Planetary Surfaces: Top

PLANETARY CURRICULUM MODULE
SURFACES: FOCUS ON IMPACTS
Most physical science units introduce students to the processes of volcanism, tectonism, and gradation (the effects of water, ice, wind and gravity on a surface); few, however, introduce the process of impact cratering. Impact cratering plays a large role in forming and modifying planetary and satellite surfaces in our solar system. The following activities are designed to introduce this important surface process.
This module includes all the necessary background information on the topic of impact cratering as well as activities designed to introduce important concepts related to impact crater formation and subsequent modification. Several sections also include a link called Teacher Feature, a teacher designed section overview with ready-made worksheets for review, wrap-up, or concept activities. The teacher feature also has suggestions for modifying the activity for other grade levels.
Scientists have studied the surfaces of the planets and
satellites in our solar system for hundreds of years. Geologists first studied
the surface of the Earth, and when telescopes were invented, astronomers began
to look at the surfaces of the Moon and Mars, and later the other planets.
Today, we have sent unmanned space probes to all the planets except Pluto, we
have Earth-observing satellites, and we have even walked on the Moon and
brought back samples to study in our labs. All these explorations have allowed
us to compare surface features on other worlds to the terrestrial ones with
which we are the most familiar. Such comparisons are crucial for understanding
more about how the Earth and other planets formed, and how they may change in
the future.
This series of activities will describe how scientists study the surfaces of
our own and other planets. First, we will discuss how to locate features on
the surface of a spherical planet. Then, we'll talk about cratering, one of
the most important surface processes in the solar system. Analysis of craters
on the surfaces of planets can help scientists estimate how old the surface is,
what its composition is, and what agents of change are important on that body.
We'll discuss how craters are made, and what can remove them from the surface
of a planet. Then, we'll look in detail at some of the planets in the solar
system, and what we can tell about the history of a planet by examining its
craters. Finally, we'll apply our understanding of the inner solar system to
try to interpret some of the new images from the Galileo spacecraft currently
orbiting Jupiter.
Teacher Feature
Student Objectives:
Direct student attention to various planet surface features
Identify student's knowledge as it relates to Earth's features
Provide overview of how knowledge is collected
Background Information:
Lesson Format: Teacher lecture, Group work, Discussion
Introduction: Have each student list at least 5 surface features in his/her state
Class Activity: Here, There, & Everywhere?
Type: Group discussion
Materials Needed: Reference materials appropriate to age (State maps)
Procedures: Here
Let volunteers share state features orally. Be sure students only include geological surface features (i.e. not forests).
List on board or have several ready on cards.
Let tables select a feature & do a fast fact find on the discovery (i.e. who found, year, etc.) using references.
Let students share info.
There
Extend discussion on how distant features are discovered (explorers, observations, etc.).
Extend to moon, planets
Everywhere
Relate discussion to how we learn about processes forming the features (theory, experiment, comparisons, simulations)
Evaluations:
Have students summarize various surface features discussed, how info. was attained, and how we learn about other planetary features.
Let students answer orally or record in log format.
Other Activities, Misc. Information, etc.:
Given state maps and blank state outline, have students identify, draw, & label surface features
Possibly include addition of longitude/latitude link as intro. to next lesson
Research & report on the 7 Natural Wonders of the world. Illustrate them as postcards.
Research earliest observations/stories of the planets
I. LATITUDE AND LONGITUDE
Concepts:
Geography
Coordinate systems
Origin
Angles
2-d and 3-d geometry
In order to discuss features on the surfaces of planets, we first need a way to
describe where they're located. On the Earth, we use a system of
latitude and longitude, lines which divide up the spherical
surface of the Earth into a grid, based on angles.
Latitude represents how far north or south of the equator a point is. The
latitude of a point is the number of degrees in an angle made by the equator,
the center of the Earth, and the point. The latitude of Tucson [or insert
your city here], for example, is 32 degrees north, meaning that Tucson is
32 degrees north of the equator. The latitude of points on the equator is 0
degrees (north or south).
While with latitude, the obvious place on Earth from which to measure north or
south is the equator, there is no similarly obvious choice from which to
measure east or west. An imaginary line passing through Greenwich, England was
arbitrarily defined, and longitude represents how far east or west a point is
from this line. So Tucson, at longitude 111 degrees W, is therefore 111 degrees
west of Greenwich, England.
The origin of a coordinate system is the starting point for
measurements. The origin for the latitude-longitude coordinate system on Earth
is where the equator and the line through Greenwich, England, meet, at zero
degrees latitude, zero degrees longitude.
Interpretation:
Given a map or globe, find the origin (0 degrees,0 degrees).
Given a list of cities and their latitude and longitude, find them on a
globe.
What is the latitude of the north pole? the south pole? What is the
longitude of the north pole?
Trace a circle of constant latitude on a globe.
Q: Where is such a circle the largest?
A: It is largest at the equator.
Q: What is the radius of a circle of constant latitude at the equator?
