Scientists estimate the age of planetary surfaces by counting the number of impact craters. The age of craters can be estimated by examining their shape. The oldest craters on the Moon are estimated to be more than 2 billion years old.
Impact craters on Mars
Scientists have mapped the major impact craters on Mars (larger than 4 km diameter) - each red dot is an impact crater. A more detailed count revealed 635,000 impact craters with a diamater larger than 1 km.
Impact craters on Earth
Scientists have also identified the major impact craters on Earth. There are about 150 known impact craters (of any size). The image below is Wolfe Creek crater in Western Australia - it is .875 km in diameter, and estimated to be 300,000 years old.
Where have all the impact craters gone?
TASK 3: All these impact craters are on Earth. See if you can place them in their chronological order - oldest to youngest. Explain your reasoning.
Where do we find impact craters?
Impact craters are not uniformly distributed on Earth. The older surface found in places like Scandinavia, Canada and Australia have collected more impacts, while younger surfaces, or surfaces that have been covered by sediments (or ice) have few craters (Amazon, Greenland and Antartica).
TASK 4: Suggest why there are so many impact craters in the western half of Australia.
Unlike the Moon, on Earth, wind and rain erode craters, but it is another force that buries them - plate tectonics.
Next, we'll compare volcanoes on Mars and Earth.