Voyage: A Journey Through Our
Solar System
Grades 5-8
Lesson 10: Impact Craters:
A Look at the Past
On October 17, 2001, a one to ten billion scale model of the Solar System was
permanently installed on the National Mall in Washington, DC. The Voyage exhibition stretches nearly half a mile from the National Air and Space Museum to
the Smithsonian’s Castle Building. Voyage is a celebration of what we know
of Earth’s place in space and our ability to explore beyond the confines of this
tiny world. It is a celebration worthy of the National Mall. Take the Voyage at
www.voyageonline.org, and consider a Voyage exhibition for permanent installation
in your own community.
This lesson is one of many grade K-12 lessons developed to bring the Voyage experience to classrooms across the nation through the Journey through the Universe program. Journey through the Universe takes entire communities to the space frontier.
Voyage and Journey through the Universe are programs of the National Center for Earth
and Space Science Education, Universities Space Research Association (www.usra.
edu). The Voyage Exhibition on the National Mall was developed by Challenger
Center for Space Science Education, the Smithsonian Institution, and NASA.
Copyright June 2006
JOURNEY THROUGH THE UNIVERSE
Lesson 10: Impact Craters:
A Look at the Past
Lesson at a Glance
Lesson Overview
In this lesson, students discover that it is possible to learn a lot about
objects in the Solar System and their history, as well as the history
of the entire Solar System, just from looking at the craters on their
surfaces. Students simulate how impact craters are formed, and how
craters can look different based on the amount of energy they had
during impact. Students then examine pictures of cratered surfaces
of other worlds in the Solar System and discover that impact craters
can provide a lot of information about a world’s history, and the
history of the entire Solar System.
Lesson Duration
Two 45-minute class periods
Education Standards
National Science Education Standards
Core Standard
Standard D2: The earth processes we see today, including erosion,
movement of lithospheric plates and changes in atmospheric composition, are similar to those that occurred in the past. Earth history is
also influenced by occasional catastrophes, such as the impact of an
asteroid or comet.
Related Standard
Standard D1: Land forms are the result of a combination of constructive
and destructive forces. Constructive forces include crustal deformation,
volcanic eruption, and deposition of sediment, while destructive forces
include weathering and erosion.
AAAS Benchmarks for Science Literacy
Benchmark 4C2:
Some changes in the earth’s surface are abrupt (such as earthquakes and
volcanic eruptions) while other changes happen very slowly (such as
uplift and wearing down of mountains). The earth’s surface is shaped
in part by the motion of water and wind over very long times, which
act to level mountain ranges.
JOURNEY THROUGH THE UNIVERSE
Essential Question
w
What can craters tell us about the history of objects in the
Solar System?
Concepts
Students will learn the following concepts:
w Impact craters are depressions or pits on the ground formed
by impacts by objects falling from space.
w The surface of the Moon has more impact craters than the Earth
because there are processes on the Earth that erase and retard
this type of cratering.
w Scientists can tell a lot about an object simply by studying the
craters on the surface of objects.
Objectives
Students will be able to do the following:
w Examine impact craters and determine some properties of the
object which created it.
w Decipher the history of objects in the Solar System based on
the nature of its craters.
Impact Craters:
A Look at the Past
JOURNEY THROUGH THE UNIVERSE
Science Overview
What is an Impact Crater?
Impact craters are geologic structures formed when a meteoroid, asteroid, or comet smashes into a planet, a moon, or another Solar System
object with a solid surface. A meteoroid is a piece of stone-like or
metal-like debris that travels in outer space. Most meteoroids are no
bigger than a pebble. Large meteoroids are believed to come from the
asteroid belt. This is a large belt of objects that are in orbit around the
Sun between Mars and Jupiter. Some of the smaller meteoroids may
have come from the Moon or Mars. When a meteoroid hits the Earth’s
surface, it is called a meteoritre. Comets are sometimes called dirty
snowballs or “icy mudballs.” They are a mixture of ices (both water
and frozen gases) and dust that for some reason did not get incorporated into planets when the Solar System was formed. Most comets
orbit the Sun in a path that comes close to the Sun and then travel far
beyond the orbit of Pluto.
All the inner planets (Mercury, Venus, Earth, and Mars) in the Solar
System have been heavily bombarded by meteoroids throughout their
history. This may also be true of Pluto (currently the outermost defined
planet), since it is a solid planet like the inner ones, even though we do
not know for sure at present time, since there are not sufficiently good
pictures of Pluto to see features on its surface. Craters can also be found
on the moons, asteroids, and possibly even on comets. It appears that
most objects in the Solar System with a solid surface show evidence for
meteoroid bombardment at some point in their history.
On Earth craters are continually erased by erosion as well as by volcanic
resurfacing and tectonic activity. Thus only about 160 terrestrial impact
craters have been recognized, the majority in geologically stable areas of
North America, Europe, southern Africa, and Australia, where most exploration has taken place to date. Spacecraft orbital imagery has helped
to identify structures in more remote locations for further investigation
in the future. As an example, scientists found the Chicxulub crater
near Cancun in Mexico using seismic monitoring equipment designed
to search for oil. Much of the crater lies under the ocean, and all of it
is hidden under 65 million years’ worth (or about 1 km; 0.6 miles) of
sediment. The crater is estimated to be 145-180 km (90-110 miles) wide.
The Chicxulub impact is thought to have triggered a mass dinosaur
die-off, through massive, long-term environmental changes. However,
it is rare that impacts occur on Earth. The meteoroid would need to be
large enough not to be vaporized as it entered Earth’s atmosphere. In
addition, the orbit of a meteoroid would need to cross the Earth’s orbit
at the same time as the Earth is at that same position.
