Where does the Moon come from? Does anyone know?
But astronomers have several theories. The most widely accepted explanation today is the so-called Giant-impact hypothesis. This model says that the Moon was created 4.5 billion years ago as a product of a big collision between an early Earth and a Mars-sized object.
A collision in the young solar system
The Sun was born 4.6 billion years ago, so the impact is thought to have happened immediately after. The formation of the Sun left a large disk of space rocks and star dust around the newly born Sun. The dynamics in the disk caused the rocky material to grow into objects such as planets.
The Giant-impact hypothesis states that two of those planets were Gaia and Theia. Gaia is the name of the young Earth before it was impacted by Theia: a planet roughly the size of Mars.
20-100 million years after the Sun was born, Gaia and Theia are believed to have smashed together in a violent collision causing rocky debris from Gaia’s outer layer and Theia to be thrown into space.
The debris would gravitate into a spherical object, which we know today at the Moon. The collision created a lot of heat so in the beginning the Moon was molten.
The Moon got its crust when the magma ocean would cool down during the next 100 million years. Rocks would float to the surface, and as the surface would cool the lunar crust formed.
The resulting object from the Gaia-Theia collision was planet Earth.
Giant-impact hypothesis solves only a part of the puzzle
Several models for the Moon’s formation have been proposed through time, but Giant-impact hypothesis is the current favorite. It answers most of the questions that arise from observations and measurements.
For instance, the model explains why the Moon has lower density than Earth and why the Moon is spherical. It also explains the low amount of Iron in the Moon: Theia’s core is thought to have been mixed with Earth’s core, which during the collision was untouched.
However, scientists have not figured out why the Moon is enriched in Aluminum and Titanium. It is also still up for debate why the Moon and Earth have similar Oxygen-isotope ratios. One explanation could be that Theia had the same Oxygen-isotope ratio as Gaia, or maybe heat from the collision created an ‘equilibrium ratio’ on both Moon and Earth. This is still unanswered.
Another headache arises from knowing that Venus experienced similar impacts during its early years, yet Venus hosts zero natural moons. It is also still not completely understood why the Moon has so few elements that evaporate easily, compared to Earth.
The Moon might be our nearest space neighbor, but it still holds many mysteries.
The Earth’s Moon at NASA
Giant-impact hypothesis at Wiki (Oct 1st, 2019)
How many moons does Venus have? at Phys.org