A lunar crater is a characteristic, bowl-shaped depression caused by a hypervelocity collision with an asteroid, meteoroid, or comet. Determining the exact number of these features is not straightforward, as the total count depends entirely on the size threshold used for definition. The overall count shifts from a few thousand to many billions based on whether one includes only the largest, named features or the microscopic pits scattered across the lunar regolith.
The Challenge of Counting
No single, fixed number exists for the total population of lunar impact features because the count increases exponentially as the size decreases. Scientists use different size categories to provide meaningful estimates based on high-resolution mapping data. For instance, the number of large craters 20 kilometers or more in diameter totals around 5,000 features.
This count rises dramatically when considering smaller formations. Estimates suggest there are at least 1.3 million craters across the Moon’s surface that are larger than one kilometer in width. The International Astronomical Union formally recognizes and names over 9,000 of the largest and most prominent craters. However, the true number is far greater when moving down to smaller scales.
Current statistical models estimate that the Moon hosts over half a billion craters exceeding 10 meters in diameter. These figures are derived from statistical sampling of high-resolution images from spacecraft like the Lunar Reconnaissance Orbiter, not a literal crater-by-crater tally. Advanced techniques, including machine learning, are consistently improving these estimations by identifying thousands of previously uncataloged small features. The number of microscopic craters, often referred to as “zap pits,” is effectively infinite, covering nearly every grain of lunar soil.
The Mechanics of Lunar Crater Formation
An impact event begins with the compression stage, where the high-speed impactor makes contact with the lunar surface. The force of this collision generates a powerful shockwave that travels through the target rock, crushing and vaporizing the material beneath the point of impact. The immense kinetic energy is converted into heat and pressure.
This is followed by the excavation stage, where the compressed material rebounds, pushing rock and debris outward to form the raised rim and ejecta blanket. A temporary bowl-shaped cavity, known as the transient crater, is created during this phase. The ejected material, called ejecta, settles around the newly formed feature.
The final phase is modification, which dictates the crater’s ultimate appearance. Smaller impacts result in simple craters, which are smooth, bowl-shaped depressions generally less than 15 kilometers across. Larger impacts transition into complex craters, where the ground collapses inward under gravity, forming terraces on the inner walls and often creating a central peak or a flat floor.
Why the Moon Retains So Many Impact Features
The Moon’s environment is the primary reason for the extreme preservation of its impact record compared to Earth. The Moon’s near-total lack of an atmosphere means there is no wind, weather, or water to cause erosional processes. On Earth, these forces quickly erase impact scars, but on the Moon, a crater can remain virtually unchanged for billions of years.
The Moon is also geologically inert, lacking the vigorous plate tectonics that constantly recycle Earth’s crust. On Earth, subduction and mountain-building processes destroy or bury old impact structures. However, the Moon’s surface has remained largely static for over three billion years, preventing the resurfacing that would otherwise erase ancient collisions.
The only significant ongoing degradation process is impact gardening, caused by a continuous rain of micrometeorites. This constant bombardment slowly churns the lunar surface layer, gradually softening the sharp features of older craters. The high density of craters is particularly visible in the heavily scarred lunar highlands, which are ancient crustal material.
In contrast, the dark, smoother lunar maria are relatively younger, having been flooded by immense lava flows approximately three to four billion years ago. These volcanic events paved over older craters in those regions. This results in a lower density of impact features in the maria compared to the older highlands, providing a visual timeline of lunar history.
Using Craters to Date the Lunar Surface
The number of craters provides planetary scientists with a powerful tool for determining the age of different lunar surfaces, a technique known as crater counting. The underlying principle is that an older surface has been exposed to continuous bombardment for a longer time, resulting in a higher concentration of impact craters. This method allows for the relative dating of different regions.
The principle of superposition is applied: a surface unit with more craters per unit area must be older than one with fewer craters. For example, the crater-saturated highlands are much older than the crater-sparse maria. This technique is calibrated using the absolute ages determined from the radioactive dating of rock samples returned by the Apollo missions.
By linking the density of craters in a specific region to the absolute age of the corresponding Apollo samples, scientists established a chronology for the entire Moon. This calibrated system allows researchers to estimate the absolute age of any planetary surface, even those not directly sampled, by counting the density of impact craters.