The Moon’s surface is heavily marked by countless craters. These lunar features are predominantly circular, which might seem counterintuitive since objects strike the Moon from various angles. The reason for this common circularity lies in the immense forces unleashed during these extraterrestrial collisions.
The Energy of Cosmic Collisions
Lunar craters are formed by hypervelocity impacts, where asteroids or comets strike the Moon at speeds far exceeding the speed of sound in rock. This extreme velocity means the incoming object possesses enormous kinetic energy. Upon impact, this kinetic energy is converted into an explosive force. The collision is less like an object simply digging a hole and more like a high-energy detonation. This sudden release of energy often vaporizes or shatters the incoming projectile itself. The resulting crater is typically many times larger in diameter than the impacting object.
How Shockwaves Create Circular Shapes
The explosive energy from a hypervelocity impact generates powerful shockwaves that propagate outwards in all directions from the point of contact. These shockwaves travel through the lunar material, compressing and accelerating it. As they expand, they excavate material radially, regardless of the initial trajectory of the incoming object. This process creates a bowl-shaped depression known as a transient cavity.
The radial excavation continues until the shockwaves lose enough energy to no longer displace the lunar rock. The transient cavity often becomes unstable and collapses under the Moon’s gravity. Material from the cavity walls slumps inward, and the floor may rebound, sometimes forming a central peak in larger craters. This dynamic process, driven by spherically propagating shockwaves and gravitational adjustments, inherently leads to the circular rim and bowl shape of most lunar craters.
Why Impact Angle and Other Factors Have Little Effect
The overwhelming explosive energy released during a hypervelocity impact largely negates the effect of the incoming object’s angle of approach. Even if an object strikes the Moon at an oblique angle, the explosive forces expand symmetrically outwards from the point of impact, overpowering the directional kinetic energy of the projectile. Craters formed by angled impacts generally still appear round. Only extremely shallow, glancing impacts (less than about 10 to 20 degrees from the surface) might produce elongated or irregular craters.
While the initial formation mechanism favors circularity, other factors can modify a crater’s final appearance. Subsequent impacts can create overlapping or complex structures. Larger craters may exhibit features like terraced walls or central peaks due to gravitational collapse. These modifications, along with ejecta blankets, alter the overall morphology but do not change the fundamental principle that initial explosive excavation results in a round depression.