CRATERS in the SOLAR SYSTEM
REMOTE ACCESS ASTRONOMY PROJECT
UNIVERSITY OF CALIFORNIA, SANTA BARBARA
and
CENTER for PARTICLE ASTROPHYSICS
Craters in the solar system
Jatila van der Veen, UCSB Physics Department and Adolfo Camarillo High School
Philip Lubin, UCSB Physics Department
THE FOLLOWING MAY BE REPRODUCED IN ANY FORM FOR USE IN A CLASSROOM,
BUT UNDER NO CIRCUMSTANCES MAY ANY PART OF THIS BE REPRODUCED FOR
PUBLICATION WITHOUT THE WRITTEN CONSENT OF THE AUTHORS.
Modified by WAC at UNC-CH 6/12/02
References: "The Collision of Solid Bodies", Eugene and Carolyn Shoemaker, in THE NEW SOLAR SYSTEM,
Beatty and Chaikin, ed., Sky Publishing, 1990.
"Killer Crater in the Yucatan", J. Kelly Beatty, in SKY AND TELESCOPE, v.82, no.1, July, 1991.
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In the following activity, you will:
* Compare the sizes of craters on some of the satellites of the planets in our solar system;
* Calculate the energy released by impacts at an average speed based on crater sizes.
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Background: Impact cratering is one of the most widely observed geologic processes in the solar system. In fact,
there is strong evidence that asteroid or comet impacts have played a role in some of the major extinctions of plant
and animal life in the Earth's history. It is estimated that at least 20 craters of diameter 10 km or larger form on the
Earth per million square kilometers every billion years, based on craters found on Earth and the number of observed
asteroids and comets whose orbits pass close to Earth.
On a geologically active planet like Earth, impact craters get eroded relatively rapidly and are difficult to find.
On bodies with no atmosphere, water, or active volcanoes, ancient impact craters are preserved. The surfaces of our
Moon, Mercury, Venus, Mars, and many of the moons of Jupiter and Saturn are covered with craters.
IN THIS ACTIVITY YOU WILL COMPARE IMPACT CRATERS ON TWO OF THE SATELLITES OF
SATURN, OUR MOON AND THE EARTH ITSELF.
FIGURE 1 IS A PICTURE TAKEN BY VOYAGER I OF MIMAS, ONE OF THE MOONS OF THE PLANET,
SATURN.
Notice the very large crater in the northeast quadrant of Mimas. This crater is named Herschel, after the astronomer
who first discovered this moon of Saturn.
Figure 1
I. Estimating the size of the crater Herschel
Using Figure 1 measure the diameter of the image of Mimas in centimeters:
Diameter of Mimas = _______________________________________________ cm
We know from measurements taken by Voyager I that the actual diameter of Mimas is 390 km. Now we can find the
scale of this image in km/cm:
Scale factor = (diam. in km)/(image diam. in cm) = ___________________________km/cm
Now we can estimate the diameter of the crater....
Find the widest part of the crater, and measure the diameter in the same manner as you measured the diameter of
Mimas itself.
Diameter of crater = ____________________________________________ cm
Now multiply by the scale factor to find the diameter of Herschel crater in kilometers:
diam.(cm) x scale factor (km/cm) = diam. = _____________________________km
What fraction of the diameter of Mimas is the diameter of Herschel crater?
____________________________________________________________
Herschel crater is one of the largest craters observed on any body in the solar system. If the impact had been
much larger, Mimas would have broken apart.
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II. Craters on Dione:
FIGURE 2 IS AN IMAGE OF DIONE, ANOTHER SATELLITE OF SATURN
Figure 2
This image does not show the entire moon, but you can get a fairly good estimate of where the center of Dione is by
observing the curvature on the sides in the image.
Once again, we will need to estimate the scale of this image in kilometers/cm.
Try to find the center of Dione from the image. Get as accurate a measurement of the diameter of Dione’s image in
centimeters as you can, using the same method as before.
Diameter of Dione’s image in cm = ____________________________________ cm.
The actual diameter of Dione is about 1120 km. Now you can estimate the scale factor for this image in the same
way you did for the image of Mimas.
