The prevailing theory regarding the origin of the Moon suggests it formed from a colossal impact event involving Earth and a body known as Theia. The degree to which materials from these two celestial objects mixed during this event is still debated. Poor mixing may leave traces of the original atomic and/or Theia composition. The sulfur isotopic makeup of the primordial materials that survived the impact can help establish parameters concerning the chemistry of the early solar nebula, the sulfur distribution in the early solar system, and the efficiency of mixing during this significant lunar impact event. In a recent study, researchers from Brown University and other institutions present intriguing sulfur isotope data derived from lunar rocks collected from the Taurus Littrow region during Apollo 17. Their analysis reveals that the volcanic material in the samples is significantly depleted in sulfur-33. This depletion sharply contrasts with sulfur isotope ratios found on Earth, suggesting the likelihood of:
Commander Eugene Cernan retrieves a drive tube from a lunar roving vehicle during Apollo 17 EVA. Image credit: NASA.
Some elements possess distinct “fingerprints” through specific isotopic ratios, revealing slight variations in atomic weights.
If two rocks share the same isotopic fingerprint, it strongly indicates a common origin.
In terms of the Moon and Earth, researchers have identified general similarities in the oxygen isotopes of both bodies.
Dr. James Dottin, a researcher from Brown University, stated:
“Previously, it was assumed that the Moon’s mantle shared the same sulfur isotope composition as Earth.”
“This was the anticipated outcome when we examined these samples, yet we observed values markedly different from those found on Earth.”
The sample under investigation was sourced from a double-drive tube—a hollow metal cylinder driven approximately 60 cm into the lunar soil by Apollo 17 astronauts Gene Cernan and Harrison Schmidt.
Upon returning to Earth, NASA secured the tube in a helium chamber to preserve the sample for future studies under the Apollo Next Generation Sample Analysis (ANGSA) program.
In recent years, NASA has begun to make ANGSA samples accessible to academic researchers via a competitive application process.
Dr. Dottin and his team chose secondary ion mass spectrometry for sulfur isotopic analysis. This precise analytical method did not exist in 1972 when the samples were initially returned to Earth.
For their research, they targeted specific samples from drive tubes believed to originate from mantle-derived volcanic rocks.
“There are two possible explanations for the anomalous sulfur,” Dr. Dottin explained.
They may represent remnants of chemical processes that took place during the Moon’s early history.
When sulfur interacts with ultraviolet light in a thin atmosphere, a diminished sulfur-33 ratio can be observed.
It is theorized that the Moon had a transient atmosphere in its early history, which could have facilitated such photochemical reactions.
If this is indeed the case, it would have interesting implications for the Moon’s evolutionary history.
“This offers evidence of ancient material transfer from the lunar surface into the mantle,” Dr. Dottin said.
“On Earth, we rely on plate tectonics for this process, but the Moon lacks such tectonic activity.”
“Thus, the idea of some form of exchange mechanism on the early Moon is thrilling.”
Alternatively, the unusual sulfur signatures could be remnants from the Moon’s formation itself.
The prevailing theory states that a Mars-sized object named Theia collided with Earth early on, with debris from that impact eventually forming the Moon.
The sulfur signatures from Theia differ significantly from those of Earth, and these differences may be reflected in the Moon’s mantle.
This study does not definitively resolve which explanation is accurate.
“Investigating sulfur isotopes from Mars and other celestial bodies may someday provide insights,” Dr. Dottin remarked.
“Ultimately, a better understanding of isotopic distributions will enhance our comprehension of solar system formation.”
study Published in Journal of Geophysics: Planets.
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JW Dottin III et al. 2025. Endogenous yet exotic sulfur in the lunar mantle. JGR: Planet 130(9):e2024je008834; doi:10.1029/2024je008834
Source: www.sci.news
