Moons of Jupiter

Since the discovery of Jupiter's four largest moons (satellites) in 1610 by Galileo Galilee, the number of discovered moons orbiting Jupiter has continued to increase. When an astronomer discovers a new moon orbiting Jupiter, it is reported to the International Astronomical Union, which gives it a temporary name until its orbit is confirmed. Once confirmed, the moon is given an appropriate name and added to the list of officially recognized moons. There are currently 50 of such officially named moons (satellites) with another 17 "temporary" moons (satellites) awaiting orbit confirmation.

Jupiter's four largest moons are Io, Europa, Ganymede and Callisto. Their discovery provided the crucial evidence that planets in the Solar System orbit the Sun rather than the Earth. It was also the first time that a moon was discovered orbiting a planet other than the Earth.

Io is the third largest of Jupiter's moons and fifth in distance from the planet. It is only slightly larger (diameter 3,643.2 km) than the Earth's moon (3,474 km). It is characterized by the tremendous amount of volcanic activity on its surface. Io is the most volcanically active object in the Solar System. Its surface is covered by a layer of sulfur and more than a dozen volcanoes release hot silicate magma. Io orbits Jupiter once every 42.46 hours (1.76 Earth days).

Europa is the smallest of the four Galilean moons and is sixth in orbit from the planet. Its diameter of 3,121.6 km is slightly smaller than the Earth's moon. Europa's unique feature is its surface of water ice. There is also evidence to suggest that this icy surface may be covering an ocean of either water or slush beneath it. Europa's watery condition is important information because it could possibly support some life forms. Europa orbits Jupiter once every 3.55 days.

Ganymede is the largest of the four Galilean moons and is seventh in orbit from the planet. Ganymede also holds the distinction of being the largest moon in the Solar System, even larger than the planet Mercury. It is the only moon with its own internally generated magnetic field. Its surface is made of silicate rock and water ice and is characterized by dark regions of craters and light regions of grooves and ridges. Ganymede completes one orbit around Jupiter in 7.15 days.

Callisto is the farthest of the Galilean moons. It is roughly the size of Mercury (4,820.6 km vs. 4,879.4 km) but is only one third of the planet's mass. Callisto's ancient and heavily crated surface is a mixture of unknown rocky material and water ice in approximately equal parts. Callisto completes one orbit around Jupiter in 16.68 days.

References:

• Jupiter's Known Satellites by Scott S. Sheppard

Image Credits:

• Courtesy NASA Planetary Photojournal