The Moon’s surface is a geological archive, scarred by billions of years of cosmic impacts. A lunar crater is a bowl-shaped depression created by a hypervelocity collision with an asteroid, comet, or meteoroid. Because the Moon lacks the atmosphere, water, and tectonic activity present on Earth, these impact features are preserved with remarkable clarity. While it is impossible to give a single, exact number for the Moon’s craters, the true count is astronomical and depends entirely on the size of the feature being counted.
Why a Precise Number is Impossible
A precise count of lunar craters is impossible because the number changes based on the minimum size threshold used for counting. The Moon is constantly being struck by tiny particles, meaning that billions of microscopic pits, often called zap pits, cover every square inch of the surface. These are craters by definition, but they are too small and numerous to be cataloged. The number of visible features increases exponentially as the size threshold decreases.
Scientific surveys, primarily utilizing data from missions like the Lunar Reconnaissance Orbiter (LRO), must establish an arbitrary minimum size for their catalogs. One recent, global database identified over 1.3 million craters with a diameter of 1 kilometer or larger. This massive census of larger features still represents only a fraction of the total impacts. If the threshold is lowered to just 10 meters, the estimated number of craters soars to over half a billion. Therefore, any precise number is a reflection of the size limit chosen by the observer.
The Mechanics of Crater Formation
The formation of an impact crater is a rapid, explosive process driven by the kinetic energy of the incoming object. This process is conceptually divided into three main stages: contact and compression, excavation, and modification. It begins when the impactor strikes the surface, instantly converting its immense kinetic energy into a powerful shock wave. This initial phase, contact and compression, lasts only a fraction of a second as the shock wave travels through the target rock and the projectile itself.
The second stage, excavation, involves the shock wave propelling material outward and downward, creating a transient cavity. During this phase, material is ejected from the cavity, forming a blanket of debris around the rim. The size of the resulting crater is determined primarily by the impactor’s velocity, not its mass, as velocity is squared in the kinetic energy equation. Finally, the modification stage sees the transient cavity collapse under gravity, shaping the crater into its final form.
Simple, Complex, and Impact Basins
Lunar impact structures are classified by their resulting morphology, which is primarily controlled by the size and energy of the impact.
Simple Craters
The smallest, most common features are simple craters, characterized by a smooth, bowl-shaped depression with raised rims. These structures are generally less than 15 to 20 kilometers in diameter on the Moon, retaining the classic bowl profile of the transient cavity.
Complex Craters
Larger, more energetic impacts produce complex craters, which exhibit distinct structural differences due to gravitational collapse. These features, typically exceeding 15 to 20 kilometers in diameter, often have terraced inner walls, a relatively flat floor, and a prominent central peak. The central peak forms when the compressed rock beneath the crater floor rebounds upward immediately after the impact.
Impact Basins
The largest impact features are known as impact basins, defined as structures over 300 kilometers in diameter. These immense events, such as the 1,160-kilometer-wide Mare Imbrium, are so powerful that they often create a multi-ring structure. The enormous South Pole-Aitken Basin is the largest known feature of its kind in the solar system, stretching over 2,500 kilometers across the lunar far side.
The Role of Craters in Lunar Dating
The distribution of craters across the Moon provides planetary scientists with a method for estimating the age of different surface regions. This technique, known as Crater Density Chronology, relies on the idea that a surface accumulates more craters the longer it has been exposed to impacts. Since the Moon lacks geological processes that erase impact scars, the craters act as a chronological marker.
By counting the number of craters of a specific size, often 1 kilometer or larger, scientists can determine the relative age of that region. For example, the heavily cratered lunar highlands are considered older than the smoother, volcanic plains known as maria, which have fewer impacts. The absolute ages of these surfaces are calibrated by comparing crater densities to the radiometrically dated rock samples returned by the Apollo and Luna missions.