Scale factor = (diam. in km)/(diam. in cm) = __________________________________ km/cm
Finding the largest crater on Dione.
Look for the largest crater you can find in Figure 2, and estimate its diameter in the same way you did for Herschel
crater on Mimas.
Diameter of largest crater in cm = _________________________________________________ cm
Diameter of crater in km = (diam. in cm) x (scale factor) (km/cm)= ________________________km
What fraction of the diameter of Dione is the diameter of this crater?
______________________________________________________________________
Which crater is actually larger, this one on Dione, or Herschel on Mimas? _____________________
Which crater is larger IN PROPORTION TO THE BODY IT IS ON? ________________________
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III. Comparing Craters on the Saturnian Moons with Craters on Our Moon
FIGURE 3 IS AN IMAGE OF THE EARTH’S MOON TAKEN FROM APOLLO 16.
Figure 3
Let’s repeat the exercise again.
Find the center of the Moon from the image. Get as accurate a measurement of the diameter of the Moon’s image in
centimeters as you can, using the same method as before.
Diameter of the Moon’s image in cm = ____________________________________ cm.
The actual diameter of the Moon is 3476 km. Now you can estimate the scale factor for this image in the same way
you did before.
Scale factor = (diam. in km)/(diam. in cm) = __________________________________ km/cm
Finding the largest crater on the Moon.
Look for the largest crater you can find in Figure 3, and estimate its diameter in the same way you did for craters on
Mimas and Dione
Diameter of largest crater in cm = _________________________________________________ cm
Diameter of crater in km = (diam. in cm) x (scale factor) (km/cm) = ________________________km
What fraction of the diameter of the Moon is the diameter of this crater?
______________________________________________________________________
Now which crater is actually larger, this one on the Moon, the largest crater on Dione, or Herschel on Mimas?
_____________________
And, now, which crater is larger IN PROPORTION TO THE BODY IT IS ON? _______________________
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IV. Comparison with Craters on Earth
The famous Barringer Crater in Arizona shown above (Figure 4) has a diameter of only 1.2 kilometers. This is tiny
compared to the large craters on moons that we have been examining. Does that mean that large bodies haven’t hit
the Earth? Of course not! It simply means that because of erosion on Earth, craters tend to be erased fairly quickly.
However, it turns out that evidence for large impacts in earlier times can now be recovered using modern techniques
for mapping the density and magnetic field variations of ancient rocks.
FIGURE 5 BELOW IS A MAP OF AN IMPACT CRATER NEAR CHICXULUB, YUCATAN PENINSULA IN
MEXICO.
Figure 5
This impact basin is buried under several hundred meters of sediment beneath the coastal waters of the Gulf of
Mexico; thus hiding it from view. The diameter of this buried crater is 170 kilometers! The best estimate of the time
of the impact was 65 million years ago (based on the depth of the sediment, dating of the rocks, etc). This is
believed to be the event, which caused the last great worldwide extinction in which half of the species on Earth
became extinct--including the dinosaurs!
How does this Chicxulub Crater compare in size with the large craters we examined on the moons?
__________________________.
And how does the Chicxulub Crater compare IN PROPORTION TO THE BODY IT IS ON?
__________________________.
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V. Going further... Estimating the Energy Released in an Impact
Bodies that crash into planets or satellites at high speeds create tremendous shock (pressure) waves that penetrate
the surface of the planet or moon. These extremely high pressures cause both the target material and the impacting
body to get so hot it is vaporized or melts. This ultra hot material moves outward from the location of the impact, as
an expanding shock front, which pushes outward to form a crater rim. After the shock wave passes, some material
from the crater rim slumps inward and converges in the center to form a peak. Also there is often an elastic rebound
at the center of the crater, which accentuates the formation of a central peak in the crater. An examination of the
larger craters in Figures 1 through 3 shows that central peaks are quite common.
