The stark, heavily marked surface of the Moon is defined by countless circular depressions commonly referred to as “holes,” which scientists call impact craters. These features are the result of cosmic collisions that have occurred over billions of years, scarring the lunar landscape. The density and size of these craters provide a detailed record of the solar system’s history since the Moon’s formation. These depressions range in size from microscopic pits to vast basins hundreds of kilometers across, creating the familiar, textured appearance visible from Earth.
How Lunar Craters Form
The formation of a lunar crater begins with a hypervelocity impact, typically involving an asteroid, comet, or meteoroid slamming into the surface at speeds averaging around 20 kilometers per second. This extreme velocity means the impacting object possesses immense kinetic energy, which is instantaneously converted into heat and shockwaves upon contact. The energy release vaporizes most of the incoming body and a portion of the surface rock, creating a massive explosion rather than a simple dent.
This explosive event generates powerful, outward-moving shockwaves that compress and fracture the lunar material beneath the impact point. The initial phase is called excavation, where the shockwave and subsequent decompression eject a massive volume of pulverized rock outward, forming a temporary cavity. This ejected material, known as the ejecta blanket, settles around the rim and extends outward, often creating secondary craters where larger fragments land.
The total diameter of the final crater is typically 10 to 20 times larger than the size of the original impactor. Much of this intense bombardment occurred during the Solar System’s early history, particularly during the Late Heavy Bombardment, roughly 4.1 to 3.8 billion years ago. The resulting ancient craters are a preserved testament to this violent cosmic era.
Why Craters Last on the Moon
The abundance of visible craters on the Moon, compared to Earth, is primarily due to the Moon’s fundamental lack of geological activity and atmosphere. On Earth, weathering from wind, rain, and water erosion constantly works to smooth, fill, and ultimately erase craters over relatively short geological timescales. The Moon has virtually no atmosphere, meaning there is no wind or water to facilitate this kind of surface erosion.
The Moon also lacks the internal geological “housekeeping” mechanisms that Earth possesses, such as active plate tectonics. Earth’s crust is continually recycled, subducted, and resurfaced by the movement of tectonic plates and volcanism, which destroys or buries old impact sites. In contrast, the Moon has been geologically quiet for billions of years, with no widespread tectonic movement or significant volcanic activity to resurface its ancient terrain.
The only continuous process of erosion on the Moon is the slow, steady bombardment by micrometeorites and smaller impacts. These tiny particles perpetually churn the surface layer, creating a powdery soil called regolith, but this process is extremely slow. Because the Moon is essentially a static environment, any feature created on its surface, whether an ancient impact crater or an astronaut’s footprint, remains preserved for eons.
Simple Versus Complex Craters
Lunar craters are classified into two main morphological types, primarily determined by their size at the time of formation. The smallest and most common type is the simple crater, which appears as a clean, smooth, bowl-shaped depression with a raised rim. Simple craters on the Moon are generally less than 10 to 15 kilometers in diameter and retain their original profile because their structure is stable under the Moon’s gravity.
When an impact exceeds this size threshold, the sheer scale of the energy release and subsequent gravitational forces cause the crater structure to collapse, forming a complex crater. The walls of these larger craters are unstable and slump inward, creating terraces along the inner rim. The enormous pressure from the initial impact causes the compressed rock beneath the crater floor to rebound upward, forming a distinct central peak or cluster of peaks.
Complex craters, typically 15 kilometers in diameter and larger, also have relatively shallow, flat floors compared to their overall width. The most massive impacts, exceeding 300 kilometers, form features called impact basins, which represent the largest and most complex structures on the Moon, sometimes featuring multiple concentric rings instead of a single central peak.