This is an image of Io.
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NASA

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Evolution of Io

Io is the smallest of the major Galilean satellites, with a size that could be that of the rocky core of one of the other Galilean moons. Io is heated through the process of tidal forcing, which causes the moon to be volcanic. Since the density indicates that the composition of Io is mostly rocky, there is the possibility that Io started out similar in character to the other icy moons and lost it's icy mantle over time due to tidal forcing.

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Amalthea

Amalthea was discovered by E Barnard in 1872. Of the 17 moons it is the 3rd closest to Jupiter. Amalthea is about the size of a county or small state. Amalthea is named after the goat in Greek mythology...more

Callisto

Callisto was first discovered by Galileo in 1610. It is the 2nd largest moon in the solar system, and is larger than the Earth's moon. It is about as big as the distance across the United States. Callisto...more

Evolution of Callisto

Measurements by the Galileo spacecraft have been shown that Callisto is the same inside from the center to the surface. This means that Callisto does not have a core at the center. This means that, unlike...more

Very Large Impact Crater

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Surface of Callisto

The surface of Callisto is deeply marked with craters. Craters are the little white marks in the picture. It looks like it might be the most heavily cratered body in the whole solar system. And some of...more

Europa

Europa was first discovered by Galileo in 1610, making it one of the Galilean Satellites. It is Jupiter's 4th largest moon, 670,900 km ( miles) from Jupiter. Europa's diameter is about half the distance...more

The Atmosphere of Europa

The Galileo mission discovered an amazing thing! Europa has its own atmosphere, although it is very, very thin. When fast moving molecules traveling through space near Jupiter hit Europa's atmosphere,...more

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