Cosmic
Collisions
by Ronald Greeley,
Arizona State University (Adapted from the NASA workbook, Activities in Planetary
Geology)
Materials
- A tray or very
strong box at least 2 feet on a side and about 4 inches deep
- A large supply
of extremely fine sand
- Four identical
marbles or small ball bearings
- Three solid
spheres about 1 inch in diameter, all the same size but made of different
materials, for example, glass, plastic, steel; or glass, wood, aluminum)
- Meter stick
- 10-centimeter
ruler
- Toy Slingshot
(Optional)
- Kitchen tea
strainer
- Dark color of
dry tempra paint (powder); for example, red or blue
- Safety glasses
or goggles
- Large pack of
assorted marbles
- One steel ball
bearing about 1/2'' in diameter
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):
- Which sphere
created the largest crater? (The most massive.)
- What is the
only difference in the way each crater was made? (The mass was varied.)
- Each sphere
represents a meteorite. What can you say about the importance of the mass
of a meteorite in making a crater? (Crater diameter increases with increasing
mass.)
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:
- In this case,
each marble (meteorite) had the same mass. What did dropping marbles from
different heights (and propelling two marbles, if the slingshot was used)
accomplish? (This varies the velocity at impact.)
- Did you measure
any difference in the diameters of the craters? (Yes, as velocity increases,
so does crater diameter.)
- Besides diameter,
do you notice any other difference in appearance among the craters? (No,
all look qualitatively similar.)
- Which do you
think is more important in creating larger craters, more mass or more velocity?
(Velocity increases have more effect on crater diameter than mass increases.
Velocity has a greater contribution to the energy of impact.)
 |
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:
- Where do you
find the thickest ejecta? (On the rim.) What do you think caused the
crater rim to form? (Sand scooped out by the impact was deposited on the
rim.)
- The colored
powder represents the most recent sediment deposited on a planet's surface.
Any material beneath the top layer must have been deposited at an earlier
time (making it physically older). If you were examining a crater on the Moon,
where would you probably find the oldest material? Why do you think so? (Near
the rim. Because the deepest material ejected lands closest to the crater,
i.e., on the rim.)
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:
- How does the
appearance of the original crater change as you continue to bombard the area?
(It loses its crispness.)
- Look at a photograph
of craters on the Moon (see photo or use one taken from a book). Do all the
craters have the same fresh, sharp, new appearance? Describe the various appearances?
(No — smooth rims to sharp rims, bowl-shaped to elliptical, etc.)
- What do you
think has happened in this area? (Long-term bombardment.)
- What do you
think is an important source of erosion on the Moon? (Impact cratering.)
- What does the
appearance of a crater tell you about its age? (The younger the crater,
the crisper the features; the older, the more subdued.)
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).
- Chapman, C.
and Morrison, D. Cosmic Catastrophes. 1989 Plenum Press. [See excerpts
in the Nov/Dec 1989 and Jan/Feb 1990 issues of Mercury magazine.]
- Goldsmith, D.
Nemesis. 1985 Walker.
- Gould, S. "An
Asteroid to Die For'' in Discover, Oct. 1989, p. 60.
- Morrison, D.
and Chapman, C. "Target Earth: It Will Happen'' in Sky and Telescope,
Mar. 1990, p. 261.
- Sinnott, R.
"An Asteroid Whizzes Past the Earth'' in Sky and Telescope, July
1989, p. 30.
- Weissman, P.
"Are Periodic Bombardments Real?'' in Sky and Telescope, Mar.
1990, p. 266.
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