The Moon’s surface is covered by roughly hundreds of thousands of craters visible from orbit, ranging from microscopic pits in individual soil grains to basins stretching thousands of kilometers across. Nearly all of them were carved by impacts, objects slamming into a world that has no atmosphere to slow them down, no rain or plate tectonics to erase them, and no ocean to fill them in. The result is a surface that has recorded billions of years of collisions in a way Earth never could, making the Moon a kind of fossil record for the entire inner solar system.
Why the Moon Has So Many Craters
Earth gets hit by space debris at roughly the same rate as the Moon. The difference is what happens afterward. On Earth, wind, water, vegetation, and the constant recycling of the crust through plate tectonics erase craters within geologically short timescales. On the Moon, none of those processes exist. A crater that formed four billion years ago can still be traced today, softened by later impacts but never truly removed. The only significant erosion comes from further impacts: tiny micrometeorites gradually churn and smooth the uppermost layer of soil, a process called gardening. Research using data from the Chang’E-4 landing site confirmed that while large impacts create the bulk of the Moon’s loose surface material, small impacts are the main agents mixing and overturning the top few meters of that layer over time.
1Icarus. The gardening process of lunar regolith by small impact craters: A case study in Chang’E-4 landing areaThe absence of an atmosphere also matters at the moment of impact. On Earth, small meteoroids burn up high in the sky. On the Moon, even tiny grains strike the surface at full speed, which is why the lunar soil is littered with craters all the way down to the micrometer scale. And because there is no air drag on ejecta, material flung out of a crater can travel enormous distances, landing far from the original site and sometimes creating secondary craters of its own.
How Shape Changes With Size
Not all lunar craters look alike, and the single biggest factor determining a crater’s shape is its diameter. The smallest craters, up to roughly 15 kilometers across, are “simple” craters: bowl-shaped depressions with smooth walls and a depth-to-diameter ratio that stays fairly constant. Once you cross a threshold somewhere in the 15 to 20 kilometer range, walls begin to slump inward and a flat floor develops. These are called transitional craters.
2Journal of Geophysical Research: Planets. Influence of Target Properties on Wall Slumping in Lunar Impact Craters Within the Simple‐To‐Complex TransitionGo bigger still and you reach complex craters, which feature terraced walls, flat floors, and central peaks or peak rings formed when the ground rebounds after the shock of impact. A detailed global survey of fresh lunar craters found that the transition from simple to transitional craters occurs at about 17 kilometers in the lunar highlands and about 14 kilometers in the darker volcanic plains known as the maria. Full complex morphology kicks in at roughly 28 kilometers in the highlands and 24 kilometers in the maria. The difference is attributed to the layered basalt of the maria, which allows crater walls to collapse more readily.
3Journal of Geophysical Research: Planets. Deriving Morphometric Parameters and the Simple‐to‐Complex Transition Diameter From a High‐Resolution, Global Database of Fresh Lunar Impact CratersAt the very largest scales, craters become basins, structures so big that the central rebound produces a ring of mountains rather than a single peak. The most dramatic example is the South Pole-Aitken basin on the far side, whose inner rim stretches about 2,000 kilometers along its long axis. Modeling suggests it was formed by an oblique impact of a projectile roughly 350 to 400 kilometers in diameter, traveling at 12 to 16 kilometers per second, excavating material from as deep as 80 to 120 kilometers below the surface.
4Geophysical Research Letters. The South Pole‐Aitken Basin: Constraints on Impact Excavation, Melt, and EjectaThe Moon as a Clock for the Solar System
One of the most consequential things about lunar craters is their usefulness for dating. The basic idea is straightforward: the longer a surface has been exposed, the more craters accumulate on it. Count the craters in a given area, measure their sizes, and you can estimate how old the surface is. This technique, called crater size-frequency distribution dating, is the primary method for remotely estimating the age of surfaces across the solar system. Its calibration comes from the Apollo and Luna missions, which returned rock samples whose ages were measured in laboratories using radiometric methods.
5Earth and Planetary Science Letters. New crater calibrations for the lunar crater-age chronologyThose anchor points tie a specific crater density to a specific absolute age in billions of years. Surfaces with many craters per square kilometer are old; surfaces with few are young. The volcanic plains, which were flooded with lava after the heaviest bombardment had subsided, typically show lower crater densities than the ancient highlands. There is no fixed minimum area you need to count, but there is a tradeoff: smaller counting areas and younger surfaces mean fewer craters, which widens the uncertainty on the estimated age.
