To solve this problem, theorists are making tweaks to their computer simulations. In a report published in April, Japanese scientists, led by Natsuki Hosono of the Yokohama Institute for Earth Sciences, described how the still-molten Earth might have sloughed off the moon, so long as Theia thwacked the planet while it was still covered in a magma ocean.

Another theory, first proposed in 2017, calls for Theia and Earth to vaporize, forming a bagel-shaped cloud of fire that is neither a planet nor a disc. The short-lived structure, called a synestia, is a scorching gas blob with outer edges rotating so fast that they essentially launch into orbit. Once it begins to cool and solidify, rock droplets condense and fall onto the embryonic planet at the center. Leftover debris in the outer edges of the bagel would also condense, but at a greater distance, forming the moon.

“We can’t use current lunar accretion models to study this,” Simon J. Lock, a postdoctoral researcher at the California Institute of Technology who developed the theory with Sarah Stewart at the University of California, Davis, said in March at the Lunar and Planetary Science Conference in Houston. “We need to include differential equations, and we need to include the physics. It’s going to be hard — I’m sorry. But if we want to understand how our moon formed, and link its chemical properties to its origin, we need to overcome these challenges.”

A primordial pummeling — or not

The moon’s younger days are also the subject of intense debate.

Mare Imbrium, where Mr. Scott and Mr. Irwin touched down, is a vast lava plain and impact crater that formed about 3.8 billion years ago, when a titanic asteroid or protoplanet collided with the moon. When scientists started measuring Apollo samples, they found most of them had experienced some kind of horrific heating around that time.