Imagine that after one of the greatest collisions in the history of the solar system, Earth was not surrounded solely by a diffuse cloud of debris. Within about five hours, a large precursor of the Moon may already have been there, held together by its own gravity. New computer simulations suggest that the outcome of the impact may have been determined by a property often overlooked in older models: the strength of hot rock.
Yes, according to one simulated scenario, a largely intact precursor of the Moon could have formed roughly five hours after the collision. However, this does not mean scientists have determined how long the Moon actually took to form. The result depends on the unknown temperatures and strengths of the young Earth and Theia, as well as the speed and angle of the impact.
What May Have Happened After Earth Collided with Theia
The most widely accepted explanation for the Moon’s formation involves a giant collision around 4.5 billion years ago. A Mars-sized protoplanet, commonly called Theia by scientists, is thought to have struck the young Earth. The impact ejected vast quantities of rock into space and fundamentally altered both bodies.
Older models often assumed that after such a high-energy event, rock would behave essentially like a liquid. These models generally produced a disk of incandescent material and debris around Earth, from which the Moon would gradually assemble under the influence of gravity.
Some more recent calculations, however, suggest another possibility. Rather than creating only a disk, the collision may have placed a large mass of material into orbit that formed a nearly intact lunar body from the outset.
Rock Strength May Have Been Decisive
In their computer simulations, a team led by C. Adeene Denton accounted for the fact that rock has a certain mechanical strength and that this strength changes with temperature. Put simply, hotter material is weaker and deforms more easily, while cooler material can be stronger.
The researchers tested bodies with different temperature profiles, including initial surface temperatures of approximately 400, 800 and 2,000 kelvins. The different configurations produced very different outcomes. In some cases, Theia broke apart and created a disk of debris; in others, the collision left a large, nearly intact precursor of the Moon in orbit.
It is therefore inaccurate to claim that hot, soft surfaces alone would always accelerate the Moon’s formation. The relationship between temperature, strength and the breakup of the bodies is more complex. The key finding is that including temperature-dependent rock strength in the model can fundamentally change the outcome of the collision.
Where the Five-Hour Figure Comes From
When the scientists replicated the specific parameters of an earlier model, including the same temperature structure for both bodies, an intact object resembling the primordial Moon appeared in the simulation after roughly five hours. This is an extreme but physically simulated scenario—not a timescale measured from lunar rocks.
Moreover, the statement that “the Moon formed in five hours” does not mean it looked as it does today after that time. The simulation describes the formation of a large, gravitationally bound precursor following the impact. It does not cover the billions of years of subsequent cooling, internal evolution, or the formation of the Moon’s present-day crust and surface.
The idea that an intact Moon formed rapidly is not entirely new. The main significance of the new study is that it demonstrates the potentially decisive role of material strength and temperature in determining which of two outcomes occurs: a large lunar body, or a disk from which the Moon would gradually assemble.
Why the Young Earth’s Temperature Matters
After forming, protoplanets gradually cooled and became more rigid. If the nature of the collision depended on the thermal state of Earth and Theia, simulations could one day help narrow down the period in which the impact occurred. First, however, scientists would need a better understanding of the actual properties of both bodies.
This is where the greatest uncertainty remains. We do not know their exact internal temperatures or strengths, nor the precise speed and angle of the collision. For now, it is therefore impossible to select one scenario from the many calculated possibilities and declare it the definitive account of the Moon’s formation.
Another unresolved question is why Earth and the Moon have such similar isotopic compositions—that is, similar proportions of different forms of the same chemical elements. One possible explanation is that Theia and the young Earth formed in the same region of the original disk of material surrounding the Sun. However, the new simulations do not in themselves confirm this possibility.
The five-hour result is therefore a fascinating demonstration of how quickly gravity could have created a lunar precursor after a giant impact. But it is not a stopwatch revealing the exact time it took our cosmic neighbor to form.
What the Collision Looks Like in the Simulation
A video from the Southwest Research Institute shows the modeled collision and the formation of the lunar body.
Sources
Marcel Macko
I’m most interested in technology, the internet, bizarre news, and stories that make you stop and think, “There’s no way this can actually be true.” I follow what’s happening both at home and around the world and pick the topics I think are worth paying attention to. On Pitchoviny, I try to present them simply, factually, and without unnecessarily sensational headlines.