A: It is the radius of the Earth.
Trace a circle of constant longitude.
Q: Where is such a circle largest?
A: They are all the same size.
Q: What points do all such circles go through?
A: They go through the north and south poles.
Such a circle on the surface of the Earth, which goes through the equator
or the north and south poles, is called a great circle.
Scientific context: A latitude-longitude system is fundamental for the
reliable location of surface features. With an origin, any place on a planet
can be located with only two numbers: how many degrees north or south of the
origin, and how many degrees east or west of the origin. The hard part is
choosing an origin. On other planets, the zero of longitude is chosen once the
surface of the planet has been mapped in enough detail to pick one. On some
planets, the origin is defined as the point on the equator known as the
"sub-Earth" point, which is the point on the surface that faces the Earth at
the time when the two planets are closest in their orbits. On other planets,
the choice is much more arbitrary. The most important criterion is that the
origin must be a point that is easy for everyone to find, whether it's the
center of a crater or some other obvious permanent geologic feature.
Introduction:
Using ABC/123 grid, have students draw their own maps, listing 5 features by name on cross lines.
You may need to give a list of formations from which to choose: mountains, plateaus, bodies of water by name, canyons, vocanoes, etc.
Class Activity: Find the Feature
Type: Mapping skills
Materials Needed:
Maps with longitude/latitude divisions appropriate for age (globe & world maps)
Procedures:
Using ABC/123 grid maps from intro. activity, have students exchange and give locations for each feature, using the grid. Discuss.
Relate to Earth's coordinate system
Have students locate cities by their longitude & latitude
Continue with interpretation questions as given using globe if possible
Let students suggest ways to decide a point of origin on planets
II. CRATER FORMATION, MODIFICATION, AND REMOVAL
Though fairly rare on the Earth, impact craters are one of the
most common, and therefore important, types of surface features in the solar
system. Craters are found on almost all the solid planets of the solar system,
but not on gas giants like Jupiter and Saturn since there is no solid surface
to preserve a record of the impact. All such impacts are governed by a set of
physical principles based on properties of the impacting body, the target body,
and the speed and angle of the impact. Craters are also affected by the
presence (or absence) of an atmosphere on a planetary body. A thick atmosphere
can cause smaller impacting bodies to burn up before impact, thus screening the
surface of craters caused by these smaller impactors.
A surface which is completely covered with craters is called saturated.
New craters on a saturated surface tend to cover older craters, so once a
surface becomes saturated with craters, the number of craters remains
approximately the same. Saturated surfaces are very old. Only geologically
inactive planetary bodies can become saturated, since on an active planet such
as the Earth, craters are quickly erased by agents of change such as
tectonics, volcanism, and erosion. Thus a saturated surface such as the Moon's
is a sign that the Moon is no longer geologically active, and regions with a
lower crater density are younger than those with a higher crater density. The
study of craters can provide much information about the history of planetary
bodies in our solar system.
This section will discuss in greater detail how craters are made, how they are
removed, and what can be learned from images of craters on the Earth and on
other planets.
IIA. HOW ARE CRATERS MADE?
Impact craters are made when an object or bolide impacts the surface of a
planet or satellite. A bolide is any falling body such as a comet or
meteorite. Many hands on impact simulation activities are available, and all
involve dropping a series of bolides, with different masses, onto a simulated
planetary surface. The planetary surface can be dry, for example made out of
flour with a dusting of cocoa powder, or wet, such as a muddy composite of dirt
or sand.
The composition of a planetary body and various factors relating to an
impact have a strong effect on the appearance of craters on the surface of a
planet. Even more important, however, are the many different agents of change
which serve to either remove craters from the surface of a planet, or preserve
them, over geologic time.
Scientific context: The most important factor in predicting how the
surface of a planet will look is the degree of geologic activity, or how
effective various agents of change are. The appearance of craters on the
surfaces of the terrestrial planets is an indication of how geologically active
the planets are. Since bolides impact all bodies in the solar system, a lack
of craters must be explained by past or current geologic activity. There are
many possible agents of change which could be responsible. Craters can be
removed by having other craters form over them, as on a saturated surface.
They can have lava flows bury them, or tectonic activity fracture them. They
can be filled in by dust, blown away by wind, or washed away by water. They
can even be obscured by vegetation. Using this indicator, we can rank the
planets from old, inactive bodies to young, geologically active worlds.
Oldest on this scale are bodies like Mercury and the Moon. These are
relatively small objects, with old, heavily cratered surfaces and little
evidence of subsequent activity which could have covered or partially obscured
their craters. Some of the large craters on the Moon, however, are filled in
with lava flows, evidence that the Moon was once active. Mercury and the Moon
are sometimes called "dead" bodies, because there is no evidence of current
geologic activity.
