Moon may have formed in hours after Theia collision, study finds

September 4, 2026 at 11:30 PM2 min

This digest was compiled by AI from multiple sources — links to the originals are below.

A new simulation published in The Astrophysical Journal Letters shows a Moon-sized body could form in about five hours after the giant impact that created the Earth-Moon system. The study adds temperature-dependent rock strength to a canonical collision model, producing a single satellite instead of a debris disk. The result is one branch of a numerical experiment, not proof of the Moon's actual formation time.

Key Facts

A study in The Astrophysical Journal Letters reran a canonical Moon-formation collision with temperature-dependent rock strength included.
In one calculation, a single Moon-sized body emerged in roughly five hours.
The standard model treats the colliding proto-Earth and Theia as strengthless fluids, ignoring solid rock's resistance to deformation.
Apollo samples show the Moon was once extensively molten, consistent with a global magma ocean.
Robin Canup and Erik Asphaug's 2001 simulations found a narrow family of collisions that matched the Earth-Moon system's mass and angular momentum.

The New Simulation

The study published in The Astrophysical Journal Letters reran a canonical version of the giant-impact collision with temperature-dependent rock strength included. In one calculation, a single Moon-sized body emerged in roughly five hours. The result is not proof that the real Moon formed on that timetable, but one branch of a numerical experiment. Earlier high-resolution models have also made a large satellite within hours. The proposed control is whether the colliding rock was hot and weak or colder and stronger, which could help decide whether the impact left one coherent body or a disk.

Why the Moon's Origin Puzzles

The Moon is unusually large compared with its planet and contains relatively little iron with a much smaller core than Earth. The Earth-Moon system carries a particular amount of angular momentum that any convincing history must reproduce. Apollo samples showed that the Moon was once extensively molten, consistent with a global or nearly global magma ocean. The Moon's isotopic chemistry is extraordinarily similar to Earth's mantle, despite expectations of a more foreign signature from an independent impactor.

The Giant-Impact Hypothesis

Near the end of terrestrial planet formation, Theia struck the proto-Earth at an oblique angle. Iron preferentially stayed with or fell back into Earth while silicate-rich material entered orbit. That debris spread into a disk and later accreted into the Moon. Robin Canup and Erik Asphaug's 2001 calculations found a narrow family of collisions capable of producing the system's broad mass and angular-momentum constraints. The result became a benchmark that later models have repeatedly revisited.