The Moon’s surface is extensively covered with circular depressions, known as impact craters. These bowl-shaped formations are created when solid objects, such as asteroids or comets, collide at high velocity with a planet or moon’s surface. In contrast, Earth, despite being a larger target, appears relatively smooth and largely free of such prominent scars. The varying geological and atmospheric conditions on each body play a significant role in determining how many of these impact features remain visible over time.
The Moon’s Unchanging Surface
The Moon’s heavily cratered appearance is a direct result of its unchanging surface conditions, which allow impact scars to persist for billions of years. A primary factor is the near-total absence of an atmosphere. Without an atmosphere, incoming meteoroids do not burn up from friction, nor is there wind to erode existing craters. This means even small impacts leave lasting marks.
The Moon also exhibits very little geological activity, unlike Earth. There are no active plate tectonics, significant volcanism, or other internal processes that would reshape or resurface its crust. While volcanic activity occurred in its early history, forming the dark maria, it largely ceased about 3.1 billion years ago. This lack of internal forces means craters are not buried or erased by crustal movement.
Another contributing factor to crater preservation is the complete absence of liquid water. On Earth, water plays a substantial role in erosion through rainfall, rivers, and ice. Without water, the Moon lacks this powerful erosional agent. The only significant surface modification comes from continuous bombardment by micrometeorites and small moonquakes, which cause very slow erosion compared to Earth’s processes.
Earth’s Dynamic Shield
Earth, unlike the Moon, has a dynamic environment that constantly reshapes its surface, erasing most impact craters. A thick atmosphere acts as a protective shield. Most smaller celestial objects burn up due to friction in Earth’s atmosphere, appearing as meteors before reaching the ground. Only larger objects create significant impact features.
Earth’s active geological processes also obscure impact craters. Plate tectonics constantly recycle and reform the planet’s surface. Craters can be subducted, uplifted, or buried by new crustal material, disappearing over millions of years. Volcanic activity can also bury impact structures under lava flows, contributing to their disappearance.
Additionally, Earth’s surface is subject to relentless weathering and erosion by wind, water, and ice. Rain, rivers, glaciers, and atmospheric processes steadily wear down and fill in craters over geological timescales. This continuous process can degrade a crater’s features until it becomes indistinguishable or is completely removed. Consequently, only about 190 terrestrial impact craters have been identified, most less than 500 million years old.
A Tale of Two Histories
The contrasting crater records of the Moon and Earth tell a story of two very different planetary histories. Both celestial bodies formed approximately 4.5 billion years ago and experienced intense bombardment by asteroids and comets in the early solar system, a period sometimes referred to as the Late Heavy Bombardment. During this era, countless impacts scarred their surfaces, creating the craters still observed on the Moon today.
Earth’s persistent geological activity and active atmospheric and hydrological cycles ensure its visible crust is geologically much “younger” than the Moon’s ancient surface. Earth’s processes continuously renew its surface, burying or eroding the evidence of past impacts. The oldest known impact structures on Earth are only about 2 billion years old, with many older ones likely erased. The Moon, lacking these dynamic forces, has preserved its early bombardment record, appearing as a frozen snapshot of the solar system’s violent past. The difference in crater density is not because Earth was hit less frequently, but because its active processes effectively erase the geological evidence of these cosmic collisions.