Labs from YSI 94 :
Craters!

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

Equipment to be gathered and prepared ahead of time:

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?

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!

[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 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

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:

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