Moon younger than previously thought

An old friend has just become 100 million years younger. Our closest neighbour in the Solar System, the Moon is now only 4.36 billion years old, Copenhagen scientists reckon

by Mike Young

A new analysis of a piece of lunar rock brought back to Earth by the Apollo 16 mission in 1972 has shown that the Moon may be much younger than previously believed.

This is according to an international team of scientists that includes James Connelly from the Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen. Their work has just been published in Nature, according to ku.dk.

The prevailing theory of our Moon’s origin is that it was created by a giant impact between a large planet-like object and the proto-Earth very early in the evolution of our solar system. The energy of this impact was sufficiently high that the Moon formed from melted material that began with a deep liquid magma ocean.

Rocks brought back with Apollo 16

As the Moon cooled, this magma ocean solidified into different mineral components, the lightest of which floated upwards to form the oldest crust. Analysis of a lunar rock sample of this presumed ancient crust has given scientists new insights into the formation of the Moon.

»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. Once we removed the contamination, we found that this sample is almost 100 million years younger than we expected,« says researcher James Connelly of the Centre for Star and Planet Formation.

According to the existing theory for lunar formation, a rock type called ferroan anorthosite, also known as FAN, is the oldest of the Moon’s crustal rocks, but scientists have had difficulty dating samples of this crust.

Same age as oldest minerals on earth

The team analysed the isotopes of the elements lead and neodymium to place the age of a sample of a FAN at 4.36 billion years. This figure is significantly younger than earlier estimates of the Moon’s age that range to nearly as old as the age of 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.

miy@adm.ku.dk

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