6Icarus. Planetary surface dating from crater size–frequency distribution measurements: Interpretation of small-area and low number countsA long-standing question in this field is whether the Moon experienced a sudden spike in impacts around 3.9 billion years ago, sometimes called the Late Heavy Bombardment. The idea originally drew support from the clustering of ages measured in Apollo samples. But recent isotopic work on Apollo 17 impact-melt rocks found no consistency between two independent dating techniques applied to the same samples. The researchers concluded that the polygenetic nature of most lunar impact-melt rocks, which are created and reshuffled by multiple impacts, makes testing the Late Heavy Bombardment hypothesis extremely difficult with current methods.
7Earth and Planetary Science Letters. A failed search for concordancy across multiple isotopic systems in lunar impactites: Implications for testing the Late Heavy Bombardment hypothesisWhat does seem clear from the broader cratering record is that the Moon was struck by two distinct populations of impactors over its history. An early population, with a size distribution resembling the present-day asteroid belt, dominated during the first billion years or so and produced the heavily cratered highlands. A second population, matching the size distribution of near-Earth objects, has dominated since about 3.7 to 3.8 billion years ago and produces craters at a much lower rate.
8Research in Astronomy and Astrophysics. The inner solar system cratering record and the evolution of impactor populationsThe Problem of Secondary Craters
Crater-counting works only if most of the craters you count were made by independent objects arriving from space. But every large impact also launches chunks of rock at high speed, and those chunks land elsewhere on the Moon and make craters of their own. These secondary craters can be a serious contaminant, especially at small diameters below about one kilometer. A review of the evidence across the solar system argued that distant secondary craters may globally dominate the population of small lunar craters, which would undermine the standard production functions used for age dating at those sizes.
9Annual Review of Earth and Planetary Sciences. THE IMPORTANCE OF SECONDARY CRATERING TO AGE CONSTRAINTS ON PLANETARY SURFACESThe reason secondaries skew small is physical: when an impact excavates a crater, the smaller fragments tend to be launched at higher speeds, meaning they travel farther and pepper distant terrain. Their size-frequency distribution is steeper than that of primary craters from interplanetary debris, so they pile up preferentially at small diameters. Accounting for these factors, including how secondaries cluster spatially around their parent crater and how their numbers scale with impact energy, is essential for getting surface ages right.
10Meteoritics & Planetary Science. Secondary craters and ejecta across the solar system: Populations and effects on impact‐crater–based chronologiesOne study of small young craters on the Moon tried to pin down how much of the observed heterogeneity in small-crater distributions could be blamed on recognizable secondaries or ejecta blankets from fresh impacts. The conclusion was that identifiable nearby secondaries could not account for the uneven distribution, pointing toward distant secondaries or poorly understood resurfacing processes as the culprits.
11Planetary and Space Science. Characteristics of small young lunar impact craters focusing on current production and degradation on the MoonWhy Most Craters Are Round Even When Impacts Are Not
Objects striking the Moon do not arrive straight down. Statistically, most impacts come in at angles well away from vertical. Yet the vast majority of lunar craters are circular, which can seem counterintuitive. The explanation is that at typical impact speeds of tens of kilometers per second, the energy released is so extreme that it generates a roughly symmetric shock wave regardless of the angle of approach. The crater is excavated by that expanding shock, not by the impactor physically scooping out a hole.
The impactor’s angle does leave signatures, though, just not in the crater rim until the angle gets very low. A detailed study of lunar oblique impacts found that as the impact angle decreases from vertical, the ejecta blanket starts to lose symmetry: a forbidden zone devoid of material develops on the uprange side. At even lower angles, material concentrates in a butterfly-shaped pattern to the sides.
12Journal of Geophysical Research: Planets. Ejecta Pattern of Oblique Impacts on the Moon From Numerical SimulationsThe actual crater rim stays circular until the incoming angle drops below about 15 degrees from horizontal, at which point the rim takes on a saddle shape. Below about 10 degrees it becomes elongated along the trajectory, and the ejecta settles into a clear butterfly distribution. These extremely low-angle craters are rare, so the overwhelming majority of impacts produce round craters even though the incoming objects are randomly oriented.