Much material is ejected from the excavated crater. While the floor of the crater is often smooth due to melting,
the ground outside the crater is often littered with debris from the impact. The ejecta consists mainly of material
blasted out from the floor of the crater, and sometimes chunks of the impacting body itself (if it was massive enough
that it did not all vaporize upon impact!). There are often secondary craters observed around large craters, which are
made by the ejecta. Rays are observed around young craters on the Moon and other planets, which are made up of
ejecta, which were shot out from the crater.
It turns out that the size of a crater depends primarily on the Kinetic Energy of the impacting body. A rough rule of
thumb is that the impact energy is approximately proportional to the cube of the diameter of a crater. For example a
crater that is twice as big will be excavated by an impact in which the energy (either by making the impact velocity
higher or the body larger) is about 8 times bigger! Thus, by studying crater sizes astronomers can estimate the sizes
and impact energies of bodies that made them.
While this is actually a rather complicated process that involves doing impact experiments under controlled
conditions in the lab, we can make some rough estimates of the amount of energy released by impacting bodies from
some simple considerations...
1. Impact velocity.
Impact velocities are estimated to range from about 5 km/sec for colliding asteroids to 40 km/sec for the body
that collided with the Earth 65 million years ago creating the Chicxulub Crater. To be on the conservative side,
however, let's assume an impact velocity of 10 km/sec.
2. Size of the impacting body.
This is a tough one. From our examination of the various sizes of craters on the Moon, Dione and Mimas we can
conclude that impactors probably come in a large variety of sizes. Furthermore, since there seem to be many more
small craters than large ones we can conclude that there must be more “small” impactors than large ones. A very
approximate rule of thumb is that an impact crater is about 10 times bigger in diameter than the impactor if the
density of the target and the impacting body are about the same. If the 1.2 km diameter Barringer Crater was caused
by a rocky body, our rule of thumb would imply that its diameter was about 0.12 km or 120 meters.
3. Mass of the impacting body.
We need to estimate the density of a chunk of interplanetary material...If it was a rocky piece of material, it could
have a density of around 3 - 5 gm/cm3, but if it was a chunk of an iron-nickel meteor, its density could have been 8 9 gm/cm3. If it was a piece of comet, on the other hand, its density could have been around 1 - 2 gm/cm3. Let's just
assume our craters were caused by asteroidal chunks of rock, with a densities of about 3.0 gm/cm3.
Now let’s try to estimate the Energy of the Barringer Meteorite!
Make sure your units are consistent:
a. Convert density in grams/cm3 to density in kg/m3 : ___________________________kg/m3
b. Find the volume in m3 for a spherical body from V = (4  r3)/3 (for the Barringer Meteorite r = 60 meters
V = ________________________________m3
c. Mass = (volume) x (density) =_________________________________________kg
d. Convert velocity to m/sec: ____________________________________________m/sec
e. Now, let's consider how much kinetic energy is dissipated as heat and other forms of energy when this body
of 120 meters diameter, density 3000 kg/m3, and moving at a velocity of 10000m/sec impacts the Earth (we can
neglect atmospheric effects).
K.E. =
1
mv 2 = __________________________________________________Joules
2
To get an idea of how much energy this really is, consider that 1-megaton of TNT (that's 106 tons of TNT) is
equivalent to approximately 4.2 x 1015 Joules.
How many megatons of TNT are equivalent to the energy released by our typical hypothetical impacting body?
______________________________________________________________Megatons of TNT
And this is just a small collision!
f. Now, the Chicxulub Crater is 170 km in diameter instead of 1.2 km. Assuming that crater diameter really is
proportional to the energy cubed we can estimate the energy of the impact based on the energy of the Barringer
event.
3
K.E. (Chicxulub) = K.E. (Barringer) x [diam (Chicxulub)/diam. (Barringer)] = ___________________Joules
Or
___________________Megatons.
As a point of reference, the combined nuclear weapons stockpiles of all the “Nuclear Powers” in the world add up to
somewhat more than 10,000 Megatons. No wonder the Chicxulub event caused a widespread extinction!
g. Assuming the Chicxulub event was also caused by a stony asteroid of density 3000 kg/m3 estimate how big it
was if it was also moving at a speed of 10000m/s.
Diameter of Chicxulub impacter __________________ km.