During the Apollo Era, one of the most important operations conducted by astronauts was sample-returns, where lunar rocks were procured and brought back to Earth. The study of these rocks revealed a great deal about the composition, structure, and geological history of the Moon. This led to profound discoveries, including the presence of water on the Moon and the fact that both Earth and its only satellite formed together.
Over time, scientists have taken advantage of new techniques and technology to conduct more in-depth analyses to learn more about the formation and evolution of the Moon. Recently, a team of researchers from Brown University and the Carnegie Institution for Sciences (CIS) examined some of these samples for sulfur isotopes to shed new light on the early history of the Moon and its evolution.
The analysis was performed by Alberto E. Saal (a professor of geology with Brown’s Department of Earth Environmental and Planetary Sciences) and Dr. Erik H. Hauri, a geochemist with The Earth and Planets Laboratory at CIS (who passed away in 2018). Using the technique known as Secondary Ion Mass Spectrometry (SISM) at the Carnie Institute, they studied the isotope signature of 67 individual samples of lunar material.
The Apollo 15 and 17 rocks contain glasses that are thought to be some of the most primitive volcanic lunar material. These glasses contain tiny bits of molten lava (melt inclusions) that were captured before sulfur and other volatile elements could be released with eruption – a process called degassing. The study of these bits of lava can allows scientists to see what the lava sources were like.
Using the SISM facility at Carnegie, Saal and Hauri measured these samples for sulfur isotopes – specifically the ratio of sulfur-32 to sulfur-34. These isotopes were selected because the rate at which they appear can reveal things about the chemical evolution of the samples, from the point where they were formed to their transportation, to the point where they finally erupted onto the surface.
Initial studies of lunar glass found that they uniformly tended to lean more towards the heavier sulfur-34 isotope, which stood in contrast to other elements and isotopes (which showed large variations). As Saal said in a recent interview with News from Brown:
“For many years it appeared as though the lunar basaltic rock samples analyzed had a very limited variation in sulfur isotope ratios. That would suggest that the interior of the Moon has a basically homogeneous sulfur isotopic composition. But using modern in situ analytical techniques, we show that the isotope ratios of the volcanic glasses actually have a fairly wide range, and those variations can be explained by events early in lunar history.”
These results were used to calibrate a model of the degassing process for all the lunar samples, which allowed Saal and Hauri to determine the composition of the priginal lava sources. This showed that the lavas originated from different reservoirs within the Moon’s interior that had a wide range of sulfur isotope ratios. Saal and Hauri found that this range in values could be explained by key events in the Moon’s early history.
For example, the lighter isotope ratio in some of the glasses is consistent with the separation of the iron core from silicate minerals when the early Moon was still in a molten state. When iron separates from silicates and other materials that make up the mantle and crust of a planetary body, it tends to retain the heavier sulfur-34, leaving the remaining magma enriched in the lighter sulfur-32.
Another key event was the cooling and crystallization process that followed, which is the likely source of the heavier isotope values found in some of the volcanic glasses and basaltic rocks returned from the Moon. This crystalization process removed sulfur from the magnum pool, leading to the formation of solid reservoirs with the heavier sulfur-34. As Saal explained:
“Once we know the degassing, then we can estimate back the original sulfur isotope composition of the sources that produced these lavas. The values we see in some of the volcanic glasses are fully consistent with models of the core segregation process. Our results suggest that these samples record these critical events in lunar history. As we keep looking at these samples with newer and better techniques, we keep learning new things.”
Saal also indicated that more research needs to be done (and more samples analyzed) before the sulfur isotopic composition can be fully understood. In the meantime, these latest results will help to clarify long-standing questions about the composition of the Moon’s interior and how it became differentiated billions of years ago. This effectively brings astronomers one step closer to understanding the early history of the Moon.
The valuable scientific returns that the Apollo lunar rocks continue to provide beautifully illustrates the value of sample-return missions. Here on Earth, studies can be performed using instruments that would be too large and cumbersome to send as part of a robotic mission. They can also be archived so scientists can go back and conducted further analysis as the technology improves.
It is for this reason that NASA’s latest robotic mission to Mars – the Perseverance rover – will collect samples and store them in a cache for future retrieval by an ESA mission. Once these are brought back to Earth, scientists will be able to study them for generations, learning more and more about what our planets have in common in the process.
The research was performed with funding provided by NASA’s Solar System Workings program.