Moon rock: Apollo 17 rock sample hints lunar surface cooled faster than we thought

A rock sample collected from the moon during the Apollo 17 mission in 1972 has been re-examined, and the results suggest the lunar surface might have cooled in just 20 million years


14 December 2021

NASA image of Troctolite 76535. NASA/Johnson Space Center photograph S73-19456. - [1] Troctolite sample 76535 brought back from the Moon by Apollo 17. This sample has a mass of 156 grams and is up to 5 centimeters across. It was collected as part of a rake sample at station 6 at the foot of the North Massif. Another view of this sample was used in Figure 7-16 of the Apollo 17 Preliminary Science Report (SP-330, 1973), which has the following caption: Coarse-grained norite (sample 76535) with fresh-appearing plagioclase (white to light gray) that has typical striations of albite twinning. Although pyroxenes (medium gray) are fractured along cleavage planes, they do not appear badly crushed.

NASA image of Troctolite 76535

NASA/Johnson Space Center

A rock collected from the moon during the Apollo 17 mission in 1972 cooled to its current state much more rapidly than we thought, according to a reanalysis of the sample. The finding highlights the value of re-examining old lunar samples.

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.”

Journal reference: Nature Communications, DOI: 10.1038/s41467-021-26841-4

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