Anatomy of an impact
The Orientale Basin is the youngest and best-preserved multi-ring basin on the Moon. Formed approximately 3.8 billion years ago, its structure resembles a massive bullseye. This feature consists of three main concentric rings of rock. The outermost ring is a mountain range called the Montes Cordillera, which defines the basin's main diameter of about 930 kilometers (580 miles). Inside the Cordillera are two more rings, the Outer and Inner Montes Rook. Unlike many other lunar basins, Orientale is not completely filled with dark volcanic basalt, leaving much of its complex structure visible to spacecraft.
The impact that created Orientale was immense. A body estimated to be 64 kilometers (40 miles) wide struck the Moon at a speed of around 15 km/s (9.3 mi/s). This collision was powerful enough to obliterate any trace of the initial, transient crater, which is estimated to have been between 320 and 460 kilometers across. Data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission revealed the subsurface structure. The gravity maps show a mass concentration, or "mascon," in the center, which indicates that denser mantle material is closer to the surface because the crust was thinned by the impact. The impact excavated at least 816,000 cubic miles (3.4 million cubic kilometers) of the Moon's crust.
The formation puzzle
The exact mechanism that forms multi-ring basins is a subject of scientific debate. For years, one idea was that the rings were simply enormous slump terraces, like those seen in smaller craters. Evidence suggests the rings formed almost instantaneously with the impact. Data from the GRAIL mission has allowed for more sophisticated modeling, providing new insights.
One leading model, supported by simulations, suggests a process of violent crustal rebound. Moments after the impact, the crater floor rebounds so forcefully that it obliterates the initial crater. The lunar crust and upper mantle behave like a fluid. Warm, ductile rock from deep below flows inward and upward. This causes the overlying crust to crack and slip along enormous faults, throwing up the cliffs that form the outer two rings (the Cordillera and Outer Rook mountains) in a matter of minutes.
The innermost ring appears to form differently. The central rebound creates a peak so massive that it becomes unstable. This towering peak of rock then collapses outward, forming the Inner Rook ring. This entire sequence (impact, rebound, and collapse) happened over the course of just a few hours, reshaping a vast region of the lunar surface.