That age-old Moon age discussion

A study just published in Nature Geoscience confirms a prior study suggesting that the Moon age may be 100 million years older than conventionally believed. 

But is it? As Aristotle famously said, “One swallow does not a summer make”, and one or two scientific studies won’t be enough, in and of themselves, to upset conventional belief. 

Given that the Moon is about 4.5 billion years old, the dispute might not sound all that important. That’s because looking backward through the eons puts it in the wrong perspective. Seen that way, 4.5 billion years versus 4.4 billion years doesn’t look like a big deal. 

But if you reverse perspective and think in terms of time since the dawn of the Solar System – 4.56 billion years ago – it becomes more important. 

Sample of an ilmenite basalt collected by the Apollo 12 mission. Credit: Maxwell Thiemens

The new research suggests that the Moon formed when the Solar System was only 50-60 million years old, rather than 150 to 200 million years later, as is conventionally believed. That’s a huge difference.

In January 2017, a team led by Melanie Barboni, an igneous petrologist now at Arizona State University, Tempe, looked at uranium decay in zircon crystals in moon rocks brought back by Apollo astronauts. They concluded that these crystals – and hence the Moon – were 4.51 billion years old, only 50 million years younger than the Solar System. 

Now, a team led by scientists at the University of Cologne, Germany, has reached the same result, this time by studying the concentration of isotopes of hafnium and tungsten in other Moon rocks. 

Hafnium is a rare, silvery metal, sometimes used in microchips. Tungsten is best known as the filament in lightbulbs.

What makes them interesting for dating ancient rocks is that, compared to the Earth, the Moon appears to have too much of a form of tungsten known as tungsten-182.

One way this could happen, the German team says, is if the magma from which these rocks formed was rich in hafnium-182, a radioisotope that decays into tungsten-182. 

But in geologic terms, that decay occurs pretty quickly – fast enough that for significant amounts of hafnium-182 to be present in the magma from which these rocks formed, they must have crystallised before the Solar System was more than about 50 million years old. Otherwise, the primordial hafnium-182 would already have been snuffed out, and not have been available to enrich the rocks with tungsten-182.

“This means that any giant [Moon-forming] impact had to occur before that time,” says Carsten Münker of the University of Cologne, one of the authors of the new study.

Trying to figure out exactly when the Moon formed is important, says Richard Carlson, a geochemist and geochronologist at the Carnegie Institution for Science in Washington, DC, who was not part of the study team, largely because if the Moon formed that early, it probably occurred while the Earth itself was still forming. 

“Estimates are that the Earth probably took a few tens of millions of years to form,” he says. 

If the Moon formed later, however, it means that the giant impactor that struck the Earth to create it had been hanging harmlessly around the Solar System for 150 to 200 million years, until it then smashed into us. Figuring out which way it happened, Carlson says, is highly relevant to understanding the mechanics of planet formation. 

He also notes that if the Moon wasn’t formed until relatively late, “then the Earth had 100 million years to be a planet.” I.e., it had 100 million years to lie undisturbed and start to mature. “It could even have developed life, and the giant impact would have wiped it out,” he says.

Turning to the new study, however, he’s not sure that it proves the Moon formed early. 

The problem, he says, isn’t with the isotopic data. The scientists on the research team, he says, “make these kinds of measurements very well.” 

But early formation, before primordial hafnium-182 had burned out, isn’t the only way to explain the observed isotope ratios, and how they compare to the Earth’s. 

In particular, he says, we don’t even know for sure what the tungsten isotope ratios on the early Earth were, as compared to those in modern rocks, because geological processes have been continually stirring up our planet, even to the extent of possibly bringing material up from its core. 

“So in the end, I don’t find this particularly convincing,” he says.