JOURNEY THROUGH THE UNIVERSE
Crater Parts
When an object slams into a planet, a moon, or another solid body in
the Solar System, it hits the surface very hard and causes the energy of
the impacting object to be transmitted to the ground by a shock wave,
and much of the object vaporizes during the process. The object that
hits the planet is called an impactor. They create a shock wave; also,
the way a crater is created depends on the size of the impactor—small
impactors do not involve shock waves and a creation of a great crater;
rather, they excavate a crater only slightly larger than the impactor; the
sequence here describes a very high-velocity impact, where the impactor has not been slowed down much by the atmosphere. The shock
waves expanding from the point of impact cause rocks and dust to fly
away from the impact side and excavate a crater. The resulting impact
forms a usually circular depression in the ground, which is called a
crater. The base of a crater is called the floor, whereas the sides are
called the wall. At the top of the crater wall is the crater’s rim. When
a meteoroid strikes a planet, debris is typically ejected from the site of
the impact. This debris is called ejecta. Sometimes rays can be seen
surrounding a crater. The ray system is created by fine ejecta coming
from the crater (and not all ejecta), and only occurs on objects that do
not have a significant atmosphere. See Fig. 1 for an illustration of the
parts of the crater.
Walls – The sides of the crater bowl. Walls can be very deep,
depending on the severity of the impact. If a crater has shallow
walls, then the crater may have been filled or eroded somehow
since its formation.
Floor – The bottom part of the impact site. It may be the shape
of a bowl, or it may be flat. This part is lower than the surrounding surface.
Rim – The edge of the crater; the rim is usually the highest
part of the crater.
Ejecta – The debris that shoots, or ejects, out of the impact site
when the crater forms. There is a lot of ejecta close to the crater,
so the layer of eject is thick there. The ejecta gets thinner the
farther away it is from the crater. The impact creates debris as
the shock wave crushes, heats, and melts the rock.
Rays – The bright streaks that start at the rim of the crater and
extend outward. Rays are created by fine ejecta coming from
the crater and are only found on worlds where there is no significant atmosphere (e.g., on the Moon but not on Earth).
Impact Craters:
A Look at the Past
Lesson at a Glance
Science Overview
Conducting the
Lesson
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JOURNEY THROUGH THE UNIVERSE
Central Peak – A small mountain that may form at the center
of the crater in reaction to the force of the impact. Only large
craters can have a central peak. The size at which craters can
have central peaks depends on the size of the world. For example, on the Earth, craters larger than 2-4 km (1.2-2.5 miles)
can have central peaks, while on the Moon, the crater must be
larger than 15-20 km (9-12 miles) in diameter.
What Changes the Shape of a Crater?
Initially, craters have a crisp rim and blankets of ejecta around the sides.
On the Earth, actions of wind, water, lava flows, and plate tectonics can
alter the appearance of a crater. Wind can blow away debris around
the crater. Rivers and floods can erode the crater’s walls and rim, or
the crater may be filled with water to form a lake. Lava flows can fill
in the crater and make the rim smoother. Another impactor may come
along and create a new crater inside an old crater. Other impactors can
partially or completely destroy an older crater. Some of these effects
can also operate on other worlds in the Solar System, depending on the
properties of the world. For example, there is no wind or water on the
Moon to erode the craters formed there, but one can see craters on the
Moon that have been filled with lava, or see craters where new craters
have been created on top of old ones.
Craters and Surface Age
The older a surface is, the more time impactors have had to hit it. Really
old surfaces have so many craters that it would be difficult to notice if
another impactor hit it. Little of the surface is smooth. Most cratering
took place right after the planets and moons formed, when there was
a lot of debris left over from the formation of planets and other large
objects in the Solar System. Places like the Earth’s Moon and the planet
Mercury have heavily cratered, old surfaces.
However, the situation is a bit more complicated if the object has a substantial atmosphere. If an atmosphere were present, most of the small
meteoroids would “burn up” before hitting the surface. The friction of
the atmosphere heats up the objects flying through it, and this process
completely destroys most of the smaller meteoroids. In addition, there
are other factors that can influence the number and the type of craters
on an object. If there is a substantial atmosphere, there are probably
winds that would tend to erode craters or fill them with dust (like on
Earth or Mars). If the object has water, volcanoes, or is subject to the
action of plate tectonics, the craters would have been covered with
water and sediment, covered with dust and rock, or destroyed by the
movement of the surface, respectively.
JOURNEY THROUGH THE UNIVERSE
Impact Craters:
A Look at the Past
Lesson at a Glance
rays
walls
Science Overview
central
peak
Conducting the
Lesson
floor
rim
ejecta
Figure 1.(Image credit: Part of Apollo 17 Metric photograph AS17-2923.)
The amount of energy involved in the impact depends upon the size
of the impactor and the speed with which it hits the surface. The larger
the impactor (more energy) the larger the crater. In addition, for two
impactors of equal size, the crater would be larger for the one that hit
with the greater speed (more energy).
Craters on the Moon range in size from over 300 km (190 miles) in diameter (larger than the state of Connecticut) to smaller than the head
of a pin. Impact structures greater than 300 km (190 miles) are called
impact basins, rather than craters, the dark regions of the Moon called
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JOURNEY THROUGH THE UNIVERSE
maria, are examples of these large impact basins. Because there is no
weather on the Moon, and little geologic activity, the only way to erode
an existing crater is to cover it with more impacts or more debris from
impacts. Young craters have sharp rims and are relatively deep. Older,
more worn craters are usually shallow and have less distinct rims than
newer craters. Scientists can estimate the relative ages of portions of
the Moon by counting the number of impact craters present. The more
craters, the longer the surface has been exposed to bombardment by
meteoroids. Look at the Moon through a pair of binoculars, and you
will see many craters. In fact, it is hard to find any place on the Moon
where there are not any craters.
