If asked, most people would tell you that craters are round—the Moon’s abundant craters certainly appear (more or less) to be circular in shape. Yet, for those early students trying to explain the origin of lunar surface features, crater “roundness” caused great consternation. In 1893, the great geologist G.K. Gilbert wrote at length trying to rationalize the angle of impact for craters. Finding that the vast majority of impacting objects collided with the lunar surface at around 45 degrees incidence, Gilbert surmised that such geometry would result in most craters being elliptical, rather than round. Yet round they were, so he came up with a complex explanation about ancient rings of debris (similar to the rings of Saturn) providing the projectiles that shaped most of the craters of the Moon.
Still, the conundrum of off-vertical impacts making round craters persisted. Then in 1960, Gene Shoemaker demonstrated that impact crater-forming events were similar to the explosion of nuclear bombs—instantly vaporized projectiles creating a point-source release of energy. Such a release results in symmetric energy flow and hence, a circular impact crater is created.
This explanation would have solved the problem except for the fact that some craters (just a few) aren’t circular, including some of the largest and oldest features on the Moon: multi-ringed basins (a crater larger than 300 kilometers in diameter, usually displaying concentric rings of mountain peaks, is called a basin). So how do these features form? Experiments with a high velocity gas gun done by Don Gault and co-workers at NASA’s Ames Research Center in the 1960s and 1970s gave us some clues. Gault found that if the angle of impact was less than 15 degrees from horizontal, the final crater shape was elliptical. Yet at very grazing incidence (5 degrees and lower), bizarre crater shapes resulted, forming elongate ellipses and sometimes double-craters, with downrange trending rays, ejecta and ricocheted, scattered debris.
These classic laboratory experiments suggested that during low-angle impacts, unusual cratering mechanics resulted in atypical morphologies. A systematic search of the lunar surface revealed several examples of elliptical craters, some whose forms uncannily mirrored the results of the laboratory experiments. A classic oblique impact is inferred from the craters Messier and Messier A (above). These paired craters formed when a projectile hit the Moon at grazing incidence, probably less than a few degrees from the horizontal. When the expected rate of occurrence of low angle impacts is compared with the observed frequency of elliptical craters over the entire Moon, results are in reasonable agreement.
It was recognized early in lunar studies that the largest craters on the Moon are lava-filled mare basins. These impact features—large, regional structures, hundreds of kilometers across and pre-dating later lava filling, which took hundreds of millions of years—threw material across the entire near side of the Moon during their formation. Although most basin rims appear roughly circular, significant irregularities of shape occur on many scales. Systematic geological mapping of the Moon in the 1970s and 1980s found that ejecta from these basins often form elongate, asymmetric patterns—a “butterfly” shape (after its resemblance to two “wings” of ejecta on opposite sides of the crater). This form resembles the shape of the ejecta blankets of smaller, elliptical craters, such as Messier. From such patterns, it was inferred that these basins formed during the oblique collision of asteroid-sized bodies with the Moon, creating a bilaterally symmetric ejecta blanket with lobes perpendicular to the impact direction.
A complex “sculpture” pattern over most of the landscape of the central, near side highlands of the Moon has long been noted. When plotted on a map, these grooves, gouges and crater chains generally align back to the center of Mare Imbrium, and were thus named Imbrium sculpture. First recognized in Gilbert’s classic description in 1893, it has also been known that not all grooves are strictly radial to the basin center. This orientation is difficult to understand, as theory suggests that material ejected from the excavation cavity appears to originate from a single point. New studies by Pete Schultz of Brown University and David Crawford of Sandia National Laboratories may have uncovered a solution to this dilemma.
Using both laboratory impact experiments with the Ames gas gun and model calculations, Schultz and Crawford have examined the physical effects of the oblique impact of a very large (hundreds-of-kilometers-sized) asteroid. Given a very low angle of impact, they find that the projectile will ricochet off the surface from initial contact, causing a splitting or “decapitation” of the object such that a cloud of projectile debris will scour the area downrange, gouging out valleys that generally point uprange but do not converge on a single point, having originated in a dispersed cloud of shock-shattered particles. The resulting morphology can yield sculpture that is generally, but not exclusively, oriented radial to the basin center.
Comparison of features produced by the process of oblique impact can show some remarkable similarities of form. One experimental crater shows features similar to the lunar basins Crisium and Moscoviense (below). In these examples, the projectile forming the main basin hit the Moon at a very low angle, perhaps 5-10 degrees.
Decapitation of the projectile resulted in a shattered object moving rapidly downrange, with individual fragments gouging out the lunar surface in a spreading pattern beyond the basin rim. The overall shape of the basin and its ejecta is fish-like—a large quasi-circular main segment, where the projectile first hit the Moon, then fanned out downrange of the impact point into a fin-like tail.
These new insights come from experimentation, modeling and mapping observations of the Moon. Many basins have unusual morphology, including Imbrium, Humboldtianum and Crisium, in addition to the Moscoviense basin. Applying the oblique impact model to these features can help explain some of their most puzzling and enigmatic features. Continued mapping of the basins of the Moon is required to fully test this new idea and determine its general applicability.