones are known to "wander." This is due to Jupiter's immense gravitational pull shifting the moon one way, while its sister moons Ganymede and Callisto, pull it the other way. Call it planetary sibling rivalry, but the effect on Io is such that a volcano can erupt suddenly anywhere Io's surface has a weak point due to this pulling. Imagine never knowing where a volcano could erupt on this planet!
Io's volcanic geysers are the result of outgassing, and send plumes of sulfur hundreds of miles into the moon's thin sulfur dioxide atmosphere. The plumes are so tall, good telescopes can see them from here. Some of the gases ejected from the moon are captured by Jupiter's incredibly strong magnetosphere, which pulls charged particles to the poles of our system's largest planet, causing an aurora effect similar to Earth's northern lights.
Next up is Callisto. What lies beneath?
Callisto: Honey, the universe has been hard on you. But don't worry. I see potential in you. Potential for life, baby!
The surface of Callisto is pretty remarkable all by itself, I've always thought. It has so many crater impacts, that a new impact from a meteor or comet couldn't help but hit another three or four old impact sites. These craters are ancient, and the surface is unchanging, unlike Io's. The moon's surface looks the same now as it did 4 billion years ago, but underneath that scarred exterior may lie our solar system's most friendly potential home for microbial life outside our own.
Roughly the size of Mercury, Callisto is made of rock and ice. Probes have revealed, however, that a vast, global saltwater ocean could exist somewhere beneath. This ocean could be as deep as 100 kilometers, making it one of the largest single sources of liquid water in the solar system.
One of the reasons we think Callisto may have oceans is because its surface is flat and free of mountains or ridges. This suggests ice. Furthermore, many of the moon's vast array of impact craters are silvery white around the edges -- also suggesting that ice crystals were stirred up by the crashing meteors and other impacts. In addition, it was found that Callisto's magnetic field flows in different directions at different times in response to Jupiter's magnetic field, further suggesting an ocean.
Next up, Ganymede, largest moon in the solar system!
Ganymede: Perhaps our system's glam-rockiest heavenly body.
First off, Ganymede is big. It's bigger than our moon. It's bigger than Mercury and almost as big as Mars. It's big enough to be considered a planet in its own right if it orbited the sun rather than Jupiter.This is one reason why Pluto is no longer considered a planet. Anti-Pluto extremists pointed to Ganymede as a good reason to beat up on poor Pluto and strip it of its status as a planet. Well, I say it's not size that counts. It's the rotation that matters.
Like Callisto, this large and in charge satellite has a rocky core with a mantle of rock and ice and may have a very thin oxygen atmosphere. The planet's surface has a fascinating blend of mountains, valleys, lava flows, peaks, plateaus and craters -- a veritable hodgepodge of the features found on the other Galilean moons.
The surface is also rich in water ice, with some of the lighter features streaked with relatively clean, fresh frost. Ganymede has a smoother side, too, and recent research indicates this may have been caused by eruptions of ice volcanoes -- a sort of water "lava" brought to the surface as the moon wrestled with the gravitational pulls of Jupiter and the other large moons.The molten ice flowed out and refroze, filling in low-lying areas.
The large, dark grey area in Ganymede's northern hemisphere is called Galileo Regio. The area contains impact craters not seen in the moon's lighter regions, indicating that the darker areas are older than the lighter ones. Galileo Regio always reminds me of our moon's lake-like features, such as the Sea of Tranquility.
Next up and furthest out, Europa! The scarred beauty of Jupiter's moons.
Europa: No, that's not Octomom's belly. That's a moon.
It's long been known that Europa harbors a hidden ocean locked in ice. Recent evidence suggest that it might be Europa's oceans, rather than Callisto's, that show more potential for the development of life. Underneath the icy crust, Callisto's oceans are tumultuous, and water plus energy could equal life.. The strong pull of Jupiter's gravity on Europa, combined with its slightly elliptical orbit, has led some scientists to think there could be waves rocking Europa's ocean.
Europa's icy surface is like a cue ball drawn upon with a magic marker. It's one of the smoothest objects in the solar system. The surface features include cracks, troughs, valleys, ridges, pits and ice floes, but none of them are more than a few hundred yards high or deep. In some areas, giant sections of the surface have split apart and separated. There are almost no impact craters on this moon, unlike on Callisto.
The dramatic-looking cracks that score Europa's surface could be caused by tidal the same tidal stresses that send the moon's oceans sloshing around. When the stress reaches the tensile strength of ice, the tides can rise as much as 30 meters, causing cracks to appear. Think of them as "stretch marks."
Scientists are eager to explore what might be underneath Europa's surface in the oceans. If the ice layer is thin enough, some scientists imagine self-directing probes that might one day be capable or drilling or melting their way through the crust and into the ocean, before floating around like a deep-sea submersible to look around for signs of microbial life.
Particularly optimistic biologists believe that complex life could exist in these seas, much like the tube worms and other organisms that live around volcanic vents at the bottom of the Pacific, or some of the life forms found beneath Antarctica's ice. If life is proven to exist on two bodies, it might be that life in the universe is more common than we thought!