To form realistic craters you need realistic materials. However, we can't reasonably fire slugs of iron or rock at speeds up to 60 kilometers per second at slabs of rock (not and expect to survive anyway). But that doesn't mean we can't come up with an analog which will demonstrate many of the characteristics of a real impact. One idea I came across is the use of fine sand and a projectile like a marble. I haven't tried this and it may work. Another idea was to use flour instead of the sand. Again, I haven't tried that one, but it sounds like it might work. Here's what I have tried.
Playing in my Pudding
You'll need the following:
- 1 box of instant pudding
- 2-1/2 cups of skim milk
- powdered sugar
- M&Ms or equivalent
- rectangular plastic container for the pudding
- plastic garbage bag to contain the mess
Pick you favorite flavor of instant pudding. Most of the small boxes are designed to make about 2 cups of pudding and require 2 cups of cold milk to make. However, that results in a pudding which is a little thick for our needs. I use 2-1/2 cups of skim milk (skim milk supposedly makes for thinner pudding in the first place). Chill the pudding for an hour or two so it is set. This will not be runny pudding, but it will certainly not hold its shape very well. I put the entire 2-1/2 cups of pudding into a rectangular plastic container so the pudding itself is 1- to 1.5-inches deep. Congratulations! You've just made your analog Earth for our impact tests.
To best demonstrate what happens with the ejecta, sprinkle powdered sugar over the top of the pudding. You want to wait until the last minute before the experiment to do this part because the pudding is moist and the powdered sugar will immediately start to soak up that moisture and stop being powdery. Note: the powdered sugar will make the experiment significantly more messy! When our "meteor" imacts you are going to get a spray of powdered sugar. You'll also get a spray of pudding, but the powdered sugar will travel quite a bit further.
Now, while the powdered sugar is still powdery, sit the pudding container on the plastic garbage bag. Fling your M&Ms at the pudding. This isn't a carnival contest, so there is no need to stand far away, but do try to hit the container!
The first batch of pudding we made was the stuff you cook. Nothing wrong with it for eating, but.... We made it according to the recipe and it was a bit thick. It also tends to form a crust on top as it cools. When Matthew and Jonathan made their first throws, the M&Ms bounced off! That's when I went out and got the instant pudding and made it a little thin.
Sometimes, perhaps even quite often, the M&Ms will manage to hit the pudding edge-wise. When this happens, instead of making a nice round entry hole with a splat, the M&M may actually sort-of cut its way into the pudding. Just ignore those as unrealistic. Our boys are both allegic to nuts, so at home we used M&M-clones from Vermont Nut-Free Chocolates. These are a bit more round than regular M&Ms and didn't have this "problem." But you probably don't want to order these more expensive candies just for the experiment.
The children have to kneel down near the pudding container to throw. That bowl looks easy to hit when standing up, but you'd be surprised at home many misses you get.
Analyzing the Results
First, its fun to just fling the M&Ms at the pudding, so its easy to let things get out of hand and not pay attention to what's happening. When we did this at home, there were several things we noticed. First, the powdered sugar goes in all directions, but more goes away from the direction of the incoming M&M. Second, the pudding itself creates a small crater rim around the impact site. Third, while the hole formed is approximately circular, it's not symmetrical like a bowl; if the M&M went in at an angle, the hole goes down at an angle. Fourth, the pudding is watery enough that if we wait a few minutes, the pudding fills in around the M&M and the bottom starts to look more bowl-like (think "simple crater").
I'll add one more thing, too. When I flung an M&M at the pudding, both of my sons standing off to the side commented that the felt splatters from the pudding. This was a good opportunity to comment on both how far the splat can go (remember Barringer Crater, ejecta spread over a 10 mile circle) and how the splat is not confined to the "downrange" direction from the impact.
Finishing the Results
Usually in science, eating your experiment is a bad idea. In this case, enjoy!