Investigating The Sun-Earth-Moon System
Going Further

Craters on the Earth and Moon

Introduction: A glance at the Moon through a small telescope shows that its surface is dominated by impact craters both large and small. On Earth, however, craters are few and far between, and not very large at that. 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 errant objects cutting through the inner solar system, drawn in by the gravity of each body. The 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 just how many craters are there on Earth? Does the Moon really have more? 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?

Directions:
1. 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. a) 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. b) Do a simple crater count by making a table with bins of increasing diameter, D: D< 1.25 km, 1.25

2. 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 one of the best collections 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 appears 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. 1) Spend some time looking at craters of different sizes in different photos and collect images of a few representative ones. 2) 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 - but accurate - way by downloading a representative "M" image and measuring each crater by hand with a ruler or digitizer. 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 most larger craters have names. The advantage is that if you scroll to the bottom of the page you can download the entire list as an Excel spread sheet or text file that you can put into a spread sheet and sort automatically by diameter. 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 and divide the number in each bin by the surface area. Graph your results. That gives you the same number of craters per square kilometer per diameter bin that you can compare directly with your Earth crater count. 3) 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.

Your Report:
1) Prepare a poster or Powerpoint 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.

2) 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.