Final answer:
Some bodies maintain indications of impacts due to lack of atmosphere and inactive geologies, which prevents the erasure of craters. An atmosphere can erode craters, while geological activity can resurface a planet, removing evidence of impacts. Crater counting is used to estimate surface ages based on these impact records.
Step-by-step explanation:
Some bodies in the Solar System maintain countless well-preserved indications of impacts on their surface even after many millions of years due to a variety of factors. These factors can include a lack of atmosphere, absence of geological activity, or both, which together affect the level of crater preservation on a celestial body's surface.
Crater Preservation
One factor contributing to the preservation of impact craters is the lack of an atmosphere. Bodies like Mercury and the Moon have very thin to no atmospheres which means there is no weathering or erosion to erode away the evidence of impacts. No atmospheric protection also means that more meteorites can strike the surface directly without being burned up.
Another factor is the level of geological activity. Bodies such as Earth have dynamic geological processes that can over time lead to the deformation, erosion, or covering up of craters. Earth's tectonic activity and planetary weathering, such as the erosive effects of water and ice, continually resurface the planet, removing evidence of ancient impacts. Conversely, celestial bodies with inactive geologies, like the Moon, do not have these surface renewal processes, which means craters can remain visible for much longer periods.
These factors combined with the constant rate of impacts over the solar system's history help explain why some planetary surfaces are covered in craters while others are not. The crater counting technique is used to estimate the age of a surface by assuming a constant impact rate over time, and noting that the absence of erasing forces allows for an accumulation of craters that is proportional to the object's exposed surface age.