The Moon likely has a very dry interior, say scientists who analysed a rusty rock collected from the lunar surface during the Apollo 16 mission in 1972. The question of the Moon’s moistness matters because the amount of water and other volatile elements and compounds provide clues to the Moon’s history and how it was formed. “It’s been a big question whether the Moon is wet or dry. It might seem like a trivial thing, but this is actually quite important,” said James Day, from University of California San Diego in the US. “If the Moon is dry – like we have thought for about the last 45 years, since the Apollo missions – it would be consistent with the formation of the Moon in some sort of cataclysmic impact event that formed it,” said Day. The results suggest that when the Moon formed it was “very, very, hot,” he said. Researchers believe it would have been so hot that any water, or other compounds and elements that are volatile under conditions on the Moon, such as zinc, would have evaporated very early in the Moon’s history. They arrived at this conclusion after analysing fragments of the “Rusty Rock,” a rock collected from the lunar surface during the Apollo 16 mission in 1972. “It’s the only rock from the Moon that came back with what appeared to be rust on its outer surfaces,” Day said.
The implications of the Rusty Rock have mystified scientists for a long time since water is one of the essential ingredients of rust. Some speculated the water could have been terrestrial, but further tests showed the rock and the rust were lunar in origin. The new chemical analyses applied to the Rusty Rock revealed that the rock’s composition is consistent with it coming from a very dry interior. “It’s a bit of a paradox. “It’s a wet rock that comes from a very dry interior part of the Moon,” Day said. Day found that the rust on the Rusty Rock is full of lighter isotopes of zinc, meaning it is probably the product of the zinc condensing on the Moon’s surface after evaporating during the sweltering period of the Moon’s formation.
“Zinc is a volatile element, so it behaves a bit like water under conditions of Moon formation,” Day said. “It’s something like clouds forming from the ocean; the clouds are rich in light oxygen isotopes, and the ocean is rich in heavy oxygen isotopes,” he said. The interior of the Moon must be enriched in the heavy isotopes and have been depleted in the light isotopes and volatile elements.