Lunar Probe
Surface guide

How Lunar Impact Craters Form and What They Reveal

From excavation and shock to central peaks, ejecta rays and the geological clock recorded by crater populations.

Key takeaways

  • Lunar craters form mainly through hypervelocity impacts, not volcanic explosions.
  • Small craters are simple bowls; larger craters develop terraces, central peaks and more complex structures.
  • Ejecta, rays and secondary craters extend the effects far beyond the rim.
  • Crater populations help estimate surface age, but interpretation requires calibration and attention to resurfacing and secondary impacts.
Lunar landscapes preserve overlapping records of impacts, volcanism and space weathering.

A hypervelocity event

Typical lunar impact speeds are high enough that the event behaves more like an explosion than a slow mechanical excavation. Shock waves compress, heat, fracture and accelerate target rock.

The final crater is much larger than the impactor. Energy, angle, impact speed, target material and gravity all influence the resulting structure.

Simple craters

Smaller lunar craters tend to have bowl-shaped interiors and raised rims. Fresh examples may show sharp edges and blocky ejecta.

Over time, micrometeoroids, seismic shaking from later impacts and small secondary impacts soften the morphology and reduce contrast.

Complex craters

Above a size threshold, crater walls collapse inward and the compressed floor rebounds. The result can include wall terraces, flatter floors and central peaks.

Copernicus and Tycho are classic examples visible from Earth. Their internal morphology is especially dramatic when the Sun is low.

Basins

The largest impacts create multi-ring basins hundreds or thousands of kilometres across. These events can excavate deep crustal material, fracture enormous regions and influence later volcanism.

South Pole-Aitken is an extreme example whose scale makes it important for studying crustal and potentially mantle materials.

Ejecta and rays

Material thrown from a crater can blanket surrounding terrain and create radial patterns. Bright rays are especially conspicuous around young craters because fresh material has experienced less space weathering.

Large ejecta blocks can create secondary craters when they fall back to the surface. Those secondaries complicate crater-count dating if they are mistaken for independent impacts.

Crater counting as a clock

Older exposed surfaces generally accumulate more impact craters than younger or resurfaced terrain. Scientists count craters over mapped areas and compare size-frequency distributions.

Absolute ages require calibration from radiometrically dated samples. The method is powerful but model-dependent, especially for very young surfaces and terrains dominated by secondaries.

See the landscape in context

Use the Lunar Surface Atlas, Feature Finder and Crater Morphology Explorer to connect the explanation with real terrain.

Sources and further reading