The in-between cases are Mars and Venus. Mars has some craters on its surface,
but also has other features like volcanoes and giant rift valleys, evidence
that the planet was once much more active than it is today. Some craters on
the surface seem to have been filled in with dust or eroded away, evidence that
while Mars' thin atmosphere is not very efficient, it does affect surface
features. There are relatively few craters on the surface of Venus, and most
seem to be preserved in pristine condition. There is also evidence of
volcanism and tectonic activity. Scientists have interpreted surface images as
indicating that Venus underwent a period of great geologic activity about 500
million years ago, which removed all older craters. Since then, however, the
planet has been relatively inactive, meaning that any craters that have
accumulated in the last 500 million years have been preserved in relatively
pristine form.
The other extreme is the Earth, which is a large, very active planet with very
few craters preserved on its surface (Meteor crater, in Northern AZ, is one of
the best-preserved.). Tectonics and volcanism are important processes on
Earth, but even more important are erosional processes caused by wind and
water. Earth is the only body with liquid surface water, which quickly washes
away most craters. It is no accident that one of the most well-preserved
craters on Earth is in a desert!
Much can be determined about the state of geologic activity on a planet merely
by examining craters and other features on its surface. A heavily cratered
surface (Mercury, Moon) indicates that the planet is not currently active, and
has not been active for (perhaps) billions of years. A non-saturated surface
is an indicator of past (Mars, Venus) or current (Earth) geologic or
atmospheric activity of some sort. Whatever the crater removal process, it is
important to understand that if the surface of a planet is not covered with
craters, there must be a reason!
Other Activities, Misc. Information, etc.:
- have students draw a cratered surface or various surfaces based on atmospheric densities. Switch and let observer determine atmospheric condition.
- teacher can prepare drawings noted above for younger children to match with atmospheres
CRATER ACROSTIC
Directions:
Enter the letters of the appropriate word in the spaces provided in the clue section. When all the clue words are done, enter the indicated letter from each clue in the numbered space in the mystery word.
This mystery word may determine whether small craters will exist on a planet or moon.
Moon 1: presence of atmosphere may limit number of small craters, probably has moderate number of craters (not saturated because of geologic activity, volcanoes and wind erosion), some craters may be filled in by lava as well as sand/dust carried by wind; crater edges may be weathered by wind; ice can be interpreted as part of the composition of the surface - lower strength means shallower craters, or may indicate seasonal surface water (erosion)
Moon 2: thick atmosphere probably limited number of craters, especially small ones (not saturated due to atmosphere); craters should be fairly pristine due to lack of geologic activity and weathering agent (no wind, water or ice)
Moon 3: moderate atmosphere probably limited number of small craters (not saturated due to atmosphere and agents of change present); extensive geologic activity in form of volcanoes and erosion (wind and lots of water) fills and erodes and maybe erases craters
Bonus Question: some ideas may include - proximity to asteroid belt (lots of impactors), life-form modification, mass of moon (extremes of gravity, landslides)
Evaluations:
Collect and check or use as discussion activity
Other Activities, Misc. Information, etc.:
- students may be allowed to use the chars constructed earlier
- students may work in groups, then present at the end of the period in a "science symposium"
- pictures may be drawn to illustrate conclusions
-have students defend others answers based on knowledge from previous lessons
LUNAR LINE UP
In the year 2056, one of NASA's deep space missions returned the following information about three satellites orbiting a gas giant planet in a distant solar system. From the information provided, predict how impact craters on the surface would look. List the overall appearance of the surface (few, some, many craters or saturated) and the appearance of individual craters (how modified are they?). Justify your predictions (use 'because of' or 'from' statements).
Data Set 1 Data Set 2 Data Set 3
composition rocky rocky rocky
atmosphere thin with wind thick no wind moderate
some wind
water some ice no water/no ice lots water
volcanoes some no lots
Open-Ended Bonus Question:
What other conditions might affect cratering on these bodies?
IIC. FOCUS ON THE GALILEAN SATELLITES
Concepts:
Geologic interpretation of images
Observing and recording data
Making comparisons
Drawing conclusions
The four largest satellites of Jupiter are referred to as the Galilean
satellites, since they were first discovered by
Galileo in 1610. These
satellites are too small and too far from the Earth to study their surfaces in
detail. The surfaces of Io, Europa, Ganymede and Callisto were first imaged at
high resolution by the Voyager spacecraft in 1979 and 1980, and are being
observed in greater detail by the Galileo spacecraft which will be in orbit
around Jupiter until December of 1997. Using the techniques discussed in the
previous section, we can examine geologic processes important on these bodies,
and determine the relative ages of their surfaces. This technique of comparing
newly-explored worlds to those with which we are more familiar is a common one
in science.
Activity:
Look at Galileo images of
Io, Europa, Ganymede and Callisto,
Compare them to the terrestrial planets you looked at in the previous activity.
Using the appearance of craters on the surfaces, and any other features you
can identify, try to rank the Galilean satellites in terms of geologic
activity.
Did you see any examples of weathering or erosion of craters?
Why or why not?
No. Weathering and erosion require wind and water, but the
Galilean satellites have no atmospheres and no liquid water on the
surface.
