Lunar craters are bowl-shaped depressions created by hypervelocity impacts from asteroids and comets. The Moon’s surface preserves a pristine record of these events because it lacks a thick atmosphere and active geological processes, such as plate tectonics or erosion, that would erase them. This preservation means lunar craters remain intact over billions of years, spanning an immense range of sizes. Impact structures range from microscopic pits found on returned rock samples up to colossal basins stretching thousands of kilometers across the surface.
The Dynamics That Determine Crater Size
The final dimensions of a lunar crater are determined by the immense kinetic energy of the impactor, a combination of its mass and velocity. Because the average impact speed is tens of kilometers per second, the resulting explosion-like event creates a crater typically 10 to 20 times larger than the original projectile.
The impact process occurs in three stages. It begins with the contact and compression phase, which transfers the impactor’s kinetic energy into a powerful shock wave radiating through the lunar rock. This is followed by the excavation stage, where the shock wave moves the target rock outward and downward, forming a temporary, open bowl called the transient crater.
The final size and shape are set during the modification phase, where the transient crater collapses under the influence of the Moon’s gravity and the strength of the target rock. The Moon’s lower gravity allows for relatively larger and shallower final crater forms compared to bodies like Earth. The geology of the impact site also plays a role, influencing the size threshold at which the crater morphology changes.
Classifying Craters by Scale: Simple, Complex, and Basins
Planetary scientists categorize lunar craters based on their morphology, which changes systematically as size increases due to gravity affecting the collapsing structure.
Simple Craters
Simple craters are the smallest and most common type, characterized by a smooth, bowl-shaped depression with a raised rim and a depth-to-diameter ratio around 1:5. These structures are typically less than 15 kilometers in diameter on the Moon, retaining the classic inverted cone shape of the transient cavity.
Complex Craters
As impact energy increases, the morphology transitions to a complex structure, generally beginning at 15 to 20 kilometers in diameter. The gravitational collapse of the transient cavity is significant, causing the central floor to rebound and form a central peak. Complex craters also feature a flat, shallow floor and terraced walls resulting from the inward collapse of the steep rim.
Impact Basins
The largest impact structures are classified as impact basins, defined as 300 kilometers or more in diameter. These massive features go beyond the central-peak morphology, exhibiting multiple concentric rings of mountains and scarps. The scale of the impact and subsequent collapse leads to the formation of a peak ring or multi-ring basin, such as the Orientale Basin, where the rings represent deep-seated structural fractures.
Record Holders: The Largest and Smallest Lunar Impact Structures
The extremes of lunar crater size demonstrate the full range of impact events that have shaped the Moon.
Largest Structures
At the upper limit is the South Pole-Aitken Basin (SPA), the largest and oldest confirmed impact structure, located on the far side. This colossal feature spans approximately 2,500 kilometers in diameter, covering nearly a quarter of the lunar surface. The basin reaches depths of up to 13 kilometers, making it one of the largest known impact craters in the solar system. Another major example is the Imbrium Basin, which created the dark region known as Mare Imbrium, with a diameter exceeding 1,100 kilometers.
Smallest Structures
In contrast to these gigantic basins, the smallest impact structures are microscopic. Scientists have observed tiny impact pits, measured in micrometers, on rock samples brought back by the Apollo missions. The smallest craters visible using high-resolution orbiter images can be less than a millimeter across, demonstrating that the bombardment of the Moon is a continuous process.