In the lunar samples brought back to Earth by the Apollo 17 mission in 1972, traces of a particular type of sulfur were found, different from that of terrestrial rocks.
When the Apollo 17 astronauts returned from the Moon in 1972, they brought with them numerous samples of lunar rock and soil. Some of these samples were sealed and preserved with extreme care, on the hypothesis that future technologies and methods could reveal new information from those materials.
Now, a research team led by James W. Dottin III, of Brown University, has dug into the rocks in search of still hidden traces and has discovered something surprising: signs of “exotic”, i.e. strange, sulfur in the lunar mantle.
The study. A recent publication on Journal of Geophysical Research: Planets describes all this, that is, how some of the volcanic materials taken in the Taurus Littrow region present unusual isotopic compositions of the element sulfur — in particular a marked poverty of sulfur-33 (³³S), a stable isotope of sulfur. These values are very different from those typical of terrestrial rocks and suggest processes or origins that call into question some ideas considered certain about the primordial history of the Moon.
The isotopic fingerprint: a “signature” in samples. To understand the importance of discovery, it is useful to recall the concept of isotopic signature. Each chemical element can exist in multiple isotopic forms — atoms with the same number of protons but different numbers of neutrons. Tiny differences in isotope ratios (for example between ³²S, ³³S, ³⁴S and ³⁶S in sulfur) can tell a lot about the geological, chemical and cosmic processes that involved that material. If two rocks show the same isotopic “pattern”, they are most likely derived from similar regions or sources.
In the case of the Moon and Earth, for many years scientists had observed substantial similarity in oxygen isotopes, which supported the idea that the Moon formed, at least in part, from Earth’s material. For sulphur, however, the evidence so far was less clear, even though many studies had come to the conclusion that the lunar mantle was also very similar to the Earth’s.
cutting-edge technology. Dottin and colleagues wanted to test this assumption using modern analytical techniques. The samples examined derive from a “double-chambered tube”, a metal cylinder that astronauts Cernan and Schmitt inserted up to about 60 cm into the lunar soil during research.
After returning to Earth, NASA sealed this tube in a helium chamber to preserve its pristine condition — part of the program known as Apollo Next Generation Sample Analysis (ANGSA).
Thanks to the ANGSA program, in recent years NASA has made those samples accessible to researchers chosen through scientific competitions. Dottin used “secondary ion mass spectrometry” (SIMS) to measure sulfur isotopic compositions with great precision, a technique that was not available in 1972.
Unexpected result. When the researchers isolated portions of the sample that appeared to have characteristics typical of mantle-derived volcanic rock, they expected to find sulfur isotope ratios similar to those on Earth. Instead, the measured values differed drastically. Dottin says his first thought was: “It’s not possible!“, but after a thorough verification of the methods, the results were confirmed.
The data show that the ³³S / ³²S ratios are strongly depleted compared to terrestrial ones, i.e. the sulfur particles in the sample contain relatively little ³³S compared to other isotopic variants. Furthermore, the data suggests internal variability: not all sulfur in the sample shows the same “signature,” implying the presence of multiple sources or processes.
What explanations? The researchers mainly propose two possible scenarios to justify the sulfur isotopic anomaly:
Photochemistry in an ancient lunar atmosphere + recycling to the mantle One hypothesis is that the primordial environmental conditions of the Moon favored sulfur photochemical reactions: when sulfur interacts with ultraviolet (UV) radiation in an optically tenuous atmosphere, an isotopic fraction that depresses ³³S can occur. If the Moon, in its early moments, had possessed a very tenuous but present atmosphere for some time, those reactions may have left traces. If these “photochemically altered” sulfur molecules were then transported (exchanged) from the lunar surface to the deep mantle — an internal recycling mechanism — this would mean that the Moon, in its past, would have had a way of mixing materials between the surface and mantle, even without the presence of plate tectonics as occurs on Earth. This idea is intriguing because it implies that the early Moon was more dynamic than is often imagined.
Traces of the origin of the Moon itself (signature of Theia or the protoplanetary disk) The other hypothesis looks even further back, to the events that led to the formation of the Moon. The prevailing explanation is that the Moon originated from a gigantic impact between the early Earth and a body the size of Mars, called Theia. The debris resulting from that impact would then aggregate to form the Moon.
If Theia had a sulfur isotopic composition significantly different from that of Earth, it is possible that this particular “signature” was somehow preserved in the lunar mantle, as a residual trace of the formation event. In this case, the sample analyzed by Dottin would have retained an “exotic” sulfur signal that was not well mixed with the terrestrial one.
Both hypotheses remain valid at the moment. The data available so far do not allow us to definitively exclude one or the other path. Dottin and colleagues suggest that future studies of sulfur isotopes in other planetary bodies—Mars, for example—could help discriminate between these scenarios.
Thank you for your wonderful article on Apollo moon samples.
It’s exciting to learn more about the moon