Lunar craters range from shallow depressions barely a meter across to the South Pole-Aitken basin, a structure roughly 2,000 kilometers wide that gouged deep into the Moon’s mantle. Even craters of similar diameter can differ significantly in depth depending on where they formed, what kind of rock they punched into, and how billions of years of bombardment have worn them down. The short answer is that no two craters are obligated to be the same depth, and the factors that set crater depth turn out to be more varied and interesting than most people expect.
The Ratio That Governs Crater Shape
Planetary scientists think about crater depth relative to diameter, a value known as the depth-to-diameter ratio. If all craters were perfect bowl-shaped holes scaled up uniformly, this ratio would stay constant regardless of size. It does not. Fresh craters larger than about 400 meters in diameter tend to have a ratio near 0.21, meaning their depth is roughly one-fifth of their width. But craters smaller than 400 meters are consistently shallower for their size, with ratios dropping to between 0.11 and 0.17.1Icarus. Relative depths of simple craters and the nature of the lunar regolith Across all small craters on the maria and highlands, the ratio ranges from roughly 0.1 to 0.2.2Icarus. Investigation of the depth and diameter relationship of subkilometer-diameter lunar craters
The reason for this has to do with what the impactor is hitting. Small projectiles strike the Moon’s loose upper layer of broken rock, called regolith, which can be meters to tens of meters thick. Punching into this granular, already-shattered material produces a shallower bowl than punching into solid bedrock. Once a crater is large enough that the excavation reaches below the regolith into competent rock, the physics shifts, and the resulting hole is proportionally deeper. Think of the difference between poking a finger into a sand pile versus pressing it into firm clay: the sand collapses inward and fills partly back in, while the clay holds a sharper, deeper impression.
When Craters Stop Being Simple Bowls
Up to a certain size, craters on the Moon are “simple” structures: clean, bowl-shaped cavities with smooth walls. But once a crater gets wide enough, the rock around the rim can no longer support itself and begins to collapse inward. Terraces of slumped material slide down the inner walls, and the floor rebounds upward to form a central peak or set of peaks.3Journal of Geophysical Research. The role of slumping in the modification of lunar impact craters These “complex” craters are much shallower relative to their width than simple ones, because the slumping and uplift partially fill the original cavity.
The transition diameter at which craters shift from simple to complex depends on the terrain. On the lunar highlands, craters start showing transitional features at roughly 16 to 17 kilometers across, while on the dark basaltic plains of the maria, the transition kicks in earlier, around 14 kilometers.4Journal of Geophysical Research: Planets. Deriving Morphometric Parameters and the Simple‐to‐Complex Transition Diameter From a High‐Resolution, Global Database of Fresh Lunar Impact Craters A more recent study that included polar craters found similar figures, placing the mare transition at about 14.2 kilometers and the highland transition at about 15.7 kilometers, with polar regions slightly higher still.5Icarus. Morphological characteristics of impact craters with diameters of 5–20 km on the Moon The layered basalts of the maria, being weaker in certain ways than the ancient highland crust, allow the collapse process to begin at a smaller size.
Early telescopic measurements from Earth dramatically underestimated the depth of small craters. Spacecraft data showed that craters under 15 kilometers across are at least 50 percent deeper than those older ground-based observations had suggested, though larger craters were not revised nearly as much.6Geophysical Research Letters. Depth/diameter relations of fresh lunar craters: Revision from spacecraft data That correction mattered: it meant the contrast between small and large craters was even more dramatic than originally thought.
Why Terrain Makes a Difference
Even among craters of the same size, location matters. A global analysis using modern elevation data found that small craters (under about four kilometers wide) on the maria are deeper than same-sized craters on the highlands.7Geophysical Research Letters. A Global Analysis of Crater Depth/Diameter Ratios on the Moon This seems counterintuitive at first. The maria are plains of solidified lava, and you might expect softer, more layered rock to yield shallower holes. But the explanation involves the highland regolith being thicker and more heavily fractured from billions of years of accumulated impacts. A fresh crater punching through deep, loose regolith collapses more readily than one excavating through relatively coherent basalt. The highlands have simply been beaten up longer and more thoroughly.
