Leading hypotheses for the origin of the Moon involve a giant impact between the proto-Earth and a separate impactor called Theia. The efficiency of mixing material between these two planetary bodies remains a matter of debate. Ineffective mixing during this process could have left behind remnants of the composition of proto-Earth and/or Theia. The sulfur isotopic composition of the primordial components that survived this impact can be used to place constraints on the chemistry of the early Solar Nebula and the distribution of sulfur components throughout the early Solar System, as well as on the efficiency of mixing during the giant impact event that formed the Moon. In a new paper, scientists from Brown University and elsewhere present data on anomalous sulfur isotopes from lunar rocks taken from the Taurus-Littrow lunar region during Apollo 17. Their analysis shows that the volcanic material in the samples contains sulfur compounds that are highly depleted in sulfur-33, one of the four radioactively stable isotopes of sulfur; these depleted samples contrast sharply with the sulfur isotope ratios found on Earth and indicate the presence of either: (i) exotic chemistry and crustal recycling during the Moon's early evolution, or (ii) material that was not well mixed during the Moon's formation.
Commander Eugene Cernan removes the drive tube from the lunar rover during the Apollo 17 spacewalk. Image credit: NASA
Some elements bear characteristic “fingerprints” in the form of isotope ratios—subtle changes in the weight of their atoms.
If two rocks have the same isotopic signature, it is a strong indication that they came from the same source.
In the case of the Moon and Earth, researchers have shown that the oxygen isotopes of these two bodies are very similar.
“It has long been assumed that sulfur isotopes would tell a similar story,” said Dr. James Dottin, a researcher at Brown University.
“Prior to this, it was thought that the lunar mantle had the same sulfur isotopic composition as the Earth.”
“This is what I expected to see when analyzing these samples, but instead we saw values that are very different from anything we find on Earth.”
The samples the authors analyzed were taken from a double drive tube, a hollow metal cylinder buried about 60 cm into the lunar soil by Apollo 17 astronauts Gene Cernan and Harrison Schmitt.
After returning to Earth, NASA sealed the tube in a helium chamber to preserve the sample in pristine condition for future research in a program called Apollo Next Generation Sample Analysis (ANGSA).
In the last few years, NASA has begun making ANGSA samples available to academic researchers through a competitive application process.
Dr. Dottin and his colleagues proposed analyzing sulfur isotopes using secondary ion mass spectrometry, a high-precision isotope analysis technique that did not exist in 1972, when the samples were first returned to Earth.
For his work, they looked for specific samples from the drive pipe, which appeared to be volcanic rock of mantle origin.
“There are two possible explanations for the abnormal sulfur,” Dr. Dottin said.
They could be remnants of chemical processes that took place on the Moon at the beginning of its history.
A depleted sulfur-33 ratio is found when sulfur reacts with ultraviolet light in an optically thin atmosphere.
Early in its history, the Moon is thought to have had a short-lived atmosphere that could support such photochemistry.
If the samples were indeed formed this way, it has some interesting implications for the evolution of the Moon.
“This would be evidence of an ancient exchange of materials from the lunar surface into the mantle,” Dr Dottin said.
“On Earth we have plate tectonics that do this, but on the Moon we don't have plate tectonics.”
“So the idea of some kind of exchange mechanism on the early Moon is exciting.”
Another possibility is that the anomalous sulfur was left behind after the formation of the Moon itself.
The main explanation for the formation of the Moon is that a Mars-sized object called Theia collided with the Earth early in its history.
The debris from this collision eventually came together to form the Moon.
It is possible that Theia's sulfur signature was very different from Earth's, and that these differences were recorded in the lunar mantle.
It is unclear from this study which of these possible explanations is correct.
“Further study of sulfur isotopes on Mars and other bodies may one day help scientists find the answer,” Dr Dottin said.
“Ultimately, understanding the distribution of isotopic signatures will help scientists better understand how the solar system formed.”
study was published in Journal of Geophysical Research: Planets.
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J.W. Dotting III etc.. 2025. Endogenous but exotic sulfur in the lunar mantle. JGR: Planets 130(9):e2024JE008834; doi: 10.1029/2024JE008834