Impacts on Earth
As stated previously, craters are formed when a large meteoroid, asteroid or comet smashes into a planet or its moon. Earth’s atmosphere
protects us from most small objects that enter it. However, once in a
while, a large object collides with Earth. The Chicxulub crater (mentioned earlier) was formed when a meteoroid hit Earth near Cancun,
Mexico, 65 million years ago. Another example is the Meteor Crater
(also known as Barringer Crater) in Arizona that was formed by another meteoroid collision about 50,000 years ago. These large collisions
happen only rarely. However, it is important for astronomers to track
large meteoroids, because a large impact could be catastrophic to life on
Earth. There are many astronomers around the world that are looking
for objects that may pass near the Earth in the future.
Impacts on Other Worlds
In 1994, comet Shoemaker-Levy 9 crashed into Jupiter, creating disturbances in its atmosphere that were visible for months afterwards.
The comet’s orbit had been disturbed when it got too close to Jupiter
to resist its gravitational pull. In fact, the pull was so strong that the
comet was split into at least 21 discernible fragments two years before
it plunged into Jupiter’s atmosphere. The largest of these fragments
was 4 km (2.5 miles) in diameter, and upon arrival produced an explosion the equivalent of 6,000,000 megatons of TNT, or about 75 times the
estimated nuclear arsenal of the entire world during the height of the
Cold War. This was the first time scientists were able to witness such
a collision in the Solar System. Many available instruments on Earth
and in space (Hubble Space Telescope, Galileo spacecraft) were used
to capture images of this amazing event. Since Jupiter does not have
a solid surface, no crater was produced. However, it is an example of
an impact on another world.
JOURNEY THROUGH THE UNIVERSE
By studying the properties of the impact craters on different worlds (or
different parts of a world), you can deduce something about the history of that object, or even about the history of the whole Solar System
(such as there was a lot of debris left over from the formation of the
Solar System, giving rise to the heavy meteoroid bombardment early
in Solar System’s history).
Impact Craters:
A Look at the Past
Lesson at a Glance
Science Overview
Conducting the
Lesson
Resources
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JOURNEY THROUGH THE UNIVERSE
Conducting the Lesson
Warm-Up & Pre-Assessment
Teachers will lead a discussion with the students about why there
are hardly any craters easily visible on Earth, but many seen on the
Moon.
Teacher Materials
w
w
Picture of the Moon and the Earth found
in the back of the lesson
Overhead projector
Preparation & Procedures
1. Make an overhead transparency of the picture of the Moon and the
Earth found in the back of the lesson. Project this onto a screen for
the entire class to see.
2. Ask students to list ways in which the Moon looks different from
the Earth. Some examples may be the lack of colors on the Moon,
the lack of clouds on the Moon, and that the Moon has many craters.
Make sure students understand what impact craters are.
3. Lead a discussion with the students as to why the two worlds look
so different, even though they are located at the same distance from
the Sun in the Solar System. Focus the discussion on craters. Ask
students to hypothesize why there are craters on the surface of the
Moon, but few visible on the surface of Earth.
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Notes:
Impact Craters:
A Look at the Past
Lesson at a Glance
Science Overview
Conducting the
Lesson
Warm Up &
Pre-Assessment
Activity 1:
Creating Craters
Activity 2:
Craters in the
Solar System
Lesson Wrap-Up
Resources
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JOURNEY THROUGH THE UNIVERSE
Activity 1: Creating Craters
In this activity, students will simulate crater impacts by dropping
pebbles or marbles into a pan of flour and cocoa. Students will identify
the characteristics of impact craters and compare them to the picture
of a lunar crater.
Student Materials (per pair of students)
w
w
w
w
w
w
w
w
w
w
w
w
Student Worksheet 1
Lunar Crater Image
1 pie pan (approximately 9 in)
Two pebbles or marbles of different sizes
Scale to measure mass of marbles
A bag of flour or a box of corn starch
Metric ruler
Sifter
Newspaper to cover the work surface
Powdered cocoa (about one sandwich-sized plastic bag full)
Meter stick
Calculator (optional)
Preparation & Procedures
1. Make copies of Student Worksheet 1 and the Lunar Crater Image
found in the back of the lesson.
2. Discuss with the class what an impact crater is and how it is formed.
You can use the following questions to lead your discussion:
There are a lot of pieces of rock floating around in space—do they
ever hit anything? (Desired answer: yes) What evidence do we have
that this has happened? (Desired answer: we see impact craters, for
example on the Moon) What happens to the piece of rock (we call
this the impactor) that hits the Moon when it impacts? (Desired answer: it can hit with such a speed that it is vaporized) Does everything
hit the Moon with the same amount of energy? (Desired answer:
probably not) What would make something hit the Moon with a
different amount of energy? (Desired answer: if the impactor was
bigger or moving faster it might hit with more energy) Years later, if we
have no remnants of the impactor, but only the impact crater, how
could we tell the amount of energy that impactor had when it hit
the Moon? (Desired answer: the impact crater might look different if it
has been hit with different impactors with varying energies) How could
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we investigate what those impact craters might look like? (Desired
answer: we can create a model and create impact craters, and examine
them to see what they look like based on different kinds of impacts with
varying energy) That’s exactly what we’re going to do!
3. Divide students into pairs and pass out
Student Worksheet 1 and the student
materials.
4. When students have completed the
activity, give them a copy of the Lunar
Crater Image so that they can answer
the questions on their worksheet.
Impact Craters:
A Look at the Past
Teaching Tip
If corn starch is used instead of
flour in this activity it will store longer and can be used again.