Terrain also matters underground. When an impact is energetic enough to deform the boundary between the Moon’s crust and its denser mantle, the local crustal thickness helps determine how much deep material gets pushed around. For craters in thick-crusted highland regions, the minimum diameter needed to uplift mantle material from below is larger than it would be in thin-crusted areas.8Journal of Geophysical Research: Planets. Investigating the Influences of Crustal Thickness and Temperature on the Uplift of Mantle Materials Beneath Large Impact Craters on the Moon So the same-energy impact can produce structurally different results depending on where it lands.
How Craters Lose Their Depth Over Time
A freshly formed crater is at its deepest the moment it finishes forming. From that point on, everything conspires to make it shallower. On a world with weather, rain and wind would do the job. On the Moon, the main agent of erosion is more impacts: a steady drizzle of micrometeorites, plus occasional larger hits whose debris blankets the surrounding terrain. This process, called impact gardening, is a combination of excavation by small craters and burial under their ejecta, and it steadily chews away at rims while filling in floors.9Journal of Geophysical Research: Planets. Secondary Impact Burial and Excavation Gardening on the Moon and the Depth to Ice in Permanent Shadow
The rate at which a crater degrades is not constant. Research on craters in the 5-to-20-kilometer range found that inner wall slopes erode quickly when a crater is still relatively fresh, but the rate slows as the crater shallows out. Once the depth-to-diameter ratio drops from about 0.25 to 0.15, degradation proceeds rapidly, but from 0.15 down to 0.05, the process decelerates.5Icarus. Morphological characteristics of impact craters with diameters of 5–20 km on the Moon There is a kind of diminishing-returns effect: a steep-walled, deep crater has lots of unstable material ready to slump, while a shallow, gently sloped crater has much less to lose.
Lava flows also buried craters wholesale during the Moon’s volcanically active period, billions of years ago. Some craters on the maria are only partially buried: their rims poke above the lava plain, but their floors are hidden beneath solidified basalt. Estimating how deep those craters originally were requires careful modeling of how much the exposed rim has eroded versus how much lava filled the interior.10Journal of Geophysical Research: Planets. Thickness of Lunar Mare Basalts: New Results Based on Modeling the Degradation of Partially Buried Craters Without accounting for rim erosion, scientists tend to overestimate how thick the lava fill is.
At the largest scales, a different kind of modification comes into play. Ancient multi-ring basins, especially those on the Moon’s far side, show signs of viscous relaxation: the crust was warm enough after the impact that it slowly flowed like extremely thick putty over millions of years, gradually flattening the basin’s topography. Modeling suggests that some far-side basins experienced enough crustal flow to substantially reduce their original depth, requiring ancient heat fluxes well above what the crust sees today.11Journal of Geophysical Research: Planets. Viscoelastic evolution of lunar multiring basins On the near side, where the crust is thinner, this process was less likely to operate efficiently, so the same basin-forming impact might have left behind a different depth profile depending on which hemisphere it struck.
South Pole-Aitken and the Extremes of Depth
The single most dramatic depth story on the Moon belongs to the South Pole-Aitken basin on the far side. Its inner rim stretches about 2,000 kilometers along its long axis, making it one of the largest recognized impact structures in the solar system. Beneath it lies a mass anomaly of at least 2.18 × 1018 kilograms, extending more than 300 kilometers below the surface, which researchers believe may be the metallic remnants of the impactor or dense mantle material dredged upward by the collision.12Geophysical Research Letters. Deep Structure of the Lunar South Pole‐Aitken Basin The basin floor sits kilometers below the surrounding terrain, and the crust there is so thin that lava was able to erupt into it more readily than into other large basins, leaving a higher concentration of lava-filled ponds.13Journal of Geophysical Research: Planets. Volumes of lunar lava ponds in South Pole‐Aitken and Orientale Basins
Basins of this magnitude do not simply form deeper versions of complex craters. Beyond a certain size, the transient cavity punches entirely through the Moon’s rigid outer layer into the hotter, more pliable rock below. Numerical modeling suggests that any lunar basin larger than Schrödinger (about 320 kilometers in diameter) should have been capable of forming multiple concentric rings, because the cavity penetrated into the asthenosphere for the thermal conditions expected at the time.14Icarus. Investigating the onset of multi-ring impact basin formation Multi-ring basins behave fundamentally differently from smaller craters in terms of depth, because the rock beneath them responds in ways that bowl-shaped craters never encounter. The floor can rebound dramatically, rings can collapse and terrace at multiple elevations, and the final topography bears little resemblance to a simple hole in the ground.
