Making Impact Craters
Children examine images of Moon craters and speculate about what caused them. Next, they model the formation of an impact crater. They do this by dropping balls of different sizes and weights from three heights into a tray with layers of different colored powders. They examine the effects of each impact and the features that each impact creates. Children measure crater sizes and draw ejecta patterns to see what effect size, weight, and velocity have on the resulting craters. Finally, children compare the patterns in their testing with the patterns of craters on the Moon to see if their modeling experience gives them additional insights into crater formation.
What's the Point?
- Impact craters are caused when an impactor collides with a planet.
- A crater's size and features depend on the mass and velocity of the impactor.
- Impact craters provide insights into the age and geology of a planet's surface.
- Impact craters on Earth older than about 200,000 years are worn by weathering, erosion, and plate tectonics.
- Models help us test different variables and understand complex phenomena.
- Pictures of craters on the Moon or Mars (or both)
- Safety glasses
- One large tub or box per group such as a dish pan, pizza box, aluminum pan, or copy paper box lid (larger pans allow children to drop more marbles before having to resmooth or resurface)
- Fine white powder, such as sand or flour or white sugar
- Very fine, dry powdered tempera paint, pudding, powdered drink mix, cocoa, or colored sand
- Sieve, sifter, large spoon, or cheese cloth to sprinkle the dark powder
- Two same-sized balls of different weights (e.g., marbles, ball bearings, gum balls, wooden beads, moth balls, clay balls, grapes; try to have one light-, one medium-, and one heavy-weight ball). You don't need a set of balls per group — groups can share.
- Two same-weight balls of different sizes (e.g., ball bearings, rubber balls, super balls, golf balls, water-filled ping pong balls, jaw breakers, large gum balls; try to have one small, one medium, and one large ball). You don't need a set of balls per group — groups can share.
- Containers (e.g., cups or plastic bags) to hold a set of balls
- Data chart
- Small irregularly shaped rocks
- Yardsticks or tape measures
- Small rulers
- 3 × 5 index card to smooth the surface of the powder
- Newspaper or drop cloths
- Large sheets of paper to record ideas from the discussions
For the facilitator:
1. To find out children's ideas of how craters form, have them look at photographs of the Moon. Ask: What are the parts of a crater? What factors might affect a crater's appearance?
To examine craters, almost any image of the Moon (or Mars) will do. Most craters have deep central depressions, raised rims, and a blanket of ejected material surrounding them. Factors that affect the appearance include the nature of the surface and the speed, size, and mass of the impactor.
2. Have the children prepare the activity's “planetary surface” by completing the following steps:
- Fill a pan with white powder (sand or flour) to a depth of about 2.5 centimeters (1 inch).
- Tap the pan on the table to settle the material and smooth the surface.
- Sprinkle a fine layer of colored powder (see materials list) evenly and completely over the white layer. (Use a sieve or sifter for uniform layering.)
- Sprinkle a fine layer of white powder (sand or flour) on top of the colored layer.
You may want to prepare the pans ahead of time, although children enjoy setting them up.
3. Divide the children into groups of two to four. Have them put on safety glasses and go to a station. Caution them that during the activity, the dry powders may be dispersed into the air and could get in their eyes, so they must wear eye protection (e.g., safety glasses).
Set up each station with a pan, yardstick, ruler, bag of balls, and data sheet. Spread newspapers under the pan(s) to catch spills or consider doing the activity outside.
4. Have children fill out the boxes on the data sheet, which requires them to do the following:
- Drop the different-mass balls from three different heights. (Balls are similar in size.)
- Measure the diameter of each crater and the distance the ejecta traveled after each impact.
- Record results on the data chart.
- Repeat the above sequence with the different-sized balls. (Balls are similar in mass.)
- Draw the surface after completing all the impacts.
Dropping balls of different mass from the same height allows children to study the relationship of mass to crater size. Dropping balls of different sizes from the same height allows children to study the relationship of volume to crater size. Dropping balls from different heights allows children to study the relationship of velocity (speed) to crater size. If the pan surface becomes highly disturbed, resmooth it. The modest amount of mixing of colored and white materials should still enable children to collect data. However, if the surface becomes so disturbed that it is hard for children to analyze the impacts, apply a new layer of colored material, covered with a layer of white material. Have children determine the depth of the craters using toothpicks, measuring this length with a ruler. Depending on the children's level, you may want to use the term "weight" instead of "mass". Weight depends on gravity while mass is a property of matter independent of gravity. Because all impacts occur under constant gravitational conditions, you can use either weight or mass.
5. When the teams finish testing, bring the group together. Have the children compare and contrast their hypotheses on what factors affect the appearance of craters and ejecta. Discuss the following questions:
- How did crater size change when balls of different mass (i.e., weight) were dropped from the same height?
- How would you state the general relationship between a ball's mass and the crater size?
- How did the size of the balls affect the crater sizes?
- How would you state the general relationship between a ball's size and the crater size?
- How did the different speeds of the balls affected the crater sizes?
- How would you state the general relationship between a ball's speed and the crater size?
The higher the drop height, the greater the velocity of the ball. At any given height, the most massive ball will have the greatest effect. Gravity accelerates all objects equally, irrespective of mass. As it falls to Earth, a more massive object develops more force than a less massive object (Newton's Second Law: force = mass × acceleration). Thus the more massive ball will gain more energy as it falls, resulting in a more forceful impact with deeper craters and ejecta spread out farther. However, a less-massive ball dropped from a greater height can develop as much force (or even more force) than a more massive ball dropped from a lower height. So, the only fair comparisons are for balls dropped from equal heights. This is consistent with experimental protocol: only change ONE variable at a time! In summary:
- The higher the ball's starting point, the greater its velocity at impact.
- The greater an object's velocity, the larger its impact crater.
- When dropped from a given height, the greater the mass, the larger the crater.
- When dropped from a given height, the greater the volume, the larger the crater.
6. Have children reexamine the images of craters on the Moon or Mars. Have them compare the patterns in their testing with the patterns in these images to see if their modeling experience gives them additional insights into crater formation.
7. Discuss the limitations of the model. How is it similar to and different from a real impact?
Compared with the velocity of real impactors (20 kilometers — 12 miles — per second!), the velocity of the balls is low. In real impacts, the impactor is vaporized or broken into small chunks. Obviously, in this activity the balls remain intact and fill the craters. Consequently, the children's craters may not have raised rims, and, because the balls still sit in the craters, they have no central uplifts or terraced walls.
Extension A: Have the children resurface the pans with a new layer of flour or sand and smooth out the top with the edge of a 3 × 5 index card or ruler. Have them try one or more of the following and then discuss the results of their testing:
- Throwing a ball at an angle and comparing the crater to one formed by a straight drop.
- Dropping rocks that are similar in mass but different in size or shape.
- Dropping irregularly shaped rocks that are similar in size but different in mass.
- Making a crater with an impactor that disintegrates upon impact. Make impactors out of moistened flour or mud. Let them dry before using. Drop from a fairly substantial height, otherwise they will not break apart upon impact.
Extension B: To show how you can tell a surface's age by the number of craters, use three packed wet sand surfaces and a sprinkler watering can with most of the holes covered. Cover the sprinkler nozzle with tape and poke out 10–12 holes. Then pour the water out of the sprinkler over the first surface for 10 seconds, over the second for 20 seconds, and over the third for 30 seconds. The number of craters made by the water droplets will be greatly increased over time.
Extension C: Encourage children to look at the Moon. With the naked eye, they can only see impact basins (features that are more than 300 kilometers — 85 miles — across). If they use binoculars (or telescopes), they will be able to see many craters.
August 31, 2009