Reflection & Discussion
1. Discuss the features of impact craters with students. Discuss with
students the formation of impact craters and their features (rays,
rim, ejecta, central peak).
2. Ask students to discuss how their craters looked different based
on the amount of energy the impactor had when it hit.
3. Ask students to pretend that they are scientists who are seeing a
crater on the Moon or another world for the very first time. Can they
tell what the impactor was like based on what the crater looks like?
(Desired answer: not completely, because the size of the crater depends on
the velocity and the size of the impactor, not just the mass. However, as a
general rule, the larger and more massive the impactor, the larger the crater. Therefore, scientists can determine the
approximate size of impactors based
on crater size.) Discuss with
Teaching Tip
the students the difference
between size (diameter,
What students do in this activity is diffor example) and mass.
ferent than real cratering events in some ways.
Both are varied simulWhen a crater is formed on a planetary surface, the
taneously in their exenergy of the impactor can break apart—even vaporperiment, but they
ize—the impactor. In addition, the energy of a crater is
are actually different
determined by its mass and its velocity when it reaches
factors. Crater size,
Earth, but in this activity students calculate the potential
however, depends on
energy from the height dropped. You may want to
both mass and size.
make sure the students understand the differences
between their model and real impact craters
at the end of the activity.
Lesson at a Glance
Science Overview
Conducting the
Lesson
Warm Up &
Pre-Assessment
Activity 1:
Creating Craters
Activity 2:
Craters in the
Solar System
Lesson Wrap-Up
Resources
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Assessment Criteria for Activity 1
4 Points
w Student completed Student Worksheet 1 and gave appropriate reasons for
the answers to the questions.
w Student answered the two Transfer of Knowledge questions correctly and
gave appropriate reasons for the answers.
3 Points
w Student completed most of Student Worksheet 1 and gave appropriate
reasons for the answers to the questions.
w Student answered the one Transfer of Knowledge question correctly and gave
appropriate reasons for both answers (this places emphasis on supporting
conclusions, no matter whether correct or incorrect).
2 Points
w Student completed some of Student Worksheet 1 and gave appropriate
reasons for the answers to the questions.
w Student answered the two Transfer of Knowledge questions and gave appropriate reasons for their answers, even if the answers were incorrect.
1 Point
w Student completed some or all of Student Worksheet 1, but did not give appropriate reasons for the answers to the questions.
w Student answered the two Transfer of Knowledge questions correctly, but did not
give appropriate reasons for their answers.
0 Points
w No work completed.
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Transfer of Knowledge
Have students complete the Transfer of Knowledge section in Student
Worksheet 1. Here, they are asked to compare two impactors that
caused two craters on the Moon.
Impact Craters:
A Look at the Past
Placing the Activity Within the Lesson
Ask student to blow gently on their impact craters (or ask them to
imagine what would happen if they were to blow on their impact craters). What happened? (Desired answer: they erode away) Ask students
to think about why we can see impact craters on the Moon if they are
so easily destroyed. Ask them to think about what sorts of conditions
may happen on other worlds that would help to erase impact craters
that once were there. This activity helped students understand the
nature of impact craters, but in the next activity, they will be looking at
pictures of surfaces on other worlds, and they will be asked why some
impact craters are still around while others have disappeared.
Extensions
Have students create a graph of Energy vs. Size of Impact Crater. Help
them analyze it to see that the correlation is positive. You can also
create graphs of impactor mass vs. impact crater or impactor size vs.
impact crater.
Notes on Activity 1:
Lesson at a Glance
Science Overview
Conducting the
Lesson
Warm Up &
Pre-Assessment
Activity 1:
Creating Craters
Activity 2:
Craters in the
Solar System
Lesson Wrap-Up
Resources
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Activity 2: Craters in the Solar System
In this activity, students will examine impact craters on different worlds
in the Solar System and discover that the craters can tell us a lot about
the world on which they formed.
Student Materials (per pair of students)
w
Student Worksheet 2
Preparation & Procedures
1. Ask students to recall the Warm-Up & Pre-Assessment activity
where they looked at the Moon and the Earth and compared the
two worlds. The Moon clearly has more craters on its surface than
the Earth. Ask students why they think this is; does it make sense
that things hit the Moon more than the Earth, even though they
are located at approximately the same place in the Solar System?
(Desired Answer: no; some students may think that things actually do
hit the Moon more often than the Earth. Ask them to explain why this
would be the case and to back up their answer. Then move on and ask for
other possibilities.)
Are there any other explanations as to why there are not as many
craters on the Earth as on the Moon? Is there anything that the
Earth has that would protect itself from getting hit that the Moon
does not have? (Desired answer: an atmosphere).
In fact, many smaller objects burn up in Earth’s atmosphere due to
friction with air and they never reach the ground to make a crater.
For this reason, do you think the craters that do appear on Earth
are very big or very small? (Desired answer: they are very big, because
all of the very small things that hit the Earth burn up in its atmosphere,
while big objects that hit the Earth may burn partially, but they are big
enough to make it through the atmosphere to the ground.)
Is there anything that the Earth has that will not let craters stick
around that the Moon does not have? (Desired answer: Earth has
weather that can erode the craters, like wind and rain, as well as lakes
and oceans, volcanoes, and plate tectonics)
What other reasons could there be for why there are not as many
craters on Earth? (Desired answer: Earth has oceans; if something hits
the ocean, a crater will not be visible on the surface but only on the ocean
floor, which may quickly be eroded away by the water)
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2. Tell students that scientists describe surfaces like Earth as “young”
and surfaces like the Moon as “old.” Why do they think this is the
case? (Desired answer: the craters on the Moon have been around for a
long time, because there is very little there to erase them or to “renew”
the surface. The Earth’s surface gets renewed all the time because of the
activity on the surface; the craters on the Earth are probably young, because
the older ones have been erased, or they are in regions where significant
erosion has not happened for one reason or another.) Some worlds may
have very young surfaces, some may be very old. Some worlds
may have parts that are one or the other.
