The Universe in the Classroom

Star Science in the Autumn Sky


Ganymede is the largest moon in our solar system, larger even than the planets Pluto and Mercury. Some scientists used to think that Saturn's moon Titan was bigger, but later measurements showed that some of Titan's apparent diameter is actually a very thick atmosphere. Ganymede is the only place, besides Earth, with lateral faults similar to California's infamous San Andreas fault. Ganymede's surface consists of two main types of terrain: old, dark polygonal units, and young, bright valleys and ridges (see figure 11). White marks on Ganymede are young craters. These bright white marks may be the natural consequence of a high-speed impact onto an icy surface. In a similar way, hitting a large block of ice with a hammer makes white marks.

Figure 11
Ganymede as seen by Voyager 2. This satellite's claim to fame, besides the fact it's the largest moon in the solar system, is its strike-slip faults. One appears in the lower left corner of this picture. These faults look a lot like the San Andreas fault in California, raising hopes that studying Ganymede will help geologists to understand terrestrial earthquakes. The dark region on the right is ancient terrain. The grooved, light-colored regions are younger. Image courtesy of NASA Headquarters.


Io looks like a giant pizza. One of the prettiest -- and strangest -- bodies in the solar system, Io is sure to be on magazine covers when the pictures arrive from Galileo. The moon is famed for its giant volcanic eruptions, discovered by NASA engineer Linda Morabito (see figure 12). Although it is only about the same size as our moon, Io is the most active volcanic body in the solar system, even more than Earth. There has been so much geologic activity that the entire planet may have passed through volcanoes at one time or other.

Figure 12
Io as seen by Voyager 1. On the limb of the planet you can see a volcanic explosion. The explosion threw solid material to an altitude of 160 kilometers (100 miles) at a speed of 2,000 kilometers per hour (1,200 miles per hour). Unfortunately this black-and-white image doesn't show Io's yummy pizza color. The reddish and orangish colors of the moon are probably due to sulfur compounds. Image courtesy of NASA Jet Propulsion Laboratory.

Some of the volcanic fountains, or plumes, cover an area larger than Texas. They are active for different periods of time. In the four months between the Voyager 1 and Voyager 2 flybys in 1979, one volcano stopped erupting, while two started up and subsequently stopped. Scientists can't wait to see which volcanoes are active during the two-year Galileo mission. Volcanism might have changed the appearance of the surface since the Voyagers, although the Hubble Space Telescope has not detected major changes.

Io is so active because it is caught in a tug of war between Jupiter's and Europa's gravity. The tug of war locks helpless Io into an elliptical orbit, so that its distance to Jupiter -- hence the effect of Jupiter's gravity -- keeps changing. The constantly changing gravity stretches, squeezes, and kneads Io. A similar thing happens when you bend a wire coat hanger back and forth: It gets hot.

The vibrant reds, yellows, and blacks on Io's surface are probably sulfur compounds. Some researchers predict that the entire visible surface is sulfur; others feel that silicate lava flows must be present. Galileo's instruments will be able to tell the difference.


Europa has the flattest surface of any body in our solar system. It has no mountains and hardly any craters. The whole moon could be encased in a layer of ice, and under the ice may be a global ocean of water. Europa is crisscrossed with lines called linea and flexus, thought to be cracks in the ice (see figure 13). Some scientists speculate that the cracks could contain primitive life forms, such as blue-green algae. In Earth's Antarctic, algae thrive beneath tens of meters of ice.

Figure 13
Europa as seen by Voyager 2. Cracks crisscross this mottled moon. They may be breaks in a thin ice crust that covers water or softer ice. Because Voyager couldn't spot any large impact craters, scientists have concluded that the moon's surface is young. Image courtesy of NASA Jet Propulsion Laboratory.

Why doesn't Europa freeze solid? Like Io, it is heated by tides produced from the interaction of Jupiter's gravity with the other Galilean moons. The tidal forces are 10 percent as powerful as those that affect Io.

Recently the Hubble Space Telescope discovered that Europa has a thin oxygen atmosphere. On Earth oxygen is a product of life, but on Europa it has a more prosaic origin: Impacts by dust and charged particles knock water molecules off the icy surface, whereupon a series of chemical reactions produce hydrogen and oxygen.

As Galileo goes through its motions, The Planetary Society will be working with the Jet Propulsion Laboratory to provide updates. This will be done on the World Wide Web and through a toll-free telephone number. More information on this special service can be obtained by calling The Planetary Society at 818-793-5100 or visiting their website.

Activity: The Size of Jupiter's Moons

Jupiter's moons fascinate planetary scientists partly because they are large, about the same size as some planets. This activity compares the sizes of the moons and some planets. With a drawing compass and the following table, draw circles to show the size of some planets and moons. With forethought, you should be able to get all of them on a single sheet of paper. Your students may enjoy drawing sample geologic features, such as volcanoes or cracks, on the Galilean satellites.

  Radius (km) Scaled radius (cm)
Earth 6,375 6.4
Mars 3,400 3.4
Mercury 2,400 2.4
Ganymede 2,638 2.6
Callisto 2,410 2.4
Moon 1,738 1.7
Io 1,816 1.8
Europa 1,563 1.6
Pluto 1,150 1.2

JAMES J. SECOSKY is a science teacher at Bloomfield Central School in Bloomfield, N.Y. He has worked with professional astronomers on Hubble Space Telescope projects, and has an upcoming article in The Universe in the Classroom on space travel.

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