When the Apollo astronauts explored the Moon from 1969 to 1972, they left behind several science experiments designed to measure the Moon's magnetic field, seismic activity, and environment. Each mission also returned with samples of rock and soil (regolith), the analysis of which revealed a wealth of information about the Moon's composition. For instance, the rocks showed that the Earth and Moon had similar structures and compositions, leading to the widely accepted theory that the Moon formed 4.5 billion years ago when a Mars-sized object (Theia) impacted primordial Earth (known as the Giant Impact Hypothesis).
This impact turned both bodies into molten rock that slowly coalesced to form the Earth-Moon system, which eventually settled into their current orbits. However, one sample of rocks returned by the Apollo 17 mission was carefully sealed and stored so that future research, using more sophisticated instruments, could lead to new discoveries. In a recent study, a research team led by Brown University has provided the first look at these samples, which showed surprising traces of sulfur compounds that were highly depleted of a particular radioactively stable sulfur isotope - sulfur-33 (33S).
The research was led by James W. Dottin III, an assistant professor from Brown University's Department of Earth, Environmental and Planetary Sciences, with support from the Lunar Structure, COmposition and Processes for Exploration (LunaSCOPE) program, Brown's lunar science research consortium. He and his team were joined by researchers from the Institute of Meteoritics (IOM) at the University of New Mexico and the Woods Hole Oceanographic Institution (WHOI). Their research is described in a paper published in the Journal of Geophysical Research: Planets (JGR: Planets).
When geologists examine rock samples, they will look for subtle variations in the weight of certain elements, known as isotope ratios. When two samples have the same ratios, it is a strong indication that they came from the same source. The examination of the Apollo rock samples has revealed broad similarities with Earth rocks, including elements such as silica, iron, and oxygen isotopes. As Dottin explained in a Brown University press release, it has long been assumed that the same would be true where sulfur isotopes are concerned:
Before this, it was thought that the lunar mantle had the same sulfur isotope composition as Earth. 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.
The samples Dottin and his team analyzed were taken from a double drive tube, a hollow metal cylinder used by Apollo 17 astronauts Gene Cernan and Harrison Schmitt to retrieve samples from the Moon's Taurus Littrow region in 1972. Unlike other samples that have been studied for decades, this tube was sealed in a helium chamber for future analysis under a NASA program called Apollo Next Generation Sample Analysis (ANGSA), of which Dottin and several co-authors are members. These samples were unsealed a few years ago and made available to academic institutions for analysis through a competitive application process.
As part of their application, Dottin and his team proposed analyzing sulfur isotopes in the Apollo 17 samples using secondary ion mass spectrometry, a form of surface analysis that uses an ion beam to erode material, creating secondary ions that are analyzed with a mass spectrometer. Dottin and his team employed this highly precise method to analyze specific samples from the drive tube that appeared to be mantle-driven volcanic rock. To their surprise, the team found that the ratios for 33S in these samples varied dramatically from those on Earth.
These results presented two possibilities. First, the depleted sulfur isotopes could be the result of sulfur interacting with ultraviolet radiation back when the Moon had a thin and short-lived atmosphere. This is consistent with the fact that the Moon was geologically active billions of years ago (as evidenced by sinuous rilles and basaltic plains) and experienced volcanic outgassing. Second, the chemical depletion could be remnants left over from the collision with Theia that led to the formation of the Moon 4.5 billion years ago. Said Dottin:
I was targeting sulfur that had a texture that would suggest it was erupted with the rock and not added through a different process. My first thought was, 'Holy shmolies, that can't be right.' So we went back to make sure we had done everything properly, and we had. These are just very surprising results. That would be evidence of ancient exchange of materials from the lunar surface to the mantle. 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.
While it is not clear which of these possibilities is correct, Dottin and his colleagues are hopeful that studies that are similarly focused on sulfur isotopes from other bodies (including Mars) might provide an answer. The distribution of isotope ratios in different bodies will ultimately reveal how elements were disseminated throughout the early Solar System, thereby showing how it formed and evolved over time.
Further Reading: Brown University, JGR Planets