On Mars and on many other bodies, scientists find the relative age of areas by comparing the number of craters in a given area. The area to the right would be considered younger than the area in the next picture. The first picture seems to have no craters, while the other picture of the same size has many craters of different sizes. Crater stikes are really rare. Can you ever remember one forming in your lifetime? Actually, probably only a few craters have appeared on the Earth's surface throughout the time that humans have been around. One of the youngest impact craters on Earth, Meteor Crater, is believed to have formed 50,000 years ago.
This dating by the number or density of craters can be understood based on the beharior of some red neck males who fire their weapons at all highway signs. New signs have few bullet holes, whereas old signs are peppered with numerous holes.
Often on bodies like Mars or the Moon, scientists can only determine the sequence of events--not the exact ages.
Near the top of the image, crater B formed before crater A. We can see that the rim of crater A sits on the floor of B--not the reverse.
For the other series of craters. Crater C came first, then D, then F. If F came before D, the impact that produced D would have erased crater F.
Layers (Strata)
The layers shown in walls represent different environments of the past. One layer may have formed when the whole area was covered by sand dunes. Another may be from an erupting volcano. Some may originated when a sea existed here.
The deeper the layers, also called strata, the further in the past the layer was created. Layers on Earth are best seen in the Grand Canyon. In the Canyon one goes back in history 20,000 years with each step down into the Canyon.
Much of Earth's history is gone--most material ends up in the ocean, then is moved deep underground to be melted in the process of Plate Tectonics. On Mars little Plate Tectonis has occurred, so a lot of rocks billions of years old can be found. That also means that Mars has a lot more stuff to look at because so little has disappeared. Mars is truly a geologist's paradise!
The image to the right shows a cliff with several layers. Some are dark in color, some light
The picture to the right,
from the Coprates region,
shows layers that can be
traced long distances.
The picture is about 2 miles in width.
The image to the left shows layers
at the top. There may be more layers
under them, but they are covered
with material like dust. At the bottom
of the cliff (upper right) are sand dunes.
The picture below includes a giant cliff that does not show layers.
The picture is about 2 miles wide, so the cliff must really be tall.
It may very well be higher than any straight cliff on Earth.
On the right, a small crater appears patially beneath an overhang. It is an example of an exhumed crater. First a large crater occured, then layers were deposited. The samll crater was than formed by an impact. It was then preserved by many thick layers. When those layers eroded away, the small crater became visible as it is today. Note that if the crater formed recently, the overhang would have been destroyed by the impact that made the crater.
Mars experts have found many cases like this in which various features were buried then much later exhumed.
Click here to see more images.
Last revised January 2007
