The Moon May Have Arrived in Five Hours
Moon-formation models usually turn Theia into an orbiting debris ring. Add realistic rock strength, and one simulation produces an intact Moon within hours.
The Collision
The standard Moon-birth story begins with violence: a Mars-sized world called Theia slams into the young Earth, breaks apart and leaves a ring of debris that slowly gathers into the Moon.
For decades, impact models treated both worlds almost like fluids. The collision was so energetic that the strength of rock seemed too small to matter. A new study in The Astrophysical Journal Letters put temperature-dependent material strength back into the calculation.
Change the rock, and the same collision builds a different Moon.
Two Outcomes From One Impact
The simulations used smoothed-particle hydrodynamics with a model of how real geological material resists deformation. In one set of otherwise identical impact conditions, the internal temperature of Theia changed the ending.
- A hot but still solid Theia deformed in a way that produced an intact Moon.
- A colder, stronger Theia was destroyed into the more familiar protolunar debris disk.
- Strength mattered most in the impactor's outer few hundred kilometres, where it changed how momentum moved through the collision.
The University of Arizona's account of the work reports that the intact Moon emerged in roughly five hours. Previous simulations have also formed moons directly, but this study isolates rock strength and temperature as controls on which path appears.
Why the Clock Matters
Young protoplanets begin hot and cool with age. That means the simulated outcome links the timing of the giant impact to the Moon's original structure: an earlier collision with a warmer Theia could behave differently from a later collision with a colder one.
This is a model result, not a recovered video of lunar birth. It does not solve the stubborn fact that Earth and the Moon have remarkably similar compositions, and it does not prove that the intact route is what happened.
What Happens Next
The useful prediction is geochemical. Different formation paths process, mix and retain material differently, so lunar samples and measurements of volatile elements may help narrow the starting conditions.
The giant impact was never one simple explosion. According to the new model, even the stiffness of a doomed planet could decide whether our Moon arrived as a world or assembled as wreckage.