So, craters, right? The Moon is covered with ‘em, as are several of the planets in our solar system and their moons. Heck, the entire Gulf of Mexico is essentially one big crater from the asteroid that killed off the dinosaurs. But not all craters are created equal: those on the Moon are jagged edged, with scattered debris betwixt them; those on Mercury are comparatively neat and tidy, with smooth edges and smooth ground between impact points. ¿Por que?
Go Big (Craters) or Go Home
The original version of this science demo called for a cereal bowl, spoon, soil, and water. It worked perfectly swell that way, but we at Demo Science wanted to make it more interesting, so we scaled up. With our version, you’ll want to do the experiment outside.
Instead of a bowl, use an old cast iron bath tub (or find a company that can fabricate a deep drawn stamping and have them make you a new one). Instead of a spoon, get yourself a shovel. You still need dirt and water, just more of each. Also, you’ll need a ladder and a small bucket (a gallon or less). And, if possible, a roof you can readily climb up on (like the one of the school where you teach, perhaps).
So: fill your tub roughly halfway with soil, then add some water and stir up the whole mess. Add more water (or dirt), as needed, until you’ve got a good mud going—it needs to be thin enough to drip and flow easily, but thick enough to have some weight behind it. “Sloppy” is the perfect consistency to shoot for.
Fill your bucket with mud, then smooth out the surface of your tub o’ mud as much as possible. Then, ascend that ladder, good sir or ma’am, with your bucket o’ mud and shovel. If possible, climb up on the roof. (Obviously, your mudtub should be set up by the side of the building in this case.)
From atop your ladder (or rooftop), scoop out a good bit of mud with your shovel, then hold it flat over the tub. Tilt the shovel slightly so the mud slides off and splatters into the tub below. Repeat your mud splattering process until the bucket is empty, moving your shovel over the tub to create mud “craters” in different places on the tub’s surface.
Clamber back down, then inspect the craters with your students. What sees ye?
A whole bunch of different craters of different sizes, that’s what! As the dripping mud hits the muddy surface in the tub, both the dripped mud and the surface mud will splatter. Gravity pulls these splatters down again quickly, where they create smaller craters of their own.
The falling mud has roughly the same effect as meteorites or asteroids striking the surface of a celestial body (albeit with much less force). When large meteorites hit a planet or moon, the impact instantly generates incredibly high temperatures, which melt the materials on the planet’s/moon’s surface. These liquefied materials splatter upward and outward, just like the mud in your tub.
All your “craters” will have roughly the same splatter pattern, and the surface of the tubmud will be not exactly uniform, but more or less the same across the board. As mentioned above, however, the craters of the Moon and Mercury (as just two examples) are wildly different. This is due to the varying gravity of different celestial bodies—Mercury has a strong gravitational pull, so material from asteroid impact splatters is pulled back to the surface quickly, resulting in minimal “splashed out” secondary craters; the Moon, however, has far lighter gravity, so splattered materials fly higher and spread out more, creating not only secondary craters but also high, jagged rims around the craters themselves.