Photo: freepik
Scientists know that the Moon formed after a massive collision between the early Earth and a planetary body called Theia. But more than 50 years after the Apollo 17 mission, astronomers still cannot fully explain exactly how Earth’s natural satellite came into existence, according to Science Alert.
Researchers believe the impact occurred around 4.51 billion years ago. However, the size of Theia remains uncertain. Some theories suggest it was as small as a proto-Mercury, while others propose it may have been roughly half the size of modern Earth.
New models indicate that a collision with a larger object could explain why Moon rocks brought back by the Apollo missions closely resemble volcanic basalts found on Earth.
“This event completely rewrote Earth’s history,” said Wim van Westrenen of Vrije Universiteit Amsterdam.
According to scientists, the newborn Moon was initially a glowing sphere of magma with temperatures reaching thousands of degrees.
“It wasn’t even rock yet,” van Westrenen explained. “It first had to cool before minerals could form.”
One of the biggest mysteries is determining how long it took after the impact for those minerals to appear — something researchers say is extremely difficult to calculate precisely.
Moon rocks collected during the Apollo missions continue to provide valuable clues. One famous sample is the “Genesis Rock,” brought back by Apollo 15 in 1971. It consists mostly of plagioclase, a light-colored mineral that tends to float to the surface of magma.
Scientists believe the Moon was once covered by a vast global ocean of magma about 1,700 kilometers deep. As the molten material cooled, lighter minerals like plagioclase rose to the surface, forming the Moon’s bright crust seen today.
A major problem for current theories is that computer simulations can reproduce the physical structure of the Earth-Moon system but fail to match its chemistry. According to van Westrenen, lunar rocks are far more chemically similar to Earth’s rocks than existing models predict.
One theory suggests Earth was already nearly fully formed when a smaller Theia struck it at a sharp angle and high speed. Another proposes that Earth was only about half its current size, and the collision with Theia contributed much of the remaining mass.
Scientists believe lighter debris from the collision eventually formed the Moon, while denser material sank into Earth and helped create the planet’s core.
However, in the classic giant-impact scenario, the Moon should chemically resemble Theia more than Earth — something that observations do not support. That contradiction remains one of the biggest unanswered questions in planetary science.
