Introduce the activity:
In the solar system, chunks of rock orbiting the sun, can collide with planets or moons.
(The asteroids, small chunks of rock and metal, are mostly concentrated in the asteroid belt, between Mars and Jupiter, but a collision between them sends an asteroid out of the asteroid belt where they might collide with planets or moons.)
Impact craters, from when a "meteoroid" hits the surface, are the dominant geographic features on many solid Solar System objects. (Early in the history of our solar system, asteroid impacts were much more common. Now, many of these stray chunks of rock have been caught by the gravity of big planets.)
Demonstrate to students how to make craters:
Shake a tray of flour back and forth so that the surface of the flour is relatively flat. Do not press the surface down - allow it to settle with its own weight.
Sprinkle cinnamon over the flour to make an even coat of brown over the flour.
Drop the marble/clay ball from a height above the tray, or allow students to do this for the first time once in their groups.
Name the parts of the crater that are formed (either in the introduction or while circulating around the classroom when students are working):
The floor of the crater (the bottom of the hole), the rim (the raised edges ringing the hole), ejecta (material projected out of the crater - it will look white in contrast to the brown cinnamon), rays (long lines extending away from the crater; a pattern made by the ejecta).
(See p.62 of http://www.nasa.gov/pdf/180572main_ETM.Impact.Craters.pdf)
Ask students to try dropping the asteroid from different heights (therefore different impact speed), and to observe the difference in the crater pattern made.
Optional: ask students to measure the heights they are dropping from and to measure the diameter of the crater from rim to rim in each case. (Note that this data is messy and there is not a huge amount of difference between high and low drop heights, but it demonstrates graphing data, and how to draw lines of best fit. See worksheet attached.)
Ask students to try dropping their asteroid onto a tray that is angled, to model an asteroid impacting a planet surface at an angle.
Show students how to stir up the flour and cinnamon to make an even colour then reapply the cinnamon. They will need to do this when there are several craters in their tray, and it gets hard to see the patterns.
1. Students should find that the crater diameter is always larger than the asteroid. When the surface of a real planet or moon is hit by a chunk of rock (more precisely called a meteoroid, or the "impactor"), the shock fractures the surface rock and makes a large cavity which is larger than the impactor. The impact sprays material ("ejecta") in all directions. (Any remaining rock pieces are called meteorites.) Asteroids hit the moon at an average of 20 kilometers per second, and make a crater 10 t0 20 times larger than the impacting object.
2. Point out to students that the marble/clay stays in the hole - for a real crater the impact rock has gone. It has shattered and been distributed with the ejecta. Or if the impact generates enough heat, the impactor melts or vapourizes.
3. Either through eye-ballng the difference in patterns, or by graphing data collected, the size of the crater and the average ray length increases with impact speed. (See graph of data - it is messy, but a line of best fit shows the trend.)
Show students an image of real craters and ask if theirs looked similar e.g. on Mercury: http://photojournal.jpl.nasa.gov/jpegMod/PIA11355_modest.jpg and on the Moon: http://www.nasa.gov/images/content/582010main_081211b.jpg. Next time students see a full moon, they might be able to find the crater rays on it: http://www.nasa.gov/centers/langley/images/content/528691main_Super_Moon...
Tell students that scientists work the other way around: they can figure out the speed of an impact by the size of the crater formed. They also look at other features that are made with the very high speeds of meteorites hitting moons and planets - sometimes craters have central uplifts and terraced walls that give more information about the impact and the rock on and under the surface of the planet.
4. If relevant, ask students what patterns they got from an angled impact, and compare to a real "oblique impact": http://lroc.sese.asu.edu/posts/595
5. Show students a crater with different features - Rampart Craters on Mars (https://en.wikipedia.org/wiki/Rampart_crater). There are no rays. Water or ice present under the impact site melted with the impact, then flowed away instead of being thrown away from the crater. This pattern of ejected material can be used as a way to identify areas with possible water or ice in the surface layers of a planet.
The number of craters on a surface can tell scientists about a planet:
Many craters indicate that there is no atmosphere to burn up the rocks as they come down. (e.g Mercury). It also may indicate that there are no plate tectonic movements or erosion turning over the planet’s surface. Earth and Venus have an atmosphere, which burns up rocks as they hit it (called "meteors" or "shooting stars"). Earth also has moving tectonic plates and erosion that recycle the rocks, so removing craters. (The craters on the Moon are good for studying crater formation, as the Moon has no atmosphere, plate tectonics, or moving water (which all erode a surface and erase all but the most recent impacts).