Science has come a long way since NASA launched the Apollo 17 mission. Over the last 50 years, researchers have developed advanced technologies and techniques that far surpass those available in 1972.
This progress is exactly what NASA was hoping for when the Apollo 17 astronauts—the last humans to set foot on the Moon—returned to Earth with more than 2,000 samples of lunar rock and dust. Some were squirreled away in the hopes that one day, better-equipped scientists could study the samples and make new discoveries.
And that’s what a team of researchers led by James W. Dottin III, an assistant professor of Earth, environmental, and planetary sciences at Brown University, just did. Dottin and his colleagues analyzed the composition of samples taken from the Moon’s Taurus-Littrow valley. The findings, published last month in the journal JGR Planets, indicate that volcanic material in the samples contain sulfur compounds that are starkly different from those found on our planet.
“Before this, it was thought that the lunar mantle had the same sulfur isotope composition as Earth,” Dottin said in a press release. “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.”
A discovery 50 years in the making
After the Apollo 17 astronauts landed in the Taurus-Littrow valley, they extracted a 2-foot-long core sample from the lunar surface using a hollow metal instrument called a double drive tube. Once returned to Earth, this sample and many others like it remained sealed inside their tubes under the protection of NASA’s Apollo Next Generation Sample Analysis (ANGSA) program.
In the last few years, NASA has begun accepting new research proposals to study the ANGSA samples. Dottin proposed analyzing sulfur isotopes using secondary ion mass spectrometry, a high-precision technique that wasn’t available when the samples were first returned to Earth.
Researchers can use this technique to measure the ratios of different isotopes in a sample. These ratios serve as a distinctive “fingerprint” that points to the sample’s origin. Thus, two samples with the same isotopic fingerprint likely came from the same source.
Previous research has shown that oxygen isotopes in lunar samples are nearly identical between Moon and Earth rocks, so Dottin assumed the same would be true for sulfur isotopes. His findings tell a very different story.
Two distinct isotopic fingerprints
Dottin and his colleagues specifically analyzed portions of the drive tube sample that appeared to be volcanic rock from the Moon’s mantle. Their analysis revealed that volcanic material in the sample contained sulfur compounds that are very low in sulfur-33, a radioactively stable sulfur isotope. This is very different from sulfur isotope ratios found on Earth.
“My first thought was, ‘Holy shmolies, that can’t be right,’” Dottin said. “So we went back to make sure we had done everything properly and we had. These are just very surprising results.”
According to the researchers, the results suggest that the sulfur formed in chemical reactions early on in the Moon’s history, or that it stems from its formation. Experts widely believe the Moon is made of debris ejected from a collision between Earth and a Mars-sized object called Theia. It’s possible that the researchers have found traces of Theia’s sulfur signature in the Moon’s mantle.
Dottin hopes that as researchers analyze sulfur isotopes from other planets like Mars they may begin to solve this mystery. Isotopic analysis has already provided key insights into how Earth and its only natural satellite came to be, and this approach will continue to help scientists unravel the history of our solar system.
