The Moon may have formed through the merging of 20 smaller ‘moonlets’ following asteroid strikes on the young Earth, a new theory suggests.
The formation of the Moon has always remained something of a puzzle for scientists.
The main theory, which has held way since the 1970s, is known as the ‘Giant Impact’ theory and suggests that a Mars-sized object named Theia smashed into Earth around 4.31 billion years ago, knocking a huge cloud of debris into space which eventually coalesced to form the Moon.
But there are problems with this scenario. Chemical analyses of Moon rock brought back by astronauts shows it is nearly identical to that of the Earth.
In other words, there is no trace of the large body that supposedly hit Earth. Now researchers in Israel have offered a solution. They suggest that if the Earth had been bombarded by a number of smaller asteroid strikes it could have allowed smaller ‘moonlets’ to form from Earth’s debris which merged over time.
It means that the Moon we see every night is not Earth’s 1st moon, but rather the last in a series of moons that orbited the Earth in the past
“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth,” said co-author Dr Hagai Perets of the Technion (Israel Institute of Technology)
“It’s likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons. “We believe the Earth had many previous moons, a previously formed moon could therefore already exist when another moon-forming giant impact occurred.”
To check the conditions for the formation of such mini-moons or moonlets the scientists ran 800 imitations of impacts with the Earth.
The collisions – which could have been with different sized space rocks – would have sent clouds of rubble, melt and vapor into orbit around the early Earth.
These, according the simulations the scientists created, would have cooled and agglomerated into small moonlets that, in time, could have merged into one.
Small, high-velocity smashes could also mine more material from Earth than a single, large one, the scientists claim, and accounting for why there is no evidence of the strike asteroids.
And if a number of different bodies collided with Earth over a period of millions of years, their different chemical signatures – for example, ratios of oxygen-16 to its heavier cousins, oxygen-17 and -18 – might even out, masking the traces of the various collisions.
The tidal forces from the Earth could have instigated the moonlets to slowly migrate outwards in the same way that the current Moon is moving at a pace of about 1 cm a year.
A pre-existing moon would slowly move out by the time another moon forms. However, their mutual gravitational attraction would eventually cause the moons to affect each other, and change their orbits.
Dr Gareth Collins of the Department of Earth Science & Engineering at Imperial College says: “The paper shows they show how a Moon that is formed largely out of Earth-derived material may be a more natural consequence of building the Moon from a number of moonlets, formed by a series of large impacts, rather than in one go.
“Since it was proposed in the mid-1970s, the giant-impact hypothesis has become the favoured explanation for how the Moon was born.
“The team has revived the hitherto largely discarded scenario that a series of smaller and more common impacts, rather than a single giant punch, formed the Moon.
“For final adjudication, we must now look for firmer evidence on each side.
The research was published in the journal Nature Geoscience.