What are the main agents of change you see modifying craters on the
Galilean satellites?
Volcanism:
Io has sulfur volcanism, and Europa has possible water outflow.
Tectonism:
Europa and Ganymede have cracks and grooves etc. which imply tectonism.
Are there other differences between the craters in the Jovian system, and
those in the inner solar system?
Ganymede and Callisto have palimpsests (relaxed craters).
These satellites are made up of a large fraction of ice. The viscosity
of this ice is such that (over long periods of time) the ice flows and
fills in craters instead of preserving the hole.
(Relate this to the cratering exercise in section IIA
with respect to the composition of the target body.)
Background Information:
The four largest satellites of Jupiter are referred to as the Galilean satellites, since they were first discovered by Galileo in 1610. These satellites are too small and too far from the Earth to study their surfaces in detail. The surfaces of Io, Europa, Ganymede and Callisto were first imaged at high resolution by the Voyager spacecraft in 1979 and 1980, and are being observed in greater detail by the Galileo spacecraft which will be in orbit around Jupiter until December of 1999. Using the techniques discussed in previous sections, we can examine geologic processes important on these bodies, and determine the relative ages of their surfaces. This technique of comparing newly-explored worlds to those with which we are more familiar is a common one in science.
A. Have groups look at Galileo images of Io, Europa, Ganymede, and Callisto.
B. Compare them to the terrestrial planets looked at previously.
C. Using the appearance of craters on the surfaces, identify any geologic activity.
D. Have a recorder keep group notes (see worksheet).
E. Have groups report their findings to class with supportive statements.
Discussion Points:
Observations and answers:
Callisto is heavily cratered and looks like our Moon and Mercury, it appears inactive.
Ganymede has heavily cratered regions, regions with fewer craters which also have cracks and grooves (implying tectonic activity), and regions which appear to have experienced resurfacing (like our Moon). This means some areas were more recently active than others.
Europa has few craters, a smooth surface and cracks (implying tectonics). All indicate that is was recently active.
Io has very few craters and clear evidence of surface activity (volcanism), it is active today.
1. No. Weathering and erosion require wind and water, but the Galilean satellites have no atmospheres and no liquid water on the surface.
2. The main agents of change are volcanism (Io has sulfur volcanism, and Europa has possible water outflows) and tectonism (Europa and Ganymede have cracks and grooves).
3. Ganymede and Callisto have large relaxed craters (palimpsests). These satellites are made up of a large fraction of ice. The viscosity of this ice is such that (over long periods of time) the ice flows and fills in craters instead of preserving the hole. (Relate this to the cratering exercise in section IIA with respect to the composition of the target body.)
Evaluations:
Students are observed taking part and can contribute when called on
Other Activities, Misc. Information, etc.:
Create drawings of each satellite
Include drawings in a "Jupiter's Family" scrapbook for younger children
Write a myth about why each satellite is so different
GALILEAN SATELLITES - DO YOU SEE WHAT I SEE?
Group Recorder Worksheet
Directions: Select a recorder for your group. Using the pictures make observations of each satellite. Then answer the questions.
Observations:
Callisto
Ganymede
Europa
Io
Questions:
1. Did you see examples of weathering or erosion of craters? Why or why not?
2. What are the main agents of change you see modifying craters on Galilean satellites?
3. Are there other differences between the craters in the Jovian system, and those
in the inner solar system? (HINT - think about temperature)
EXTRA CREDIT: Find out about PALIMPSESTS
III. CRATER IMAGE INTERPRETATION
In the previous section, we examined images of the Galilean
satellites taken by the Galileo spacecraft. Such images will be returned until
the end of 1999, and scientists will be busy for years to come analyzing them.
This data can be compared to what we already know about the Earth and the
terrestrial planets, and we can use it to learn about the physical properties
and geologic history of the satellites of Jupiter. We have previously examined
the agents of change present on the Galilean satellites. However, there is
much more we can tell about these worlds merely by carefully inspecting images
of them.
Initial Activity:
Make a list of all the things you think we can measure or
determine by
looking at images of craters on the surfaces of the Galilean satellites (or any
planets), and what such measurements indicate.
Teacher Feature
Student Objectives:
Students will be able to relate shape and size of crater to size and speed of impactor, and surface composition
Background Information:
In previous sections, we examined images of the Galilean satellites taken by the Galileo spacecraft. Such images will be returned until the end of 1999, and scientists will be busy for years to come analyzing them. These data can be compared to what we already know about the Earth and the terrestrial planets, and we can use it to learn about the physical properties and geologic history of the satellites of Jupiter. We have previously examined the agents of change present on the Galilean satellites. However, there is much more we can tell about these worlds merely by carefully inspecting images of them. There are numerous things we can measure or determine by looking at images of craters of the surfaces of the Galilean satellites (or any planet). Crater depth and diameter provide information about the strength of surface materials, the impactor size and speed; crater shape relates to the structure of the surface material and its composition. The size distribution of craters within an image allows for estimations of the age of the surface.