3. Pass out Student Worksheet 2 to each pair of students, and tell them
that they will be looking at cratering on other worlds in the Solar
System, and that they will have to come up with conclusions about
what those worlds are like, and whether they have old or young
surfaces.
4. After the discussion below, present students with the Transfer of
Knowledge question and ask them to write down their answers.
Impact Craters:
A Look at the Past
Lesson at a Glance
Science Overview
Conducting the
Lesson
Warm Up &
Pre-Assessment
Activity 1:
Creating Craters
Reflection & Discussion
1. Discuss with students the possibility of an impactor hitting Earth.
Most students are interested in the dangers of this, and you can
use the Science Overview for details.
2. Discuss why the students have done this activity in terms of “relative age” instead of absolute age. We cannot know the absolute age
of these occurrences or surfaces, unless we go on the locations and
test them with experiments that can determine the ages of rocks. If
we stay on Earth and look at these surfaces from a distance, relative
ages are the only way to measure age.
Transfer of Knowledge
Have students complete the Transfer of Knowledge section in Student
Worksheet 2. Here, they are asked to put four surfaces in order by
age.
Activity 2:
Craters in the
Solar System
Lesson Wrap-Up
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Assessment Criteria for Activity 2
4 Points
w Student completes Student Worksheet 2 correctly and justifies his or her answers.
w Student completes Transfer of Knowledge correctly and justifies his or her answer.
3 Points
w Student completes Student Worksheet 2 and justifies his or her answers.
w Student completes Transfer of Knowledge and justifies his or her answer.
2 Points
w Student completes some of Student Worksheet 2 and justifies his or her answers.
w Student completes Transfer of Knowledge and justifies his or her answer.
1 Points
w Student completes some of Student Worksheet 2 and justifies his or her answers.
w Student completes some of Transfer of Knowledge correctly and justifies his or her answer.
0 Points
w No work completed.
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Placing the Activity Within the Lesson
Students have now looked at other worlds in the Solar System and
the nature of the impact craters on those worlds. Bring the discussion
again back to Earth and discuss why there are not very many visible
impact craters on Earth. Ask students to hypothesize whether there
were impact craters in the past, and whether Earth may change in the
future. Ask students, if we were to look for life on other worlds, would
we want to look at worlds with many or few impact craters? Why?
Notes on Activity 2:
Impact Craters:
A Look at the Past
Lesson at a Glance
Science Overview
Conducting the
Lesson
Warm Up &
Pre-Assessment
Activity 1:
Creating Craters
Activity 2:
Craters in the
Solar System
Lesson Wrap-Up
Resources
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Lesson Wrap-Up
Transfer of Knowledge for the Lesson
Students must use the knowledge they have accumulated in Activity
1 about the nature of craters, and the knowledge from Activity 2 about
what craters can tell you about the age of objects in the Solar System,
and decide the history of the surface in the Transfer of Knowledge Picture
found in the back of the lesson.
Lesson Closure
Discuss with students the Transfer of Knowledge for the Lesson. In Activity 2, students decided whether surfaces were relatively young or old,
but in this section, students had to decide the order of processes that
occurred on one surface. Discuss how critical this technique could be
in deciding what happened to an object in the Solar System long ago,
and the history of those objects. For example, whether their surfaces
went through a liquid phase a long time ago or a short while ago could
help determine the conditions that the entire Solar System (or at least
in the part of the Solar System where the object is or was located) was
in at those times. Scientists can compare objects in the Solar System
as pieces in a puzzle, and begin to put together the big picture of the
history of the Solar System. In this way, we have a sequence of events
at hand, just waiting for the determination of the absolute age of one
or a few events to be able to start to talk in absolute ages rather than
relative ages.
Extensions for the Lesson


Have students research the current theory of how the Moon was
formed—by a giant impact!
Have students research the extinction of the dinosaurs and
how scientists think that it was a massive impact that was the
cause of it.
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JOURNEY THROUGH THE UNIVERSE
Assessment Criteria for the Lesson
4 Points
 Student justified their answer for parts A and B in Transfer of
Knowledge for the Lesson.
 Student’s answers for Transfer of Knowledge for the Lesson were
both correct.
3 Points
 Student justified their answer for parts A and B in Transfer of
Knowledge for the Lesson.
 One of student’s answers for Transfer of Knowledge for the Lesson
was correct.
2 Points
 Student justified one of their answers for parts A and B in
Transfer of Knowledge for the Lesson.
 One of student’s answers for Transfer of Knowledge for the Lesson
was correct.
1 Point
 Student justified one of their answers for parts A and B in
Transfer of Knowledge for the Lesson.
 Student’s answers for Transfer of Knowledge for the Lesson were
incorrect.
0 Points
 No work completed.
