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Lab 4 - Crater Counts and Surface Ages

## Age of the Lunar Maria

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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. This region should contain several craters of about the same size.

2. Use the ruler to make a rough measurement of the size of one of these craters (in km).

3. Find the number nearest 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. Weâ€™ll call this size the â€œtarget size.â€

4. Count all the craters in the region with sizes between the small tick above the target size (20 km for a target size of 16 km) and the tick below the target size (14 km for the case of a target size of 16 km). This is the observed number of craters Nobs.

5. Using the ruler, 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:

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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 You will report your age estimates as the range between these two diagonal age lines.

8. Repeat the experiment for another target size in the same region in order to obtain a second estimate of the age (not for Mars. Only once for Mars). Compare the two ages for consistency.
1. Age of the Lunar Maria
The age of the Lunar maria were 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 from Google Earth Proâ„¢ maps of the moon. To do this, start
a web browser, go to https://www.google.com/earth/versions/#earth-pro, and install the software.
Okanaganâ€™s lab computers by following the instructions found at: link. Any of the computers in
SCI 126 will have the software.
Use the switch button ( ) in the top tool bar to navigate to the surface of the moon. Select moon
and find the maria which are the very dark regions on the moon. Zoom in on an area in the maria
and find a region with some craters. You will need to use the ruler ( ) in the top tool bar to
measure the sizes of objects (in km).
Q1: Using the method described in the introduction, measure the age of the maria using crater
counts.
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 crater counts.
These regions are reportedly the oldest surface areas of the moon. Does your crater counting
bear that out?
2/3
Lab 4 â€“ Crater Counts and Surface Ages
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 â€œage linesâ€ slope
from upper left to lower right? (2) Why are the age lines for older ages above the lines for
younger ages?
3. 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 Systemâ€™s history and (2) that
the speed of impact is dominated by the orbital motion of the impactor and not the gravity of the
planet.
Q4: There are several images from the surface of Mars on Canvas. Select one image and use the
techniques you have developed above, estimate the age for this area. Submit the image showing
the measurement of the size of the craters.