Procedures:
Have students complete worksheet (either individually or in groups).
Answers are then shared and defended.
Evaluations:
All students complete worksheet and can defend their answers
Other Activities, Misc. Information, etc.:
For younger students, move words to bottom and have students cut and paste
Create an 8-frame comic of de-Terminator showing the results of each type of determiner.
de"TERMINATOR" - CRATERS ARE BACK
Directions: Fill each crater characteristic with items that would help determine it. Choose from the determiner list. You may use an answer more than once. An example has been done for you. Be prepared to de-Fend your selections.
IIIA. CRATER SIZE
Note: This activity is written for two levels. Level one is appropriate for pre-algebra students, and level two involves more sophisticated algebra skills including the
manipulation of an equation with two variables.
Crater size is related to the mass and velocity of the impacting body. Mass
and velocity can be combined to find the kinetic energy of an impactor.
Increasing either the mass or the velocity of the impactor increases the
kinetic energy of the impact. Review the results of your crater experiments in
section IIA. The size of the crater increased with the mass of the bolide, and
also with the height from which it was dropped (which is proportional to the
speed of impact). This fundamental physical relationship allows an estimate of
impactor mass to be made from crater diameter.
Activity:
Reexamine your results from activity IIA.
Graph the mass of the bolide against the diameter of the resulting crater (for
bodies dropped from the same height).
What relationship do you get?
As the mass of the bolide increases, the diameter of the
crater increases.
What are you assuming is constant, by graphing mass vs. crater
diameter for bodies dropped from the same height?
Height or velocity is being held approximately constant.
Graph the height from which the bolide is dropped vs. the crater
diameter for objects of the same mass.
What can you observe from your graph?
As the impact velocity increases, the crater diameter
increases.
What conclusion can you draw from this?
Crater size is proportional to the mass of the impactor,
and the velocity of impact. Given that the kinetic energy of the impact is
related to
mass and velocity, we can see that the experimental results support the theory
that crater diameter is related to the kinetic energy of the impact.
Crater size is related to the size and velocity of the impacting body.
These two quantities can be combined to find the kinetic energy of an
impactor, defined as
K = 1/2 m v2
where K is the kinetic energy, m is the mass of the
impacting body, and v is the velocity of the impactor. Review the
results of your crater experiments in section IIA. The size of the crater
increased with the mass of the bolide, and also with the height from which it
was dropped (which is proportional to the speed of impact). This fundamental
physical relationship allows an estimate of bolide mass to be made from crater
diameter.
Activity:
Reexamine your results from activity IIA.
Graph the mass of the bolide, m, against the cube of the
diameter,
D3, of the resulting crater (for bodies dropped from the
same height).
Describe and explain the relationship.
The results should follow an approximately straight line,
due
to the
relationship below.
The total amount of energy, K, required to form a crater is
proportional to the volume, V, of material excavated in
the impact.
Since a crater is basically a hemisphere (half a sphere),
its
volume, V, is proportional to the diameter, D, of the crater. (V = 2/3
[pi] (D/2)3 )
So the energy, K, needed is proportional to the diamete
cubed,
D3.
The energy of an impact is the kinetic energy, as defined
above:
K = 1/2 m v2.
Since the energy K, is proportional to D3, we
can
predict
D3 is proportional to
1/2 m v2
Measuring the diameters of craters allows us to
estimate the size of the impacting bolide. The diameter is proportional to
both the mass of the bolide and its impact velocity. We can measure the
estimate the size of the impacting bolide. The diameter is proportional to
both the mass of the bolide and its impact velocity. We can measure the
diameter of the crater, but unless we know either the mass or the velocity of
the bolide, we can't solve for the other. By assuming a constant impact
velocity, however, we can predict relative bolide masses for different crater
diameters.
Assuming a constant impact velocity, we know that the mass of
an
impactor is proportional to the diameter of the crater
cubed. (m
is proportional to D3)
In order to produce a crater twice as large as another, how much larger
must a bolide's mass be? What about a crater 10 times larger?
100 times
larger?
A: 8 times; 1000 times; 1000000 times.
Look at
some images of planetary surfaces. (Example images 1,
2) Measure the crater
diameters and estimate how much more massive the bolide which formed
the
largest crater in the picture was than the bolides which formed the
smaller
craters.
IIIB. CRATER DEPTH
Crater depths are indicative of both the strength of the surface material, and
the impactor size and speed. A variety of crater depth exercises are
available, such as "Long Distance Detective" in Craters!
The depth of a crater can be determined from the length of the shadow cast by
the crater rim and the angle of the incoming light source. If the angle of
incoming light and image scale are provided along with each image, students can
measure shadow lengths and calculate crater depths from the relations below.
As shown in the diagram above, using the geometry of triangles, if we
know Ø, the angle of incoming light, and can measure L,
the length of the shadow, we can calculate d, the crater depth.
