Labs from YSI 94 :
Craters!

Luisa Rebull
CARA Yerkes Summer Institute, August 1994

This is the staff copy of the lab and other brainstorming/reflections.

Equipment to be gathered and prepared ahead of time:

Collect lots of books with images of Mercury, Venus, Earth, Mars, Moon, and terrestrial satellites of gas giants. (for comparison to craters we make and discussion afterwards.) Data sheet forms for everyone, rulers (transparent, for measuring diameters of craters - the CARA-owned protractors work great).

Collect several projectiles of varying masses and diameters. (marbles, balls, wooden beads...)

Line a box lid (from a Xerox paper box) with a large garbage bag, and dump about 5 lbs of flour into it. Smooth out the flour, trying not to pack it too firmly (drag a protractor over it). For each new round of crater trials, sprinkle powdered drink mix lightly over the top of the flour. (This will provide contrast with the flour and enable us to see ejecta much easier.) Try multiple layers of drink mix for a rather cool layered effect. (Powder must not be clumpy - some cocoas are too clumpy - and have finer grains than sand - most sweetened drink mixes work well.) Do this two times - a group of about 10 kids needs two boxes to work well.

Get water balloons ready for ground impacts.

Before we start throwing things into the flour and making a royal mess, what do we expect to see?

My original thoughts on how to approach the lab:
Discuss what happens when a crater forms (qualitatively). (What are their preconceived notions? Try approach from what happens when crater forms, or what does a crater look like? maybe latter first.) Copies of strobe-light photography (by Harold Edgerton, MIT) of a milk droplet. (tiny, b&w copies found in Pasachoff, Contemporary Astronomy, p369.) Discuss how this might happen in rock. How can we get rock to flow like a liquid? (may be tough concept - bring up volcanoes if this is hard.) -> kinetic energy of projectile goes into heat, melting the rock. Discuss energy issues, mgh -> 1/2 mv^2 -> heat, motion of material.

Look at pictures of craters in the Solar System, start with Mercury and Moon. Ask them to describe a crater found on Mercury or the Moon. Talk about ejecta, center 'dimple', steep interior slope, more gradual outside slope, level of floor of crater as compared to level of surrounding ground.

What I actually did was closer to:
Start with picture of Moon. Ask them if they can identify this object. What can they observe about the craters? (nearly all round, more small than large, look explosive/powdery, ejecta...)

Where else in the solar system would they go to find craters? Show picture of Mercury in particular because ejecta work VERY well. Don't do Mars/Venus pictures yet - erosion and different ejecta come later. Ask if they expect to find craters on the Earth. Show picture of Barringer Crater (AZ). Point out scale. I should have brought up the milk droplet pictures here, but I forgot at least once. I talked about energy, etc, while making the first round of craters.

Now go make craters!

Set up experiment in front of group. Explain that the drink mix is the topsoil on our "planet." Have one person drop a projectile of their choice into flour. Ask group - how could you make a bigger crater? (bigger diameter, bigger mass, higher velocity before impact.) Pick another person to do just that. Present them with data sheets and explain the non-quantitative nature of the sheets and the fact that I want them to do at least two craters. (Much trouble with what I mean by "diameter of crater" and "diameter of crater + ejecta." Trouble also with idea of sketching crater from top and side. This is related to problem we observed with "thinking concretely" rather than "thinking abstractly.") Suggest they experiment with different heights, angles, masses. Talk one-on-one about relationship of these craters to craters on Mercury, Moon (while wandering around and helping individually). Observe similarities, differences among artificial craters. Talked about layering of craters (and thus chronological ordering) a bit when giving explicit instructions about waiting for someone to finish measuring before making a new crater, but I'm not sure they got it. Have them all take data, filling out sheets with qualitative information. Some will, by themselves, wonder about making non-symmetrical craters or ask "What happens if I throw this whole handful of projectiles into the flour?" (Crater chains are the point here.)

[other notes : More quantitative lab - measure diameter of crater+ejecta as function of height? Possibly measure depth as function of height, too? (note that in my experiment, diameter of crater was pretty much diameter of marble (unless you really throw it), so you have to use ejecta too -- need to go try this. Depth varied as expected; from 70 cm, bounced a little, messing up crater; from 80 cm, bounced completely out of crater and away from crater site. ]

(A more quantitative approach may take more time than we have - reviewing the distinctions between this experiment and reality is important, as is reviewing the craters found in the Solar System. So is just playing with the stuff - this is an important part of science too.)

