Apollo 17 rock sample hints the moon cooled faster than we thought

Most of our current knowledge of lunar evolution comes from rocks collected by astronauts half a century ago during NASA’s Apollo programme, but they can still yield new information.

William Nelson at the University of Hawaiʻi at Mānoa and his colleagues reinvestigated one of the collection’s most well-studied rocks, known as troctolite 76535. It weighs roughly 156 grams and is 5 centimetres across at its widest point. The sample is part of a group known as the magnesian suite (Mg-suite). These rocks represent some of the first stages of what is known as secondary crust formation, which happened when the lower parts of the moon’s mantle rose to the surface and crystallised.

Using high-resolution analytical techniques, Nelson and his team found that phosphorous was distributed through the sample fairly unevenly. This suggests that the rock may have cooled quite quickly, as the element didn’t have enough time to spread out uniformly within the rock before it solidified. Then, via computer modelling, the team deduced the sample must have taken around 20 million years to solidify from its initial molten state. This cooling time is significantly shorter than previous estimates, which were around 100 million years.

The result shows that early lunar evolution is more complicated than we thought, say the researchers, although they say we need further research to determine if the cooling history of troctolite 76535 represents the entire Mg-suite, given that just a single example has been reanalysed. “This is a sample size of one right now,” says Nelson.

“There’s still value to be had in looking back at old samples to try to get a good idea of how the moon as a whole formed,” says Nelson. “You can always go back and reanalyse old data sets with new techniques to pull out new nuggets of information.”

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