The tangent function relates d, L, and Ø as follows:
tan Ø = d / L
Given d and Ø, multiply both sides of
the
equation above by L to get:
L * tan Ø = d
Activity: Given a Galileo image, its scale (either in km/pixel, or the
size of a feature), and the angle of incoming light, determine the depths
pf
the craters in the image. Here is one
possible
image.
For each crater, measure:
its diameter (in pixels or centimeters, then convert to km)
the length of shadow it casts
Calculate the depth of the crater, given the angle of incoming
light,
Ø, and the shadow length, L, using
L * tan
Ø = d
Graph crater depth vs. diameter.
Interpretation:
Are there any relationships between crater diameter and
depth?
Crater diameter is related to the mass and speed of the impactor,
as
discussed in section IIIA. What other factors might influence
crater
depth?
later tectonic or volcanic activity (crater could
be filled with
lava)
relaxation of the crater (surface composition)
Scientific context: Scientists often have to do considerable amounts of
detective work when analyzing images taken of other worlds. The exercises
above demonstrate how scientists start from a single picture and extract
valuable information not only about the appearance of the surface, but also the
approximate sizes of impacting bodies and the depths of craters on the surface.
Crater depths provide clues to surface composition. A crater formed in a firm
material such as rock can last much longer than a crater formed in a softer
material, such as ice. This distinction is especially important in the outer
solar system, when examining craters on such bodies as Europa, Ganymede, and
Callisto. These bodies are part-rock, part-ice. While ice behaves almost like
rock at the very cold temperatures near Jupiter, its properties are still
different enough to let large craters flow slowly over time, eventually
resulting in large flat circular areas with almost no topography at all, called
palimpsests. (Example image)
Crater depths are also important in understanding what events might have
modified the crater since its formation. For example, a broad shallow crater
on a rocky planet could have been filled in with lava at some point after its
formation, either immediately afterwards if the impact was energetic enough to
melt the surrounding material, or long afterwards if the planet underwent a
period of volcanic activity. If part of a crater floor is higher than another
part, it's possible that some sort of fault or other tectonic activity took
place nearby, thus disrupting the crater. The simple technique of shadow
measurement discussed above also has other applications. On earth, it can even
be used to measure the height of far-off mountains or trees!
Teacher Feature
Student Objectives:
Using images the students will measure diameter and shadow length, convert to kilometer, calculate crater depth and graph results
Background Information:
Crater depths are indicative of both the strength of the surface material, and the impactor size and speed. A variety of crater depth exercised are available, such as "Long Distance Detective" in Craters! The depth of a crater can be determined from the length of the shadow cast by the crater rim and the angle of the incoming light source. If the angle of incoming light and image scale are provided along with each image, students can measure shadow lengths and calculate crater depths from the relations below.
As shown in the diagram above, using the geometry of triangles, if we know &Oslash , the angle of the incoming light, and can measure L, the length of the shadow, we can calculate d, the crater depth. The tangent function relates d, L, and &Oslash as follows: tan &Oslash = d / L (or L * tan &Oslash = d ). Crater depths provide clues to surface composition. A crater formed in a firm material such as rock can last much longer than a crater formed in a softer material, such as ice. This distinction is especially important in the outer solar system, when examining craters on such bodies as Europa, Ganymede, and Callisto. These bodies are part-rock, part-ice. While ice behaves almost like rock at the very cold temperatures near Jupiter, its properties are still different enough to let large craters flow slowly over time, eventually resulting in large flat circular areas with no topography at all, called palimpsests [have students brainstorm other modifications here] Crater depths are also important in understanding what events might have modified the crater since its formation. For example, a broad shallow crater on a rocky planet could have been filled in with lava at some point after its formation, either immediately afterwards if the impact was energetic enough to melt the surrounding material, or long afterwards if the planet underwent a period of volcanic activity. If part of a crater floor is higher than another part, it's possible that some sort of fault or other tectonic activity took place nearby, thus disrupting the crater. The simple technique of shadow measurement discussed above also has other applications. On Earth, it can even be used to measure the height of far-off mountains or trees.
Lesson Format:
lecture, individual work, class discussion
Introduction:
Have students go outside and measure heights and shadow lengths for group. Complete the sheet and answer questions, then apply formula to crater measurements.
Procedures:
Work introductory part of activity, then present background information in form of lecture.
Work through formula with class, possibly even using data collected to demonstrate formula and calculation.
Hand out crater images and continue with activity.
Demonstrate conversion to kilometers (scale of photos) as necessary.
Other Activities, Misc. Information, etc.:
If available, play the intro. to the classic radio show "the Shadow".
With younger children, just use the intro. activity (provide a structured data table for collecting and recording measurements) and relate to other items [flagpole, building, teacher]. Then discuss the craters.
THE SHADOW KNOWS Worksheet
Introductory activity
Directions:
A. Go outside and measure the heights of everyone in the group, record data in form of a chart along with each student's name.
B. Measure the length of each person's shadow, record in the chart with height information.
C. Record time of day.
Questions:
1. Explain why time of day would be important in shadow length.
2. At the same time of day, would all shadows be in proportion to the height
of the objects being measured? Explain.
