By Milo Moran
We don’t know where the Moon came from. For all of human history, people have been able to look up at the night sky and see it. The Moon is intrinsically part of our story as a species: it has been personified as a deity by many prehistoric cultures, and to this day we see the face of the so called ‘Man in the Moon’. However, we are still unsure of its origin.
The three main theories are called Fission, Capture and Giant Impact. Fission is the now discredited belief that the Earth was once larger, but some of its mass was broken off and formed the Moon. Capture suggests that the Moon formed somewhere else in space and was passing by when the Earth’s gravity caught hold of it and pulled it into orbit. Giant Impact, the most popular theory, says that the Earth was once smaller until a body the size of Mars collided with us. Some of this debris joined the Earth, while some formed the Moon. This collision tilted the Earth’s axis, causing the seasons.
Finding data about the Earth and Moon from such a long time ago is difficult, but the Center for Lunar Science and Exploration have announced that some rocks on the Moon originated on Earth. A lunar rock obtained by astronauts from Apollo 14 has been identified as coming from Earth about 4 billion years ago, during the Hadean eon. At this time, the Earth’s atmosphere was thinner, making it easier for comet or asteroid impacts to dislodge rocks from the planet. The Moon was also three times closer to us than it is now, making eclipses more common and increasing the likelihood that rocks jettisoned from the Earth would land there.
This has provided a terrifying insight into the early Earth: this rock was formed 20 kilometres beneath the surface, yet the impact events of the Hadean eon were so strong they blasted it out of the ground, and it ended up on the Moon. Asteroid impacts at the time have left craters thousands of kilometres in diameter. The Hadean eon is named after Hades, the Greek mythological hell: with a toxic and volatile atmosphere of hydrogen, methane, and ammonia, high levels of radioactive elements, and extremely high volcanic activity it’s easy to see why.
A high number of collisions in the Hadean eon is evidenced, not just on the Earth and Moon, but on other solar system bodies. The more we learn about this period of history, the closer we get to discovering the origin of our lunar companion.