The formation of a multi-ring lunar crater
Two new studies based on data from the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft mission have painted a clearer picture of the Orientale impact basin, one of the largest, youngest and best-preserved craters on the Moon. Clearer insights into this structure, which has been difficult to observe to date, could help researchers better understand the formation of the multi-ring basins on the Moon and other planets. In the first study, Maria Zuber et al. used high-resolution maps of the Moon's gravitational field to depict the structure of the crater's three rings. Before the GRAIL mission, the rings, which formed soon after the impact event, were only partially resolved, but since the spacecraft flew as low as two kilometers above the crater's surface, it was able to acquire data on these structures with unprecedented resolution. The data suggest that while Orientale as a whole has a diameter of 930 kilometers, the transient crater – which formed shortly after impact – has a smaller diameter, somewhere between 320 and 460 kilometers, meaning the transient crater does not correspond to any ring visible today, and has been hidden by later geological relaxation. Based on their analyses, the authors estimate a minimum amount of material that was redistributed from the lunar crust during the Orientale impact. Approximately one-third of this material was deposited back on the outskirts of the impact basin, they say, leading to a thicker lunar crust in this region. Results that probe the crater's subsurface region also show that Orientale's asymmetry in surface structure appears to extend to the subsurface, accompanied by faulting. The faulting, the authors suggest, predates the basin's formation.
In the second study, Brandon Johnson et al. use the GRAIL data to reconstruct a model of how Orientale formed, including its multiple rings. Their simulations match the GRAIL data when the impact is of a 64-kilometer diameter body traveling at 15 kilometers per second. The authors suggest that the impact first produced a bowl-shaped crater with a maximum depth of about 180 km, but this cavity was unstable and experienced gravitational collapse. As the initial crater collapsed, their simulations suggest that existing faults in the rock allowed warm, weak mantle material to flow inward. This resulted in a sequence of collapses that formed the two outer rings of Orientale, after ejected material from the initial impact had already settled. The simulations suggest that three hours after initial impact, the center of the basin lay roughly 7 kilometers deeper than it currently sits, and that the center has gradually risen to its current position in response to the post-impact stress states in the rocks. This work improves scientists' understanding of how Orientale formed and will inform studies of large impact craters throughout the solar system, including those on Earth.
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