How can pupils estimate surface ages by counting craters?
Pupils count craters in equal-sized areas on images of the Moon or Mars and compare regions, learning that more craters usually means an older surface.
Crater counting is a real method planetary scientists use to estimate the ages of surfaces, and pupils can try a simple version.
The principle. Surfaces collect craters over time. A surface with many craters has been exposed longer than one with few. On the Moon, the bright highlands are heavily cratered and old, while the dark maria, flooded later by lava, have fewer craters.
Activity. 1. Provide high-quality images of the Moon showing both highlands and maria, or of Mars showing different terrains. 2. Pupils lay a grid of equal squares over the image, perhaps on tracing paper or digitally. 3. In several squares from each region, they count craters above a chosen size. 4. They calculate the average number of craters per square for each region. 5. They compare: which region is older?
Calibration. Explain that Apollo samples allowed scientists to link crater counts on the Moon to actual ages. For example, mare surfaces with certain crater densities were dated to about 3 to 3.8 billion years, and China's Chang'e 5 samples gave an age of about 2 billion years for a younger region. These dates help estimate ages on other worlds, such as Mars.
Complications. Secondary craters from ejecta, crater erosion, overlapping craters and uncertainty in impact rates make real crater counting more complex.
Extension. Pupils compare crater densities on the Moon, Mercury, Mars and icy moons, and discuss why Earth has so few craters.
This links data handling, sampling and geological reasoning.