When astronauts Gene Cernan and Harrison Schmitt returned from NASA’s Apollo 17 mission in 1972, they carried with them a treasure trove of lunar rocks and soil.
Some of these samples were sealed away immediately, untouched for decades, waiting for the day when scientists would have the tools to study them in greater detail.
That moment has now arrived, and the results are surprising: hidden inside one of the samples is sulfur unlike anything we find on Earth.
A new study, led by James Dottin of Brown University and published in the Journal of Geophysical Research: Planets, reveals that certain volcanic rocks from the moon’s Taurus Littrow valley contain a highly unusual sulfur signature.
The researchers discovered that these rocks are depleted in sulfur-33, one of four stable forms, or isotopes, of sulfur. The finding is striking because sulfur isotopes on Earth follow very different patterns.
Scientists often use isotopes as “fingerprints.” If two samples share the same ratios of isotopes, it suggests they came from the same source.
Earth and the moon, for instance, share nearly identical oxygen isotope fingerprints, which has long supported the idea that the moon formed from Earth’s material. Until now, many assumed sulfur would follow the same rule.
“That’s what I expected to see when I analyzed the samples,” Dottin explained. “But instead, we saw values that were very different from anything on Earth. My first reaction was basically, ‘Holy shmolies, that can’t be right.’ But it was.”
The samples he tested came from a long metal tube, called a double drive tube, that Cernan and Schmitt hammered about 60 centimeters into the lunar soil.
NASA sealed the core sample in a helium-filled container when it arrived back on Earth, preserving it in perfect condition for decades.
Through the Apollo Next Generation Sample Analysis (ANGSA) program, NASA has recently begun allowing researchers to study these untouched samples with modern technology.
Using a highly precise tool called secondary ion mass spectrometry, Dottin and his colleagues were able to measure sulfur isotopes in tiny mineral grains from the sample. The unusual sulfur signature they found could have two possible explanations.
One possibility is that the sulfur reflects chemical reactions that occurred early in the moon’s history, when it may have briefly had a thin atmosphere.
In such conditions, ultraviolet light could have altered sulfur atoms, producing the distinctive depletion in sulfur-33. If so, this would suggest that material from the lunar surface somehow made its way into the mantle, deep inside the moon.
On Earth, this kind of recycling happens through plate tectonics, but the moon does not have tectonic plates, so such an exchange would point to a mysterious, ancient process unique to our satellite.
Another explanation is that the sulfur is a remnant from the giant collision thought to have formed the moon. According to the leading theory, a Mars-sized body named Theia slammed into the early Earth, and debris from the impact formed the moon.
If Theia’s sulfur isotopes were very different from Earth’s, the lunar mantle might still preserve those chemical fingerprints.
For now, researchers cannot say for certain which scenario is correct. Further comparisons with sulfur isotopes from Mars and other planetary bodies may eventually solve the puzzle. But either way, the discovery deepens our understanding of the moon and, more broadly, how the solar system was shaped.
As Dottin put it, “Understanding these isotope signatures gives us clues about the processes that built the planets. Each new piece of information helps us better understand where we came from.”
Source: Brown University.
