Tutorial
Impact Craters
The impact cratering process is the most ubiquitous phenomena in our solar system. The Martian surface has some of the most impressive impact craters and thus makes studying these features an important task to many planetary scientists. Impact crater's form almost instantaneously when a cosmic projectile makes contact with the ground releasing a high-pressure shock wave that then causes the ground to be ejected into a characteristically circular shape. Impact craters are of particular interest because they may be breading grounds for life. Right now the Mars rover Curiosity is exploring an impact crater named Gale crater, scientists believe it may be a site for past water activity! There are two main types of impact craters (simple and complex) depending on the size of the projectile.
Simple Impact Craters
These craters are small bowl shaped depressions with an uplifted overturned rim. The transition from small to complex craters is ~ 7km on Mars.
Practice identifying a simple crater
-
A Martian volcano in daytime IR
-
Colour view of Martin, a complex impact crater
-
Simple crater in the Libya Montes
This is actually a volcanic caldera and not an impact crater.
From looking at the shadow you can see that this "crater" is a on the top of a cone of material. An impact crater would be at the same level as the ground surrounding it.
This is actually a complex impact crater.
You can tell it is not a simple crater because it is greater than 7 km in diameter, has terraces around the perimeter, and a central uplift or peak. Compare this to the two simple impact craters just to the bottom right of the large crater. They are smaller and have a simple bowl shape.
Yes this is a simple crater!
You can tell this is an impact crater and not a volcanic cone because the surrounding area is somewhat flat with just a thin rim of raised material. A volcanic cone would be raised up with the central depression on top of the cone above its surroundings. It is not a complex crater because it is less than 7 km in diameter and has a simple bowl shape.
Complex Crater
These impact craters are larger than simple craters and have terraced walls and central uplifts. Some of these craters can be as large as 200 km in diameter! These craters are of interest because they may contain hydrothermal deposits. On Earth these systems contain small colonies of life.
Practice identifying a complex crater
-
Santa Fe, a complex impact crater, viewed in daytime infrared
-
Pingos and polygons
-
Skylight on a lava tube
Correct! This is a complex crater.
There is a skirt of debris (called the "ejecta blanket") around the impact centre and a partially collapsed outer rim with terraces around the inside of the rim. The crater also exhibits a central peak, likely formed from molten material pushing up in the centre of the crater during its formation. The two simple craters to the bottom and left of the main crater show contrast with their smaller size and simple bowl shape.
This is not an impact crater.
The shadows here reveal that this is actually a hill and not a crater. It is believed that this structure is a pingo. Pingos, which are are a perglacial feature, are raised mounds formed when underground water freezes to form ice, which expands and pushes up the overlying ground.
This is not a complex impact crater.
At first glance this looks like an impact crater, but you can tell it isn't because there is no raised rim. The edges drop away from a relatively flat surface. It is believed that this small (~200 m in diameter) depression was formed when the material overlying an empty lave tube collapsed, leaving large boulders littering the floor of the tube.