Martian Crater Density Data Table
Northern Hemisphere
Number of Craters Number of Craters
in Image
in 1,000,000 km2
Southern Hemisphere
Astronomy
110 / 111 Laboratory Number
4: Crater
and Surface Ages
Crater size
of CratersCounts
Number of Craters
range (km)
in Image
in 1,000,000 km2
Chart From ASTR150 @ U.W.
In this
< 8 laboratory, you will use cratering counts on the surface of the Moon to develop
estimates of surface age. Crater counts are an essential tool in studying the geology of
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planets since we are not able to get samples from all the planets and then use
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radioisotopes
to determine their ages. Instead, we use a calibrated rate of cratering
based
on a few isotope samples from the moon in the same places that we can
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estimate
crater counts. This calibrates an empirical relationship which we then can
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generalize to other areas of the Moon and indeed the rest of the Solar System, though
the latter is with some caution.
Astronomy 150
0–3
The Planets
The
chart above is a crater count-age
graph which you
will use through this laboratory.
You use it as follows:
1. Find the region for which you want to measure the age.
2. Find several craters of about the same size. Use the scale bar to make a rough
measure of that size (in km).
3. Find a number near that size on the x-axis of the graph above. For example, if you
find a crater with size ~15.5 km, look for 16 km in the graph. Weʼll call this size the
“target size.”
4. Count all the craters in the region with sizes between the small tick above the target
size and the tick below the target size (14 km for the case of a target size of 16 km)
Astronomy 110/111 • Laboratory 4 • Page 1 of 3
and the small tick above the target size (20 km for a target size of 16 km). This is the
observed number of craters Nobs.
5. Using the scale bar, estimate the observed area for your region in square km. This is
the variable Aobs
6. Calculate the number of craters that would be observed in 1,000,000 square km,
Npredicted:
Npredicted = Nobs
106 sq km
Aobs
7. Find the predicted number on the y-axis of the chart above and trace over to the
intersection of that number and the target size. Find the two diagonal age lines that
the intersection is in between. Estimate the age of the surface based on the ages
indicated next to the chart in Gyr.
8. Repeat the experiment for another target size in the same region in order to obtain a
second estimate of the age. Compare the two ages for consistency.
1. Age of the Maria
The age of the Lunar Maria was measured by radioisotope dating and serves as the
basis for the crater count experiments. In this section, we will measure the age of the
maria using crater counts. We will use imaging data on the Google(tm) maps data on
the moon. To do this, start a web browser and go to http://moon.google.com/ Use the
navigation tools in the left-hand corner to find the maria which are the very dark regions
on the moon. Zoom in on the maria and find a region with some craters. You will need
to use the scale bar in the lower left-hand corner to measure the sizes of objects.
Q1: Using the method described in the introduction, measure the age of the maria using
crater counts.
Hints: You will need to define a target area, but you will frequently zoom in on the area
and move it around. To be able to always return to the same area, click on the “Link to
this page” link above the navigation tools and bookmark the URL.
The “Visible” images are the default and a good way to identify the maria, but the
craters stand out best in the “Elevation” images.
The Apollo button shows where the moon landings occurred. Note that all of the
landings were on the maria and so all our sampling is from this area.
2. Age of the Lunar Highlands
The next area for exploration is the Lunar Highlands which are primarily on the far side
of the moon. They are the light colored regions near the equator of the moon, opposite
the maria. These regions have only been observed by satellite so the ages are only
from cratering count. These regions are reportedly the oldest surface area of the moon.
Does your crater counting bear that out?
Astronomy 110/111 • Laboratory 4 • Page 2 of 3
Q2: Use the crater counting method to estimate the age of the lunar highlands. The
same hints as before apply.
Q3: The crater counting chart has a lot of information encoded in it. Based off your
reading in the textbook and class notes, answer the following questions: (1) Why do the
“age lines” slope from upper left to lower right? (2) Why are the age lines for older ages
above the lines for younger ages?
Concluding Investigation:
We can also use the crater counting chart above to estimate the ages of other surfaces
in the Solar system. The approximation is rather rough but relies on assuming (1) that
the number of impactors of a given size is uniform for a given period in the Solar
Systemʼs history and (2) that the speed of impact is dominated by the orbital motion of
the impactor and not the gravity of the planet.
The TA has several images from the surface of Mars. Select one image and use the
techniques you have developed above, estimate the age for this area.
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