A new study led by a Scripps Institution of Oceanography researcher contradicts a recently published paper, which suggested the interior of Earth’s only permanent natural satellite is wet.
The Rusty Rock (bottom left) was chipped from a small boulder (center). Image credit: NASA.
“It’s been a big question whether the Moon is wet or dry,” said study lead author Dr. James Day, a geochemist at Scripps Institution of Oceanography.
“It might seem like a trivial thing, but this is actually quite important.”
“If the Moon is dry — like we’ve 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.”
“Our results suggest that when the Moon formed it was ‘very, very hot.’ Essentially an ocean of magma,” he said.
“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.”
Dr. Day and his colleagues from the Université Paris Diderot, the University of New Mexico, the Institut Universitaire de France arrived at this conclusion after analyzing fragments of the ‘Rusty Rock,’ a rock collected from a boulder on the rim of a 10-m crater at the base of Stone Mountain during the Apollo 16 mission in 1972.
The Rusty Rock, also known as lunar sample #66095, is one of the most volatile-rich rocks from the Moon and the only rock from Earth’s natural satellite that came back with what appeared to be rust on its outer surfaces.
The implications of this specimen have mystified planetary researchers for a long time — water is one of the essential ingredients of rust, so where could that water have come from?
Some speculated the water could have been terrestrial, but further tests showed the rock and the rust were lunar in origin.
This is a close-up photo of metallic salts, or ‘rust,’ on surface of the Rusty Rock. Note the appearance of a crust under the colored salts. Magnification – 10x. Image credit: NASA.
The new chemical analyses Dr. Day and co-authors 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,” Dr. Day said.
The team found that the rust on the Rusty Rock is full of lighter isotopes of zinc, meaning it’s probably the product of the zinc condensing on the lunar 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. 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,” Dr. Day said.
“In the same way, the interior of the Moon must be enriched in the heavy isotopes and have been depleted in the light isotopes and volatile elements. Meaning: dry.”
The insights the authors have gleaned from one lunar artifact run contrary to recently published results from another.
In a recent study, Dr. Ralph Milliken of Brown University and Dr. Shuai Li from the University of Hawaii analyzed glass deposits found on the Moon’s surface and concluded that the presence of water in these deposits suggests that the Moon’s interior is actually wet. But Dr. Day is skeptical of their results.
“Their study says that all of the glass bead deposits on the lunar surface are ‘wet,’ which is a great observation. However, they cannot elucidate the mechanism of their formation,” Dr. Day said.
The findings were published online this week in the Proceedings of the National Academy of Sciences.
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James M.D. Day et al. Late-stage magmatic outgassing from a volatile-depleted Moon. PNAS, published online August 21, 2017; doi: 10.1073/pnas.1708236114
