The Moon probably formed when a Mars-sized world struck the young Earth
About 4.51 billion years ago, the favoured story goes, a body the size of Mars, nicknamed Theia, struck the infant Earth at a glancing angle. Molten debris thrown into orbit gathered into the Moon. Simulations published in 2023 even suggest pieces of Theia could still be buried inside our planet.
Rival explanations all stumbled. Spinning off from Earth's crust would have needed an impossibly fast rotation; capturing a passing Moon required an implausibly extended atmosphere to slow it; forming the pair side by side fails to explain why the Moon is poor in metals. None could account for the system's high angular momentum. The impact model can, and its simulations match the size of the lunar core. The awkward part is chemistry: Earth and the Moon have nearly identical oxygen and tungsten isotopes, while Mars and Vesta differ sharply, so either most of the Moon came from Earth or the debris mixed thoroughly afterwards. A 2022 study with high-resolution simulations found an impact could place a Moon-like satellite directly into a distant orbit.
The new Moon circled much closer, looming large, causing frequent eclipses and strong tides. Tidal interaction has since pushed it outward to an average of 384,399 kilometres and locked it so the same face always points at us, though wobbles called libration let us see about 59 percent of its surface. Its diameter of 3,474 km is roughly a quarter of Earth's, making it the largest moon relative to its planet, and its gravity is about a sixth of ours.
The dark patches, the maria or seas, are basalt plains from volcanism that ran until 1.2 billion years ago, mostly erupting 3.3 to 3.7 billion years back. They cluster on the near side, perhaps because the crust there is thinner. With no weather to erase them, craters like Tycho and Copernicus still display bright rays. Despite looking brilliant at night, the surface reflects about as much light as asphalt.
The Soviet Luna 1 flew past in 1959 and Luna 9 made the first soft landing in 1966. Apollo 8 reached lunar orbit on December 24, 1968, and Apollo 11 landed on July 20, 1969; by 1972 twelve people had walked there. The English word moon traces back to a Proto-Indo-European root for month, possibly linked to measuring time.
Source: Moon
