Meet the Moons of Jupiter
The planet Jupiter is the largest world in the solar system. It has at least 67 known moons and a thin dusty ring. Its four largest moons are called the Galileans, after astronomer Galileo Galilei, who discovered them in 1610. The individual moon names are Callisto, Europa, Ganymede, and Io, and come from Greek mythology.
Although astronomers studied them extensively from the ground, it wasn't until the first spacecraft explorations of the Jupiter system that we knew how strange these little worlds are. The first spacecraft to image them were the Voyager probes in 1979. Since then, these four worlds have been explored by the Galileo, Cassini and New Horizons missions, which provided extremely good views of these little moons. The Hubble Space Telescope has also studied and imaged Jupiter and the Galileans many times. The Juno mission to Jupiter, which arrived in summer 2016, will provide more images of these tiny worlds as it orbits around the giant planet taking images and data.
Explore the Galileans
Io is the closest moon to Jupiter and, at 2,263 miles across, is the second smallest of the Galilean satellites. It is often called the “Pizza Moon” because its colorful surface looks like a pizza pie. Planetary scientists found out it was a volcanic world in 1979 when the Voyager 1 and 2 spacecraft flew by and captured the first up-close images. Io has more than 400 volcanoes that spew out sulfur and sulfur dioxide across the surface, to give it that colorful look. Because these volcanoes are constantly repaving Io, planetary scientists say that its surface is "geologically young".
Europa is the smallest of the Galilean moons. It measures only 1,972 miles across and is made mostly of rock. Europa's surface is a thick layer of ice, and underneath it, there may be a salty ocean of water about 60 miles deep. Occasionally Europa sends plumes of water out into fountains that tower more than 100 miles above the surface. Those plumes have been seen in data sent back by Hubble Space Telescope. Europa is often mentioned as a place that could be habitable for some forms of life. It has an energy source, as well as organic material that could aid in the formation of life, plus plenty of water. Whether it is or not remains an open question. Astronomers have long talked about sending missions to Europa to search for evidence of life.
Ganymede is the largest moon in the solar system, measuring 3,273 miles across. It's made mostly of rock and has a layer of salt water more than 120 miles below the cratered and crusty surface. Ganymede's landscape is divided between two types of landforms: very old cratered regions which are dark-colored, and younger areas containing grooves and ridges. Planetary scientists found a very thin atmosphere on Ganymede, and it's the only moon known so far that has its own magnetic field.
Callisto is the third-largest moon in the solar system and, at 2,995 miles in diameter, is nearly the same size as the planet Mercury (which is just over 3,031 miles across). It's the most distant of the four Galilean moons. Callisto's surface tells us that it was bombarded throughout its history. Its 60-mile thick surface is covered with craters. That suggests the icy crust is very old and hasn't been resurfaced through ice volcanism. There may be a subsurface water ocean on Callisto, but conditions for life to arise there are less favorable than for neighboring Europa.
Finding Jupiter's Moon's From Your Back Yard
Whenever Jupiter is visible in the nighttime sky, try to find the Galilean moons. Jupiter itself is quite bright, and its moons will look like tiny dots on either side of it. Under good dark skies, they can be seen through a pair of binoculars. A good backyard-type telescope will give a better view, and for the avid stargazer, a larger telescope will show the moons AND features in Jupiter's colorful clouds.