General Directions for Craters and Surface Processes

Pages for this activity: General directions, surface processes experiment, moons, impact experiment.

Overall: Drop objects into an inch or three of flour that's been coated with a colored substance to simulate craters. Use pictures of Jovian moons and some simplistic techniques to study the four planetary surface processes. Focus on cratering and determine the effects of one variable; compare your results with others in your team who have studied other variables.

Note: These directions have been assembled from in-person labs and teacher workshops, and a distance-learning course for teachers. Until I figure out how to organize it better, you'll have to ignore the parts that don't apply to you.

Materials:

1. A bin or other container. I've used 15-quart cheap plastic bins with covers and put a 10-lb bag of flour in each; I can snap on the covers, stack them to haul around, and re-use them almost indefinitely. They're a nice size. For a single use, 2-5 lbs of flour in the lid of a copy-paper box or large bowl does nicely. The surface you put the bin on can also matter - a hard surface is best, but you might experiment with putting the bin on a stack of newspaper or other springy surface. If the medium is deep, the surface under the bin generally has less effect.
2. Flour or equivalent medium. Flour works very well. Plaster of Paris makes even better craters, but it tends to get everywhere and you must be especially careful not to breathe it. Sand or loose dirt can work well, but you'll need heavier and generally denser objects; consider moistening the sand slightly. Salt does not work well. Baking powder is okay, but not as good as flour. Clay doesn't work with dropped objects. The depth of the medium matters - you might try different depths, but 1-2" is generally pretty good.
3. Objects to use as impactors. I have an assortment of balls - hollow, foam, plastic, glass, and metal - ranging from about 1 cm to about 5 cm. I also use a few irregular objects, and have some hollow plastic eggs into which fishing weights can be tucked. Generally, lightweight objects work best on fluffy surfaces and medium to dense objects work better on packed surfaces. For sand or dirt, start with dense objects such as glass or metal. You'll need a few different kinds to figure out what will make the best craters in your setup. If you want to experiment with mass or size, you might want a pattern of impactors - multiple sizes of the same material, or one size with multiple materials. (Do a test run to establish the best parameters before you go out to buy things!) See impact experiment for suggestions about classroom sets of impactors.
4. Colored medium to coat the surface. The traditional recommendation cocoa powder works well, but can be expensive for classroom use. I can't use it if I'm hungry - the aroma drives me bonkers. Powdered tempera paint is effective but don't breathe it. For use with students or in teacher workshops, I buy bags of table salt and tint it with food color. (Pour into a plastic zip bag, drop in color, close, mush the color lumps until well-distributed. It'll clump if left for a few days, but mush to break the clumps.) Sugar colors well, too, but is expensive and more likely to attract pests in storage. If you're using something dark instead of flour (cement mix, for example), you can use flour as the color coating. I put my colored salt into a cheap sugar shaker; small strainers also work well to help make a thin coating.
5. 2-meter stick or other height-measuring apparatus. In lab, a 2-meter stick allows drops from reaching-up heights. For home & travel use, I've taped two tape measures together and weighted the end so it hangs vertically. A tape measure or yardstick against a wall or other vertical surface also works.
6. Ruler + 2 blocks, vernier calipers, or other size-measuring apparatus. Vernier calipers are best. A ruler is very useful for longer distances, such as crater rays. Raw pasta can help you measure vertically or in tight places - break to fit if needed, mark with a felt-tip pen, then pull it to a more comfortable position and measure to the mark. You can get the diameter of a sphere by using two blocks, each at least half the height of the sphere, if you catch the sphere between them. Shove the whole thing against a straightedge so the blocks are parallel, and measure the distance between the blocks.
7. Small scale. You need to be able to measure the mass of the dropped objects. There are other ways to do this, such as water displacement, but a scale is the most straightforward.
8. Optional: grabbers or a magnet on a string. It's useful to be able to remove a dropped object without having to poke your fingers into the flour. I found a three-wire syringe-like gizmo meant for extracting olives from tall jars. Tongs are useful. Magnets work for steel objects.
9. Optional: newspaper or dropcloth to keep the floor clean. Flour or other fine powder on the floor can be very slippery, so I strongly recommend using a dropcloth. The flour tends to work its way around newspaper. Cheap plastic tablecloths from a dollar store work pretty well; if you use a dark one, you'll also be able to see any rays that go past your container. When done, shake it out and fold it so the flour (up) side is inside if you plan to use it again.
10. Optional: wooden spoon and/or damp and dry cloths for mixing the flour. I keep woodens spoons with my kits. Mostly, though, I end up running my hands through the flour to mix it and remove any lost objects, and then the damp and dry cloths - or sink and towel - come in handy.

How to set up and make a crater:

1. The flour in the bin will represent the upper layer (regolith) of a world's surface. Start by making sure it is mixed well. Try to get it even in "fluffiness" - fluffed up, packed, or something in between. (I don't recommend fluffy unless you're willing to fine-tune the depth and impacting objects.) Try varying the degree of packing to see what makes the best craters. Record what you do and the result. In general, record all methods you use! There are many different combinations, and it's not a good idea to depend on memory. However, brief qualitative descriptions are fine if all you're doing is ruling out poor combinations and identifying potentially good combinations.
2. Gently smooth the top. (It may take a few tries to decide how best to do this.)
3. Coat the top of the regolith with a very thin, fairly even layer of a different color. You need a different color in order to be able to see the "ejecta" from the craters you'll make.
4. To make a crater, drop something (an "impactor") onto the surface. Try several impactors of different sizes and compositions. Drop a known impactor from a known height to make a crater. At first, experiment with heights, packing amount, and impactors to get good craters; you only need rough estimates and descriptions of the results. "Waist height", "reaching up", and "a few inches" may be good enough for height, "dime-sized" and "palm-sized" may be good enough for sizes. Pay attention to what changes most and what tends to stay the same. (Use the experimental cratered surfaces for the "surface processes" experiment before you re-do the surface.) Later, when you're doing systematic cratering experiments, you should measure the impactors and the heights from which you drop them. Do not throw the impactors - it is difficult to judge how much energy they have, and you should consider safety.
5. There are many different kinds of measurements you can make to characterize the resulting craters. The best qualities to measure are the ones that vary most! Use the "parts of a crater" diagram for guidance. A list of possible parameters is in impact experiment. In general, make what measurements you can make before removing the impactor. You should also record qualitative descriptions, and definitely make a few sketches or diagrams. (Flat pasta can be a convenient measuring stick for awkward places - you can mark it, then remove & measure the marks.)
6. Use the surface for as many craters as you can* before you mix, fluff or pack, and re-coat the surface. When you re-make the surface like this, you are essentially using a different surface. What can you do to check if it can be reasonably compared to the previous or standard surface? (Do it for the systematic experiments.)

* then do the "surface process" experiments if you haven't yet.