A crater is simple, right? Well, not really, they are not just holes in the ground (well they are really but they do differ depending on the size of the crater). Once we have become experts on the different types of craters and why they are classified differently then I’ll go through the different ways of spotting a crater.
The way those intelligent scientists classify craters depends on-
- The size of the thing hitting the surface.
- The speed of the thing (I’ll call that impactor to sound intelligent).
- The things that the impactor are made up of.
- The composition of the rock that is being impacted (the target rock).
- The angle that the impactor hits the surface.
- The gravity of the target planet.
- How porous or strong the impactor is.
Microcraters
Working from the smallest to the largest, microcraters can be as small as no 0.0000001 metres in diameter. These are made by hypervelocity (over 3000 m/s and allows metals to behave like a liquid).
Simple craters
Simple craters are next up on the scale and are a bit more of a serious beast. Meteor crater in Arizona is a typical example of a simple crater. A simple crater is up to several kilometres in diameter and has no central uplift and no terracing. Generally, if you wanna get slightly technical the rim to the floor depth is 1/5 of the diameter.
Simple crater
Complex craters
These are the most common ones in the solar system and are seen on the moon and other bodies. The main difference between complex craters and simple craters is the gravity of the body being hit and the strength of the target material. These two things will produce craters that are two kilometres in diameter if they land in soft sediments on the Earth and 4 km if they land in much more substantial rock on the Earth.
On the moon it is a bit of a different story as the moon has only has one sixth of the Earth’s gravity. Therefore a larger asteroid or comet is needed to reduce a complex crater with the correct characteristics. The size that is needed is 10 km to about 20 km. If you saw one of these coming towards you on the Earth it would be a good idea to put your head between your legs and kiss …
On Mercury the gravity is one third that of the Earth’s so smaller asteroids or comets are needed to create the characteristics of a complex crater. The impactor needed tends to be around 7 km in diameter.
complex crater
Elongate craters
These are special type of crater that depends on the angle of the impact of the asteroid or comet with the surface. Most impact craters are circular as described in my previous post but at 10° to the surface the impactor tends to glance off and cause a crater that is elongated. The objects that produce elongated craters aren’t small and tend to be bigger than you would think.
elongate crater
Multi-ring basins
These are some of the biggest craters that are found in the solar system. They are produced by the impactor hitting the hard, rigid surface and causing ripples in the layer beneath the surface (the asthenosphere). The asthenosphere is plasticky and viscous and moves in response to the impact. The layer above the asthenosphere, the lithosphere, ripples but is much firmer and rigid and therefore cracks form. These cracks form in circles around the impact.
multiring basin
That’s all the different types of craters that you may come across in the solar system. Outside of the solar system, who knows? It would be nice to spot some of these craters with your telescope or binoculars or even your eyes. On the Earth craters are hard to spot because of erosion and weathering but it is possible although in some cases you have to have quite an imagination.
Poor a list of all the craters on earth that is regularly updated you might like to try the impact cratering database.
Some of the indicators to look for when looking for an impact crater-
The first is basically flat quartz and the second is only produced by comets, asteroids and meteorites in the Earth and forming impact craters. These features are not shown by any other geological process so this area is certain to be an impact crater. Glass is obviously found on Earth but is a very good indication of an impact crater and is formed by the shock of the impact melting the rock.
It is possible to see craters on the moon with the naked eye but if you get a pair of binoculars or even a telescope much more can be seen. One of the very best places to look is the Internet as there are superb images as for most of us places such as mercury and Uranus are inaccessible. Now that you are a crater expert you can really interest your friends telling them about the latest craters and get some street cred in the, err, crater geek world.
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