After half a century, samples collected during NASA’s 1972 Apollo 17 Moon mission are still revealing new secrets about the lunar surface.
Fortunately for today’s scientists, the Apollo Moon missions had the foresight to store samples for exploitation by technologies and methodologies not yet known at the time. Because of that, a new study in JGR: Planets brings to light a surprising sulfur isotope hiding in the lunar regolith that may also provide clues to the origin of our solar system.
Apollo Sample Preservation
Apollo 17 astronauts Gene Cernan and Harrison Schmitt collected these samples of volcanic material from the Taurus Littrow region, after pushing a hollow metal cylinder 60 centimeters into the regolith. Upon their return, the NASA Apollo Next Generation Sample Analysis (ANGSA) program preserved the entire tube full of material in a helium chamber for future scientists to study.
After fifty years on the shelf, NASA has finally begun allowing researchers to apply modern scientific tools to ANGSA samples. Gaining access to the precious samples is no easy feat, though. A rigorous and competitive application process is in place to ensure that ANGSA releases materials only to the most deserving projects.
One of the recipients of the lunar samples was James Dottin of Brown University, whose work proposed using ion mass spectrometry on the lunar soil for the first time, in an effort to explore the sulfur in what resembled mantle-derived volcanic rock.
“I was targeting sulfur that had a texture that would suggest it was erupted with the rock and not added through a different process,” Dottin explained.
Sulfur-33
The unexpected sulfur compounds found in the samples were strange for their high depletion of sulfur-33, which is among the four most radioactively stable sulfur isotopes. Compared to sulfur-33 samples recovered on Earth, the lunar samples are far different, as terrestrial sulfur-33 is typically far less depleted.
“Before this, it was thought that the lunar mantle had the same sulfur isotope composition as Earth,” Dottin said. “That’s what I expected to see when analyzing these samples, but instead we saw values that are very different from anything we find on Earth.”
Isotope ratios form a sort of “fingerprint” from which rocks can be identified as originating from the same source. Existing research has shown similarities between oxygen on the Moon and Earth, leaving scientists to expect a resemblance between sulfur isotopes as well.
“My first thought was, ‘Holy shmolies, that can’t be right,’” Dottin exclaimed. “So we went back to make sure we had done everything properly, and we had. These are just very surprising results.”
Exploring Lunar Origins
Dottin suggests two possibilities behind the unexpected sulfur. One is that the early moon briefly held a thin atmosphere, just enough to allow for reactions between ultraviolet light and sulfur. This could have depleted the isotope early on, carrying significant implications for lunar evolution.
“That would be evidence of ancient exchange of materials from the lunar surface to the mantle,” Dottin said. “On Earth, we have plate tectonics that does that, but the Moon doesn’t have plate tectonics. So this idea of some kind of exchange mechanism on the early Moon is exciting.”
Another possibility is that the depleted isotope arrived from the Moon’s initial formation. The most commonly accepted lunar origin story is that Theia, an object roughly the size of Mars, impacted the ancient Earth, sending off debris that eventually formed the Moon. If this were the case, the anomalous sulfur isotope may be remnants of Theia, which could have had a far different sulfur signature than the Earth.
While both theories are possible, no clear evidence from the isotopes themselves currently supports one over the other. Dottin hopes that continued analysis of sulfur isotopes from different bodies in our solar system, such as Mars, will help identify the correct explanation and provide a better understanding of how our solar system formed as a whole.
The paper, “Endogenous, yet Exotic, Sulfur in the Lunar Mantle,” appeared in JGR Planets on September 10, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.