Panoramic composite image of Apollo 16 landing site with a white arrow pointing to the sample collecting site. The black shadow is the Apollo 16 lunar landing module.
Credit: Lars Borg
PARIS: The Moon may be younger than previously thought and may even have lacked a magma ocean in its early life.
According to new analyses of lunar rock published in Nature today, current hypotheses of when the Moon formed may be incorrect. The Moon contains large amounts of a type of white rock called ferroan anorthosite, and according to current lunar formation theories, these anorthosites are the oldest crustal rocks on the Moon, formed when the planetary body first solidified.
New analyses by a team of scientists from the U.S., Denmark and France that measure three different isotopes in these rocks now suggest that the ferroan anorthosites crystallised as late as around 4.36 billion years ago.
"If the current model of lunar formation is correct - that is, that ferroan anorthosites were the first materials to solidify on the Moon - our results imply that the Moon must be young," said lead author Lars Borg from the Lawrence Livermore National Laboratory in California. "Alternatively, the Moon could still be older but then the ferroan anorthosites could not have formed during the early stages of lunar solidification."
Analysing isotopes
The ferroan anorthosite rocks are thought to have floated on a global magma ocean at the early stages of the Moon's formation and subsequently became trapped in the cooling liquid. In principle, determining the ages of ferroan anorthosites can therefore be used to estimate the age of the Moon.
However, previous attempts to do this - using so-called isochron methods - have produced rather ambiguous results with ages ranging from the very old (around 4.55 billion years) to the very young (4.29 billion years).
The team analysed pyroxene mineral samples from the lunar collection at the Johnson Space Centre in Texas. that they carefully washed using acids to remove small amounts of contamination. This ensured that the samples only contained isotopes of lead (lead-207 and lead-206) that had been produced by radioactive decay on the Moon, which allowed the age of these pyroxenes to be very precisely determined.
The researchers also used a very pure neodymium tracer to calculate the ages of samarium-146-neodymium-142 and samarium-147-neodymium-143 in the same samples. "Ultimately, agreement between all three isotopic systems is what gave us confidence that we were dating the rock at the moment it solidified," said Borg.
The results also suggest that samples thought to have come from the solidifying magma ocean might actually have been produced by another process. This implies that the Moon need not have had a magma ocean to account for the formation of its crust - something that would completely overturn current lunar formation theories.
Incompatible data
"These results are an extremely important contribution to lunar science," commented Thomas Lapen of the University of Houston. "The new data would also seem to be incompatible with many previous calculated ages of ferroan anorthosites - for example samarium-neodymium whole rock dating."
He says that many more experiments like these need to be conducted on lunar samples to see if consistent young ages can be obtained through such multi-chronometer approaches.
"Indeed, this new study highlights the importance of accurate and precise age determinations of lunar rocks because models of planetary formation and early evolution depend crucially on these data."
The team now plans to do just this - apply their approach to other lunar rock samples from different sources. "If we obtain a range of young and old ages, it might be that at least some ferroan anorthosites did not come from a primordial magma ocean solidifying," said Borg. "But, if all our analyses yield ages near 4,360 million years, then we may be dating the real age of the Moon and precisely when its magma ocean solidified."
