A View of Jupiter

Last night was the clearest night in months around here, so I hauled the telescope out in the yard and pointed it skyward. I only had one image turn out well... This one:

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If you look carefully you will see all four of what are referred to as the "Galilean" moons of Jupiter (Io, Europa, Ganymede and Callisto) . They are called that because they were initially discovered by Galileo Galilei on January 7th 1610.

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Interestingly enough, it was not Galileo who gave these moons their names. At roughly the same time (1614), another astronomer named Simon Marius published these names in a publication, named Mundus Jovialis (The moons of Jupiter). There were others who suggested other names for these moons. One interesting suggestion was forwarded by Giovanni Batista Hodierna. His suggestion for naming these moons was Principharus, Victipharus, Cosmipharus and Ferdinandipharus. These were based on the names of the four Medici brothers. As for Galileo, he referred to them by the imaginative names I, II, III and IV, numering them from nearest to furthest. This system of naming moons is still used today in parallel with the official naming designation. Moons of planets are generally named for mythological figures with the exception being the moons of Saturn and Uranus which are named for characters from the works of William Shakespeare or Alexander Pope.

Here's a little overview of these four moons:

Starting with Io, Jupiter's nearest moon. Io is an interesting icy body. The tidal forces from Jupiter's imense gravity and the gravity of the other moons tug on the little moon making it a very active place. In the picture above you see a plume erupting from an ice geyser. The surface of Io can flex as much as 100 meters (@ 300 feet) in response to this gravitational tug of war. The ice volcanoes of Io can spew plumes of material as far as 300KM (@170 Miles) out into space.

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Europa is a world with a global ocean. Current figures estimate that the entire surface of Europa is covered by a layer of water 100KM(@57 Miles) thick. Of course at the low temperatures on Europa, one would think that this layer of water would be frozen solid. It is however thought that again, because of the gravitational tug on Europa that enough heat is generated to keep the water fluid underneath of an outer layer of ice. The outer surface of Europa is covered in a network of cracks and fault lines in the floating ice. There's been various proposed missions to Europa to bore or melt through the ice and investigate if life could have taken hold in this hostile environment.

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Ganymede is Jupiter's largest moon. In fact, it is the largest satelite of any planet in the solar system. At 5262KM (@2990 Mi.) in diameter it's nearly half as large as the Earth itself (@40%). In fact Ganymede is larger than both Mercury and Pluto.

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Callisto, the most distant Galilean moon sports one of the most cratered surfaces in the solar system. It is also another satelite with an ocean. Callisto's liquid ocean however is only 10KM(@5½Miles) thick, but it is covered by a 150KM(@85 Miles) thick sheath of ice.

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