Moon may be millions of years younger than thought

Posted by Kate Taylor

A new analysis of a piece of lunar rock indicates that the moon is much younger than previously thought - or else scientists have been dramatically wrong about the way in which it was formed.

The accepted theory is that the moon was created through a giant impact between the Earth and another large planet-like object  very early in the evolution of our solar system. The moon formed from melted material that began with a deep liquid magma ocean.

As the Moon cooled, this magma ocean solidified into different mineral components, the lightest of which floated upwards to form the oldest crust.

And it's an analysis of a piece of this crust - brought back to Earth by the Apollo 16 mission in 1972 - that's led the University of Copenhagen team to its conclusion.

"We have analysed a piece of lunar rock that was brought back to Earth by the Apollo 16 mission in 1972. Although the samples have been carefully stored at NASA Johnson Space Center since their return to Earth, we had to extensively pre-clean the samples using a new method to remove terrestrial lead contamination," says researcher James Connelly.

"Once we removed the contamination, we found that this sample is almost 100 million years younger than we expected." 

A rock type called ferroan anorthosite, also known as FAN, is generally believed to be the oldest of the moon’s crustal rocks, but scientists have had difficulty dating samples of this crust.

But the team used newly-refined techniques to determine the age of its sample. They analysed the isotopes of the elements lead and neodymium - and placed the age of their FAN sample at 4.36 billion years.

This is significantly younger than earlier estimates of the Moon’s age - indeed, some place it as nearly as old as the solar system itself at 4.567 billion years.

The new, younger age obtained for the oldest lunar crust is similar to ages obtained for the oldest terrestrial minerals - zircons from Western Australia - suggesting that the oldest crust on both Earth and the moon formed at approximately the same time.

"The extraordinarily young age of this lunar sample either means that the moon solidified significantly later than previous estimates – and therefore the moon itself is much younger than previously believed - or that this sample does not represent a crystallisation product of the original magma ocean," say the researchers.

"Either scenario requires major revision to previous models for the formation of the moon."