• NewScientist.com news service
  • Maggie McKee

It’s a good thing the Moon doesn’t have any feelings to hurt. New research suggests it is actually 30 million years younger than anyone had thought, and that it is merely a ‘chip off the old block’ of Earth rather than being made up of the remnants of a Mars-sized body that slammed into Earth billions of years ago.

That violent impact was thought to have taken place 30 million years after the solar system began to condense from a disc of gas and dust 4.567 billion years ago. The event was thought to have melted the Earth, generating a magma ocean that covered the planet and allowed iron and other metals to sink to its centre, forming a core.

At the same time, the Moon was thought to have coalesced from a disc of molten debris blasted off the Earth and the Mars-sized interloper.

But new research led by Mathieu Touboul of the Swiss Federal Institute of Technology in Zurich suggests that picture is not so simple. The researchers base their analysis on studies of an isotope of the metal tungsten in lunar rocks.

That isotope, tungsten-182, is produced by the decay of two other elements: hafnium-182, which has a half-life of 9 million years, and tantalum-182. Tantalum-182, however, is not an intrinsic component of the Moon – it forms when energetic charged particles from space, called cosmic rays, slam into the lunar surface.

Previous estimates of the Moon’s age were based on tungsten measurements that did not subtract the effect of the decay of tantalum. “It is crucial to remove all the tungsten-182 coming from the cosmic-ray production,” Touboul told New Scientist. “Otherwise, the age one gets is too old.”

Lengthy formation

When Touboul’s team accounted for tantalum, they found that the giant impact had to have occurred at least 50 million years after the solar system began to form, and that the Moon had completed its formation within the next 10 million years – about 30 million years later than thought.

The revised timing of the impact implies the terrestrial planets, such as the Earth and Mars, took longer to build up from the collision of smaller ‘planetesimals’ than previously thought. “The age of the Moon is also the age of Earth because the Moon-forming giant impact was the last major event in Earth’s formation,” says Touboul.

Alan Brandon, a scientist at NASA’s Johnson Space Center in Houston, Texas, US, agrees. “It may mean that Earth and Mars took at least 50 million years, and possibly hundreds of millions of years, to reach their final mass,” he comments.

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The researchers also found that the composition of the Moon appears identical to that of the Earth’s rocky mantle, “such that a major portion of the Moon must have been from proto-Earth”, Brandon told New Scientist.

Similar makeup

He says this runs counter to some computer models showing that at least 80% of the Moon is made up of material from the Mars-sized world, which is expected to have a different makeup from the Earth. “I think the Moon-forming impact models will have to be redone to try to get an explanation for why the Earth and Moon are so compositionally similar,” he says.

Intriguingly, the new work suggests the Moon formed at least 16 million years after the Earth’s core formed. That raises questions about how the planet’s iron-rich core could have coalesced in the absence of global magma oceans produced by the Moon-forming impact.

“It could be that there were several generations of magma oceans in Earth,” Brandon says. “My guess is … that the Earth probably had a magma ocean at the time the Earth’s core formed,” he says, adding that the giant impact may have re-melted the material millions of years later.

Journal reference: Nature (vol 450, p 1169 and 1206)

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