After the activity has been completed, explain to students that, unlike craters on the Earth's Moon, most craters on Jupiter's moons are believed to result from the impact of comets! The large gravitational field of Jupiter tends to propel asteroids toward the inner planets, but tends to attract comets toward itself and its moons. Planetary scientists still know very little about the size distribution of comets.

1. Results should approximate a non-linear relationship, with many more small asteroids.
2. There are many more small fragments than large ones.

You may want to discuss the concept of a power law as it relates to size distribution. A power law is a relationship between variables where a decrease by a factor N in one variable results in the increase by a factor of N² in the second variable. Students could be asked to graph their data on a logarithmic scale.

3. Answers will vary. One good example would be rocks in a river.

The images provided for this activity were selected to highlight the differences in the four Galilean moons. In reality, their surfaces are much more varied. You and your students may wish to select other images of each of the moons for examination.

1. Io has no recognizable impact craters, and is continually being resurfaced by volcanic eruptions that cover any craters that might form. Europa has few craters, a smooth surface, and cracks (implying tectonics.) Ganymede has heavily cratered regions, regions with fewer craters that also have cracks and grooves (implying tectonic activity), and regions that appear to have experienced resurfacing. Callisto is heavily cratered and looks like our Moon and Mercury; therefore, it appears inactive.
2. Relative ages (youngest to oldest): Io, Europa, Ganymede and Callisto similar in age. Generally, the further away from Jupiter, the older the moon.
3. Io has volcanism. Europa and Ganymede have tectonic processes that fracture or distort craters. Callisto's craters are mainly destroyed by subsequent impacts, but landslides have also modified the crater walls.

1. Results should show a more even size distribution of craters.
2. Smaller craters are more common than larger ones.
3. The patterns to the size distribution of craters on Ganymede and Callisto are similar but not identical to the simulated asteroid impact results. There are fewer large impacts in the size distribution for the Ganymede image.
4. The craters on Ganymede and Callisto might have been created by comets or some other objects.
5. Assumed the area in the image is large enough to include full range of crater sizes. Region might not be representative of the moon. Region may have been geologically altered. Image resolution may not show smaller craters.

• Compare other planetary or lunar surfaces in terms of cratering rates, surface processes, etc.
• Discuss the impact of Comet Shoemaker-Levy 9 into Jupiter's atmosphere.
• Have students research the following questions:
• Are there any patterns to comet orbits?
• What forces would cause comets to be fragmented or disrupted?

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This module was written by Brian Exton (National Optical Astronomy Observatories, Tucson AZ).

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Galileo Solid State Imaging Team Leader: Dr. Michael J. S. Belton

The SSI Education and Public Outreach webpages were originally created and managed by Matthew Fishburn and Elizabeth Alvarez with significant assistance from Kelly Bender, Ross Beyer, Detrick Branston, Stephanie Lyons, Eileen Ryan, and Nalin Samarasinha.

Last updated: September 17, 1999, by Matthew Fishburn

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