The depth of craters on the Moon is highly variable, resulting from a complex interplay of physics and geology. A lunar crater is an impact feature created by the hypervelocity collision of an asteroid or comet with the surface. The resulting depth is not uniform but depends on factors present at the moment of impact and processes that occur over billions of years. Understanding these differences provides insight into the Moon’s history and the mechanics of impact cratering.
Impact Energy and Initial Depth
The initial depth of a newly formed crater is primarily determined by the kinetic energy of the impacting object. Kinetic energy is a function of both the mass of the impactor and the square of its velocity, which is typically around 20 kilometers per second for objects striking the Moon. This energy release vaporizes and excavates the target rock, forming an initial bowl-shaped depression known as the transient cavity.
The size of the impactor directly dictates the resulting crater’s dimensions; the final diameter is typically 10 to 20 times larger than the object that created it. Scientists use the depth-to-diameter ratio (d:D) to characterize the shape of pristine craters. Smaller, fresh craters tend to have a relatively high d:D ratio, meaning they are deeper relative to their width, often around 0.2.
This proportional relationship changes as the impact energy increases. For example, a simple crater with a diameter of a few kilometers might have an initial d:D ratio of about 1:5. The initial excavation phase is governed by shock waves and the outward movement of material, but the final observed depth is only the first step in a dynamic process.
Crater Morphology: Simple and Complex Forms
The most significant factor causing depth variation among similarly sized craters is the structural reaction of the lunar crust to the impact force. Craters are categorized into two main types based on their final structure: simple and complex. Simple craters are typically small, bowl-shaped depressions with smooth inner walls, retaining the high d:D ratio established during excavation. On the Moon, the transition from a simple to a complex crater occurs at a size threshold of approximately 15 to 20 kilometers in diameter.
Once the transient cavity exceeds this threshold, the impact force and the Moon’s gravity cause the unstable structure to collapse immediately after formation. This process involves the crater floor uplifting and the walls slumping inward, a structural transformation that dramatically alters the depth. The inward collapse of the rim forms terraced walls, while the central floor rebounds upward to create a prominent central peak or a cluster of peaks.
This structural modification results in complex craters being disproportionately shallower than their smaller counterparts. For instance, a complex crater may have a d:D ratio closer to 1:10 or less, despite being much larger than a simple crater. The largest complex structures, known as impact basins, can exceed 300 kilometers in diameter and feature multi-ringed uplifts instead of a single central peak, complicating the depth profile. The resulting flat floors and terraced walls mean that the final depth is a product of the gravitational collapse of the surrounding rock, not a simple measure of the initial impact energy.
Modification Over Geologic Time
Even if two craters were formed with identical initial depths and structures, their current depth would likely differ based on their age and subsequent modification. After formation, the depth of a crater is subject to long-term degradation processes that reduce its apparent depth over billions of years. On the Moon, where there is no atmosphere, water, or active tectonics, this modification is slow but constant.
One primary process is infilling, where ejecta from subsequent impacts lands within the existing crater, gradually raising the floor. This “lateral sedimentation” slowly smooths the internal topography and reduces the measurable depth. Additionally, the constant bombardment by micrometeorites, known as space weathering, slowly grinds and churns the upper layer of lunar soil, or regolith.
This slow erosion causes the rims and interior walls to soften and slump, a process more pronounced in older craters. The cumulative effect is that an older crater, even a large one, will have a shallower depth-to-diameter ratio and less distinct features compared to a younger crater of the same size. The longest-lived craters have been degraded to a point where their original depth is irrelevant, existing only as subtle, highly-degraded depressions on the lunar surface.