3. As the angle of the sun changes throughout the day, would the shadows
change in proportion?
4. What are the two determining factors in shadow length?
The Shadow Knows Activity
Directions:
A. For each crater, measure the shadow length (L) and record in the space provided.
B. Assuming the angle of incoming light , &Oslash, is 34 degrees, use your measurements and the formula L * tan &Oslash = d to calculate the depth, d, of all the craters. Show your work.
Crater 1 Crater 3
Crater 2 Crater 4
IIIC. CRATER SIZE DISTRIBUTION AND SURFACE AGES
Concepts
Measurement
Data recording and organization
Bar graphs
Data interpretation
Drawing conclusions
The size distribution of craters on a planetary body can be used to estimate
relative, and even absolute, surface ages. As discussed in part II, in the
absence of agents of change such as erosion, tectonics, and volcanism, a
planetary surface tends to become saturated with craters. Since even currently
inactive bodies like the moon were active at some point in their past, surface
processes have removed craters at some point in every planet's past. Thus
regions on a body with higher crater densities tend to be older than regions
with lower crater densities. This activity will investigate crater densities
and size distributions, and interpret those in terms of relative and absolute
surface ages.
Decide on appropriate size intervals and plot your results in a bar graph
Interpret results:
Small craters are much more common than large ones
Repeat exercise for different regions on each satellite. Compare
results.
Some regions are older (have more craters) than others
Compare crater density (craters per area) on different
satellites
Callisto has the highest crater density, Ganymede the next.
Europa
has very few craters, and Io has almost no recognizable craters.
Estimate the relative ages of individual craters and other features in
the images
New craters cover older ones.
Fresh sharp craters are younger than old, smooth, relaxed
ones.
Scientific context: The first part of the activity above
revealed that small craters are much more common than large ones. This is
because small bolides are more common than large ones, and, as seen in activity
IIIA, crater diameter is proportional to bolide size. Smaller bolides would be
more common than larger ones if most craters are caused by impacts of asteroid
fragments. The asteroid belt is a region of small planetesimals, most of
which orbit the sun between the orbits of Mars and Jupiter. Asteroids come in
sizes from very small to very large, but the size distribution is not random.
Rather, it is governed by the fact that asteroids often collide with each
other. In such a collision, both asteroids break into smaller pieces. The
size of these pieces follows a predictable distribution (which can be simulated
in a laboratory) made up of many small fragments and a few larger ones. Over
geologic time, therefore, asteroids continue to collide with each other and
produce many small fragments. These bodies may eventually collide with the
planets, producing a crater size distribution dominated by smaller craters.
The activity above also involved measurements of crater densities, which can be
used for relative dating of planetary surfaces. Assuming that bolides strike
all regions of a planet at approximately the same rate, all areas of the
surface should have the same crater density unless agents of change have
removed some of them during the planet's geologic history. A common way for
large numbers of craters to be removed is through volcanism. Early in the
Moon's history, for example, large lava flows flooded portions of the surface,
completely burying any craters which were there at the time. These areas were
essentially wiped clean of craters about 3.5 billion years ago andnd thus their
crater density dates back to those lava flows. The lunar highlands, in
contrast, were not flooded by lava flows. Their crater density dates back to
when they were formed, about 4.1 billion years ago, and thus is much higher.
Io, in contrast to the ancient Moon, is one of the most geologically active
bodies in the solar system today. Its surface has no recognizable impact
craters, and is continually being resurfaced by volcanic eruptions which cover
any craters which might form. Regions of a surface with a higher crater
density are older than regions with a lower crater density, and a surface like
Io's, with no observable craters, is extremely young and implies current
geologic activity.
Crater size distributions can also be used to estimate the absolute age of a
surface. The cratering rate decreased with time in the early solar system,
beginning when the planets finished forming about 4.5 billion years ago and
there was much leftover interplanetary debris to cause impacts. The amount of
debris decreased over time as collisions and impacts swept it up, thus
decreasing the frequency of impacts. By measuring the crater density on
different areas of the Moon, and measuring the actual ages of rocks returned
from different regions by Apollo astronauts, scientists can calibrate cratering
rate to actual surface ages. This relationship of cratering rate vs. time can
then be extrapolated to other places in the solar system, since it can vary
based on distance from the asteroid belt (Mars, located nearer to the asteroid
belt than the Earth, may have a rate of crater formation roughly twice that at
earth) and proximity to large planets such as Jupiter, whose gravitational
field attracts impactors.
Materials Needed:
'answers' preprinted (see list below), mounted on colored paper and placed around the room in plain sight
copy of the worksheet for each student
Procedures:
A. Have students read through worksheet with blanks.
B. Explain that the words (answers) that fit in the blanks are in the room.
C. Allow students to walk around the room to gather information (words) - tell the students not to move the words or point them out to others. There is no talking.
