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Grade Level: Middle School

Description: How are craters formed? How do we interpret the origin of craters on distant surfaces like the Jovian moons? What do craters tell us about the age of the surface on which they are found?

Objectives: First, students will first simulate the effects of collision between asteroids, and describe the resulting size distribution. Next, using images of Jupiter's four largest moons obtained by NASA's Galileo spacecraft, students will investigate crater densities and size distributions, and interpret those in terms of relative surface ages and how the craters might have been formed.

Materials: Large plastic or paper bag; large paper plates or a flat desk; hard-baked cookies; graph paper; images of Jovian moons (To print images, click here); rulers or calipers for measuring crater sizes

Vocabulary: Asteroid, comet, Chicxulub crater, caldera, crater density, crater size distribution, bolide, planetesimals, saturation, tectonism, ground truth, impactor

The Earth and Moon have been continuously pelted by asteroids and comets ever since their formation. Just look at the Moon through a small telescope or a good pair of binoculars, and you will see that its surface is covered by craters. It is now believed that a large impact near the Yucatan Peninsula of Mexico, called the Chicxulub crater, caused the dinosaur extinction 65 million years ago. Could it happen again?

Craters are one of the most interesting and important types of surface features in the solar system. They are found on almost all the solid planets, satellites, and asteroids, but not on gas giants like Jupiter and Saturn since there is no solid surface to preserve them. Preserved craters are also fairly rare on Earth, but only because weathering, erosion, and other geological processes have removed them.

Scientists believe that most craters are formed by the impact of asteroids and comets, but these aren't the only ways that large circular depressions can be made on a surface. For example, some might actually be calderas, created by erupting volcanoes. How do we identify the origin of craters on distant surfaces like the Jovian moons? What important information can crater density, or the number of craters in a given surface area, tell us? Besides observing each and every impact or eruption as it occurs, is there a way to determine the likely culprits of these scars?

The asteroid belt is a region of small planetesimals between the orbits of Mars and Jupiter. Although they range in size from very small to very large, the number of asteroids of different sizes (known as their size distribution) is governed by the fact that when asteroids collide with each other, as they sometimes do, the fragments that are produced are dominated by certain sizes. Which size is more prevalent? To answer this question, let's model the effects of collisions on the size distribution of asteroids!

Place one or two hard-baked cookies in a plastic or paper bag. Seal the bag tightly, and then slap it against the top of your desk three times. When you are done, carefully pour the broken cookie pieces onto a paper plate. Count or estimate the number of fragments for each size range and record your results in the data table below.

1. Create a graph for your data table, with the number of fragments on the y-axis, and their relative sizes on the x-axis.
2. Describe any pattern to the size distribution of your simulated asteroid fragments.
3. Can you think of other common objects that might have a similar size distribution?

It follows that craters produced by asteroid impact should mimic the asteroids' size distribution, and this is exactly what we see on the Earth's Moon; the numbers and sizes of lunar craters closely match that of the asteroids. Could the size distribution of craters on other planetary bodies provide us with clues to crater formation, as well? How might impacts near Jupiter be different from impacts near Earth? In Part 2 of the next activity you will compare the results of your asteroid simulation with real crater data from Jupiter's moons.

If we assume that comets and asteroids strike all regions of a planetary body at approximately the same rate (an assumption that may or may not be correct), every region should have the same number of craters, or crater density. Regions with higher crater densities, therefore, tend to be older than regions with lower crater densities. For example, if you stand in the rain for a long time you will get soaked, but if you make a quick dash to shelter you will only get spattered with a few raindrops. Just as you can get completely soaked, a surface can become totally covered (saturated) with craters after a certain amount of time. After that point it retains roughly the same crater density no matter how many more impacts occur. New craters are simply created on top of older ones.

On the other hand, if the planet or moon is geologically active or has an atmosphere, then processes such as weathering, erosion, tectonism, and volcanism can partially or completely erase craters. Early in the Moon's history, for example, lava flows flooded large portions of the surface. These areas were essentially wiped clean of craters about 3.5 billion years ago, and thus their crater density dates back to the time of those lava flows. In other words, the lava flows reset the crater clock. The lunar highlands, in contrast, were not flooded. Their crater density remained high, and dates back to when they were formed about 4.1 billion years ago.

What other factors might influence crater density? Beginning when the planets formed about 4.5 billion years ago, the amount of debris began to decrease as collisions and impacts swept it up. By measuring the crater density on different areas of the Moon, and then measuring the actual ages of rocks returned from different regions by Apollo astronauts (data referred to as ground truth), scientists can figure out what density of craters corresponds to the actual surface age (in millions or billions of years). There may also be a relationship between cratering rate and distance from the asteroid belt. Mars, for example, is located nearer to the asteroid belt than the Earth is, and may have a rate of crater formation roughly twice that of Earth. We might also want to consider the sizes of the planets; large planets such as Jupiter possess greater gravitational fields that attract more impactors. Higher gravity also means that objects hit with more speed, resulting in larger impact explosions and bigger craters.

Using the images provided, calculate the crater density (craters per area) for each of the Galilean moons by making a simple count of all identifiable craters. If you are not sure a particular feature is an impact crater, assume that it is not. Record your results in the data table below.

1. Describe the surface of each of the Jovian moons in terms of crater density. Are there differences within any of the images?
2. What can you say about the relative ages of the moons? Is there a relationship between relative age and distance from Jupiter? If so, what is it?
3. Have any craters been altered? If so, what forces do you think are responsible?

In this activity you will compare the results of the asteroid collision simulation with real data from Jupiter's moons Ganymede and Callisto. How do the size distributions compare? If they are similar, does it mean that asteroids have caused most of the impact craters on Jupiter's moons? If they are different, what other objects might have caused them?

Using the images provided, determine the crater size distribution for Ganymede and Callisto. A scale bar is provided for each. Measure the diameters of the identifiable craters in the image, and record your results in the data table below. If you are not sure a particular feature is an impact crater, assume that it is not.

1. Create two graphs (for Ganymede and Callisto) from your data table, with the number of craters on the y-axis, and their relative sizes on the x-axis.
2. Describe any patterns to these crater size distributions.
3. How do these patterns compare to that of your simulated asteroid size distribution? To each other?
4. How might you explain these similarities or differences? In other words, how would you interpret the origin of craters on Ganymede and Callisto?
5. What are the problems with using only one image to determine the relative surface age and possible crater origin for an entire planetary body?

Teacher Notes are available here. 

Planetary Surfaces Module

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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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