Educator's Guide to Impact Craters
Courtesy of the Jet Propulsion Laboratory
Cratering in your Classroom
Impact cratering is a process found everywhere in the solar
system except on the giant gaseous planets. Earth has been
heavily impacted but erosion has removed most of the craters.
Perhaps the finest surviving impact crater on Earth is the
Barringer Meteor Crater
near Winslow, Arizona. It is 1.2
kilometers (0.75 miles) across and 200 meters (650 feet) deep.
It was formed about 49,000 years ago when a 50 meter (150 foot)
nickel/iron meteorite struck the desert at a speed of 11
kilometers per second (25,000 miles per hour).
An examination of actual craters, almost any image of the Moon
will do, will prepare the students for this activity. Just about
all craters have deep central depressions, raised rims, and a
blanket of ejected material surrounding them.
You and your students can observe the Moon directly during
daylight. Check your newspaper for the phases of the Moon and
observe it in the afternoon during "first quarter" and in the
morning during "third quarter." The Moon will be separated from
the Sun by 90 degrees to the east (left) at first quarter and 90
degrees to the west (right) during third quarter. The large dark
regions are the remains of very great impacts and many retain
their circular boundaries. Binoculars on a tripod provide a
spectacular view.
You can create craters in the classroom with a box, lined with a
trash bag, with sides at least 4 inches high (the lid to
photocopier paper boxes is perfect); flour (3 to 4 inches deep
with at least an inch of clearance to the box rim), some dry
(powdered) tempra paint (red or blue), and some marbles.
Place the flour in the box and smooth and firmly pack it
(experiment with different firmnesses). Place a dusting of the
paint powder over the flour (colored water in a spray bottle
works, but not as well). Use the marbles to bombard the surface
(one at a time). Look for classical cratering features: basin,
raised rim, ejecta blanket (material excavated from the crater
and dumped around it, visible as white flour on the colored
powder), and rays (material shot out at high velocity forming
lines pointing directly away from the impact site).
Students should keep careful records and can do top and profile
drawings of the craters and compare craters formed by different
size projectiles, different velocities, and different angles of
impact. Different size projectiles can be dropped from measured
heights so that they will have common velocities. They should
also remember that the quality of their tests is more important
than quantity.
After several craters, the flour and tempra can be mixed and
re-smoothed without changing the white of the flour too much.
Then a new layer of tempra can be applied and additional
experiments conducted. In real impacts the impacting object is
destroyed or broken up into small chunks. Of course the marble
will not do this and will remain whole in the crater.
Vocabulary
- Central Peak
- A mountain found in the center of large craters.
It is formed by a "rebound" of the rock at the impact site (the
marble will be sitting there in this activity).
- Crater
- A (usually) circular depression in a surface caused by
an impact.
- Ejecta
- Material tossed out of the crater.
- Ejecta Blanket
- Ejecta tossed out at low speed. The material
lies like a blanket around the crater.
- Floor
- The interior of the crater. It is flat in large craters
(the marble will be there in this activity).
- Rays
- Ejecta tossed out of the crater at high speed. The
material forms long lines pointing directly away from the crater.
- Rim
- The raised edge of the crater. It is formed by the
outwards and upwards compression of the crater walls, not ejecta.
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