Jupiter's Moon Ganymede
Ganymede is one of Jupiter's four largest satellites. Jupiter has at least 61 moons in orbit around it, but only four are of a significant size.
Jupiter's four largest moons are collectively called the Galilean Moons because they were discovered by Galileo. Ganymede was discovered by Galileo in 1610. Like Jupiter's other three large moons it is visible from Earth with a telescope.
The Galilean moons are Io, Europa, Ganymede, and Callisto, with Io being the closest and Callisto being the furthest from the planet. This makes Ganymede the third furthest of the Galilean moons from Jupiter with a distance of 1,070,000 (Chaisson and McMillan, 2005).
The top image on the right was taken by NASA's Cassini spacecraft and shows planet Jupiter and its moon Ganymede. Ganymede has a diameter of 5268 km (3273 miles), which makes it larger than the planets Mercury and Pluto. In fact, Ganymede is the largest moon in our solar system, being even larger than Saturn's largest moon, Titan.
All four of Jupiter's large moons, including Ganymede, have almost circular orbits around the planet. It takes Ganymede slightly more than seven Earth days to make one complete orbit around Jupiter.
One interesting aspect about Jupiter and its moons is that Jupiter just missed being a star like our Sun. Jupiter is a massive planet and a giant ball of hot gas (called a gas giant). Because of this, scientists believe that the moons around Jupiter may have formed in much the same way as the planets formed around the Sun in our solar system.
The surface temperature on Ganymede is cold, with daytime temperatures ranging between -297 to -171 °F (-183 to -113°C). The cold temperatures on Ganymede are due to its distance from the Sun and that it doesn't have an atmosphere to trap heat. Nighttime temperatures on Ganymede range from -315 to -279 °F (-193 to -173°C).
Scientists believe that Ganymede has geologic activity similar to that on Earth. The Voyager spacecraft found evidence of tectonic plates, which are shifting surface masses that are responsible for the quakes on Earth.
The close up of Ganymede's surface taken by Voyager 1 shows a cratered and fractured surface. The lighter regions are made of water ice. Scientists believe that objects impacting Ganymede's surface and volcanic activity brought liquid water to Ganymede's surface from its interior, where it froze on the surface.
Ganymede's Internal Structure
As depicted in the image, Ganymede has a metallic core made of iron and nickel (shown in gray), surrounded by a layer of rock (shown in brown), surrounded by a thick layer of liquid salt water (shown in blue), which is surrounded by another thin layer of rock (brown).
Ganymede is the only moon known to have a magnetosphere, which is produced by its metal core.
Another interesting aspect of Ganymede is its layer of liquid water under its surface. The presence of liquid water is of interest to scientists because they believe liquid water is a prerequisite for life formation.
Chaisson, E. and McMillan, S. (2005). Astronomy Today. Pearson Prentice Hall: Upper Saddle River, NJ.
Photos of Ganymede and Jupiter: Courtesy of NASA/JPL-Caltech