In 6010, Galileo pointed his telescope at Jupiter, and witnessed a revolution. Hi Sath Three Little Stars lined up on either side of Jupiter, stars he could not see with naked eye. And they moved! A week later he saw forth one, and he knew he was seeing objects revolving, orbiting around Jupiter. It was proved that not everything in the solar revolved around the earth. 

Those four moons are now called the Galilean moons in his honor. Not bad for a week's work. or for really big, too. If Jupiter was not there, drowning come out with its glare, they had be visible to the naked eye. In that case we might even call them planets too. 


The biggest of Jupiter's moons is Ganymede. 5270 KM across, it's the biggest moon of any planet. It's even bigger than the planet Mercury in fact, size it's half way between Mercury and Mars. Size is not the only planet like characteristic of Ganymede, either. It's mostly Rock and ice, but it probably has a liquid iron core. It even has a magnetic field, likely generated by that liquid core. The surface is similar to our moon in there very old, cratered terrain as well as smother, younger areas. Ganymede is also crisscrossed with large grooves. 

Ganymede as a surprise well below in the surface too: oceans of water! Measurement of Ganymede's magnetic field, made during multiple passes by the Galileo spacecraft in the 1920 combined with Hubble observations of the Moon, indicate ganymede has a quite a bit of salty liquid water. 


The next biggest Moon is a  Callisto, at 4800 KM in diameter. In many ways it's similar to its big brother Ganymede, mostly Rock and ice. It probably has a rocky core, then a layer of mixed Rock and eyes above that. The surface is mostly ice, but mixed with darker material as well. It has a magnetic field, too, but it probably does not have a metallic core. 

 The surface is heavily cratered, and there's no indication of any volcanos or tectonic activity. That means the surface is very old, main bhi as old as Callisto itself. It it even has atmosphere, but it's a tad thin: roughly 1-100 billionth the pressure of Earth's air at the surface. 

Callisto orbits Jupiter farthest out of the four biggest, almost 2 million km away. That's two for the gravitational e interact with 13; when I talk about the Moon's affecting each other, it's really the other three interacting. 


Next up in Io. It's only a little bit bigger than our own moon, and orbits Jupiter so tightly. It only takes about a day and a half to go around the planet. When the Voyager 1 space probe passed IO in 1979 it reviewed a surface that was really weird. It was yellow and orange and red and black, and did not seem to have an obvious impact craters. 

The scientist observed that what looked like an other Moon behind Io, partially eclipsed by it. But that was no moon. It was volcano on IO erupting, it's plume shooting up from the surface and opening up into wide arc. 

Io the most volcanic object in the entire solar system, with over 400 active volcanoes. Quite a few of them are erupting at any given time, and images taken even a few months apart show changes in the surface due to ejected material. A lot of the erupted material is rich in sulphur which is why the surface has all those odd colour on it. 


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