Unit Two: Investigating the Sun-Earth-Moon System
3. Craters on the Earth and Moon
A glance at the moon through a small telescope shows that impact craters both large and small dominate its surface. On Earth, however, craters are few and far between, and many are not very large or conspicuous. Meteor (Barringer) Crater in northern Arizona, perhaps the best-known impact crater in the U.S., is only a kilometer in diameter. It would be utterly insignificant among the lunar craters. Yet, impact craters on both the Earth and moon are caused by large, rocky objects cutting through the inner solar system, drawn in by the gravity of each body these objects strike. Our Earth is a bigger target than the moon and has six times the gravity, so one would expect even more impacts on Earth than on the moon. So, how many craters exist on Earth? Does the moon really have more or less than Earth? How do craters on Earth compare with craters on the moon in size, age and appearance? Why do the Earth and moon look so different?
First, explore the known craters on Earth using the Earth Impact Data Base: http://www.unb.ca/passc/ImpactDatabase/index.html
Click on one of the continents at the top of the page. A map will appear with all the known impact craters on that continent. Data and links to images of each one are found in a table below the map.
- Follow the links on 6�10 craters of different sizes to see what they look like. Collect a few representative images of craters of different sizes and age.
- Do a simple crater count by making a table with bins (intervals) of increasing diameter, D: D<1.25 km, 1.25<D<2.5 km, 2.5<D<5 km, 5<D<10 km, 10<D<20 km, 20<D<40 km, etc. Now count and record all the craters in each diameter bin. Make a bar graph of your results.
- Make a second table of crater counts, but in this one bin the craters according to age. Use the same intervals on the x-axis, but label them millions of years (Ma): Age<1.25 My, 1.25<Age<2.5 My, etc.
- Repeat your image collection, crater size and age counts for the other continents so that you have a count for all the craters on Earth's major landmasses. Sum up the area of the six continents (km2) and divide the total number of craters in your table by the total area so that you derive the number of craters per square kilometer in each bin.
Second, explore the craters on the Moon. Go to: http://www.lpi.usra.edu/resources/lunar_atlases/.
Select Lunar Orbiter Atlas of the Moon. This atlas is the best collection of lunar images for exploring the nature of lunar features. It can be accessed in a number of ways. For this project, start with Search by Feature Name and enter "mare" in the box on the next page. This will generate a list of images of the various flat, dark areas on the moon called maria. The image (photo) names end in M for "Medium resolution," and H for "High resolution." Click first on one of the "M" photos. A page will appear that has a thumbnail of the image, the image with larger features outlined and named in red, and a locator map. The named features are found in a list at the bottom of the page. Click on the thumbnails and explore the craters in the image. Large craters are named in the "M" images; smaller craters are named in the "H" image.
- Spend some time looking at craters of different sizes in different photos and collect images of a few representative ones.
- Do a count of lunar craters by diameter using the same bin sizes that you used for the craters on Earth. You can do this the hard (bt accurate) way by downloading a representative "M" image and measuring each crater by hand with a ruler or in ImageJ. Or you can do an easier, but somewhat less accurate count by going to: http://planetarynames.wr.usgs.gov/.
Click on the Moon button and then click on Crater, Craters in the list of feature types. This will generate a page of all the named craters on the moon (there are thousands of them!), with their locations and diameters. The list is not complete because not all craters have names, but it is mostly complete for larger craters since they are all named. If you scroll to the bottom of the page, you can download the entire list as an Excel spread sheet or text file. That way, you can put the data into a spreadsheet and sort automatically by diameter. Now all you need to do is use the beginning and ending number of the largest and smallest crater in each diameter bin to get the count. Find or calculate the surface area of the Moon in square kilometers (4Πr2) and divide the number in each bin by the surface area. Make a bar graph of your results. Compare your new graph with your Earth crater count.
- Ages have not been determined for most individual lunar craters (though some have been) though statistical estimates have been made using crater counts. Search the Internet or your textbook to find estimates of ages of most lunar craters. (Note: bright-rayed craters like Copernicus are very young compared to most lunar craters.) Compare the ages of lunar impact craters with those you found for the terrestrial craters.
- Prepare a poster or power point comparing the appearance, crater counts, and ages of craters on the Earth and moon. Use your images and graphs of counts and ages in your presentation. Describe the differences. Use the information you have gathered and what you know of the geologic history of the Earth to construct a hypothesis why craters on the Earth and Moon might be so different.
- Crater counts are commonly used tools for unraveling the geologic histories of the other planets and moons. Use the techniques you have developed above to compare crater statistics and ages of crater on the surface of another object like Mars, Venus, or Europa. Compare your data with Earth, and use the differences to infer differences in geologic history of the objects.