The moon “hatched” from the early Earth like a chick from an egg, a new theory has suggested.
Most experts believe the moon was created when a Mars-sized object smashed into the Earth, spinning large amounts of ejected material into space.
But the more colourful idea put forward by a team of US planetary scientists suggests that instead the moon grew from a “seed” within the “Earth”, which at the time consisted of a cloud of vaporised rock.
It then emerged to take its place as the Earth’s only natural satellite.
A so-called Blue Moon over Bolsover, Derbyshire. How the moon was formed is still hotly debated (Aaron Chown/PA)
The theory is said to solve a number of outstanding puzzles, such as why the isotopic signature – or atomic strain – of Earth and moon rock is almost identical, and the lack of volatile elements on the moon.
Professor Sarah Stewart, from the University of California at Davis, who took part in the research, said: “This new work explains features of the moon that are hard to resolve with current ideas.
“The moon is chemically almost the same as the Earth, but with some differences. This is the first model that can match the pattern of the moon’s composition.”
The new theory still involves a massive collision between planet-sized bodies more than four billion years ago, but in this case the result is a huge spinning “synestia”, a “doughnut” of vaporised and molten rock.
Within the synestia, which was destined to become the Earth, the moon formed under colossal pressure and temperatures of up to 3,300C.
Lead researcher Harvard University graduate student Simon Lock said: “It’s huge. It can be 10 times the size of the Earth, and because there’s so much energy in the collision, maybe 10% of the rock of Earth is vaporised, and the rest is liquid.”
Over time the Earth-synestia shrank as vaporised silicate rock condensed and rained on the proto-moon at a rate 10 times that seen during a hurricane, he said.
Eventually the moon escaped from the synestia, which continued to condense until it created the Earth we know today.
The whole process was incredibly quick, with the moon “hatching out” in just a few years and the Earth forming about 1,000 years later.
Mr Lock, whose work appears in the Journal of Geophysical Research – Planets, added: “You just don’t think of a satellite forming inside another body, but this is what appears to happen.”