13Meteoritics & Planetary Science. The shape and appearance of craters formed by oblique impact on the Moon and VenusBright Rays and the Aging of Craters
Some lunar craters, like Tycho and Copernicus, are surrounded by bright streaks of material radiating outward for hundreds of kilometers. These rays are composed of freshly excavated rock and soil that has not yet been darkened by space weathering, the gradual bombardment of the surface by micrometeorites and solar-wind particles that chemically alters the top layer of soil. Over time, rays fade as the ejected material matures and blends in with the surrounding terrain.
Researchers have used an optical maturity index derived from orbital spectral data to classify large rayed craters into age groups. Craters with rays as fresh as Tycho’s are inferred to be younger than about 109 million years, while those with rays as faded as Copernicus’s are around 810 million years old, and older rayed craters fill the spectrum beyond that.
14Journal of Geophysical Research: Planets. Optical maturity of ejecta from large rayed lunar cratersA recent census of young lunar craters found that the overall population of albedo-rayed and rocky craters is consistent with an aggregate age of about 1.2 billion years, meaning the visible-ray population as a whole reflects the last billion-plus years of bombardment history.
15The Planetary Science Journal. The Population of Young Craters on the Moon: New Catalog and Spatial and Temporal AnalysisThermal observations add another layer. Using over a decade of surface temperature data from the Lunar Reconnaissance Orbiter, researchers classified young crater ejecta into three thermal types: blocky regions rich in boulders larger than a meter, clastic ejecta with varying density layering, and impact melts with high-thermal-inertia material buried under a less dense blanket. These categories track how quickly a crater’s ejecta disintegrates and mixes into the background soil.
16The Planetary Science Journal. Thermophysical Diversity of Young Lunar Crater Ejecta Revealed with LRO Diviner ObservationsWater Ice Hiding in Permanent Shadow
Near the lunar poles, the Sun sits so close to the horizon that the interiors of some craters never receive direct sunlight. Temperatures in these permanently shadowed regions drop below 100 kelvin, cold enough to trap water ice for billions of years. Multiple independent lines of evidence point to ice being present in at least some of these craters.
Radar data from the Lunar Reconnaissance Orbiter’s Mini-RF instrument found that certain polar craters whose interiors are both permanently dark and colder than 100 kelvin show unusually high radar reflectivity consistent with ice. If the interpretation is correct, both poles could harbor several hundred million tons of water ice within the upper couple of meters of the surface.
17Journal of Geophysical Research: Planets. Evidence for water ice on the Moon: Results for anomalous polar craters from the LRO Mini‐RF imaging radarUltraviolet albedo measurements from the same orbiter showed a sharp change in spectral behavior at locations where annual maximum temperatures stay below about 110 kelvin, consistent with surface frost. The estimated concentration was modest: roughly 0.1 to 2 percent water ice by mass if mixed with dry soil, or up to about 10 percent of the surface area covered in patches of pure ice at the 250-meter scale.
18Icarus. Evidence for exposed water ice in the Moon’s south polar regions from Lunar Reconnaissance Orbiter ultraviolet albedo and temperature measurementsLaser altimeter albedo data told a similar story: permanently shadowed crater floors showed brightness anomalies that could not be explained by other factors and were most consistent with surface frost, though concentrations were heterogeneous, ranging from negligible to about 6 percent.
19Earth and Space Science. Analyses of Lunar Orbiter Laser Altimeter 1,064‐nm Albedo in Permanently Shadowed Regions of Polar Crater Flat Floors: Implications for Surface Water Ice Occurrence and Future In Situ ExplorationNot all the data agree on magnitude, though. Analysis of dual-frequency synthetic aperture radar data from India’s Chandrayaan-2 mission found that the radar reflectivity inside 14 selected polar craters was not significantly different from their surroundings. Only about 2 percent of pixels showed values consistent with homogeneous ice, though the possibility of ice mixed into the subsurface regolith could not be ruled out.
20Icarus. Exploring water-ice deposits in lunar polar craters with Chandrayaan-2 DFSAR dataThe picture that emerges is one of patchy, thin deposits rather than solid sheets of buried ice. Still, even modest concentrations could be significant for future exploration, since extracting water from lunar soil would be far cheaper than hauling it from Earth.
Craters Modified From Below
Not every strange-looking lunar crater was shaped purely by the physics of impact. A class called floor-fractured craters shows anomalously shallow floors cut by radial, concentric, or polygonal fractures, along with interior moats, ridges, and patches of dark volcanic material. These craters are thought to have been modified after formation by magma that intruded beneath the crater floor, lifting and cracking it.