Impact Craters:
A Look at the Past
Lesson at a Glance
Science Overview
Conducting the
Lesson
Warm Up &
Pre-Assessment
Activity 1:
Creating Craters
Activity 2:
Craters in the
Solar System
Lesson Wrap-Up
Resources
22
JOURNEY THROUGH THE UNIVERSE
Resources
Internet Resources & References
Student-Friendly Web Sites:
How Craters Age animation
clickworkers.arc.nasa.gov/training/how-craters-age.html
Interactive Map of Terrestrial Impact Craters
www.lpl.arizona.edu/SIC/impact_cratering/World_Craters_
Web/intromap.html
Teacher-Oriented Web Sites
American Association for the Advancement of Science Benchmarks
for Science Literacy
www.project2061.org/publications/bsl/online/bolintro.htm
Barringer Crater Web site
www.barringercrater.com/
Earth Impact Database
www.unb.ca/passc/ImpactDatabase/
National Science Education Standards
www.nap.edu/html/nses/
Terrestrial Impact Craters
www.solarviews.com//eng/tercrate.htm
Voyage Online
www.voyageonline.org/
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JOURNEY THROUGH THE UNIVERSE
Teacher Answer Key
Student Worksheet 1
Impact Table
Students’ answers in the impact table will vary. Make sure their measurements and calculations were correct based on their materials and
their data.
Impact Craters:
A Look at the Past
Lesson at a Glance
Questions & Conclusions
Questions about the experiment
1. There was an area where the marble hit the flour, which became
white because it blasted away the cocoa, leaving the flour exposed.
There were lines of white as well coming away from the middle.
2. The cocoa really helps to see how the surface is affected because
you can see that more area is affected than just the surface where
the marble hit.
3. The bigger the impactor, the bigger the crater (when dropped from
the same height).
4. The higher the impactor, the bigger the crater (when using the same
size impactor).
5. The energy is really the deciding factor in the size of the crater. No
matter what, the higher the energy, the larger the crater.
6. This may or may not be true depending on the size of the impactors
the students used. Look at the Impact Table to check the accuracy
of their answers.
Science Overview
Conducting the
Lesson
Resources
Internet Resources
& References
Teacher Answer
Key
24
JOURNEY THROUGH THE UNIVERSE
Questions about craters
1. It is fairly accurate, in that it has most of the features from the Lunar
Crater Image, except that it does not have a central peak.
2.
Walls
The walls are formed from the material that
was not blasted out when the impactor hits the
surface (and not from any indentation of the
impactor—the walls are much farther out than
the original impactor). They show the limit of
the material blasted out of the crater.
Floor
The floor is formed because the material above it
has been blasted up and out.
Rim
The rim is formed where the affected material
meets the undisturbed material.
Ejecta
The ejecta is formed from the material that was
blasted out from the crater when the impactor
hit.
Rays
The ejecta can form rays of material. (Note: In
real craters, rays should form when there is no
atmosphere, which is not the case in the experiment. Be lenient in grading this section because
rays may not form and therefore students may
not understand how they form.)
Central
Peak
The experiment did not form a central peak, but
it was probably formed in reaction to the force
of the impact.
Transfer of Knowledge
Impactor A hit the Moon with less energy than Impactor B. I can tell
this is true because, even though Impact Crater A may look deeper than
Impact Crater B (though this may also be just the effect of shadows
covering most of the floor of the smaller crater), Crater B is definitely
bigger than Crater A. From my experiment, I have found that the more
energy an impactor has, the bigger the crater it creates.
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JOURNEY THROUGH THE UNIVERSE
Student Worksheet 2
These answers are written with the assumption that the students’ only
evidence toward their conclusions are the pictures they see. You may
decide that students should have a more sophisticated understanding
of what is actually going on in the picture if they have past classroom
experience. Some leeway should be given to the students answers. If
they are basing their answers only on the pictures, a lot of conclusions
are possible without knowing anything else about the planet or its
moon(s).
1. Mars – There is no evidence of horizontal or vertical motion in any
of the photos. There are no folded mountains or places where parts
of the surface have moved in relation to other parts. If Mars had
volcanoes, they could cover older craters with dust and rocks. We
do not see any evidence of volcanoes in these photos, so we can
only offer that as a hypothesis. If Mars had an atmosphere, there
could be winds that have covered older craters (like the ghost craters) with dust or eroded parts of the crater. If Mars had water at
some time, the action of the water would have eroded older craters
(like the ghost craters). Since one of the photos shows fresh craters,
these must have been formed more recently than those in the other
two photos. It appears that dust moved by wind appears likely,
maybe some water too (though this is less clear)—for the others,
there is not enough evidence to suggest they have been factors.
2. Venus – There seems to be evidence of buckling of the surface in
some areas, but these could have been caused by other geologic
events besides plate tectonics, too. This action would destroy older
craters. There are carters on top of these areas so they must have
occurred after the buckling. If Venus had volcanoes, they could
cover older craters with lava. Some of the features could be old lava
flows. If Venus had an atmosphere, there could be winds that have
covered older craters with dust or removed parts of the crater. If
Venus had water at some time, the action of the water would have
eroded older craters. Some of the features could be old river flows.
There are several craters in the buckled area, so they occurred after
these areas were formed.
3. Ganymede and Callisto – Callisto appears to have only fresh
craters. This would indicate that there is or was no atmosphere
and, therefore, no wind. There is no evidence for volcanoes or
plate tectonics. There is also no evidence for presence of water in
the past or in the present. Ganymede appears to have a buckled
surface indicating it could be plate tectonics (but it could also be
Impact Craters:
A Look at the Past
Lesson at a Glance
Science Overview
Conducting the
Lesson
Resources
Internet Resources
& References
Teacher Answer Key
26
JOURNEY THROUGH THE UNIVERSE
caused by something else). This action would destroy older craters.
There are craters on top of these areas so they must have occurred
after the buckling. There seem to be channels that were cut by lava
or water (but we cannot tell which). There are some ghost craters
that could have become ghosts as the result of volcanic activity or
material that was shot up by other crater hits or by weather.
4. Mercury – No atmosphere or water. There are some ghost craters
that could have become ghosts as the result of volcanic activity or
material that was shot up by other crater hits or by weather. There
is a cut on the surface that goes through some older craters. [The
students would not know what caused this. It is believed that this
is the result of surface compression.] There is no evidence of plate
tectonics.
