Asteroids and comets are small solar system bodies (SSSBs). They are speeding through our solar system all the time. Sometimes, one of these SSSBs collides with a larger planetary body, such as a planet or moon. These collisions are called impact events. They can cause large depressions on the surface of a planet, moon or other body in the solar system. They can even cause large depressions on other asteroids.
Did you know?
The largest asteroid is called Ceres. It is about one-quarter of the size of Earth’s moon
A comet is a chunk of ice mixed with rock that comes from the outer part of our solar system. An asteroid is a piece of rock in orbit generally between Mars and Jupiter.
What is an impact crater?
Impact craters tend to be circular. This does not mean the object that created the crater was circular. Instead, the circular shape happens because there is typically a massive explosion upon impact. This explosion causes material to fly out in all directions. This material is called ejecta.
Sometimes, an elongated (oval) crater can form. This happens if an object strikes the surface at a very low angle.
What are the parts of an impact crater?
Impact craters have the following structures.
- Floor: The floor is the bottom of a crater. It may be flat or shaped slightly like a bowl. The floor is usually below the level of the surrounding ground.
- Walls: The walls are the interior sides of a crater. They are usually quite steep. Over time, step-like areas may form. This happens as the walls collapse due to gravity.
- Rim: The rim is the top edge of a crater. It is usually above the level of the surrounding ground. This happens because ground material gets pushed up during the impact.
- Central peak: The central peak is the higher area in the center of larger craters. It happens when the impacting object is large. Some of the material pushed towards the edges of the crater slides back into the centre, creating a peak.
- Ejecta: Ejecta is the rock material thrown out of a crater during impact. Usually, you can see it all around the crater. The ejecta is thickest closest to the crater and thinnest further away.
- Rays: Rays are the bright streaks of ejecta that extend away from a crater, just like rays from the Sun.
Illustration - Labels
Parts of an impact crater include 1:floor, 2: walls, 3: rim, 4: central peak, 5: ejecta, 6: rays. This is the Copernicus Crater on the Moon.
What are the different types of impact craters?
Simple craters are small, bowl-shaped craters with smooth walls. Complex craters are larger craters. They have features such as central peaks and stepped sides. The Moon’s Copernicus Crater is a complex crater. Simple and complex craters can range in diameter. Some are a few dozen metres. Some are about 300 km!
Impact basins are craters that are bigger than 300 km in diameter. The Moon has several impact basins. From the Earth, they look like large dark areas. Impact basins on the Moon were created when huge objects struck its surface.
Photograph - Additional Information
Full moon with impact basins. The distinct bright ray crater at the bottom of the image is the Tycho impact basin. The dark areas are lava rock-filled impact basins. The image was taken by the Galileo spacecraft in 1992.
What factors determine the size and shape of a crater?
The size and shape of a crater depend on several factors:
- the mass of the impacting object;
- the density of the impacting object;
- the velocity of the impacting object; and
- the geology (type of rock) of the surface the object strikes.
The greater the mass and velocity of the object, the larger the diameter of the crater will be.
On the Moon and other planets, craters look very much like how they looked when they were formed. But on Earth, craters tend to get worn away because of weathering and erosion. They also get destroyed through plate tectonics and volcanic activity. The processes on Earth’s surface can eventually erase impact craters. But as of 2015, 128 terrestrial impact craters have been identified on Earth.
Where can you find impact craters in Canada?
Some of the world’s largest impact craters are in Canada. There is a very interesting pair of impact craters in Quebec, near the eastern shore of the Hudson Bay. Both of these impact craters are now filled with water. One is West Clearwater Lake. It is 36 km in diameter. The other is East Clearwater Lake. It is 26 km in diameter.
Scientists think the impacts that caused these craters happened about 290 million years ago. Scientists used to think that the lakes were created by a double impact. A double impact is two impacts happening at the same time. But there were many geological studies of the area. Now, scientists now think the craters were formed about 4 million years apart!
The Mistastin crater in Labrador also contains a lake. The Mistastin Crater is the result of an impact that happened about 36 million years ago. Scientists think the original impact crater was about 28 km wide. Since then, the crater has become much smaller because of glacial erosion.
There is an island In the middle of the lake. Scientists think this island is a central peak. That would make it a complex crater! Scientists believe that the impact event that created this crater generated surface temperatures of 2 370 degrees Celsius. That’s the hottest temperature ever recorded on Earth’s surface!
The Pingualuit Crater is a young impact crater in the Ungava Peninsula of Quebec. In French, it is called the Cratère des Pingualuit. Pingualuit is an Inuktitut word meaning where the land rises. The Pingualuit Crater is approximately 1.4 million years old. This crater has also become a lake. It is called Pingualuit Lake. Although Pingualuit Crater is only 3.4 km in diameter, it is 400 m deep. Pingualuit Lake is 267 m deep. That makes it one of the deepest lakes in North America! Because the lake is so clear, the local Inuit to refer to it as the “Crystal Eye of Nunavik.”
Did you know?
The largest impact crater in Canada is the Sudbury Basin in Ontario. It is about 62 km long and 30 km wide.