How does crater diameter change as the mass increases? How does the crater diameter change as velocity increases? Which do you think is more important to the energy of impact, mass or velocity? (This may be more for the 3 or so kids who will present this material.)

Post-experiment discussion

Look for any non-circular craters in pictures of Mercury or the Moon. There aren't any, but we were able to create non-circular craters. Why? This is a crucial difference between our experiment and reality. In reality, the forces involved cause the projectile to explode on impact, which demand that all the craters be circular. (Toss a water balloon on the grass/mud to demonstrate. Must be explicit in instructions to kids - throw it in that direction.) In reality, the grass was too dry/thick/brittle for this to work in the sense I originally thought it would, but it was good for demonstrating that you don't look in the center of a crater to find a meteorite. Suggest a "meteorite hunt" to make sure we get all the plastic bits (and beads etc.) back from the grass.

Look at a Moon-globe or atlas. Can they determine a distinction between two sides? (The near side is much less cratered than the far side.) Recall that the same face of the moon faces us all the time. They needed to be told which side is the nearside, and which side is the far side, but they got the idea that the Earth protects that side of the Moon, and that the Moon shields us too.

Things hit us from outer space all the time, but they usually burn up in the atmosphere as meteors (shooting stars). If we get a big enough thing that it doesn't abrade entirely by the time it reaches the ground, it will leave a crater. (Sometimes there is a meteorite left for us to examine - pass around meteorite loaned by Yerkes Obs. (there's a huge exhibit of meteorites in the Field Museum). REALLY big things have hit us in the past. The forces of erosion usually disguise these craters, so they're hard to find. Show map of all the (currently) known craters on Earth. Two of the three groups asked good questions about craters, starting discussions of crater chains, period of lots of impacts early in the Solar System, other stuff. My reporting group needed extra help getting the vocabulary down. (I can understand the problem with meteoroid/meteor/meteorite, but they were getting crater mixed up too(!?!)...)

Discuss that erosion as we think of it can't happen on the Moon/Mercury due to lack of atmosphere, water, wind, so craters look different. They didn't know about the word "erosion" and it wasn't obvious to them that craters on planets with atmospheres would be more eroded. Showed picture of Venusian crater here. Talked about milk droplet here. They had to be helped to get the idea of what would melt rock.

Now look at craters on Mars. How do the ejecta compare to those on Mercury? The permafrost that is present on Mars melts on impact so the ejecta flow instead of spray out.

How do I know this is a crater due to an impact, and not due to a volcanic eruption? What are the differences between the two kinds of "craters''? Talk about walls, mouth, level of interior floor, frequency and location of craters of both kinds, central peak, impact crater diameters much larger than largest terrestrial volcanic craters known. They didn't know about volcanic craters. Some didn't know what caused them at all, and needed more explanation about their origin, and distinctions need to be made.

Sundial folks might have enjoyed/understood the idea of the length of the shadow of the crater walls enables you to measure the height of the wall, but I forgot entirely.

Things they definitely should be able to answer after today:

  1. What happens during the formation of a crater?
  2. What are the different kinds of energy involved in the formation of a crater?
  3. In terms of our experiment, what is at least one difference between the formation of a big crater and the formation of a small one?
  4. Where in the Solar System can you find craters? Can you find them on Earth?
  5. Why don't craters on the Moon erode away like craters on the Earth?
  6. Why are most craters in the Solar System round even though we were able to create ones that weren't round?

Extra things I hope they might take away:

  1. How can you distinguish between volcanic craters and impact craters?
  2. How can you distinguish between the near side of the moon and the far side?
  3. How are craters on Mars different than craters on the Moon or Mercury?
  4. How can you order craters chronologically?
  5. What kinds of craters might Shoemaker-Levy 9 have left behind if it had hit a terrestrial moon of Jupiter?

Sample Data Sheet

(Takes up 1/2 sheet of paper, so each person observes two and uses a whole sheet of paper.)
Data Sheet    				Name (or initials):_______________________

object used: light --- heavy

example: (bead, marble)

diameter of object: tiny -- medium -- large
example: (smallest bead, marble, ball)

thrown --> approximate angle: 0 degrees --- 45 degrees -- 90 degrees
example: (skim along flour, \, straight down)

dropped --> approximate height: lower --- waist -- head --- higher

approximate diameter of crater: ___________________

approximate diameter of crater + ejecta: __________________

Sketch crater from side and top and add any additional comments:

Important Disclaimers and Caveats: