The Universe in the Classroom

Cosmic Collisions

Activity: Impact Cratering

by Ronald Greeley, Arizona State University (Adapted from the NASA workbook, Activities in Planetary Geology)

Materials

Procedure

Pour sand into the tray to a depth of at least 3 inches. Smooth the surface of the sand with the edge of the meter stick. Divide the surface into two equal areas.

Importance of Mass of the Impacting Object on Craters

From a height of 2 meters (6 feet), drop each of the large spheres (three different types) into one area. Carefully measure the diameter of the craters formed by the impact without disturbing the sand. Students should then be asked to answer the following questions (answers in parentheses):

Importance of Velocity of the Impacting Object on Craters

Drop the four identical marbles into the second area, each from a different height, from 10 cm up to 2 meters. If desired, the third and fourth marbles can be launched from an extended slingshot 23 cm (9 inches) and 36 cm (14 inches) above the sand, and aimed directly down into the sand. CAUTION: THE SLINGSHOT IS A POTENTIALLY HAZARDOUS DEVICE. USE EXTREME CAUTION WHEN IT IS EMPLOYED IN THIS ACTIVITY. UNDER NO CIRCUMSTANCES SHOULD IT BE AIMED HORIZONTALLY. Without disturbing the sand, carefully measure the crater diameter. Students should then be asked the following questions:
map view & side view of crater
An ideal example of a fresh crater.

The Structure of a Crater

Remove all marbles and spheres from the sand and smooth the surface well. Again divide the tray into two areas. Sprinkle a very fine layer of dry tempra color over the sand using the tea strainer. The layer of colored powder should cover the surface just enough to conceal the sand. CAUTION: WEAR SAFETY GOGGLES AND BE SURE THAT NO GLASS OR BREAKABLE MATERIALS ARE IN THE VICINITY OF THE ACTIVITY.

Use the slingshot to shoot the 1/2'' ball bearing vertically into the sand. DO NOT DISTURB THE RESULTING CRATER IN THE FOLLOWING STEPS. Draw two pictures of the crater, one looking down from above (map view), and one as seen from ground level (side view). Label the drawings with the words rim, ejecta and impact crater (see sample diagrams). Notice the sharp details of the crater. Ask the students the following questions:

Cratering on the Moon

Craters in the Tycho-Clavius region of the Moon.
In the second area create another crater using the 1/2" ball bearing. Drop each marble from the pack of assorted marbles from an arbitrary height into the second area so that each one impacts at a different speed. Be careful to drop the marbles near but not directly on top of the crater formed by the slingshot method. Watch the process very carefully as you do it. Ask the students the following questions:

A note on procedure

This activity was developed for a high school science students. Impact craters can be demonstrated with younger or less advanced students using mud instead of sand and ball bearings. Add water to dirt until the mud has the consistency of thick cake batter, or until it slowly drips off a spoon. Then drop spoonfuls of mud onto a pie pan full of the thick mud to create craters. For more details on this variation, see Ranger Rick's Naturescope - Astronomy Adventures by the National Wildlife Federation (1989), or Astronomy for Every Kid, by Janice Van Cleave (John Wiley and Sons Publishers, 1991).

For further reading about cosmic collisions

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