When large meteorites strike a planet's surface, they leave impact craters. Meteor Crater in Arizona is the most famous of the 150 impact craters known on Earth. During a meteorite impact, rocks from deep in a planet are gouged up and thrown onto the surface, so impact craters can be used like a mine or drill hole to show us rocks from underground. Also, the abundance of impact craters on a surface shows its age--the more craters on a surface, the older it must be.
- Crater Excavations: Laboratory Experiment. Start with a flat sand surface: A playground sandbox is ideal, but any unbreakable box with a surface bigger than about 2 feet by 2 feet will do. Smooth the sand surface, and cover the sand with a layer of fine, contrasting powder: different sand, tempera paint powder, or colored sugar work well. Cover this layer with about a few millimeters of sand. Then throw marbles or gravel into the sand, and see if your crater can excavate the contrasting layer. How deep is your crater? How far was the contrasting powder thrown by the impact? This experiment can be expanded and quantified by experiments with different types of sand, different depths of burial, marbles of different sizes and weights, and different angles of impact. Using a slingshot to shoot the marbles will permit harder impacts and bigger craters, but careful supervision is required. BE SURE TO USE PROTECTIVE EYEWEAR.
- Craters Old and New: Laboratory Experiment. Make many craters on a smoothed sand surface by throwing gravel or marbles until the sand is evenly peppered with craters. Then smooth out half the sand surface, erasing all its craters. Resume throwing gravel or marbles at the sand, but only throw about half as many as before. Now, half the sand surface should be heavily cratered and the other half moderately cratered. If you hadn't seen it happen, could you tell which part of the sandbox was smoothed during the experiment?
- Resurfacing: Some Thought Questions. Many processes on planets can erase, or smooth out, earlier landscapes. The word for this is resurfacing, literally putting a new surface on the land. What processes on Earth act to resurface its land? Compare the photo below with a map or aerial photo of a place you know. Why does Mars have more craters than your place? Find a globe or map of the Moon. What resurfacing processes act on the moon? Can impacts resurface a landscape?
Ancient cratered highlands of Mars, east of the Hellas Basin. Scene is about 300 kilometers across.
- The Sandbox of Mars. The figure shows an area in Mars' southern hemisphere. On the figure or a photocopy, sketch or trace out all the circular rim craters you find (also outline incomplete circles). Then, draw a boundary line that separates areas with many craters from areas with few or no craters. Which of the two areas is younger? Remember that liquid water cannot exist on Mars' surface now. What processes that don't require water could have resurfaced Mars? Look at the long, twisting feature that goes from the upper right corner to the middle of the left side of the figure. Does anything on your state map have the same kind of swerving path? The feature might be a river bed, now bone dry (of course). What was Mars' climate like when water flowed in that river? What happened to the water that once flowed in the river bed? Where is it now?
More about impact craters and cratering experiments (PDF Format).
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©Lunar and Planetary Institute (1996)
