Impact Cratering
IMPACT CRATERING – TEACHER NOTES AND HANDY HINTS
The Impact Cratering module can take one of many forms, depending on time available,
equipment, and the nature of the students. The lesson could be entirely non-practical,
with the teacher leading a discussion using slides and pictures from books or the
Internet, and encouraging participation from the students. If the teacher wished, this
could be interspersed with demonstrations from the activity sheet. These could be used
to demonstrate to the students a) the effect of larger projectiles on the final crater, b)
ejecta patterns and rays, c) the effect of oblique impacts, d) the effect of velocity on the
final crater form and e) the effect of target material on the crater. Alternatively, the
teacher could opt to have the students carry out the full activity themselves. In this
case, it is recommended to use two lessons (depending on the working speed of the
students). The choice is yours.
Activity sheet - aims and benefits
This activity has been taken from the Hawai’i Space Grant College who should be
acknowledged if your results are placed in public access areas (i.e. displays, web pages
etc).
They also have many more classroom activities that you can try (see
http://imina.soest.hawaii.edu/SPACEGRANT/class_acts/ for details).
In its full form, this experiment requires many scientific skills from a student. Good
experimental practices are needed, and to obtain accurate results the students must:
a)
b)
c)
d)
work methodically
take good notes
record information accurately
transfer written data into graphical form
In addition, the students will find some of the measurements difficult to take, and will
need to think carefully about how to get the most accurate readings. Hints for these
problems are below. The students will also be encouraged to think about the
experimental setup and consider where possible errors may occur.
Hints
1. This practical really can be messy - ensure that precautions are taken to stop the
flour ending up all over the floor and students. Certain coloured powders may stain
clothes - if this type of material is chosen, aprons are strongly recommended.
2. When carrying out the experiment, the target (i.e. flour and paint) may have to be reprepared several times. There is no need to start out with fresh materials each time;
as long as the top layer contrasts the lower layer strongly enough, the materials can
be used again and again.
3. Measuring the diameter of the craters can be difficult, particularly if the bowl is
smaller than the ruler. Use a compass instead to span the diameter and then
measure the compass separation. The same method can be used to measure the
diameter of the projectiles. If you have an able class, increase the complexity by
Impact Craters (Teacher Notes)
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using two or three different projectiles of different size/mass and get them to repeat
the experiment and compare the results of each.
4. (Q2) The graph should show that the diameter of the crater and length of rays
increases with increasing velocity (i.e. increasing projectile height).
5. (Q4) The compaction of the flour is very important. You will get very different results
from flour that has been sieved into the bowl and flour that has simply been dumped
into the bowl from the bag. Results will be better if the flour is sieved, or at least
"fluffed up" with your fingers/hands. Discrepancies may result if a particularly thick
layer of coloured powder is put on top. Also, it may sometimes be difficult to
determine where the rays end, particularly if the rays of one crater overlap with
another. You should encourage the students to discuss these points.
6. (Q5) The students may or may not have a raised rim at the edge of their crater. All
fresh impact craters in reality have this feature. The student version will also lack a
central peak or terraced walls. The main reasons for this is the lower velocity of the
impactor in the experiment and the small size of the impactor. In real impacts, the
velocity of the projectile will be of the order of 10km/s or more, and the projectile is
vaporised on impact. The student's projectile of course will still be there.
Extensions
1. Discuss the possibility of asteroids/projectiles impacting the Earth. There has been
a lot of interest in this recently. Get the students to predict what would happen if an
asteroid were to hit the Earth - what difference would an impact into water make
compared to an impact onto solid ground?
2. Look at a succession of craters that have been weathered to different degrees on
Mars/Moon/Earth. This can lead to discussions of weather patterns on the different
planets, superposition of craters, and the effects of surface material on the
degradation of craters.
3. Take the discussion of the effect of water on Martian craters a bit further. Get the
students to experiment with different kinds of wet material - flour/sand/plaster of
paris and see if they can reproduce any of the Martian features. Water is not the
only liquid that can be used for this - perhaps a liquid like sunflower oil would also
work (the difficulty in this experiment is to get the final shape to hold). UNTESTED
BY US SO TRY THIS OUT BEFORE THE LESSON!! Please let us know if it works!
4. Try multiple layering. In a real impact event, the material from below the surface
ends up on top in the ejecta blanket - this is shown nicely in the experiment here.
Further to this try adding another colour on top of the current experimental setup and
see if the reverse layering still holds. UNTESTED BY US – please let us know if it
works!
