Materials You Will Need
• Two contrasting colors of sand (or similar; see Activity 1)
• Box or tray to contain sand (higher sides will help
... [Show More] contain the sand best!)
• Various small objects to simulate impactors (rocks, a marble, small ball, dice, etc., and at least one irregular shaped object)
Prelab
Answer these questions:
1. What factors could affect an impact crater's shape and size?
2. What effect do you expect varying these factors will have on the craters?
3. Explain how you could test these hypotheses.
Introduction
One look at the surface of the Moon should convince you that "empty space" is not so empty after all. There is actually a wide range of objects floating between the planets, from tiny particles to asteroids that can be a hundred miles across, debris left behind when the planets were formed. These objects can be perturbed from their orbits (by a close passage by a planet, a passing star, any number of things) and onto paths that cross ours -- or any other planet or moon. When that happens, a collision occurs and an impact crater is formed.
The size and shape of the crater depend on the impactor: its size, shape, speed, and the angle is hits the ground with. Specifically, the size of the crater depends on the energy of the impactor. However, the relationship is not linear, but rather is a power law:
where D is the diameter, E is the energy of the impactor when it hits the ground, n is the power, and k is a constant.
The energy when the impactor strikes the ground is all kinetic,
where m is the mass of the impactor, and v is the speed it's going when it hits the sand. Unfortunately, v is inconvenient to measure in our classroom. Fortunately, energy is conserved, so we can give the impactor a known energy and know it will hit the sand with that amount of energy. The total energy of a falling object is the sum of the kinetic and potential energy. If you drop the impactor so it starts with v = 0, the total energy is just potential (called gravitational potential energy):
where m is still the mass, g is the acceleration due to gravity = 9.81m/s2 at the surface of the Earth, and h is the height above the ground.
Activity 1: History of Cratering
Craters can be used to find out information about conditions on the planet or moon. An active planet will have few craters because tectonics and volcanism recycle the planet's surface. On a planet with an atmosphere, craters can be worn away due to wind or water erosion. A geologically dead planet with no atmosphere has no way to remove craters, except through more cratering.
To demonstrate cratering, you'll have a box of sand. There should be a thick base of white sand which you'll add a thin regolith of colored sand to. (Note if the sand in the box isn't white, you should get colored sand with a good contrast). Be sure to make the layer of colored sand very thin, since you want to see the pattern of ejecta when you make the crater. [You may use other, similar materials, e.g. flour and cinnamon, flour and fine layer of potting soil are combinations some have used.]
Smooth the surface of the sand and apply a thin layer of colored sand. Try throwing a ball bearing sideways into the sand and see what shape it makes. Get some of the other objects and throw them in. Make a bunch of craters without wiping the sand clean, to see how they pile up on top of each other (be careful not to mess up your craters pulling the objects back out!) Look carefully at the patterns in the sand and in the colored sand on top of the white sand -- around the crater you should see a crater rim and a little further away, rays of ejecta thrown out by the objects.
Activity #1 Questions:
1. As you dropped the marbles from different heights, how did the ejecta (material tossed out of the crater) change?
2. When you dropped non-spherical objects, or threw the marble at an angle, how did the shape and ejecta change? How does this relate to craters seen on other planets/moons?
3. If you were to look at another group's sandbox, could you tell which craters were made first (‘older’)? How?
4. Describe the transformation of energy that takes place during the formation of an impact crater (from approach of the meteorite to after the crater is formed). [Show Less]