Impact Craters

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Impact craters are the result of an object hitting a larger surface at hypervelocity creating a circular depression in the surface. Impact craters have been found on many of the planets in our solar system (including Earth) as well as on larger asteroids and moons orbiting the planets nearest to us.

Impact craters were mistakenly thought to be remnants of ancient volcanic explosions for many years until a dedicated group of scientists looked closer at the features that distinguished impact craters from other similar geologic formations. Impact craters are circular depressions with rims and floors that are usually lower than the terrain around them; they can be simple circular indentations or larger, jagged depressions that show geographic evidence of the impact. Impact craters are caused by objects, in Earth’s case, from space, coming through the atmosphere and impacting onto the surface of the Earth. This impact coming in at incredibly high velocities causes the impacting object to explode or vaporize, and the impacted surface to remain dramatically changed by the force of the impact.

Artist rendering of a planetoid crashing into earth.  Artist: Don Davis (work commissioned by NASA)

Artist rendering of a planetoid crashing into earth. Artist: Don Davis (work commissioned by NASA).

Finding Impact Craters

With the moon as a guide to see how many estimated impact craters should be found on the surface of the Earth- the moon, being in the same region of space as the Earth, would theoretically be hit with roughly the same amount of interstellar bombardment as Earth- scientists realized that they weren’t finding as many impact craters here as they thought.

This is because of a couple of reasons: one, scientists are still unable to find impact craters in one region of the world that remains relatively invisible to us, the bottom of the ocean. The sea floor could reveal more impact craters than have so far been found on land, but scientists are unable to accurately map underwater features at this time.


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Secondly, surface processes at work on Earth as well as other planets like Mars and Venus are so great that the evidence of impact craters has simply been wiped away.  These surface processes include wind and water erosion, plate tectonic movement, and the general wear and tear of a universe constantly in motion. Scientists often call the original topography of the impact crater as it has been changed by the above factors the impact structure or the astrobleme.

By studying the evidence of impact craters on Earth as well as on other planets in the solar system scientists are able to come up with a general outline of what interstellar activities were going on at certain times in the universe’s long history. Much like tree rings can tell a researcher about floods, droughts, and fires in the surrounding areas, impact craters around the solar system can tell scientists about periods of time where more impacts were made, and times when interstellar bombardment was slower.

Determining What is an Impact Crater

Impact craters are often confused with volcanic formations, calderas and ring dikes, which have similar features. All of these geological formations can be identified as different from one another by some of their physical features as well as the materials they are made up of. For instance, different rock materials will be found at a volcano than are found at an impact crater site. These two kinds of geological upheaval can look similar from the outside, but their effects on the earth around them are very distinct.

One distinct mineral feature of an impact crater are materials that have gone through shock-metamorphosis. Shock-metamorphic materials (often rocks) are materials that have been changed because of the impact on the geography around them. This often involves melting or changing the composition of a material; evidence of this is shocked quartz, a shock-metamorphic material found uniquely at impact crater sites on Earth. The supersonic shock waves that emanate from the point of contact of the meteor and Earth’s surface create these geologic changes that can allow scientists to identify an impact crater thousands of years after its creation.

Approximately 183 impact craters have been found and identified on Earth, the newest one recorded as being created in 1947. These impact craters are often hard to identify given surface processes, inability to look under the ocean, and the fact that not all impact craters look the same. (see: Geography of Impact Craters).

The impact of an asteroid or comet several hundred million years ago left scars in the landscape that are still visible in this spaceborne radar image of an area in the Sahara Desert of northern Chad. The concentric ring structure is the Aorounga impact crater, with a diameter of about 17 kilometers (10.5 miles). The original crater was buried by sediments, which were then partially eroded to reveal the current ring-like appearance.

The impact of an asteroid or comet several hundred million years ago left scars in the landscape that are still visible in this spaceborne radar image of an area in the Sahara Desert of northern Chad. The concentric ring structure is the Aorounga impact crater, with a diameter of about 17 kilometers (10.5 miles). The original crater was buried by sediments, which were then partially eroded to reveal the current ring-like appearance. Image: NASA.

The shape and composition of an impact crater can vary based on the kind of material that hits the surface as well as what the surface is made of. If a high density meteor hits the Earth in a basalt canyon, this will make a different kind of impact crater than it would if the same meteor hit in the middle of the desert in Arizona. Typical impact craters are circular, dish-like depressions in the surface of the Earth, while others are multi-layered or terraced based on the material that makes up the impacted surface. An example of this type of crater is the Nested Craters on Mars.

Barringer Meteor Crater in Arizona.

Barringer Meteor Crater in Arizona. Photo: D. Roddy, USGS.

A unique impact crater site can be found on Hyperion, a moon of Saturn, where the porous surface material causes asteroids to bury themselves in the soft surface of the moon. This causes much different impact crater formations than on other, harder surfaces.

Impact craters are an important feature of our Earth; studying them can help us determine our history as well as the history of the solar system around us.

Siljan impact crater. Several lakes trace the remnants of the eroded impact crater that was formed by a meteorite impact about 370 million years ago. With a diameter of 55 km it is the largest impact structure in Europe. Lake Siljan is the large lake in the south of the ring. Source: NASA.

Siljan impact crater. Several lakes trace the remnants of the eroded impact crater that was formed by a meteorite impact about 370 million years ago. With a diameter of 55 km it is the largest impact structure in Europe. Lake Siljan is the large lake in the south of the ring. Source: NASA.

References

Wikipedia. Impact Crater. http://en.wikipedia.org/wiki/Impact_crater

Wikipedia. List of Impact Craters on Earth. http://en.wikipedia.org/wiki/List_of_impact_craters_on_Earth

Earth Impact Database. Introduction to Impact Craters. 2014. http://www.passc.net/EarthImpactDatabase/

Views of the Solar System. Terrestrial Impact Craters. 2011. http://www.solarviews.com/eng/tercrate.htm