The visual difference between the heavily cratered Moon and the relatively smooth Earth is striking. Both bodies exist in the same cosmic neighborhood and have been subject to the same bombardment of meteoroids, asteroids, and comets for billions of years. Despite being a much larger target, Earth displays fewer than 200 confirmed impact craters, while the Moon’s surface records nearly every impact it has ever received. This disparity exists because Earth possesses a powerful, multi-layered system of erasure mechanisms that the Moon completely lacks.
Earth’s Atmospheric Shield
Earth’s atmosphere functions as the first line of defense, acting as a frictional shield that prevents the vast majority of smaller space debris from reaching the surface. Objects entering the atmosphere at high speeds generate immense heat due to friction, causing them to vaporize or explode high above the ground. Space rocks smaller than about 25 meters in diameter often burn up completely before they can leave a lasting scar on the planet’s geology.
This protective effect is absent on the Moon, which has virtually no atmosphere. On the lunar surface, even the smallest fragments, known as micrometeoroids, strike at high velocity, creating countless tiny craters. The Moon’s lack of an atmospheric blanket means that the entire spectrum of cosmic debris successfully reaches the ground. This allows craters of all sizes to accumulate, resulting in a surface saturated with impact scars.
Geological Recycling of Surface Material
The most significant factor in Earth’s low crater count is its dynamic internal heat engine, which drives geological recycling. Earth’s outer layer is broken into tectonic plates that are constantly moving and being remade. In subduction zones, one plate slides beneath another, carrying old crustal material and any craters deep into the mantle where they are melted and reabsorbed.
Volcanism also plays a role, as molten rock rises from the mantle and erupts onto the surface. These lava flows cover vast areas of the crust, effectively burying existing impact structures. This constant creation and destruction of crust means that approximately 80% of Earth’s surface rock is less than 200 million years old. In contrast, the Moon is geologically dead, having cooled rapidly, with volcanic activity largely ending over 3 billion years ago. Its surface has remained unchanged for eons, allowing craters to accumulate indefinitely.
The Impact of Weathering and Erosion
For the few impact structures that survive atmospheric entry and tectonic forces, a relentless cycle of external forces works to erase them over time. The presence of water in all its forms—liquid, solid, and vapor—is the primary agent of this surface modification. Rain, rivers, and oceans continuously wear down elevated crater rims and fill depressions with sediment, gradually smoothing the feature until it is no longer recognizable.
Wind acts as a persistent abrasive, carrying particles that scour rock surfaces, and glacial ice sheets can scrape away or bury impact structures. Biological activity, such as plant roots and the chemical breakdown of rock, further contributes to the degradation of surface features. The combined effect of these processes means that nearly all of Earth’s preserved craters are less than 650 million years old. The Moon, lacking an atmosphere, liquid water, or biological processes, remains a static environment where impact craters are preserved for billions of years.