Name __________________________________________ Date _____________
Partners ___________________________________________________________
Comparative Planetology
by Mary Lou West after Paul Johnson and Ron Canterna
Purpose : to become familiar with the major features of the planets of the solar system,
especially the Earth, Moon and Mars. This is also an exercise in comparing different
estimates of the same quantity and analyzing the assumptions supporting each one.
Introduction : After the planets of the solar system formed together about 4.6 billion years ago
they were modified by several different processes such as impact cratering, plate
tectonics, volcanism, and erosion by ice, wind, and water. These processes did not act
uniformly on all the bodies, and we would like to find out why.
Equipment:
Laminated planet images, ruler, Earth images, Mars images, colored pencils, maps of NJ,
newspapers
Procedure :
1. Categorizing Images
Examine the laminated images of planetary features. Place them into multiple categories
such as clouds, rivers, mountains, whole planets, etc. List your categories and their
member images.
Tell which solar system body you think is pictured on each image:
Image
1
2
3
4
5
6
7
8
9
Planet
Image
10
11
12
13
14
15
16
17
18
Planet
2
2. Cratering Density
Since direct dating of rocks (by their radioactive decay) is possible for only a small
sample of material from other worlds we will calibrate and use the indirect method of
crater density to get a rough idea of the ages of areas on the moon, Mars, and the Earth.
A. Moon
Data Table 1: the Moon
Mission
Apollo 14
Fra Mauro
Apollo 15
Hadley Rille
Apollo 16 Descartes
Apollo 17 TaurusLittrow
Fresh surface
Area
100
km2
100
km2
100
km2
100
km2
100
km2
N craters +/- N1/2
craters/area
Age, Gyr
4.0
3.3
3.7
3.9
Q: The largest number of craters/area is ______________________ in the
__________________________ location.
Plot a graph of crater density vs. age, and put error bars on the crater density points by +/- N1/2 .
Draw a smooth curve through the points' error bars.
Q: How has the cratering rate (slope of the curve) changed with time?
Q: Why might this happen? (Is the supply of meteors steady?)
Q: Draw a straight line through the first two points of your curve.
See where it crosses the time axis. When was the major cratering bombardment finished?
Your curve of craters/km2 vs. age will be your model of the "aging" of planetary surfaces. You
can approximate the recent part of the curve by a straight line with a much lower slope. Using a
ruler, draw a straight line to approximate the curve from 2 Ga to 0 years ago.
On the curve at age= 2 Ga the number of craters/km2 is N = _________. This calibrates the
straight line so that the age of any location with fewer craters/unit area than N is given by the
proportion
Age = 2 * 10^9 years * (the location's craters/km2)
N
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B. The Earth
Local: On the Earth find a region with impact craters, and calculate the crater density for
it. Enter this in Data Table 2.
Data Table 2: Earth
Location
Area, km2
N +/- N 1/2 craters
Craters/area
Approximate age
Q: Is this an over-estimate or an under-estimate of the true crater density? (Hint: Does the area
you used go halfway to the next crater?)
As an approximation we can use your moon's cratering curve to infer the ages of this
region. Enter this calculate value in Data Table 2.
Global: So far humans have found 178 craters on the Earth's dry land. Since the Earth's
radius is roughly 6000 km and the surface area of a sphere is 4πR2 , the Earth's total area
is roughly _______________km2. About 30% is dry land, or about _____________km2.
This means that the rough average for the Earth is about = 178 craters/ dry area =
_______________ craters/km2.
Q: Is this more or less than the cratering density on various places on the moon?
C. Mars
On Mars find a region with impact craters and also linear dimensions listed, and calculate
the crater density for it. Fill in Data Table 3. Again use your Moon's cratering curve to
estimate the age of this surface.
Data Table 3: Mars
Location
Area, km2
N +/- N 1/2 craters
Craters/area
Approximate age
On Mars the largest crater density found in the class is _______________craters/km2.
D. Comparison of Worlds
One way to compare these three worlds is to look at their most heavily cratered regions.
The largest number of craters/km2 is _____________ on the Moon, __________ on Earth
(the over-all average), and _____________ on Mars.
Q: Now list the three worlds in order of oldest (many craters) to youngest (few craters)
with respect to their surfaces:
Q: Give two reasons for this.
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3. Geological Maps
Use the various books and maps available to locate on Earth and on Mars all examples of the
following landforms. Use colored pencils to sketch them in on your maps.
| blue | Ice
| red | Volcanic mountains
|green | Rift valleys
|yellow| Pushed up or folded mountains (may be rims of huge craters)
|brown| Heavily cratered terrain.
4. Scaling a lunar crater to the Earth
The lunar crater Eratosthenes is 60 km wide. On your map of New Jersey find the scale
and figure out how big this would be. Tear out a rough newspaper circle to match the
crater Eratosthenes to this scale.
Place your paper crater on the NJ map as if the crater had formed in Trenton. Would
Montclair be destroyed? ______________
If the crater was in New York, would Montclair be destroyed? ______________
If the crater was in Philadelphia, would Montclair be destroyed? ______________
5. Discussion
Q: Discuss briefly why the Earth, Moon, and Mars are so different in surface age and landforms.
Give at least three reasons.
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