Wednesday, September 24, 2008

The Carnival in space- Accretionary Wedge 13

This month, Good Schist ask about our interest in geology off planet. I am cheating somewhat since my post is about a phenomenon that happens here on earth as well as in space. That is the idea of impact craters.

Looking up at the moon the cratered surface is pretty easy to make out even with the naked eye. With a small telescope you can easily see that indeed those shapes up there are little pock marks. Boy, did that observation get Galileo in trouble when he suggested that a heavenly body might just have zits.

We can be reasonably sure that an old surface will have a greater chance of being hit by objects flying in space. the older the surface the greater the density of craters there should be, conversely, new surfaces should be somewhat clean of impacts. Using this idea, I have had many of my high school and undergrad Astronomy students counting craters to determine relative ages of different surfaces. The maria regions having fewer craters indicate a newer surface and the upland regions being just pockmarked with craters indicate an older surface.

Airless bodies, like the moon are pretty straight forward. However, the Earth and any other planets with active weathering are a little harder to study. We want to look not only at the density of craters, but examine how quickly they are obliterated by geology. The Earth then ups the ante with active plate tectonics so that the old craters are not just weathered away, but completely recycled. And then there is Venus. The old debate between catastrophism and uniformitarianism has been getting a work out on that planet. . I won't get into the Venus mystery as hypo-thesis covers it in great detail.

Craters have now been counted all over the solar system, the inner terrestrial planets and the moons of the outer gas giants. It is fun to show students how much information can be gleaned from a bunch of pictures sent back from our robotic explorers.

I think the overall favorite impact crater we ever studied was Herschel on Saturn's moon Mimas. Every student of mine has called this the death star, interesting because the movie was made a few years before we had any imagery from Saturn showing us the moon.

This unit always brings up the idea concerning the chances of a repeat K-T impact on our modern world. Most kids don't believe it could happen again until we pull out the Earth Crossing Asteroid list. But as I often tell them, Geology happens and that includes impacts from space.

1 comment:

David Bressan said...

Fascinating - it’s a pity that meteoric metamorphism and geomorphology is not (enough) often to be teached on geology courses.

Maybe this come handy: