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Why are absolute age estimates for cratered regions on other planets uncertain?

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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.

Absolute age estimates for cratered regions on planets other than the Moon are uncertain because they rely on indirect inferences rather than direct laboratory dating of surface samples.

While scientists can determine the relative age of a surface by counting the number of craters per unit area—as older surfaces naturally accumulate more impacts—converting these counts into absolute ages in years requires precise knowledge of the rate of crater formation. Specifically, researchers must estimate:

The frequency of asteroidal and cometary impact rates.12
The size distribution of the impacting objects.1
The relationship between the size of an impacting object and the resulting crater diameter.1

Because there is very little empirical data available to calibrate these estimates, particularly regarding historical impact rates, the resulting absolute ages for planetary surfaces carry large uncertainties relative to the age of the solar system. In contrast, the Moon’s absolute chronology is more reliable because surface samples collected during Apollo missions were returned to Earth and dated in laboratories.12