The prevailing scientific postulations regarding the Moon’s genesis posit a colossal collision between proto-Earth and a celestial body designated as Theia. The degree to which material was intimately intermingled between these two planetary entities remains a focal point of ongoing scientific discourse. An incomplete amalgamation during this cataclysmic event could conceivably preserve vestiges of the original compositions of either proto-Earth or Theia, or both. The isotopic signatures of sulfur within primordial matter that endured this impact could furnish crucial insights into the chemical milieu of the nascent Solar Nebula, the distribution of sulfurous compounds throughout the early Solar System, and the efficacy of material mixing during the monumental lunar formation impact. In a recent publication, a collective of researchers from Brown University and affiliated institutions has unveiled distinctive sulfur isotope data derived from lunar geological specimens collected from the Taurus Littrow valley region during the Apollo 17 expedition. Their meticulous analysis reveals that volcanic ejecta within these samples harbor sulfurous compounds significantly impoverished in sulfur-33, one of the four isotopically stable forms of sulfur; these depleted samples stand in stark contrast to the sulfur isotope ratios observed on Earth, suggesting the presence of either: (i) unusual chemical processes and crustal turnover during the Moon’s formative stages, or (ii) constituents that were inadequately homogenized during the lunar accretionary phase.
Commander Eugene Cernan retrieves a drive tube from the Lunar Roving Vehicle during an Apollo 17 EVA. Image credit: NASA.
Distinct chemical elements are characterized by unique isotopic profiles—subtle variations in the atomic masses.
When two geological samples exhibit identical isotopic signatures, it strongly implies a shared origin.
In the comparative analysis of lunar and terrestrial materials, considerable congruence has been identified in their oxygen isotopic compositions.
“It has long been presumed that sulfur isotopes would offer a comparable narrative,” remarked Dr. James Dottin, a research scientist affiliated with Brown University.
“Prior to this investigation, the prevailing understanding was that the lunar mantle possessed an identical sulfur isotopic makeup to that of Earth.”
“This was precisely the outcome I anticipated during the analysis of these specimens, but instead, we encountered values that diverged significantly from any terrestrial analogues.”
The samples subjected to examination were extracted from a dual-tube core sampler—a cylindrical conduit that was emplaced approximately 60 centimeters into the lunar regolith by Apollo 17 astronauts Gene Cernan and Harrison Schmitt.
Upon their return to Earth, NASA meticulously sealed this core sample within a helium-filled chamber, ensuring its pristine preservation for future scientific scrutiny under the auspices of the Apollo Next Generation Sample Analysis (ANGSA) initiative.
In recent years, NASA has commenced the distribution of ANGSA samples to academic researchers through a rigorous competitive proposal process.
Dr. Dottin and his collaborators proposed to undertake sulfur isotope analysis utilizing secondary ion mass spectrometry, a high-precision analytical technique for isotope determination that was not technologically available in 1972, the year the samples were initially recovered on Earth.
For the purposes of their research, they specifically targeted samples from the core tube that appeared to originate from mantle-derived volcanic material.
“There are two plausible explanations for the observed anomalies in sulfur composition,” stated Dr. Dottin.
These anomalies could represent residual evidence of chemical transformations that occurred on the Moon in its early history.
Reduced sulfur-33 ratios are frequently associated with the interaction of sulfur with ultraviolet radiation within an atmospherically tenuous environment.
It is theorized that the Moon possessed a transient atmosphere in its nascent stages, which could have facilitated such photochemical reactions.
If this hypothesis accurately explains the formation of these samples, it carries significant implications for our understanding of lunar evolution.
“This would serve as corroborating evidence for past material exchange between the lunar surface and its interior,” Dr. Dottin elaborated.
“On Earth, such exchanges are facilitated by plate tectonics, a geological process absent on the Moon.”
“Consequently, the notion of an early lunar exchange mechanism is quite compelling.”
The alternative explanation posits that the anomalous sulfur is a vestige from the Moon’s very formation.
The most widely accepted theory for the Moon’s origin involves a cataclysmic impact between a Mars-sized protoplanet, named Theia, and the early Earth.
Subsequent accretion of the resultant debris led to the formation of the Moon.
It is conceivable that Theia possessed a distinct sulfurous isotopic signature compared to that of Earth, and these disparities have been imprinted within the lunar mantle.
The current research does not definitively elucidate which of these potential scenarios is the accurate one.
“Further investigation of sulfur isotopes from Mars and other celestial bodies may eventually aid scientists in resolving this question,” Dr. Dottin suggested.
“Ultimately, a comprehensive understanding of isotopic distribution patterns will enhance our comprehension of Solar System formation processes.”
The investigation has been published in the Journal of Geophysical Research: Planets.
_____
J.W. Dottin III et al. 2025. Endogenous, yet Exotic, Sulfur in the Lunar Mantle. JGR: Planets 130 (9): e2024JE008834; doi: 10.1029/2024JE008834
