Most craters on the Moon look round because the explosion that forms them expands equally in all directions, regardless of the angle or shape of the incoming object. When a meteoroid slams into the lunar surface at speeds between 20 and 72 km/s (45,000 to 160,000 mph), it carries so much energy that the impact behaves less like a rock hitting dirt and more like a bomb going off. That symmetrical release of energy is the core reason nearly every crater appears circular from above.
Why the Impact Acts Like an Explosion
At lunar impact speeds, even a small object releases an extraordinary amount of energy. NASA’s Lunar Impact Monitoring Program notes that a meteoroid weighing just 5 kg (about 10 lbs) can excavate a crater over 9 meters (30 ft) across, launching 75 metric tons of soil and rock into the air. That energy-to-mass ratio is key: the impactor doesn’t just push material aside like a ball landing in sand. Instead, it vaporizes and compresses the surface so violently that a shockwave radiates outward from the point of contact in every direction, much like the blast wave from an explosive charge buried underground.
Because the shockwave expands as a sphere beneath the surface, the resulting cavity it carves is roughly hemispherical. When the roof of that cavity collapses, the rim left behind is circular. The final crater can be 10 to 20 times wider than the object that created it, which means the shape of the impactor itself, whether it’s round, oblong, or irregular, gets completely overwritten by the physics of the explosion. By the time the crater finishes forming, the original object’s geometry is irrelevant.
Why Impact Angle Rarely Matters
Objects hit the Moon from all directions and at all angles, so you’d expect many craters to be elongated. But experiments on oblique impacts show that a meteoroid’s trajectory needs to be extremely shallow, typically below 15 to 20 degrees from the surface, before the resulting crater stretches into a clear ellipse. Since incoming trajectories are randomly distributed across all possible angles, the vast majority strike at angles well above that threshold. Statistically, only about 5% of random impacts come in at such a grazing angle.
Even at moderately low angles, the shockwave still dominates. The energy release is so fast and so intense that it “resets” the geometry almost immediately. The impactor may gouge a brief channel on first contact, but the expanding shockwave quickly overwhelms that initial asymmetry and opens a roughly circular bowl. Only at the most extreme grazing angles does the energy travel forward enough to produce a noticeably elongated shape.
The Craters Aren’t Perfectly Circular
Here’s the nuance: when scientists measure lunar craters precisely, most of them are technically ellipses rather than perfect circles. One detailed global survey found that over half of all cataloged craters have an ellipticity greater than 1.1, the standard threshold planetary scientists use to classify a crater as elliptical. This is especially true for smaller craters under about 30 km in diameter, where subtle asymmetries from impact angle, surface slope, or subsurface rock layers are more visible.
So when people say lunar craters are “round,” they mean they look round to the eye. A crater with an ellipticity of 1.1 is only 10% longer in one direction than the other, which is nearly impossible to notice in a photograph. The overwhelming dominance of the shockwave keeps craters close to circular, even if they aren’t mathematically perfect.
How the Impactor’s Shape Gets Erased
Real meteoroids are not smooth spheres. They’re jagged, lumpy, and often elongated. Research into how impactor shape affects crater formation confirms that shape does influence the volume of material ejected and the fine details of the debris pattern. At very shallow angles, oddly shaped impactors can produce craters with distinctive “butterfly wing” ejecta patterns. But at typical impact angles, the crater dimensions converge to roughly the same result regardless of whether the incoming body was disk-shaped, rod-shaped, or spherical. The energy of the collision dwarfs any geometric influence the impactor might have had.
Why the Moon Preserves Its Craters So Well
The Moon’s near-total lack of atmosphere plays an indirect role in why its craters look so consistently round. On Earth, wind, rain, tectonic shifts, and vegetation erode craters over thousands or millions of years, often distorting their original shape or erasing them entirely. The Moon has none of these forces. Some lunar craters have remained virtually untouched since the day they formed, billions of years ago.
This preservation matters because it means the circular shape created by the initial shockwave is the shape you still see today. There’s no weathering to selectively wear down one side of a rim, no river cutting through the wall, no sediment filling in the floor unevenly. The geometry you observe through a telescope is, in many cases, the original geometry of the explosion.
What the Surface Adds to the Picture
The lunar surface itself contributes to crater symmetry. The Moon is covered in regolith, a layer of loose, broken rock and dust created by billions of years of impacts. This granular material responds to shockwaves in a relatively uniform way compared to, say, a surface with layers of hard rock, soft sediment, and underground water like Earth’s. When the target material is fairly homogeneous, the shockwave spreads more evenly, and the resulting crater is more symmetrical.
That said, bedrock strength beneath the regolith does matter, especially for smaller craters. Research into regolith formation shows that the strength of underlying rock significantly affects how material is shattered and redistributed. Where the regolith layer is thicker, more debris ends up outside the crater rim, subtly altering the crater’s profile. But these effects mainly change the crater’s depth and rim height rather than making it less circular.