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How can crater counts help scientists compare the ages of planetary surfaces?

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four impact craters — Meteorite crater Four impact craters of the same size (30 km [20 miles] in diameter) imaged by spacecraft on different solid bodies of the solar system and reproduced at the same scale. They are (clockwise from upper left) Golubkhina crater on Venus, Kepler crater on the Moon, an unnamed crater on Jupiter's moon Ganymede, and an unnamed crater on Mars. The images are oriented such that the craters appear illuminated from the left; the Venusian crater is imaged in radar wavelengths, the others in visible light.

Scientists use crater counts to determine the relative ages of planetary surfaces based on a straightforward principle: older surfaces have accumulated more impact craters over time, while younger surfaces have fewer.12

By performing simple counts of craters larger than a specific size per unit area, researchers can compare different regions of a solid body to gain insight into its geologic history. A relative absence or sparseness of craters in a particular region indicates that the surface has been recently altered or "resurfaced" by geologic activity, such as volcanism or plate tectonics, which effectively erases older impact scars.12

While determining relative ages is conceptually simple, assigning absolute ages in years is more complex. For the Moon, absolute dating is possible because surface samples were collected during Apollo missions and dated in laboratories. For other planetary bodies, scientists must infer absolute ages by estimating impact rates from asteroids and comets, though these estimates often carry significant uncertainties due to a lack of direct data.12