ORIGIN OF THE IMBRIUM SCULPTURE (Invited Presentation)

In 1893, G. K. Gilbert published his seminal work on the origin of lunar craters. The Imbrium Sculpture provided key clues for his argument that the largest craters on the Moon must have formed by impacts. Yet detailed mapping from telescopic observations revealed that this sculpturing had multiple orientations and sources, which prompted other suggestions for a structural origin. While subsequent studies agreed with Gilbert, this distinctive non-radial pattern remained enigmatic. Crater chains and secondary craters do not always indicate impact trajectory, but breached downrange rims of secondaries reflect the retained downrange momentum, hence tracing ballistic-ejecta trajectories. When mapped as great circles on an Imbrium-centered projection, such secondaries and grooves reveal a systematic evolution of source regions: one set converging uprange; another, near the center of the oblong (NW-SE) offset inner massif ring; yet still another, near the geometric center of the outer basin ring. In some regions, different sets overlap each other, thereby also recording a sequence of formation. The first set originates from a region to the northwest of center; the second, from the basin center. Oblique impacts in laboratory experiments establish that this pattern reflects an evolving flow field from the downrange-directed shock at first contact to radial excavation at late stages. At large scales, early stages of excavation comprise a large fraction of crater growth due to reduced cratering efficiency, especially for oblique impacts. As a result, an evolving flow field becomes more evident. For Imbrium, distant grooves originate from low-angle debris created at first contact and include projectile fragments that set constraints not only for the trajectory (NW to SE -- in agreement with Urey and Baldwin) but also impactor size. The same patterns occur around other large oblique impact basins on the Moon (e.g., Crisium, Moscoviense, Orientale), Mars, and Mercury.