The moon may have formed in just five hours following a giant collision between Earth and a Mars-sized planet 4.5 billion years ago, scientists at the Southwest Research Institute and University of Arizona have found.
For decades, planetary experts believed the moon was created when a Mars-sized planet named Theia smashed into a young Earth, blasting vast amounts of molten rock into space. That debris was thought to have gradually clumped together over years to form the lunar satellite.
Using powerful computer simulations, researchers discovered that in certain scenarios, a largely intact moon emerged astonishingly quickly after the crash, with internal body temperatures acting as the deciding factor.
"Depending on how hot the Earth and Theia are prior to the collision, an impact can destroy Theia and produce this massive disk of debris that eventually forms the moon," Dr Adeene Denton said. "But when I used the same parameters as original impact modelling - down to the equal temperature structures inside both bodies - within around five hours, an intact moon emerged."
Material strength and temperature in planetary collisions
The research team found that the geological properties of colliding worlds play a crucial role in post-collision outcomes. Previous models largely treated planets as fluid-like bodies and ignored the inherent strength of their rocks.
When scientists factored in material strength alongside temperature, they observed dramatically different results. Hotter planets are weaker and tear apart easily, whereas cooler worlds are stronger and more likely to survive an impact in one piece.
"Models have evolved to include material strength, something that's really important when you're studying collisions between smaller bodies like asteroids or for my previous paper about the formation of the Pluto-Charon system," Dr Denton said. "We weren't sure if it would matter for the moon or not. When we did the simulations, we found it actually matters quite a bit."
Chemical composition and thermal state of early Earth
The findings, published in the peer-reviewed Astrophysical Journal Letters, could help solve one of planetary science's greatest mysteries: determining the precise timing of the moon-forming impact.
Researchers say the work could also explain puzzling lunar characteristics, including variations in composition and the amount of volatile material the moon contains.
"These surprising and exciting new results imply a potential connection between the physical properties of the moon today, including perhaps its volatile content, and the thermal state of the Earth and Theia at the time of the giant impact," Dr Robin Canup, who conducted an earlier study on the moon's origin, said. "This in turn might help scientists better constrain when the Moon-forming event occurred."
Explaining the nearly identical chemical makeup of Earth and the moon remains an open scientific question. One possible explanation is that Theia and the early Earth formed within the same region of the protoplanetary disk, a pancake-like ring of dense gas and cosmic dust surrounding the young sun.
By contrast, Mars is compositionally distinct from both Earth and the moon, indicating it initially formed further away in the solar system.
"Because Earth and Mars formed in the same neighbourhood of the solar system, they are like siblings," Dr Denton said. "The moon and Earth are like fraternal twins."
SpaceX rocket collision on the lunar surface
Human technology continues to alter the lunar surface. Last month, an out-of-control SpaceX Falcon 9 rocket second stage careened into the moon at 5,400 miles per hour (8,690 km/h), leaving a visible crater.
Scientists estimate that the 9,920lb (4,500kg), 39ft-long (12m) rocket struck with a force equivalent to 15,000 sticks of dynamite, or three tonnes of TNT.
Pictures captured by a NASA satellite revealed the outcome of the collision, showing a 60-foot-wide (18m) crater surrounded by streaks of lunar rock thrown up by the impact.
Apollo missions and human walks on the moon
A total of 12 people have walked on the moon across six NASA Apollo missions between 1969 and 1972.
Neil Armstrong made history on July 21, 1969, becoming the first person to set foot on the lunar surface during Apollo 11, before being joined by crewmate Edwin "Buzz" Aldrin.
Pete Conrad and Alan Bean walked on the moon during the Apollo 12 mission on November 19 and 20, 1969, after their Saturn V rocket survived two lightning strikes shortly after launch.
Alan Shepard and Edgar Mitchell landed on February 5, 1971, for Apollo 14, having launched on January 31, 1971. They touched down in the Fra Mauro region, the original destination of the aborted Apollo 13 mission.
Dave Scott and James Irwin landed on July 31, 1971, as part of Apollo 15, staying on the lunar surface for three days until August 2.
John Young and Charlie Duke reached the surface on April 21, 1972, during Apollo 16, after a problem with the main engine on the command and service module nearly forced commanders to abort the mission in lunar orbit.
Eugene "Gene" Cernan and Harrison "Jack" Schmitt became the final humans to walk on the moon during Apollo 17 on December 11, 1972. Before leaving, Cernan scratched the initials of his daughter Tracy into the lunar regolith. Because the moon lacks weather conditions such as wind or rain to cause erosion, her initials are expected to remain preserved for a very long time.
