The unusual structure of one of the Moon’s largest impact basins may have been created by ejected pieces of an impacting proto-planet, a study published in Nature suggests. New calculations indicate that the object was around 250 kilometres in diameter, larger than previously thought. The findings provide insights into the size of objects in the E-belt of asteroids that bombarded planets and shaped the nearside of the Moon around 4 billion years ago.

Patterning of the lunar region Mare Imbrium, known as the Imbrium Sculpture, is made up of grooves surrounding a 1,250-kilometre-diameter crater. Some of these furrows appear to have been ploughed out by fragments of the body that created the giant crater rather than by material excavated from the impact site, Peter Schultz and David Crawford propose. Assessment of these patterns, informed by numerical and laboratory impact simulations, allow them to estimate the object’s size - almost half the diameter of the proto-planet Vesta.

The authors conclude that the impactor was once part of a population of proto-planets in the E-belt, and that large pieces of the object would have escaped the impact to create further craters throughout the inner Solar System. They infer from the estimated size of the impacting object that a larger mass of material than previously thought seems to have battered the Moon and other planets during the Late Heavy Bombardment between 4.1 and 3.7 billion years ago.