D. After sufficient time looking, students should be able to find and place most words in the proper places in the worksheet.
Answer Words (make sure to eliminate corresponding number when placing around room)
SATELLITE SIGHT Worksheet
In order to gather information, satellites orbit various objects. Using this sheet, circulate around the room and observe the various answers for these questions. You may not move the answers or point them out to others. Talking is not permitted. Once you have your answers, complete this sheet. Look for context clues to help you.
1. _____________ craters are much more common than 2. ______________ ones. This is because 3. _______________ bolides are more common than 4. ________________ones, and crater diameter is proportional to bolide size. Smaller bolides would be more common than larger ones if most craters were caused by impacts of 5. ________________ fragments. The 6. ________________is a region of small planetesimals, most of which orbit the sun between the orbits of 7. __________________ and 8. ________________.
Asteroids come in size from very small to very large, but the size distribution is not random. Rather, it is governed by the fact that asteroids often 9. _________________ with each other. In such a 10. __________________, both asteroids break into smaller pieces. The size of the pieces follows a 11. __________________________________ (which can be simulated in a laboratory) made up of many small fragments and a few larger one. Over the 12. _________________ time, therefore, asteroids continue to collide and produce many small fragments. These bodies may continue to collide with the 13. ____________________, producing a crater size distribution dominated by smaller craters.
Measurements of crater densities can be used for 14. ____________________ of planetary surfaces. Assuming that bolides strike all regions of a planet at approximately the same rate, all areas of the surface should have the same crater density unless 15._______________________ have removed them during the planet's geologic history. A common way for large numbers of craters to be removed is through 16. ____________. 17. _________________ , in contrast to the ancient Moon, is one of the most geologically active bodies in the solar system today. It's surface has no recognizable impact craters, and is continually being 18. __________________ by volcanic eruptions which cover any craters which might form. Regions of a surface with a higher crater density are 19. ______________ than regions with a lower crater density, and a surface like Io's, with no observable craters, is extremely young and implies current geologic activity.
Crater size distributions can also be used to estimate the absolute age of a surface. The cratering rate decreased with time in the early solar system, beginning when the 20. ________________ finished forming about 21. ____________________ ago and there was much left over interplanetary 22. ________________ to cause impacts. The amount of debris decreased over time as collisions and impacts swept it up, thus decreasing the 23. ___________________ of impacts. By measuring the crater density of different areas of the Moon, and measuring the actual ages of rocks returned from different regions by 24. ______________________________, scientists can calibrate cratering rate to actual 25. _____________________ ages.
Conclusion
Images of the surfaces of the Galilean satellites, or of any planets in the
solar system, reveal a general history of the planet, and the state of the
planet's interior through time. For example, volcanoes require a hot interior.
If we know from looking at images of a planetary surface that the last period
of volcanism was hundreds of millions of years ago, we know that the interior
of the planet must have been hot until that time. This type of information
helps to constrain models of the interior structure and evolution of the
satellites, as well as provide information about their formation.
This set of activities has shown the wealth of information obtainable from the
simplest black and white images of a planetary surface. Much of the first wave
of reconnaissance of the solar system was done in just this way, with
scientists working to understand the little information they had from the early
planetary spacecraft. The Galileo spacecraft, which will remain in orbit in
the Jovian system until late 1997, not only has a camera capable of taking
black and white images of the surfaces of the satellites, but also has a wealth
of other instruments to augment this information. The camera has filters in
six different colors, allowing color images to be taken and analyzed. This can
yield valuable information about the chemical composition of surface materials.
Other instruments on Galileo allow it to measure properties of Jupiter and its
satellites at a variety of near infrared wavelengths, investigate the radiation
and magnetic environments, and obtain more precise measurements of the sizes
and densities of the satellites. Galileo's two-year tour through the Jovian
system should provide information for scientists to study for years to come.
Sources: Some information in this module was adapted from Craters: A Multi-Science Approach
to Cratering and Impacts, by W.K. Hartmann and J. Cain. A Joint Project of the National
Science Teachers Association, The Planetary Society, and NASA. Published by the National Science
Teachers Association, 1995.
Procedures:
Have students work in groups to:
A. Share own 5 facts from lessons
B. Tally or record all facts -- As groups decide on a way to demonstrate the various collections (graphs, prioritizations, continuums, info webs, etc).
C. Have groups display info on large paper, including any questions they would like to have answered.
Evaluations:
Visual check of charts/papers.
All students should be able to explain all points on the collection paper
Other Activities, Misc. Information, etc.:
Materials can be notebooked so each student has a collection of unit work.
This module was written by Cynthia Phillips, Dept. of Planetary
Sciences, University of Arizona, Tucson AZ, and funded in part by the
NASA Spacegrant program.
The SSI Education and Public Outreach webpages were originally created and
managed by Matthew Fishburn and Elizabeth Alvarez with significant assistance
from Kelly Bender, Ross Beyer, Detrick Branston, Stephanie Lyons, Eileen Ryan,
and Nalin Samarasinha.