Polar Craters and the Question of Ice
Near the Moon’s poles, some crater floors never see sunlight. These permanently shadowed regions are among the coldest spots in the solar system and are thought to harbor deposits of water ice delivered by comets and meteorites over billions of years. A natural question is whether that ice buildup has altered crater depth in measurable ways. A recent systematic analysis compared the depth-to-diameter ratios of craters hosting permanent shadow, cold traps, and craters suspected of being ice-rich with the general population of polar craters. The result was clear: no distinct depth trends related to water ice appeared.15The Planetary Science Journal. Lunar Polar Dichotomy: Insights from Crater Morphometry Whatever ice exists in those shadows, it is not thick enough to significantly change the shape of the craters holding it. The ice deposits are real, but they are thin veneers or mixed into the regolith, not deep frozen lakes filling crater floors.
How Gravity Sets the Rules on Other Worlds
The Moon’s relatively low surface gravity is itself a factor in crater depth. Across the terrestrial planets, both the depths of complex craters and the diameter at which craters transition from simple to complex vary inversely with surface gravity.16Icarus. Formation of complex impact craters: Evidence from Mars and other planets On a body with stronger gravity, crater walls collapse more easily and central peaks rebound more aggressively, producing shallower complex craters at smaller diameters. The Moon, with about one-sixth of Earth’s surface gravity, allows craters to remain simple bowls to larger sizes and to stay relatively deep before the transition to complex morphology kicks in.
Gravity also controls final crater diameter for a given impact energy. A theoretical and observational study of craters across the inner solar system found a consistent negative relationship between final crater width and the target body’s gravitational acceleration: the stronger the gravity, the smaller the crater for the same punch.17Icarus. Surface gravity and crater diameter as proxies of extra-terrestrial impact But while gravity shapes the crater’s size and collapse behavior, it does not scale equally with every impact process. The amount of rock melted by an impact, for example, depends on impact velocity and energy but not on the target’s gravity, which leads to mismatches between crater dimensions and melt volume from one planetary body to another.18Advances in Space Research. Planetary differences in impact melting Craters that look similar in width on two different worlds can have very different amounts of melted rock pooled on their floors, changing the final floor elevation and the effective measured depth.
Collapse Features That Are Not Impact Craters at All
Not every hole on the Moon was made by something slamming into it. Lunar pits are small collapse features, typically 10 to 300 meters wide, with vertical or even overhanging walls.19Journal of Geophysical Research: Planets. Lunar Pit Morphology: Implications for Exploration They form when the roof of an underground void, often a lava tube, gives way. Their geometry is nothing like an impact crater: instead of a gentle bowl, you get a near-vertical shaft that can be deeper than it is wide. Some of these pits open into subsurface caverns that may extend hundreds of meters horizontally, making them targets of intense interest for future lunar bases because they could offer natural radiation shielding.
Lumping these pits in with impact craters would be like comparing a sinkhole to a meteor crater on Earth. They are produced by entirely different processes and follow none of the depth-diameter scaling relationships that govern impacts. Their existence is a reminder that the Moon’s surface is not just a passive target for incoming rocks; it has its own geological personality, shaped by ancient volcanism and structural collapse as well as by bombardment.
How Scientists Actually Measure All This
Modern crater-depth measurements come primarily from the Lunar Orbiter Laser Altimeter aboard the Lunar Reconnaissance Orbiter, which has been mapping the Moon’s surface since 2009. The instrument fires laser pulses at the surface and times their return, building up a three-dimensional elevation model with vertical precision measured in meters. For smaller craters, scientists also use stereo imagery from cameras on the same spacecraft and from Japan’s Kaguya orbiter to construct detailed terrain models.20Journal of Geophysical Research: Planets. The transition from complex craters to multi‐ring basins on the Moon These datasets have transformed crater science. Before spacecraft, astronomers relied on shadow measurements through Earth-based telescopes, which substantially underestimated depths for smaller craters, as noted earlier.
Measuring depth sounds straightforward, but it requires careful decisions. Do you measure from the rim crest to the lowest point on the floor? From the average surrounding terrain to the floor? The rim of a degraded crater might be partially missing on one side, and the floor might have a central peak that rises above the surrounding flat. Different measurement conventions can shift the reported depth by a meaningful amount, which is one reason published depth-to-diameter ratios for the same crater can vary between studies. The high-resolution global datasets have helped standardize these measurements, but they have not eliminated all disagreement, especially for ancient, heavily degraded structures where the original rim is barely detectable.