The Moon’s heavily scarred surface is the result of billions of years of impacts, creating lunar craters. These features represent the primary geological structure, recording bombardment by asteroids and meteoroids throughout solar system history. The sheer number of craters is staggering, with estimates suggesting millions of features larger than one kilometer in diameter cover the lunar surface. The sizes of these structures range drastically, from microscopic pits to vast basins spanning thousands of kilometers.
The Vast Size Spectrum of Lunar Craters
The size range of lunar craters is immense, extending from microscopic pits to features larger than continents on Earth. The smallest impact features are tiny pits measured in micrometers on rocks returned by the Apollo missions, formed by continuous bombardment from micrometeoroids. These minuscule impacts demonstrate that the Moon’s surface is constantly being modified.
The Moon hosts over 83,000 craters larger than five kilometers in diameter, with over 6,900 exceeding 20 kilometers across. Mid-sized craters, like Copernicus, measure about 93 kilometers wide. They feature dramatic terraced walls and central peaks, representing impacts from objects that were likely several kilometers in diameter.
The largest impact structures are classified as impact basins. The largest confirmed feature is the South Pole-Aitken (SPA) basin, located on the far side of the Moon. This immense structure spans approximately 2,500 kilometers in diameter, nearly a quarter of the Moon’s circumference. The deepest parts of the SPA basin reach depths of over 8 kilometers.
Classifying Crater Structures by Size
The size of a lunar crater correlates with its physical shape, leading to three main structural classifications. Smaller impacts create simple craters, which are typically bowl-shaped depressions with smooth inner walls. This structure is characteristic of craters less than 15 to 20 kilometers in diameter. Moltke crater, about seven kilometers wide, exhibits this classic bowl form.
When the impact energy exceeds a threshold, the structure changes dramatically, forming complex craters. These larger features, generally starting around 15 kilometers in diameter, are characterized by a flat floor, terraced inner walls, and a central peak. The central peak forms as the compressed rock beneath the crater floor rebounds immediately after the impact event.
The largest impacts produce impact basins, defined as structures exceeding 300 kilometers in diameter. These basins often display a unique multi-ring structure, resulting from the massive gravitational collapse of the initial cavity. The Orientale Basin is a prominent multi-ring example, with an outer ring spanning about 950 kilometers across.
What Determines a Crater’s Final Size?
The final size of a lunar crater is dictated by the kinetic energy of the incoming impactor and the strength of the Moon’s surface materials. The kinetic energy is the most influential factor, determined by the object’s mass and velocity at collision. Since typical impact velocities can be around 20 kilometers per second, even a relatively small object carries enormous energy.
The initial size is also influenced by the density and composition of the target rock. Impacts into the Moon’s dense, ancient highlands often produce different sized features than those striking the less dense, younger volcanic plains, or maria. Generally, the resulting crater diameter is roughly 10 to 20 times larger than the diameter of the object that caused it.
The entire process involves three phases: compression, excavation, and modification. Once the cavity is excavated, the final size and structure are modified by gravity. Gravity causes the steep walls to slump inward and the floor to rebound. This gravitational collapse transforms a simple bowl into a complex structure featuring terraced walls and a central peak, establishing the final dimensions.