Photo Credit: www.nasa.gov

1. A Look atJupiter’s Largest Moons Largest Moons Jupiter Io Europa Callisto Ganymede Photo Credit: www.nasa.gov

2. Ganymede [GAN-ee-meed] is the largest moon of Jupiter and is the largest in our solar system with a diameter of 5,262 km (3,280 miles). If Ganymede orbited the Sun instead of Jupiter it could be classified as a planet. Like Callisto, Ganymede is most likely composed of a rocky core with a water/ice mantle and a crust of rock and ice. Its low density of 1.94 gm/cm3, indicates that the core takes up about 50% of the satellite's diameter. Ganymede's mantle is most likely composed of ice and silicates, and its crust is probably a thick layer of water ice. Ganymede has no known atmosphere, but recently the Hubble Space Telescope detected ozone at its surface. The amount of ozone is small as compared to Earth. It is produced as charged particles trapped in Jupiter's magnetic field rain down onto the surface of Ganymede. As the charged particles penetrate the icy surface, particles of water are disrupted leading to ozone production. This chemical process hints that Ganymede probably has a thin tenuous oxygen atmosphere like that detected on Europa. Ganymede has had a complex geological history. It has mountains, valleys, craters and lava flows. Ganymede is mottled by both light and dark regions. It is heavily cratered especially in the dark regions implying ancient origin. The bright regions show a different kind of terrain - one which is grooved with ridges and troughs. These features form complex patterns and have a vertical relief of a few hundred meters and run for thousands of kilometers. The grooved features were apparently formed more recently than the dark cratered area perhaps by tension from global tectonic processes. The real reason is unknown; however, local crust spreading does appear to have taken place causing the crust to shear and separate.

3. Jupiter's moon Io [EYE-oh or EE-oh] is one of the most exotic places in the solar system. It is the most volcanic body known, with lava flows, lava lakes, and giant calderas covering its sulfurous landscape. It has billowing volcanic geysers spewing sulfurous plumes to over 500 kilometers high. Its mountains are much taller than those on Earth, reaching heights of 16 kilometers (52,000 feet). Io orbits closer to Jupiter's cloud tops than the moon does to Earth. This places Io within an intense radiation belt that bathes the satellite with energetic electrons, protons, and heavier ions. As the Jovian magnetosphere rotates, it sweeps past Io and strips away about 1,000 kilograms (1 ton) per second of volcanic gases and other materials. This produces a neutral cloud of atoms orbiting with Io as well as a huge, doughnut shaped torus of ions that glow in the ultraviolet. The torus's heavy ions migrate outward, and their pressure inflates the Jovian magnetosphere to more than twice its expected size. Some of the more energetic sulfur and oxygen ions fall along the magnetic field into the planet's atmosphere, resulting in auroras. Io acts as an electrical generator as it moves through Jupiter's magnetic field, developing 400,000 volts across its diameter and generating an electric current of 3 million amperes that flows along the magnetic field to the planet's ionosphere.

4. Callisto [kah-LISS-toe] is the second largest moon of Jupiter, the third largest in the solar system, and is about the same size as Mercury. It orbits just beyond Jupiter's main radiation belt. Callisto is the most heavily cratered satellite in the solar system. Its crust is very ancient and dates back 4 billion years, just shortly after the solar system was formed. Callisto lacks any large mountains. This is probably due to the icy nature of its surface. Impact craters and associated concentric rings are about the only features to be found on Callisto. The largest craters have been erased by the flow of the icy crust over geologic time. Two enormous concentric ring, impact basins are found on Callisto. The largest impact basin is Valhalla. It has a bright central region that is 600 kilometers in diameter, and its rings extend to 3000 kilometers in diameter. The second impact basin is Asgard. It measures about 1600 kilometers in diameter. Callisto has the lowest density (1.86 gm/cm3) of the Galilean satellites. From recent observations made by the Galileo spacecraft, Callisto appears to be composed of a crust about 200 kilometers (124 miles) thick. Beneath the crust is a possible salty ocean more than 10 kilometers (6 miles) thick. Beneath the ocean, is an unusual interior that is not entirely uniform nor does it vary dramatically. Prior to Galileo, scientists believed that Callisto's interior was totally undifferentiated, but Galileo data suggests that the interior is composed of compressed rock and ice with the percentage of rock increasing as depth increases. Meteorites have punctured holes in Callisto's crust, causing water to spread over the surface and forming bright rays and rings around the crater. Callisto has no known atmosphere.

5. Europa is the fourth largest moon of Jupiter and is a little smaller than the Earth’s moon. It has a thin outer layer of ice. Most of the surface is smooth with very few craters but there are dark streaks and pits on its surface. Some scientists think that Europa has a deep liquid ocean under its icy shell. It is possible that life could exist in the water there. This picture is from information taken from NASA’s Galileo missions. http://galileo.jpl.nasa.gov Photo Credit: NASA/JPL/University of Arizona/University of Colorado

6. Jupiter has at least 63 moons.  Most of these moons are very small, and were probably once asteroids that got too close to Jupiter.  Jupiter's large size means that it has very strong gravity.  This powerful gravity sometimes grabs onto objects that wander too close.