The Moon’s surface is dominated by countless impact craters, ranging from microscopic pits to vast basins. The overwhelming majority of these formations are circular, regardless of their size. This seems counterintuitive, as an object hitting at an angle might be expected to create an elongated feature, similar to a stone skipping across mud. The explanation for this consistent circularity lies in the physics of space collisions, where extreme speeds transform a simple physical impact into an explosive event.
Defining Hypervelocity Impact
The mechanics of lunar crater formation are defined by “hypervelocity,” meaning the impact speed far exceeds the speed of sound within the target material. Unlike low-speed collisions, a hypervelocity event is dominated by the projectile’s immense kinetic energy, not its mass or shape. Impactor velocities striking the Moon typically range from 10 to 70 kilometers per second. At these extreme speeds, the strength of the rock material becomes negligible compared to the stresses generated upon contact. This high-energy transfer causes both the impactor and the target rock to behave almost like a fluid. The impact event is essentially an instantaneous, localized explosion deep within the lunar surface. The sheer energy involved drives the crater’s creation, not the physical momentum of the impactor itself. The projectile is consumed and destroyed within milliseconds, before its initial trajectory can influence the resulting structure.
The Role of Energy and Shock Waves
The circular shape results from the symmetrical nature of the energy release following hypervelocity contact. Upon impact, kinetic energy is instantly converted into thermal energy and pressure, generating an intense, high-pressure shock wave. This shock wave propagates outward from the contact point, traveling faster than the speed of sound in the lunar rock.
The shock wave expands spherically into the target material, much like the pressure wave from a buried explosive charge. This spherical expansion occurs because the impactor is destroyed and its energy is released at a point source beneath the surface. The direction of the incoming object becomes irrelevant to the resulting geometry, unless the angle of entry is extremely shallow (less than about five degrees from the horizon).
As the initial compression wave moves through the material, it is followed by a decompression wave that sets the rock into motion. This outward and upward motion is called the excavation flow, which carves out the crater cavity. Since the driving force—the shock wave—is spherically symmetrical, the excavation flow is pushed out equally in all horizontal directions.
This process results in the formation of a circular transient cavity, the immediate, bowl-shaped hole created by the explosion and material ejection. The process is incredibly fast, often completed in a matter of seconds, even for very large craters. Material ejected from the cavity blankets the surrounding terrain, forming the crater’s rim and an ejecta blanket.
The energy release is so rapid and complete that the projectile is vaporized or melted, transferring its kinetic energy to the shock wave. This process effectively neutralizes the directional component of the impact, resulting in a final circular depression that is a signature of symmetrical energy dissipation.
Post-Impact Shaping and Variations
While the initial formation mechanism guarantees a circular transient cavity, secondary processes can modify the final appearance of the crater. For larger craters on the Moon, typically those exceeding 15 to 20 kilometers in diameter, gravity becomes a major factor immediately after the excavation phase.
The steep walls of the newly formed cavity are unstable and cannot be supported by the rock strength under lunar gravity. This instability causes the walls to collapse inward, a process known as gravitational slumping. Slumping forms the characteristic steep, terraced interior walls seen in complex craters, but it does not destroy the overall circularity of the rim.
Another common modification in complex craters is the formation of a central peak. After the initial material is ejected, the highly compressed rock beneath the crater floor rebounds upward, similar to how water splashes up after an object is dropped into it. This rebound creates a mound of uplifted material at the center of the crater floor.
True non-circular craters are exceptionally rare among primary impact features. They are limited to impacts that occurred at extremely oblique angles, where the trajectory was less than five degrees from the surface. In these few cases, the resulting feature is slightly elliptical. Most other non-circular features are secondary craters, which are smaller, often irregular depressions formed by large blocks of ejecta raining down after a major impact event.