5. Try oblique impacts. Get the students to predict what will happen to the ejecta
before they carry out the experiment. The ejecta will be thrown preferentially
downrange. We haven't worked out a way of measuring the angle yet, but 90° vs.
non-90° is good enough to demonstrate the effect. If you can find a way of
Impact Craters (Teacher Notes)
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measuring the angle, you should be able to undertake a more scientific study by
plotting graphs of angle vs. ray length.
Answers to questions from the background information sheet and "What can
impact craters tell us?"
1. The picture with the larger number of craters is older. The students should try to
give a reason why the age of one part of Mars would be younger than another. The
answer to look for is something covering over the older impact craters and
"resetting" the crater count to zero. Lava flows can cause this on Mars, for example.
2. The main difference between the two craters is the appearance of the ejecta blanket.
The reason for the difference is that the ejecta in a) was emplaced ballistically, i.e. it
was thrown out, and it stayed where it landed, while that in b) flowed along the
ground once it landed. The flow resulted from the presence of melted ice in the
ground, which created a mud-like flow. Therefore, by studying this crater we can tell
that water was once present beneath the surface in this area.
3. Again, there could be many answers to this question. The obvious answer is to go
to a place that had water (and hence where life may once have existed). Make a list
of the answers you get for future reference - they may be useful during mission
planning!
Impact Craters (Teacher Notes)
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IMPACT CRATERING – STUDENT NOTES
What is an impact crater and how do they form?
An impact crater is basically a large hole in the ground caused by a meteorite
smashing into the surface of a planet or moon. Every solid body in the Solar System
suffers from the effects of impact cratering, and you can see this when you look at our
own Moon through binoculars. The projectiles (i.e. meteorites) that cause impact
craters travel at very high speed, faster than anything we see in our everyday lives.
They travel so fast that they are vaporised upon impact, and very little is left of the
projectile after the impact event has taken place.
What do impact craters look like?
Impact craters are normally circular features, and range in size from the microscopic to
the very large (>2000km diameter). The size and shape of an impact crater depend on
the size of the projectile, its velocity and the type of material that it hits. Small impact
craters (like Meteor Crater, Arizona) have simple shapes like a bowl. Larger craters
have a different shape (see Figure 1). They have mountains in the centre of the crater
called a central peak and huge landslides occur on the inside of the crater to form what
we call terraces. When an impact occurs, lots of material is thrown out around the
crater, and this material is called ejecta. Ejecta is made up of rock and soil from the
surface, and sometimes forms bright rays (you can see this around the bright crater
Tycho on the Moon).
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Figure 1: Some features of an impact crater.
What can impact craters tell us?
1. The age of a surface.
The only way to be certain about the age of the surface of another planet is to get a
sample of rock from that surface and date it in a laboratory on Earth. Unfortunately, this
is not yet possible for most of the planets and moons in our Solar System. To date, it
has only been achieved with rocks from the Earth, Moon and a bit of Mars, so we have
to use other techniques to work out the age of most planetary surfaces. Impact craters
can give us an idea of whether a surface is old or not simply by counting the number of
craters in a certain area. The older a surface is, the more impact craters it will have.
Question 1: Look at Figures 2a and 2b, which show two different regions on Mars.
Which area is older and why? Why would two areas on Mars have different numbers of
craters - what could have caused their difference in age?
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2. The geology of an area.
The shapes of impact craters can vary due to the type of material they form in. A solid
surface will have different looking impact craters to those in a softer, powdery surface.
The presence of water can also affect the way a crater looks and by studying the shape
and appearance of impact craters from photographs, scientists can get some idea of
what the area is like.
Question 2: Look at Figure 3, which shows pictures of two different Martian craters.
Label the different features you see on each, using Figure 1 as a guide. Name some of
the differences between two craters. Can you suggest what may be present under the
surface of crater 3b that is not present under crater 3a?
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What happens to a crater after it forms can also tell us something about the area it lies
in. For example, if a crater has formed on very loose material, then the winds on Mars
will erode the crater quite quickly. Other craters on Mars look as though they have
been in the middle of a river of flowing water (they have a streamlined appearance,
similar to that of small islands in rivers on Earth).
Impact Craters (Student Notes)
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3. Rocks below the surface
When an impact crater forms, it throws out material in a particular way. The ground is
‘flipped upside down’ so that the layers from deep in the ground end up being on top
(you will see this in your experiment). This means that by looking at the ejecta of
craters, you are looking at the material that is present below the surface. This is
interesting for scientists who want to know about the history of Mars. Digging down into
the ground is a little like going back in time. Events that happened long ago may have
been covered over by later events such as lava flows. Only by probing into the surface
will we be able to see what the surface was like billions of years ago. Perhaps some
rocks will be able to tell us what the atmosphere was like, or contain fossils of early life.
