Density of Craters
Summary
The student simulates impact cratering to understand the effect of variable
cratering rate on the inferred relative age of a surface and to recognize the
limitations of this method.
Background and Theory
Impact cratering is an important geological process on almost all the worlds
of the solar system. On airless worlds like the Moon and Mercury, impact
cratering dominates all other surface processes. We can use the surface
density of impact craters to establish the relative age of a planetary
surface.
In the lab, dropping water into sand simulates impact cratering on a
planetary surface. The two most important differences between this simulation
and nature are:
- in this experiment the rate of impacts is constant through time and
- the size of the impactors is approximately constant.
The rate of impacts onto actual planetary surfaces has decreased
through time, and there has been a great variety of impactor sizes.
Procedure
- Fill the petri dish with dry sand. Appoint team members as time keeper,
water-dropper, dry-sand getter, crater-counter, quality-control manager.
- With the cup about one foot above the dish, let the drops fall onto
the sand for 5 seconds. Move the cup around while you are doing this so
all the drops do not fall onto the same place.
- Place the card with the square hole in it over a random part of the
dish. Count how many craters you can see. Record this number in column
1 of the data table.
- Repeat steps 3 and 4 until you have a total of sixty seconds of data.
- Dump the sand out and repeat the experiment twice more, filling columns
2 and 3. You should have 3 runs of data total. Do not dump the last dish
of sand yet (see below).
Table 1: Cratering Data
Time
(seconds) |
Crater Density |
| Run #1 |
Run #2 |
Run #3 |
Average |
| 5 |
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| 10 |
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| 15 |
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| 20 |
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| 25 |
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| 30 |
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| 35 |
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| 40 |
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| 45 |
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| 50 |
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| 55 |
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| 60 |
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Before you dump your last dish of sand, take a look at this image of
a region of the highlands of the Moon. List two similarities and two
differences between your model surface and the actual surface. What are the
(most important) reasons for the differences?
- Plot your data on the graph and draw a smooth curve through the
data. Don't forget the data point at (0,0).
| |
Crater Density |
| 20 |
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| 15 |
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| 10 |
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| 5 |
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| 0 |
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| 0 |
10 |
20 |
30 |
40 |
50 |
60 |
| | Time (sec) |
Questions
- According to your graph, how "old" are the surfaces pictured
below?
- You noticed that after a while, it becomes hard to tell how old a surface
is. The craters start to overlap, and it is hard to tell when new craters
are added. This is called saturation. Look at your graph. When
did saturation occur for your experiment? That is, when did your line start
to be horizontal?
- If you were to repeat this experiment, but at some random time somebody
erased all your previous craters, how would this affect your results? This
is a similar effect to resurfacing. How does resurfacing affect how you
interpret crater densities?
- A previous group of students counted 25 craters after 15 seconds. How
does this count compare to yours? If the count is different, explain how
this could have occurred even though they were doing the same experiment.
- In real life the rate of cratering is not constant over time. There
were many more impacts early on (billions of years ago) than there are
today. If your drops fell much more frequently in the beginning than at
the end, how would your results change? What would your graph look like?
Explain briefly here. Using a red pen or pencil, lightly draw a new line
indicating what this graph would look like.
