It is commonly accepted that the Moon was created by the impact of a large planet-like object and our 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. 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.
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.
A new analysis has more accurately determined the age of the sample of a FAN that was returned by the Apollo 16 mission and has been stored at the lunar rock collection at the NASA Johnson Space Center.
Ferroan anorthosite FROM Apollo 16 mission. Credit: University of Copenhagen
"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.
The team analyzed 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.
This study is the first in which a single sample of FAN yielded consistent ages from multiple isotope dating techniques. This result strongly suggests that these ages pinpoint the time at which this sample crystallized. 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 crystallization product of the original magma ocean. Either scenario requires major revision to previous models for the formation of the Moon.