Transfer of Knowledge
1. Two of the most likely orders are: 2,3,4,1 or 2,3,1,4. Photos 2 and
3 show sharp crater rims. This would indicate that the craters in
these photos are younger than those in photos 4 and 1. In addition,
photo 3 could come before 2, if the object in 2 had some kind of
activity in the distant past (but which ceased at some point) that
would have obliterated older craters. Therefore, two other orders
could be 3,2,4,1 or 3,2,1,4. The order of the last two in each sequence
is in question. Both show weathering, but it would be difficult to
choose which was older. Again, this would depend on the activity
that had occurred on the object in the past.
2. Photo 1 could be Mercury or the Moon because there are many
craters ; it looks like the images of the Moon or Mercury shown
in this Activity and in Activity 1. This implies that there has been
little activity on either body to obliterate craters after their surfaces
solidified.
Photo 2 is Mars because it shows fairly fresh craters but not as many
as in photo 1.
Photo 3 is Ganymede or Venus because it looks like pictures of
their surfaces that were shown previously. Sharp features but no
weathering due to wind or water. Photo 4 is Mars because it looks
a lot like the NASA/Viking 1 Orbiter photo that was shown previously. They both show either water or lava flows.
3. No. The reasons are explained in the answer to question 1.
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JOURNEY THROUGH THE UNIVERSE
4. No. The characteristics (an atmosphere, oceans or other bodies
of water, weathering, volcanoes, and plate tectonics) noted at the
beginning of Activity 2 play a large part in determining the age of
the surface, but the planet may be much older than its surface. The
amount of cratering on a world does give an indication of the age
of the surface, but only of the surface.
Transfer of Knowledge Picture
Part A: Oldest to youngest: B, A, C. The river has eroded away the edge
of crater B, and you can see the river flowing through it, so it must have
gotten there before the river did. But crater C is not eroded away and
appears to be on top of the old riverbed, so it must have gotten there
after the river had dried up.
Part B: Answers will vary. The basic idea is that Mars went through a
history of cratering, with large craters first. Then river beds formed,
perhaps Mars warmed up and allowed rivers to flow on the surface.
After the river beds dried up, Mars continued to be hit and craters
were created.
Impact Craters:
A Look at the Past
Lesson at a Glance
Science Overview
Conducting the
Lesson
Resources
Internet Resources
& References
Teacher Answer Key
Warm-up & Pre-Assessment
Picture of Moon and Earth
Images from photojournal.jpl.nasa.gov
page 1 of 1
Student Worksheet 1
Creating Craters
Name ______________________________________________ Date ___________
Student Materials (Per pair of students)
w
w
w
w
w
w
w
w
w
w
1 pie pan
Two pebbles or marbles of different sizes
Scale
Bag of flour or box of cornstarch
Sifter
Metric ruler
Newspaper
Powdered cocoa
Meter stick
Calculator (optional)
Directions
1. Use the scale to measure the mass of your two impactors (pebble or marble) and record the data in the
Impact Table on the next page.
2. Cover your area of the floor with newspaper.
3. Fill a pie pan with a thick layer of flour. Smooth out the flour so that it is as flat as possible.
4. Cover the top of the flour with a light dusting of cocoa. Use a sifter if you have one available.
5. Place the pie pan on the floor or on the ground.
6. Place one end of the meter stick on the floor next to the pan and measure 30 cm above the pan.
7. Drop one of your impactors into the pan from the 30 cm height.
8. Remove the impactor, and repeat steps 5 and 6 with a different size impactor, creating a second impact
site in the pan so that it is not too close to the first impact. Remove the impactor, and draw a picture in
the Impact Table on the following page.
9. Measure the size of your impact craters and record their diameters in the Impact Table.
10. Smooth the flour, sift on a now layer of cocoa, and repeat steps 5-7 for impacts from heights of 20 cm and
10 cm. Calculate the amount of energy for each impact.
page 1 of 6
Impact Table
Height
Impactor Mass
Mass 1 _______
Mass 2 _______
Energy *
(E = mgh)
Size of crater
Draw a picture
30 cm
(0.3 m)
30 cm
(0.3 m)
20 cm
(0.2 m)
20 cm
(0.2 m)
10 cm
(0.1 m)
10 cm
(0.1 m)
* You need to calculate the amount of energy in the impactor when it is dropped toward the pie pan. The amount of energy
(E) is equal to the impactor mass (m) times the gravitational constant (g) times the height at which you dropped the impactor
(h). This energy corresponds to how much energy the impactor has at the time of impact, so it is a good way to characterize
how strong each impact is.
Energy (measured in joules) = mass (measured in kilograms) x gravitational constant x height (measured in meters)
Gravitational constant g = 9.8 m / s2
page 2 of 6
Questions & Conclusions
Questions about the experiment
1. How did the appearance of the surface of the flour change after it had been hit by the impactor?
2. What does the cocoa reveal about how impacts change the surface?
3. How does the size of the impactor affect the crater?
4. How does the height from which the impactor is released affect the crater?
5. How does the overall energy of the impactor affect the crater?
6. Are there any cases where a big impactor created a smaller crater than a small impactor, depending on where they were
dropped from?
page 3 of 6
Ask your teacher for the Lunar Crater Image and use it to help you answer the questions below.
Questions using Lunar Crater Image
1. Compared to the Lunar Crater Image, was your model an accurate representation of lunar craters? Explain why or why
not.
2. Look at the Lunar Crater Image. Examine each part of the image that is listed, and write a description about how each
feature occurs, based on your findings from this activity.
