Easily the most prominent observable geologic features on the Moon and the other terrestrial planets are impact craters. And to a geologist, craters are useful features, because they allow us to make an assessment of the age of a planetary surface and even the nature of its interior. Below, we're going to do a little first-order "crater geology" so you can see how geologists come to "know" what they do about the insides of planets they've never visited.

Answer your questions on a separate sheet of paper, to be handed in the first meeting after the exam (10/29)

The above image of the Martian surface includes the two most common crater types. Simple Craters are typically small, and show a simple morphology with a raised rim and rounded bottom. These are by far the most common craters we have. Complex Craters (the larger one) are bigger, have less prominently raised rims (due to erosion of the rims), and show a raised zone in the center, associated with rebound in the rocks below.

First activity: Find for me and provide the Web addresses for images of simple and complex craters from three Solar System bodies. List the addresses for my perusal.

Having searched up pictures of complex craters from three bodies, note the morphological features that differ about them. Large craters are associated with the ejection of a lot of material, which distributes itself around the crater in different ways. Some Craters have Rays which extend out from the site of impact (best example: Tycho on the Moon), while others show splash-like features, like the Martian crater above. The difference in ejecta profiles can tell us about the makeup of the material on the planet's surface.

Second activity: Surf about and find several pictures of large craters from the Moon, Mars and Venus that show ejecta fields (rays, "splash" or "surge" patterns, or other). Do different planets have different ejecta patterns? If so, hypothesize as to what might cause the difference (i.e., is the planet colder or hotter? Does it have an atmosphere that might hinder ejecta flight? Is there water on the planet, which might make ejecta behave in a more fluid fashion?_

Very large craters show still more complexity: some, like the Mare Orientale from the Moon (shown above) form Ringed Basins. Other impacts are so immense that they trigger melting in the planet's interior, and the crater basins fill with lava. The Lunar Maria are a fine example of craters filled with melt.

Activity 3: Look through the NASA images of the planets and moons for pictures of either Ringed Basins or large craters filled with melt. On which worlds do they occur?

What can craters tell us about the age of a place? Well, craters build up through time, so a more heavily cratered terrane is necessarily older. Of the images above, which represents an older terrane?

The two images above are from Ganymede, a moon of Jupiter. Presumably, Ganymede was made at the same time as the Earth and the Moon. And the meteor flux that is responsible for cratering in the Solar System has declined in a regular way through time - lots of impactors early on, but fewer as time passes, because with every impact on a larger body, you have one less rock flying around the Solar system trying to hit something.
So, if there are differences in the amount of cratering on different parts of Ganymede, what does that say about when Ganymede's current surface formed? (ie., was it all made at the same time, or not. And if not, what does that imply about the INTERIOR processes in Ganymede (Big hint: Why AREN'T there so many craters on the Earth, while the Moon is covered with them?)

Judging from the abundances of craters on their surfaces, organize the terrestrial planets (Mars, Venus, Earth, Mercury, and the Moon) in terms of the mean age of their surfaces. This will take a little browsing, as you need to look at SEVERAL images of each planet to get an idea of how cratered up they are. We'll discuss this listing, and the other answers, in class.

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