The Major Moons of Jupiter

Sometime around the end of 1609 or the beginning of 1610, Galileo peered through his newly improved telescope and discovered 4 moons. These were the 4 large moons of Jupiter and have been called the Galilean Moons, after their discoverer. Galileo called these moons Cosmica Sidera (Cosimo's Stars), in honor of Cosimo II de' Medici, the Granduke of Tuscany. The Grandduke suggested he call them Medicea Sidera in honor of the Medici brothers- Cosimo, Francesco, Carlo, and Lorenzo. It was also suggested the four moons be named Principharus, Victipharus, Cosmipharus and Ferdinandipharus after the Medici brothers. Oddly enough, Simon Marius, also an astronomer, claimed to have discovered them at the same time as Galileo and called them Io, Europa, Ganymede and Callisto. They are the Galilean Moons, but have the names given them by Simon Marius!

Io is the fourth largest moon in our solar system, yet it is only slightly larger than Earth's moon. It has more active volcanoes than any other object in our solar system. Io also has huge mountains, some higher than the highest mountains on Earth.

The surface temperature of Io is extremely cold, but because of the volcanoes, some of the hottest temperatures outside the sun is found here. Io has a thin atmosphere of sulfur dioxide, which can exist as a liquid, gas, or solid, much like water (just don't drink it). Io is also surrounded by an ionized cloud of alkali metals, chlorine, sulfur monoxide, sulfur dioxide, oxygen, and sulfur. This cloud is called the plasma torus.



The orbit of Io is irregularly elliptical because of the pull from Europa and Ganymede. Io always has the same side of itself facing Jupiter, and that combined with it's irregular orbit moving Io closer, then farther from Jupiter, and then back again, subjects Io to tremendous tidal forces. These forces cause Io's surface to bulge up and down by as much as 330 feet!

The image to the right is a closer view of the terrain and was taken by Voyager I with much deserved credit going to NASA.

Io has over 400 active volcanoes, and is the most geologically active object in the Solar System. Jupiter is a bit to blame for Io's volcanoes, because of it's gravitational pull on the moon. Some of Io's volcanoes can shoot plumes as much as 180 miles high into the air, and it's lava can flow as far as 300 miles! All the sulfurous volcanoes are responsible for painting Io's surface with different shades of yellow, red, white, black, and green. Some say it resembles a pizza with onions, olives, and pepper, with lots of cheese...Yum!

You can see one of the volcanoes in the image to the right.

Europa is the smallest of the Galilean moons. Europa is the sixth largest moon in our solar system, but is only slightly smaller than Earth's moon. Europa is an extremely 'heavy' moon, made of silicate rock with a possible iron core. Europa has one of the smoothest surfaces of all the objects in the atmosphere and is covered by ice. Some scientists believe there may be an ocean underneath the icy surface, and hope it may contain some forms of life. This is only speculation; however, since there has been no contact with the actual surface. Like the other Galilean moons, Europa always has the same side facing Jupiter, and orbits the planet in only 3 1/2 days. Somewhat like Io, Europa is also being flexed in and out because of Jupiter's pull. This motion help to heat Europa's inner core, perhaps keeping the oceans underneath the ice in liquid form.

One of the first things you notice when you look at Europa are the lines crisscrossing all over the moon. There is one hypothesis that these lines form when geysers under the icy crust erupt, cracking the surface ice like an eggshell, exposing the warmer layers beneath. Another hypothesis is that the gravitational pull of Jupiter is tugging at the surface of Europa, separating it from the liquid oceanic layer beneath and the inner solid core, causing the outer shell to rotate faster than the inner core.

Another oddity is the 'Chaos' spots on Europa's surface. This chaos is represented by geological features of domes, pits and smooth dark spots, or splatterings of rough texture. One hypothesis states that these lenticulae (meaning freckles-the name of these geological features) were formed by diapirs (no, not diapers) of warm ice rising up through the colder ice of the outer crust.

So what's a diapir? Think lava lamp: the magma-like bubbles that rise to the top would boil over if it could escape the glass. On Europa, a diapir of warmer material would break through the surface melting some areas and causing small fragments of ice to become refrozen, leaving the terrain in much the same way as seen in the 'chaos' areas

Ganymede is the largest moon in our solar system. It takes 7 days to complete its orbit around Jupiter, and as with the other Galilean moons, the same side always faces Jupiter. Ganymede is mostly made up of silicate rock, water, ice, and probably an iron core. One-third of the surface of Ganymede is covered with impact craters, with the remainder covered with ridges in a crisscross pattern. Ganymede has a very thin oxygen atmosphere, and is the only moon in our solar system to have a magnetosphere. Ganymede is believed to have a saltwater ocean in it's interior, but it is probably sandwiched between thick layers of ice, and therefore, the possibility of life is unlikely.

Ganymede's surface is made up of old dark cratered terrain containing organic and clay materials, and younger light terrain with the crisscross ridges and grooves. While the cratered surface is easy for scientists to explain, the lighter ridged surface isn't. Scientists believe it could be tectonic in nature, but are uncertain. The grooves and ridges in the lighter terrain are extremely long, and sometimes run through the craters, proving they are newer than the old impact craters. These craters are also rather flat, easily allowing the crisscross of the grooves and ridges over these impact craters. An example of this mixed terrain is evident is the upper right image of the sharp contrast of the dark Nicholson Regio from the bright Harpagia Sulcus.
Another feature on Ganymede are the polar caps which are thought to be composed of water frost.

Callisto is the third largest moon in our solar system. Callisto is the only Galilean moon that does not experience much tidal heating, which is the frictional heating caused by the gravitational pull from, in this case, Jupiter and neighboring moons. As with the other Galilean moons, Callisto always has the same side facing Jupiter, but does not experience the bulging effects as the other 3 Galilean moons because of its distance from Jupiter. Callisto is basically half rock, half ice. The surface of Callisto is severely cratered, and shows no internal activity such as volcanoes, plate tectonics and earthquakes. Callisto has an intense ionosphere and a thin atmosphere of carbon dioxide, and possibly molecular oxygen (a molecule composed of two oxygen atoms). In the image associated with this text, the dusting of white spots you see on the moon are frost deposits atop high elevations. Callisto is considered by scientists to be the most logical place for a surface base.

The surface of Callisto is one of the most cratered objects in our solar system. It is in fact, so cratered that any new impact has a great chance to either completely erase an existing crater, or change the appearance of any number of surrounding craters. Callisto has no large mountains or volcanoes, and the only elevation is from the upper ridges formed from the craters.

Another formation on Callisto are ridges that form rings on the surface. The image associated with this text is of Valhalla, the name given this large area of rings around the crater (the light colored circular center). Exactly how this was formed is unknown, but scientists believe it could have been formed by semi-liquid material sludging inward towards the crater, leaving behind the folds and ridged rings. Valhalla is the largest such area with another smaller ringed structure called Asgard.

Our Astounding Universe
The Planet Saturn
The Rings of Saturn
The Major Moons of Saturn
The Major Moons of Neptune
The Major moons of Uranus
Our Earth's Satellite, the Moon