Walls
Floor
Rim
Ejecta
Rays
Central
Peak
page 4 of 6
Transfer of Knowledge
Let’s assume that a long, long time ago, Impactor A hit the Moon and created Impact Crater A. Around the same time,
Impactor B hit the Moon and created Impact Crater B. Now, scientists want to know something about the impactors that
created these craters, but the only thing they have to go on is what the impactors left behind—their craters. Look at the
picture below of Impact Craters A and B. Based on the impact craters, compare the differences between the two impactors
(A and B) that created these craters.
A
B
Impactor A hit the Moon with (more, less, the same amount of) energy than Impactor B had when it hit the Moon. I can tell
this is true because:
______________________________________________________________________________________________________________
______________________________________________________________________________________________________________
____________________________________________________________________________________________________________
page 5 of 6
Lunar Crater Image
2923.)
Crater Parts
Walls
The sides of the crater bowl. Walls can be very deep,
depending on the severity of the impact. They may
look like steps, or walls can be shallow. If a crater has
shallow walls, then the crater may have been filled or
eroded somehow since its formation.
rays
walls
central
peak
Floor
The bottom part of the impact site. It may be the shape
of a bowl, or it may be flat. This part is lower than the
surrounding surface.
Rim
The edge of the crater; the rim is usually the highest
part of the crater.
floor
rim
ejecta
Figure 1.(Image credit: Part of Apollo 17 Metric photograph AS17-
Ejecta
The debris that shoots, or ejects, out of the impact site
when the crater forms. There is a lot of ejecta close to
the crater, so the layer of ejecta is thick there. The ejecta
gets thinner the farther away it is from the crater. The
impact creates debris as the shock wave crushes, heats,
and melts the rock.
Rays
The bright streaks that start at the rim of the crater and
extend outward. Rays are created by fine ejecta coming
from the crater and are only found on worlds where
there is no significant atmosphere (e.g., on the Moon
but not on Earth.)
Central Peak
A small mountain that may form at the center of the
crater in reaction to the force of the impact. Only large
craters can have a central peak. The size at which craters can have central peaks depends on the size of the
world. For example, on the Earth, craters larger than
2-4 km (1.2-2.5 miles) can have central peaks, while on
the Moon, the crater must be larger than 15-20 km (9-12
miles) in diameter.
page 6 of 6
Student Worksheet 2:
The Older You Are...
Name ______________________________________________ Date ___________
If a surface is young, it may be because the world may have...
an atmosphere
oceans, rivers, or other bodies of water
weathering (wind, rain, etc.)
volcanoes
plate tectonics
1. Look at the three pictures below of the Martian surface. These pictures show how one planet can have
parts that are older than others. (Image credits: NASA/JPL/Malin Space Science Systems)
Figure 1. This picture shows fairly
fresh craters on Mars. Fresh craters usually have a sharp rim, an
ejecta blanket, and central peaks.
Notice that this part of Mars has
many impact craters.
Figure 2. This picture shows degraded craters on Mars that have no ejecta blanket,
the rim is eroded, and interior features are
gone.
Figure 3. This picture shows ghost craters on Mars that are faintly visible and
have been eroded away almost completely.
Ghost craters are those that one can still
barely see when looking at the surface of an
object, and that have probably been there
for a long time, getting more and more
faded away. Notice that this part of Mars
also does not have many impact craters.
page 1 of 5
Fresh craters usually have a sharp rim, an ejecta blanket, and central peaks (if they were created).
Degraded craters usually have no ejecta blanket, the rim is eroded, and interior features are gone.
2. Look at the picture below of Venus. Venus has mostly large craters, but numbers vary across
the surface. Venus has a number of fresh, degraded, and ghost craters, but also surfaces with
no craters whatsoever.
Figure 4. Venus. Image credit: NASA
From this picture and the description of Venus’s craters above, what can you conclude about the
characteristics of Venus that may result in these kinds of craters? Pick three characteristics from
above and explain your answer
page 2 of 5
3. Look at the picture below of two moons of Jupiter, Ganymede and Callisto. These Moons have
many craters of all kinds (fresh, degraded, and ghost) and sizes.
Figure 5. Ganymede
Image credit: NASA/JPL
Figure 6. Callisto
Image credit: NASA/JPL
From these pictures and the description of Ganymede’s and Callisto’s craters above, what can
you conclude about the characteristics of these moons that may result in these kinds of craters?
Pick three characteristics from above and explain your answer.
4. Look at the picture at the right of Mercury. This
planet has many craters covering its whole surface, and they are mostly fresh, although there
appears to be some ghost craters.
From this picture and the description of Merucy’s
craters above, what can you conclude about the
characteristics of this planet that may result in
these kinds of craters? Pick three characteristics
from above and explain your answer.
Figure 7. Mercury.
Image credit: NASA/JPL/Northwestern University
page 3 of 5
Transfer of Knowledge
1. Put the following cratered surfaces (numbered 1 through 4) in order from youngest to oldest.
2. What objects in Activity 1 or 2 might these be images of?
3. Can you be sure of the order you picked?
4. Does an older surface mean that the planet is older than other planets? Why or why not?
1
2
Image credit: NASA
Image credit: NASA/JPL/Northwestern University
4
3
Image credit: NASA/JPL
Image credit: NASA
page 4 of 5
Transfer of Knowledge Picture
The picture below is one of craters and river beds. Use it to answer the following questions.
A
C
B
Image credit: NASA/Viking 1 Orbiter
Part A. Put these objects (A, B, and C) in order of age, and explain why you believe this is the case.
Be sure to look at the crater rims to tell you if there has been any erosion.
Part B. Based on the picture above, write a description of the environment around Mars through
time. Explain your answer.
page 5 of 5