Giant-impact hypothesis

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"text": "The giant-impact hypothesis is the leading scientific explanation for how our Moon formed.

This theory suggests that a massive collision occurred between the early Earth and another celestial body. This event likely happened about 4.5 billion years ago during the Hadean eon. At that time, the Solar System was only about 20 to 100 million years old. The collision was so powerful that it sent debris into space. This ejected material eventually gathered together to create the Moon. Scientists consider this an astrogeology hypothesis because it explains the physical history of our planet and its satellite.\n\nThe collision involved a Mars-sized protoplanet often called Theia.

Theia likely originated from the L4 or L5 Lagrange points of Earth's orbit. These are stable points in space where an object can stay near a planet. During the impact, Theia struck the Proto-Earth at an angle. Some models suggest an angle of about 45 degrees, but other evidence implies a much steeper, more direct hit. This intense energy caused the fragmentation and thorough mixing of both parent bodies. The impact was so energetic that it likely turned parts of the planets into liquid.

Theia's heavy iron core likely penetrated deep into the Earth and fused with Earth's own core. This left the Moon with a much smaller iron core than Earth. The remaining lighter material, consisting of crust and mantle fragments, was thrown into orbit.\n\nScientists believe the formation of the Moon happened in several distinct stages. First, material was ejected beyond the Roche limit. The Roche limit is the distance within which a planet's gravity prevents a moon from forming from loose debris. Material outside this limit could coalesce into a moon. Inside this limit, the debris formed a disk around the Earth. This inner disk slowly spread outward over hundreds of years. As the disk material moved beyond the Roche limit, it began producing new objects. These objects continued to grow and merge until the Moon reached its current size.

Some models even suggest a second, smaller moon may have formed in a Lagrange point. This smaller moon might have eventually collided with the main Moon, adding material to its far side.\n\nOur understanding of this event has changed through many scientific discoveries. In 1898, George Darwin suggested the Moon was spun off Earth by centrifugal forces. He used Newtonian mechanics to show the Moon was drifting away from Earth. Later, American and Soviet experiments used laser ranging to confirm this drift. However, Darwin could not explain the mechanics of how the Moon left Earth's surface. In 1946, Canadian geologist Reginald Daly proposed the impact idea instead. His theory gained significant attention after a 1974 conference on satellites. By 1984, at a conference in Kona, Hawaii, the giant-