Current Event:

The United States Geological Survey recently published a new map that shows the surface of Ganymede, the largest moon in the solar system, in colorful detail. The map is based on images taken by NASA’s Voyager 1 and Voyager 2 spacecraft, which flew by Jupiter and its moons in 1979, and the Galileo probe, which orbited Jupiter from 1995 to 2003. The map is the first global geologic map of Ganymede, Jupiter’s seventh moon, or any other icy moon, for that matter. The new false-color map provides the first solid evidence for three major geological periods in the moon’s history. During the oldest period, represented by brown areas on the map, asteroids and comets bombarded the surface, leaving many impact craters. Forces released by the biggest impacts also created large cracks in the moon’s crust. Lighter colors on the map represent two somewhat younger periods. During one of these, changes in the temperature of the moon caused the surface to expand and crack. The new terrain created by these tectonic forces was marked with parallel sets of ridges and valleys. During the third period, geologic activity declined. As the crust settled, new ridges and valleys appeared.

The false colors in a new map of Ganymede reveal geologic features created during different times in the moon's history. (USGS Astrogeology Science Center/Wheaton/NASA/JPL-Caltech)

Objective:

Jupiter is the largest planet in our solar system. It has a mass that is greater than the masses of all the other planets in the solar system added together. Jupiter is called a gas giant because the planet consists mostly of hydrogen and helium and has no solid surface. Astronomers have made detailed observations of Jupiter for centuries. It was one of the first planets studied by the famous Italian astronomer Galileo in the early 1600′s. Beginning in the 1970′s, several spacecraft explored the Jovian system in great detail. Jupiter, the fifth planet from the sun, has at least 67 moons. Sixteen of the moons measure at least 6 miles (10 kilometers) in diameter. Ganymede not only ranks as Jupiter’s largest moon, but also is the largest moon in the solar system. With a diameter of 3,270 miles (5,262 kilometers, Ganymede is even larger than the planet Mercury. The presence of water on Ganymede makes it a prime candidate in the search for extraterrestrial life. The Behind the Headlines news stories and related World Book articles explore Ganymede, Jupiter, and other astronomical topics.

Words to know:

• Asteroid
• Comet
• Crater
• Extraterrestrial life
• Galileo Galilei
• Ganymede
• Jupiter [planet]
• National Aeronautics and Space Administration (NASA)
• Planet
• Satellite
• Solar system
• United States Geological Survey
• Voyager [space probes]

Discussion Topics:

1. Excluding Jupiter, five other planets in the solar system have satellites. Ask your students to name the other five planets with satellites. (Students should say Earth, Mars, Saturn, Uranus, and Neptune. Dwarf planet Pluto also has satellites.)

2. Ask your students to debate, “People will one day come into contact with intelligent extraterrestrial life.”

3. Private companies have begun launching people into space. Ask your students if they would want to visit Earth’s moon or any other celestial body. Would they go if they knew they could never return to Earth?

4. Ask your students to use the World Book’s Timelines feature to create a timeline of Jupiter discoveries. (Students may wish to use the “History of Jupiter study” section of World Book’s Jupiter article for help.)