Impact Craters

My first thought upon seeing Meteor Crater? That's one BIG hole.

On Earth, it is somewhat rare to come across craters caused by impacts, especially ones that are recognizable as such. So why is Meteor Crater (in Arizona) so well preserved?

The impact that formed Meteor Crater was fortunate enough to happen in the middle of a desert. Here there are fewer of the weathering and erosion processes, such as rain and frost, that would wear away the features of the crater. You can find out more about the crater here. The meteorite which created this 1-mile wide crater was thought to have been only 150 ft (46 m) across. So, how does such a small object create such a large hole?

Try throwing a rock into dry, loose sand. What does the mini-crater it makes look like? What happens why you throw it harder? By throwing the rock harder, you are giving it more energy (which is why it moves faster). Upon colliding with a much larger object (the Earth), the rock stops moving. Because energy can't be created or destroyed, the energy that the rock had while moving must go somewhere. Some of this energy goes directly into the Earth and will cause the sand directly under the collision to become compressed. The rock's collision will also create a mini-shock wave in both the ground and the air. A shock wave in air is similar to wind and will blow the loose sand particles away from the collision site, causing a crater to form that is larger than the rock. The faster the rock is moving, the more energy it has and the larger the crater it will create. This same general process is what formed Meteor Crater and all other impact craters - the only difference is the scale.

While impact craters may be hard to come by on the Earth, they are very easily found all around the solar system. In fact, we see impact craters on nearly every rocky body in the solar systems from planets to asteroids. The easiest place to view impact craters is the moon - all you need is a clear night and a decent pair of binoculars. However, most of the craters on the moon can't be seen from Earth. They are located on the "dark side" of the moon that always faces away from Earth. If craters are so easy to find, why aren't there many on Earth?

Most impact craters were formed billions to millions of years ago when the solar system was still very young. There were more asteroids around that crossed paths with the larger objects. As these smaller objects continued to collide with the larger ones, the solar system was cleaned up and fewer small bodies remained in the paths of the planets and moons. However, collisions do still happen such as when a comet hit Jupiter in 1994. While many planets and moons still bear the visible scars from these violent times, the Earth seems to stand out. You might be thinking of reasons why the Earth avoided these collisions. But, it didn't. When scientists started to wonder about this, they also started looking for evidence of impact craters on Earth. Unsurprisingly, they found them. The Earth is more geologically active than many solar system bodies. Craters get eroded and weathered away, covered up by ocean, filled in as lakes, or recycled into the Earth through plate tectonics. Our planet also has something else that's special: life. It is difficult to recognize a crater from the ground or from space if it is covered by trees and plants. There is also a theory that the Earth was the victim of a giant collision while it was still forming. This collision, it is thought, is what created our unusually large moon.

There is another solar system object that stands out even more than the Earth when it comes to impact craters. Jupiter's volcanic moon Io has few, if any, impact craters on it. Not only is Io geologically active, it is also the most volcanically active body in the solar system. The volcanoes here erupt so often, that it is entirely re-surfaced on roughly a daily basis. This means that any impact craters would be filled in with lava and disappear relatively quickly.

Have you ever seen a meteor shower? None of the objects will create a crater like the one in Arizona. The meteors that we see today often start off as solar system dust particles, not even big enough to be considered a rock. Astronomers have done a very good job of keeping track of all of the known large asteroids that come near the Earth. However, asteroids are very hard to see. This means that astronomers are still discovering new ones quite often. That said, we will likely have at least a few months warning before any large collision!


References and Further Reading
Weathering by Pamela Gore at Georgia Perimeter College
Meteor Crater - Fun Science
The Explorer's Guide to Impact Craters by the Planetary Science Institute
Comet Shoemaker-Levy Collision with Jupiter by NASA's Jet Propulsion Laboratory
Moon Formation Theory by NASA
Solar System Exploration: Io, Overview by NASA