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The Moon

Moon origin theory suggests huge collision, while China’s lunar lander returns to Earth

Image credit: Guillermo Ferla | Unsplash

Supercomputer simulations could unlock the mystery of the Moon's formation billions of years ago, just as China's Chang’e 5 spacecraft returns to Earth with lunar samples.

Astronomers have taken a step towards understanding how the Moon might have formed out of a giant collision between the early Earth and another massive object 4.5 billion years ago.

Scientists led by Durham University, UK, ran supercomputer simulations on the DiRAC High-Performance Computing facility to send a Mars-sized planet - called Theia - crashing into the early Earth.

Their simulations produced an orbiting body that could potentially evolve into a Moon-like object. While the researchers are careful to say that this is not definitive proof of the Moon's origin, they add that it could be a promising stage in understanding how our nearest neighbour might have formed.

The Moon is thought to have formed in a collision between the early Earth and Theia, which scientists believe might have been an ancient planet in our solar system, approximately the size of Mars.

Researchers ran simulations to track material from the early Earth and Theia for four days after their collision, then ran other simulations after spinning Theia like a ball.

The simulated collision with the early Earth produced different results, depending upon the size and direction of Theia's initial spin. At one extreme the collision merged the two objects together, while at the other there was a grazing hit-and-run impact.

Importantly, the simulation where no spin was added to Theia produced a self-gravitating clump of material with a mass of about 80 per cent of the Moon, while another Moon-like object was created when a small amount of spin was added.

Still image from a cross section of a 3D simulation run by researchers investigating how a collision between the early-Earth and a Mars-size object might have led to the formation of the Moon

Image credit: Sergio Ruiz-Bonilla

The resulting clump, which settles into an orbit around the post-impact Earth, would grow by sweeping up the disc of debris surrounding our planet. The simulated clump also has a small iron core, similar to that of the Moon, with an outer layer of materials made up from the early Earth and Theia.

Recent analysis of oxygen isotope ratios in the lunar samples collected by the Apollo space missions suggests that a mixture of early Earth and impactor material might have formed the Moon.

Lead author Sergio Ruiz-Bonilla, a PhD researcher in Durham University's Institute for Computational Cosmology, said: "By adding different amounts of spin to Theia in simulations, or by having no spin at all, it gives you a whole range of different outcomes for what might have happened when the early Earth was hit by a massive object all those billions of years ago.

"It's exciting that some of our simulations produced this orbiting clump of material that is relatively not much smaller than the Moon, with a disc of additional material around the post-impact Earth that would help the clump grow in mass over time.

"I wouldn't say that this is the Moon, but it's certainly a very interesting place to continue looking."

The Durham-led research team now plan to run further simulations altering the mass, speed and spinning rate of both the target and impactor to see what effect this has on the formation of a potential Moon.

Co-author Dr Vincent Eke, of Durham University's Institute for Computational Cosmology, said: "We get a number of different outcomes depending upon whether or not we introduce spin to Theia before it crashes into the early Earth.

"It's particularly fascinating that when no spin or very little spin is added to Theia that the impact with the early Earth leaves a trail of debris behind, which in some cases includes a body large enough to deserve being called a proto-Moon.

"There may well be a number of possible collisions that have yet to be investigated that could get us even closer to understanding just how the Moon formed in the first place."

The team's findings are published in the journal Monthly Notices of the Royal Astronomical Society.

Meanwhile, a Chinese spacecraft carrying a collection of lunar rock samples has left the surface of the Moon, as it begins the first stage of its return to Earth, according to China's government space agency.

Chang’e 5 - the third Chinese spacecraft to land on the Moon but only the first to take off again - is the latest in a series of ambitious missions for Beijing’s space programme, which also has an orbiter and rover headed to Mars.

Chang’e 5 touched down on the Moon on Tuesday in the area known as the Sea of Storms, situated on the Moon’s near side. Its mission was to collect about 2kg (4lbs) of lunar rocks and bring them back to Earth - the first return of such samples since a Soviet spacecraft in 1976. The US Apollo astronauts had brought back hundreds of pounds of Moon rocks before that.

The landing site for Chang’e 5 was near a formation called the Mons Rumker, which it is thought may contain rocks billions of years younger than those retrieved by previous Moon missions.

Chinese officials said the capsule with the samples - sealed inside a special canister to avoid contamination - is due to land on Earth around the middle of the month. Touchdown is planned for the grasslands of Inner Mongolia, where China’s astronauts have made their return in Shenzhou spacecraft.

Before the ascent vehicle lifted off from the lunar surface, the lander unfurled what the space administration called the first free-standing Chinese flag on the Moon. The agency posted an image, apparently taken from the lander, of the ascent vehicle firing its engines as it took off.

Chang’e 5’s lander module, which remained on the Moon, is equipped to both scoop samples from the surface and also drill two metres down to retrieve materials that could provide clues to the history of the Moon, the Earth, other planets and space features.

The apparent success of Chang’e 5 has revived talk of China one day sending astronauts to the Moon and possibly building a scientific base there, although no timeline has been proposed for such projects.

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