A scientific research team led by Brown University in the United States recently announced the latest research results: they opened lunar core samples that have been properly sealed since they were brought back to Earth by Apollo 17 in 1972, and discovered an unprecedented sulfur isotope signal in them. This result challenges the traditional understanding of the moon's formation and internal evolution. The research paper has been published in the Journal of Geophysical Research: Planets.
In 1972, when the last astronauts of the Apollo program returned from the moon, some of the collected samples were sealed and preserved, hoping to leave them for in-depth analysis by scientists with more advanced technology in the future. More than fifty years later, this vision finally came true. A team led by James Dottin, assistant professor in the Department of Earth, Environmental and Planetary Sciences at Brown University, reanalyzed a section of lunar soil core collected by Apollo 17 in the Taurus-Littrow region and identified an abnormal sulfur isotope composition.
The sulfur in these volcanic materials is significantly depleted in sulfur-33 (33S), one of the four stable isotopes of sulfur. The team noted that these values differ significantly from sulfur isotope ratios typically measured in Earth rocks. In studies of Earth and other planets, an element's isotope ratio is considered a "fingerprint" that reflects its origin and how it was formed: if two samples have the same isotope pattern, it usually means they originated from the same "mother parent."
For a long time, the scientific community has confirmed that the Earth and the Moon have highly similar characteristics in terms of oxygen isotopes, so it is generally speculated that the sulfur isotope composition in the Moon's mantle should also be close to that of the Earth. However, the results of this study were quite different. Doting said that he originally expected to see values similar to those on Earth, but ended up with results that were "very different from any known sample on Earth." So much so that when he first saw the data, his reaction was: "This is impossible, we must have done something wrong somewhere." After repeated checks, the team confirmed that the experimental process was correct and could only accept that this was a "very surprising" real signal.
The sample analyzed this time came from a so-called "double drive tube": Apollo 17 astronauts Gene Cernan and Harrison Schmitt inserted this hollow metal tube about 60 centimeters into the lunar surface to obtain a relatively in-situ, undisturbed lunar soil profile. After the samples returned to Earth, the National Aeronautics and Space Administration (NASA) sealed them in a helium environment as part of the "Apollo Next Generation Sample Analysis Program" (ANGSA) to reserve the "cleanest" lunar materials for future research.
In recent years, NASA has made these precious samples available to scientific research teams through competitive selection. With support from Brown University's Lunar Research Consortium LunaSCOPE, Doting used secondary ion mass spectrometry (secondary ion mass spectrometry) technology to conduct high-precision measurements of sulfur isotopes in the samples - a method that was not available when the Apollo samples were first brought back. He specifically selected those parts of the core that were determined to be derived from volcanic material deep in the moon, focusing on looking for sulfur phases that were formed when the rocks erupted, rather than being introduced by other processes later.
For these unexpected 33S signals, the research team has currently proposed two main explanation paths. One is related to the early surface environment of the moon: in the thin atmosphere, if sulfur participates in specific photochemical reactions under the action of ultraviolet radiation, it may form the characteristics of depleted 33S. The scientific community generally believes that the early moon briefly had a thin atmosphere, and the sulfur isotope signature this time may be a relic of the surface chemical processes of that period. If this explanation is true, it means that these sulfur materials originally located on the surface were transported deep into the lunar mantle under some mechanism.
Dotting pointed out that this will constitute evidence of "surface-interior material exchange" in the early moon. On Earth, plate tectonics can subduct and recycle surface material into the mantle, but there is no similar plate tectonics system on the moon. Therefore, if there is indeed some mechanism that can send surface materials into the early moon, it will be very important and attractive for understanding its internal dynamic process.
Another explanation draws the perspective back to the origin of the moon itself. The mainstream theory is that the early Earth had a huge collision with Theia, a celestial body the size of Mars, and the debris thrown out gathered in orbit and eventually formed the moon. If Theia itself had a very different sulfur isotope composition from Earth, its material lingering deep in the lunar mantle might also be detectable in today's lunar samples.
At this time, the available data are not sufficient to make a clear decision between the two explanations. Doting hopes that in the future, through systematic comparison with isotope data from other lunar samples and more planetary bodies in the solar system, the true source of this "heterogeneous sulfur signal" can be further clarified. Researchers believe that in-depth analysis of these isotope fingerprints will not only help reconstruct the formation and evolution history of the moon itself, but will also provide new clues to the early material distribution and planet formation process of the entire solar system.