21Journal of Geophysical Research: Planets. Lunar floor‐fractured craters: Classification, distribution, origin and implications for magmatism and shallow crustal structureFloor-fractured craters tend to cluster near the boundaries between the highlands and the maria, where magma was most likely to migrate laterally underground. Their distribution maps out the plumbing system that fed the Moon’s ancient volcanic activity. Studying them gives geologists a way to probe the shallow crustal structure of the Moon without drilling.
How Lunar Craters Compare to Mercury’s
The Moon is not the only heavily cratered body in the inner solar system, and comparing craters across worlds reveals how gravity, impact speed, and surface composition shape the outcome. Mercury, with about twice the Moon’s surface gravity and a much higher average impact velocity (because of its proximity to the Sun), produces craters whose exterior features scale in predictable ways. A comparison of fresh complex craters on the two bodies found that the ratios of ejecta blanket size and secondary-crater size between same-diameter mercurian and lunar craters followed the predictions of gravity-regime scaling laws.
22Icarus. Comparisons of fresh complex impact craters on Mercury and the Moon: Implications for controlling factors in impact excavation processesOne visible difference is that rays on Mercury are more prominent at radar wavelengths than lunar rays, suggesting Mercury’s secondary craters are larger and excavate coarser debris. Rayed craters on Mercury look superficially like the youngest craters on the Moon, but the apparent youthfulness is better explained by the inherent differences in impact velocity, gravity, and target properties rather than by actual age.
23Journal of Geophysical Research: Planets. A comparison of rayed craters on the Moon and MercuryCraters as Future Infrastructure
For decades, engineers have discussed using lunar craters as ready-made structures for human habitats. A crater’s walls provide natural shielding against galactic cosmic rays, the slow drizzle of high-energy particles from beyond the solar system that poses a long-term health risk. Simulations of radiation transport in craters of different depths found that placing a habitat alongside the wall of a 100-meter-diameter crater at 15 meters depth reduced the total radiation dose by about 45 percent compared to an unshielded surface location during solar minimum conditions, and by about 11 percent at just 5 meters depth.
24Aerospace. Comparison of Doses in Lunar Habitats Located at the Surface and in CraterBeyond radiation, crater geometry reduces the amount of construction material needed for a protective dome compared to building an equivalent structure on flat ground, because the walls already provide partial enclosure and support for regolith shielding layers.
25Scientific Reports. Concept and preliminary structural analysis of a crater-covering dome for future lunar habitatsCraters on the far side of the Moon have also been proposed as sites for radio telescopes. The far side is permanently shielded from Earth’s radio noise, and a crater’s bowl shape could serve as a natural reflector. Modeling of a lunar crater radio telescope operating in the 4.7 to 47 megahertz band found that for over 95 percent of simulated observations, the measurement bias stayed below 20 millikelvin, well under the predicted 42 millikelvin depth of the cosmological Dark Ages signal the telescope would be designed to detect. The foreground sky temperature at those frequencies can reach millions of kelvin near the galactic plane, so the shielding a crater provides is not trivial.
26The Royal Society Publishing. Modelling science return from the lunar crater radio telescope on the far side of the moonThe South Pole-Aitken Basin and What Lies Beneath
The South Pole-Aitken basin deserves its own attention not just for its size but for what it exposed. At roughly 2,000 kilometers across, it is one of the largest confirmed impact structures in the solar system. The collision likely stripped away most of the pre-existing crust in the basin interior, yet gravity and seismic data show that the remaining crust is still at least 16 kilometers thick in undisturbed areas, indicating substantial crustal rebound or later volcanic infilling.
27Geophysical Research Letters. Deep Structure of the Lunar South Pole‐Aitken BasinBuried beneath the basin floor sits a large mass anomaly detected by orbiting spacecraft. Possible explanations include metal from the core of the differentiated impactor that created the basin, still embedded in the mantle, or dense oxide minerals left over from the final stage of the Moon’s magma ocean crystallization. Either way, the anomaly makes the basin a high-priority target for future landers and sample-return missions, since it offers a window into both the deep interior of the Moon and the nature of the objects that bombarded it early in solar system history.
28Geophysical Research Letters. Deep Structure of the Lunar South Pole‐Aitken Basin