Impact Cratering Details and discussion

(draft 10/8/98)

Elizabeth E. Roettger, interpreting and synthesizing a standard activity from many sources. See also http://www.soest.hawaii.edu/SPACEGRANT/class_acts/CrateringDoc.html for other suggestions. I'm sure there are many more, but this particular one is linked to images and some good discussion of variables.

Containers:

What you use as a container depends a lot on where you're doing the experiment. For classroom use, aluminum roasting pans or the tops of copier-paper boxes work rather well, especially if you put them on top of large trash bags or cheap plastic tablecloths as drop-cloths. (The trash bags can be turned inside-out around the tubs for fast cleanup.) Large buckets can be good, but you can usually only get one good impact at a time. A child's sandbox or a hole in the ground outside can work quite well, if available. (Try to pick a relatively calm day.)

Surface medium:

This can be just about anything. Dry plaster-of-paris makes the best craters I've seen, but it's expensive. Sand and flour are more typical. I've tried salt and sugar, but these tend to make craters whose centers collapse into cones -- interesting for variety, but not for the main experiment. Cornstarch (or powdered sugar) alone will fly all over, so don't use it indoors.

If you're using flour, the old standard, then practice briefly with your impactors to test the following:

depth needed for good craters
whether to pack or leave fluffy
size of typical ejecta blanket (rays shooting out from impact site), so you can judge whether several craters can be made, or if the flour needs to be re-set before each impact.

Surface coloring:

Typical is chocolate drink mix, sprinkled lightly over the surface for contrast. This assumes that the surface medium is light in color. Use a strainer, a shaker, or a sifter to distribute a very thin layer. Other options include colored sand, colored sugar (just mix a few drops of food coloring with some sugar, then spread it out an hour or so to dry), powdered drink mix (grape is generally pretty dark), or cornstarch, flour, sand, etc. on a medium-to-dark surface.

If you're patient, you can also try spraying a fine mist of water and letting it dry, so that you have a slight crust on your surface. I haven't found a good way of doing this yet.

I have seen sand layered with various colors in turn, so the medium is striped as you go down. This was used with an impactor and a slingshot. You could see colored rings, in order, in the ejecta. It was quite impressive as a demonstration, but not something I'd let most kids try on their own.

Impactors:

Steel balls are marvelous. Not only are they dense enough to make generally good craters, but you can retrieve them with a magnet without mucking up the crater too much. Marbles and stones also work, as do hexagonal nuts (misc. hardware). Use your creativity, though - nuts in their shells, broken pottery, balls of various density, etc. (one person I know used several objects of the same size, one of which was a mini pumpkin...). Light impactors generally don't work very well unless the medium is very fluffy -- but perhaps your participants would like to learn that for themselves. Objects that have shape can give you information about whether the crater is due to the object size or the energy it imparts.

In the real world, impactors tend to hit hard and explode upon impact, creating a shock wave in the surface medium. Adler Planetarium used paint balls to simulate this -- it gave very satisfactory craters, although it's beyond my means to duplicate. I've tried dropping pellets and blobs made up of the surface medium, but so far, haven't had much success. Drops of water, when released from a good height, did give some interesting results.

Other useful items:

Measuring instrument (ruler, and also a thin rod to measure the depth of the crater. It can also be useful to take photos (especially if you have an electronic camera or scanner, and can dump the image into an image processing program for detailed measurements) -- they're most impressive when the craters are lit from a relatively low angle, rather than from above. Videos (especially slow motion) of the actual impacts can be fascinating, and if you can separate and print the frames, can even make rather nice flip-books.

I've tried several times to simulate the slightly-melted craters and ejecta blankets found on Mercury and other places, but haven't found a satisfactor means yet. Wet flour is definitely not the way to go -- the gluten in the flour makes it sticky rather than fluid, even when coated by a layer of dry flour. I'm told mud works, but I haven't tried it.

Images:

Some sources of images of impact craters (also volcanic "craters"):

http://photojournal.jpl.nasa.gov/

Also check out Terrestrial Impact Craters, such as http://www.hawastsoc.org/solar/eng/tercrate.htm

Comments welcome, as always -- preferably via e-mail.

Attributed, non-commercial distribution of these notes is encouraged. Comments and suggestions are appreciated.

Web version 8 November 1998
by Elizabeth E. Roettger.
URL: http://nthelp.com/eer/HOAimpact2.htm