Origin and implications of non-radial Imbrium Sculpture on the Moon

The widespread rimmed grooves, lineations and elongate craters extending from the Imbrium impact basin on the Moon, termed the Imbrium Sculpture, includes a non-radial component that is used to infer that the Imbrium impactor was the size of a proto-planet—about half the diameter of Vesta. Carving the Moon's Imbrium Sculpture The pattern of rimmed grooves, lineations and elongate craters around Mare Imbrium on the Moon has been termed the Imbrium Sculpture. Rather than radiating from the centre of Imbrium, the grooves come from different locations reflecting the consequence of a giant oblique collision. Peter Schultz and David Crawford use the non-radial trends in these patterns to infer that the Imbrium impactor was a proto-planet with a diameter about half that of the asteroid Vesta. On the basis of laboratory impact experiments, shock physics codes and the observed groove widths, the authors conclude that multiple fragments from each oblique basin-forming impactor, such as the one that formed Imbrium, should have survived planetary collisions and contributed to the heavy impact bombardment between 4.3 and 3.8 billion years ago. Rimmed grooves, lineations and elongate craters around Mare Imbrium shape much of the nearside Moon. This pattern was coined the Imbrium Sculpture 1 , and it was originally argued that it must have been formed by a giant oblique (~30°) impact, a conclusion echoed by later studies 2 . Some investigators, however, noticed that many elements of the Imbrium Sculpture are not radial to Imbrium, thereby implicating an endogenic or structural origin 3 , 4 . Here we use these non-radial trends to conclude that the Imbrium impactor was a proto-planet (half the diameter of Vesta), once part of a population of large proto-planets in the asteroid belt. Such independent constraints on the sizes of the Imbrium and other basin-forming impactors markedly increase estimates for the mass in the asteroid belt before depletion caused by the orbital migration of Jupiter and Saturn 5 . Moreover, laboratory impact experiments, shock physics codes and the groove widths indicate that multiple fragments (up to 2% of the initial diameter) from each oblique basin-forming impactor, such as the one that formed Imbrium, should have survived planetary collisions and contributed to the heavy impact bombardment between 4.3 and 3.8 billion years ago.