Session 5A: Craters!

This session is mostly going to be a "lab" activity. But first a little background.

Everyone "knows" that the moon is covered with craters. But in fact, until the invention of the telescope, this was not at all obvious. While the craters on the moon are large, the moon itself is so far away that it is hard to tell that the brightness differences across the face of the moon are due to terrain changes. But ever since Galileo turned that first telescope onto the moon and discovered that it has craters, craters have been turning up everywhere.

Whether Weather

How hard it is to find craters on a planet all depends on the weather. No, not whether or not it is cloudy or raining, but how much weather it has in the first place. The normal weathering processes which cause erosion here on Earth slowly erase craters from the terrain. On the Moon, with no atmosphere and no weather, craters are more-or-less forever. Mercury also has craters. So do most of the moons of other planets. Venus is shrouded in clouds (made of sulfuric acid!) but radar images of the surface reveal craters. Earth has a few crater remnants, mostly in arid regions, but even sandstorms will erase them. Mars has craters. We can't even see the surface of the gas-giant planets (Jupiter, Saturn, Uranus, Neptune), but they probably don't have any craters left from any impact they had in the past. Jupiter was smacked hard by the fragments of comet Shoemaker-Levy in 1994 but whether or not it made any craters is anyone's guess.

Old Craters

Barringer Crater, Arizona, United States

The first meteor crater I learned about was Barringer Crater in Arizona. Before people realized that a meteor impact was probably a major cause for the extinction of the dinosaurs, this was probably the most famous meteor crater on Earth and the first crater to be clearly identified as a meteor impact. Barringer Crater is about 1,200 meters in diameter, the rim rises about 45 meters above the surrounding plain, and the crater center is about 170 meters deep. The force of the impact vaporized the meteor and all that remains are several tons of iron chunks scattered across an area 8--10 miles in diameter centered around the crater.

Barringer Crater is not very old, only about 50,000 years or so. While this is fairly recent in geologic terms, it is probably too long ago for there to have been people in North America. Most scholars believe there were no humans in North America before about 20,000 years ago although one archeological dig1 has found human artifacts in South Carolina which were dated to about 50,000 years ago. It's fascinating to imagine what a person might have seen (assuming they were far enough away to survive!) but there probably wasn't anyone around to witness the event.

Lonar Crater, Deccan Plateau, India

 

The Lonar Crater was initially assumed to be like any other crater in the area, which is to say, it was a volcanic crater. The Deccan Flats are a region with a long history of volcanic activity and large-scale volcanic activity there may have also played a role in the dinosaurs demise. Howver, the Lonar Crater is meteor impact between 35,000 and 50,000 years ago. It is somewhat larger than Barringer crater at 1,800 meters in diameter but is also shallower, possibly due to the harder rock (basalt) where the impact occurred.

Tenoumer Crater, Sahara Desert, Mauritania

 

This impact crater is 1,900 meters across and is one of three craters which occur in almost a straight line in the region. Sometimes, a meteor will break up as it enters the atmosphere and result in several craters all lined up. Although that can happen, it turns out that that is not the case here. All three of the craters date to different periods. The Tenoumer Crater is "only" about 10,000 to 30,000 years old again raising the possibility that there were actually humans around to see it.

Kebira Crater, Western Desert, Egypt

 

This is a very old crater and is, in fact, nearly impossible to identify as a crater. It dates to probably 100 million years ago and shows a double ring structure. The crater is about 31 kilometers across and mostly buried in desert sand which has eroded much of the structure. While the area has an abundance of "desert glass," a form of melted sand that occurs when a meteor impact causes such high temperatures that then rock and sand melts and then is flung across the region, it wasn't until after 2006 that anyone thought to use satellite images to try to find the crater remnant itself.

Pingualuit Crater, Quebec, Canada

 

This crater was noticed by a US Air Force crew when flying over the region in 1943 but wasn't explored until the 1950s. The crater is about 3,440 meters across and 267 meters deep. This is also an "old" crater by human standards, having been formed by an impact about 1.4 million years ago. The discovery and exploration of this crater eventually led to identifying more than 20 other impact structures in eastern Canada.

Others

The lower part of the Chesapeake Bay, about 85 kilometers across, was also formed by a meteor impact about 15 million years ago. Some have wondered if the Hudson Bay may have been formed by an ancient impact, but an analysis of the rock structures do not show the normal signs of a meteor impact. The best source for finding confirmed meteor impact sites is the University of New Brunswick's Earth Impact Database.

Resources

The University of New Brunswick's Earth Impact Database, http://www.unb.ca/passc/ImpactDatabase/, has maps and lists of verified impact sites around the Earth.

The Lunar and Planetary Institute, http://www.lpi.usra.edu/, has research material on impact crater formation, including the figure used on page two of the class handout.

http://www.barringercrater.com/ has a complete history of Daniel Moreau Barringer's misguided efforts at mining to retrieve the meteorite that caused the crater.

References

1 The Topper archeological site in South Carolina has been reported to have artifacts of human origin that date to 50,000 years ago, but the evidence not conclusive due to natural weathering which has eroded the artifacts.

 

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