Our Moon formed about 4.5 billion years ago when the proto-Earth was blasted by another, somewhat smaller but quite large protoplanet. The excavated material that remained in orbit around the Earth then coalesced into the Moon. The time of the formation can be quite reasonably determined through the dating of the oldest rocks that came into existence. As the material of the Moon cooled down, lighter minerals floated to the top and formed a solid crust, creating the rocks that tell the story of the first times. According to calculations, the upper layers solidified in only a thousand years, creating a lid that slowed down the cooling of the lower layers dramatically to ten million years. Though events like massive impacts and volcanism may've altered the age of the surface materials here and there, almost all of the oldest rocks formed during these mere ten million years. Or did they?

Rock sample 60025, collected by the crew of Apollo 16, while still on the Moon.

Such ancient minerals, called anorthosite, and its different subtypes are also found among the 382 kg (842 lbs) of samples collected during the Apollo missions. A subtype called ferroan anorthosite have been researched for a while now by Lars Borg (Lawrence Livermore National Laboratory, USA) and his colleagues. And the rock sample 60025, collected by Apollo 16, revealed something unexpected: it is only 4.36 billion years old, 70 million years younger than results from previous experiments that yielded 4.43 billion years as the Moon's age. Although such difference may seem as marginal, conditions in the young, primordial Solar System changed rapidly: for example, 70 million years later much less protoplanets remained to slam into the proto-Earth, rendering the whole process less likely.

The same piece of rock, now on the Earth. The rate of very slowly decaying radioactive elements and decay products, like uranium, lead, samarium, and neodymium, are used to date such samples.

Our train of thought could lead us in two different directions from here. One, followed by Borg and his team, is to assume that this sample may represent the age of the Moon itself. That would mean that the Moon is either quite younger than we thought or her geochemical history, her geochronology has to be extensively revised. There are other measurements however, tiny bits of the mineral zircon in some Apollo 17 samples, considered as quite reliable clocks, whose analysis stated that the Moon was already there 4.42 billion years from now, directly contradicting the FAN results. So we have to consider some other ways of thinking: is there a possibility to force the magma ocean to remain molten for longer periods of time? A study done last year proposed exactly that: they found that tidal forces from the Earth – which the Moon was orbiting significantly closer than today – were sufficient to lengthen the solidification to about 200 million years (pdf summary here). The thin, tidally „massaged” crust could've burst from time to time, allowing younger material to erupt from the deep to the surface. With such minor amendment, the theory is now consistent with the above, younger results and the ages of other anorthosite samples that showed quite a scatter which was slightly uncomfortable from a theoretical point of view.

Chronology of the Moon. The black line at 4.57 billion years is the age of the oldest known samples (the Allende meteorite and other carbonaceous chondrites) that represent the formation of the Solar System. Reddish lines show the formation and differentiation of the Moon while the two yellows are the ages of the anorthosite samples. Blue bands display the fast and slow processes to solidify the magma ocean: it's evident that the slower fits the samples much better.

Finally, just two remarks: one, the age of the Moon remains the same for now, and hopefully the GRAIL mission will help to sort out the sequence of events by mapping her inner structure. And number two is: scientific research in action – I love this stuff!

László Molnár

Image sources:

1.) NASA / LPI

2.) NASA  / JSC / LPI

3.) L. Elkins-Tanton & others / EPSL / S&T