But to find these things out we need rock and soil samples from that time, and
sometimes that can only come from deep down. Impact craters give us a chance of
finding out about the rocks deep below the surface without the need to land there. This
is one of the reasons impact craters are so important to study.
Question 3: Write a summary of how impact craters can help you understand more
about an area. If you were selecting a site on Mars for a human landing, what kind of
area would you like to go to and how could impact craters help you decide where to go?
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GLOSSARY
CENTRAL PEAK - a mountain formed in the centre of large impact craters. These do
not occur in small impact craters.
EJECTA - a sheet of material (rocks, soil) surrounding the crater that was thrown out
during the impact event.
ERODE - the process by which something is worn away over time.
FLOOR - the floor of an impact crater (usually either flat or bowl-shaped), lying below
the surrounding ground level.
IMPACT CRATER - a hole in the ground (usually circular) caused by the impact of a
meteorite onto the surface of a solid body.
METEORITE - a lump of rock or ice that hits the surface of a planetary body.
PROJECTILE - an object that strikes a surface to produce an impact crater. These can
include dust particles, pebbles, rocks, comets and asteroids.
RAISED RIM - this is the edge of the impact crater and is composed of rocks and
material piled up on top of one another.
RAYS - bright lines of ejecta coming from an impact crater. These can extend for vast
distances in the case of large impacts.
TERRACED WALLS - these only occur in large impact craters. After the crater forms,
the walls of the crater slip, creating huge landslides inside the crater.
Impact Craters (Student Notes)
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IMPACT CRATERING
Figure 2a
Impact Craters (Student Notes)
Figure 2b
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IMPACT CRATERING
Figure 3a
Impact Craters (Student Notes)
Figure 3b
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IMPACT CRATERING – ACTIVITY SHEETS
This experiment is a slightly modified transcription of the web pages of the Hawai’i
Space Grant College. You may find more information about the experiment from their
pages (http://imina.soest.hawaii.edu/SPACEGRANT/class_acts/CrateringDoc.html).
Aims and objectives
To investigate the way the diameter of a projectile affects the final form of an impact
crater. To compare the results from this activity with real craters observed on the Moon
and Mars.
Apparatus (for each group)
1 large high-sided bowl (~30cm diameter)
2 bags of flour
1 pot of dry powder paint (not white!)
1 flour sifter
1 sieve (optional)
projectile (i.e. large gobstopper or projectile)
1 ruler
1 compass
1 pair of tweezers
1 spoon
data tables/paper/pens/graph paper
Aprons and old newspaper/table
covering
Experiment, observations and measurement
1. Preparation
-
-
Put on your apron and roll up your sleeves - this could get messy! Put down some
old newspaper or a table covering to catch any overspill from the bowl.
Put an even layer of flour in the high-sided bowl to a depth of approximately 6cm.
Cover with a thin layer of dry coloured powder paint (any colour will do as long as it
contrasts well with the flour underneath. A flour sifter often helps at this point to get
an even layering).
Measure the diameter of your projectile, and record this on your data chart.
2. The Experiment
-
Take the projectile and drop it onto the prepared surface from a height of 30cm.
Make a note of the diameter of the crater, the number of rays and their length. The
rays will be visible as white streaks radiating from the crater. Measure the length
each ray and take an average value. Record all information in your data table.
Record any other observations you make about the crater.
Repeat the above, this time dropping your projectile from 60cm, 90cm and 2 metres.
Remember to record all information in your data tables.
Data analysis
1. Plot graphs using the data you recorded in your tables. Plot Graph 1 as the crater
diameter vs. projectile height and Graph 2 as average ray length vs. projectile
height.
Impact Craters (Activity Sheets)
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2. Explain what each graph tells you about the relationship between the projectile and
the crater.
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3. If the projectile was dropped from a height of 6m, estimate the size of the final crater
from your experimental data.
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4. Describe and discuss where possible errors could occur in your experiment. In
particular, how could your experimental set-up and procedure differ from those of
another group carrying out the same experiment with the same instructions?
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5. From your experimental observations, describe the appearance of an impact crater.
Give reasons why your craters differ from the real craters observed on the Moon and
Mars.
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Impact Craters (Activity Sheets)
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IMPACT CRATERING DATA TABLE
Sheet No. _____ of _____
Class:
Name:
Date:
Other members of group:
Projectile diameter (mm):
Trial
No:
Height
(cm)
Diameter
of crater
(mm)
Impact Craters (Activity Sheets)
No. of
rays
Length of
Ray 1
Length of
Ray 2
Length of
Ray 3
Length of
Ray 4
Length of
Ray 5
Length of
Ray 6
Average
length
Notes
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