The Moon — Crater Anomalies: Too Shallow Too Circular Too Convex
The Moon — Crater Anomalies: Too Shallow, Too Circular, Too Convex
[edit | edit source]The Expected Impact Crater Profile
[edit | edit source]When a high-velocity impactor strikes a solid body, it excavates a bowl-shaped crater. The depth of the crater is proportional to its diameter — large impactors create proportionally deep craters. The floor of an impact crater should be concave — curving inward toward the deepest point of the excavation.
On rocky bodies across the solar system, crater depths scale with diameter in a predictable way. Large craters are deep; small craters are shallow; but the ratio of depth to diameter remains roughly consistent across a wide range of sizes.
The Lunar Crater Depth Anomaly
[edit | edit source]On the Moon, this relationship breaks down for the largest craters. Large lunar craters are anomalously shallow relative to their diameter:
| Crater | Diameter | Expected depth (terrestrial scaling) | Actual depth | Depth deficit |
|---|---|---|---|---|
| Copernicus | 93 km | ~7.5 km | ~3.8 km | ~3.7 km too shallow |
| Clavius | 225 km | ~12 km | ~3.5 km | ~8.5 km too shallow |
| Ptolemaeus | 153 km | ~9 km | ~2.4 km | ~6.6 km too shallow |
| Mare Imbrium | ~1,300 km | Very deep | ~1.5–2 km | Dramatically too shallow |
The largest impact basins on the Moon — the maria — are almost perfectly flat, despite representing the sites of truly enormous ancient impacts. Mare Imbrium, created by an impactor that must have been hundreds of kilometres in diameter, is only a few kilometres deep.
The Convex Floor Phenomenon
[edit | edit source]Related to the shallow depth anomaly: many lunar craters have floors that are not concave (curving inward) but flat or even slightly convex (curving outward). From the centre of a large lunar crater, looking toward the rim, the floor actually curves upward toward the observer — the opposite of what impact excavation physics predicts.
The convex floor phenomenon is particularly pronounced in the largest craters. Some researchers have noted that the convex curvature of the crater floors follows the curvature of the lunar surface itself — as if the crater floor is a section of a sphere concentric with the Moon's surface, at a depth determined not by the impact energy but by some subsurface barrier.
The Mainstream Geological Explanation
[edit | edit source]The mainstream explanation for both anomalies is isostatic rebound:
- Large impacts excavate deep basins
- The excavated rock is removed, reducing the mass at the impact site
- The underlying mantle, under pressure, flows laterally into the low-pressure zone
- The basin floor rises (rebounds) as the mantle material compensates for the removed crust
- For very large impacts, the rebound can nearly fill the crater, leaving a flat or convex floor
This isostatic rebound model is well-established for Earth and other planetary bodies, and it accounts for many features of the lunar basins.
What Remains Anomalous
[edit | edit source]Even within the isostatic model, some aspects of lunar crater morphology are not fully explained:
- The extent of the floor elevation in some craters seems to exceed what isostatic models predict
- The near-perfect circularity of many large craters — even at oblique impact angles that should produce elliptical craters — has prompted the suggestion of a subsurface rigid structure that constrains the crater shape
- The metallic mass detected beneath the South Pole-Aitken Basin (the deepest large basin) adds a new dimension to the crater depth question — is the anomalous mass related to the limited depth of the excavation?
The Vasin-Shcherbakov hypothesis offers a direct explanation: there is a rigid metallic shell approximately 20 miles thick beneath a thin regolith layer, and impactors cannot penetrate the shell regardless of their size, producing shallow, flat or convex craters with a maximum depth determined by the shell's top surface.
