Jupiter is the largest planet in the solar system. It is so big that 1300 Earth-size planets would easily fit inside of it. Scientists have discovered seventeen moons in orbit around Jupiter, and while each one is unique, none is more fascinating than one of its innermost moons, Io. Io is one of four large moons discovered by Galileo in 1610 using one of the first telescopes ever built.
For centuries, very little was known about Jupiter and its moons. Because they are so far away, they are very hard to study from Earth. In recent years, however, spacecraft equipped with special instruments have been sent to study Jupiter and its moons in detail, and these have provided us with a great deal of new information.
The first spacecraft to fly past Io was Voyager in 1979, and scientists were amazed at the first pictures that were sent back to Earth. Io's surface was discovered to be a patchwork of red, yellow, orange, and blackish brown colors which make the moon look somewhat like a gigantic pizza. Scientists soon discovered that the surface of Io is almost completely covered with sulfur.
So why is Io's surface so colorful? The reason is that sulfur changes color depending on its temperature. At very high temperatures, sulfur is black. As it gets cooler, the color changes to brown, and then to bright orange as it cools even more. At even lower temperature sulfur is yellow, and when it gets very cold it turns white. Look at this picture of Io and you can easily locate the hottest and coldest areas. Sulfur is commonly found on Earth near areas of volcanic activity, and the source of the sulfur on Io was also found to be from volcanoes. You can always tell when there is sulfur around. It has the nasty smell of rotten eggs. Although scientists thought there would be volcanic activity on Io, they were surprised to see how widespread it was.
Io is similar in size to our own moon but that is the only way that they are similar. Pictures of our moon show no sign of recent volcanic activity. In fact, there has been no volcanic activity on our moon for several billion years. Io not only has active volcanoes, it is the most volcanically active of all the moons and planets in the solar system.
Why does Io have active volcanoes while our moon does not? Jupiter is a giant planet and because it is so big it places a huge gravitational pull on its closest moon, Io, far greater than Earth's pull on our moon. Also, beyond Io there are three other moons in close orbit around Jupiter--Europa, Ganymede, and Callisto. Although these moons are small, they too exert some gravitational pull on Io. Io is caught in the middle of this tug-of-war. Jupiter pulls Io toward itself, while the outer moons pull it in the opposite direction. Because of this Io is squeezed and stretched. This causes its interior to heat up to very high temperatures. Eventually cracks form on Io's surface and molten (melted rock) material from below erupts onto the surface.
Some of the first images from Io showed material being thrown upward from the surface into space. At first scientists thought these might be similar to explosive volcanic eruptions on Earth such as the eruption of Mt. St. Helens. After further study, however, they decided that the eruptions are more closely related to geysers on Earth. Geysers are found in volcanically active regions on Earth such as in Iceland, New Zealand, and Yellowstone National Park in Wyoming. Geysers occur when water located in open spaces beneath the Earth's surface comes into contact with hot rocks. As the water heats up, pressure builds up within the confined space. Eventually the water expands and is forced upward through cracks in the overlying rock This results in an eruption of steam and water from the crack.
On Io, the process is similar but instead of water the material that comes in contact with the heat source is sulfur. These geyser-like features on Io are called plume eruptions. Compare this picture with the one above. The top picture is of a geyser in Yellowstone National Park, Wyoming. The second picture is of the Masubi plume eruption on Io.
With so much volcanic activity on Io, it is not surprising that its features have been given the names of gods and goddesses of fire and volcanoes from ancient mythologies of different cultures. The large, orange, ring-shaped feature is called Pele, named for the Hawaiian volcano goddess. Another large feature on Io is a black horseshoe-shaped lava lake called Loki, named for the Norse god of fire and magic. Due to all the volcanic activity, Io's surface features change. You can see some of the changes that occurred between April 1997 and July 1999 at the volcanic center, Pele in these pictures. Notice in the second picture that a new, large, dark spot can be seen within Pele's red ring which cannot be seen in the first.
Many of the moons and planets in the solar system show signs of impact craters. These form when meteorites, chunks of interplanetary material, strike the surface of a planet or moon leaving behind a large hole in the ground. This picture shows a large impact crater on our moon. Although meteorites must also strike Io's surface, there are no signs of impact craters. Because material is constantly erupting from its interior onto the surface, impact craters are filled in soon after they appear.
In 1989, NASA sent another spacecraft to study Jupiter and its moons. This spacecraft was given the name Galileo in honor of the man who discovered Io and its neighboring moons. Since its arrival at Jupiter in 1995 Galileo has given us a tremendous amount of new information about Io, and still today is helping us to find answers to the many questions we have about this fascinating world!
Photos and Illustration Credit: JPL/Nasa
Geyser Photo Credit: S.R. Brantley, USGS
Now that you have learned about Io, try these fun activities and puzzles!
- Kids Activities - Make your own volcano, geyser, and lava flow!
- Io for Kids Interactive Crossword Puzzle - Solve a puzzle on-line!
- Io for Kids Crossword Puzzle - Print out a puzzle to share with your friends!