A half-century-old lunar sample opened in 2022 has upended a foundational assumption about how Earth and the Moon formed. Researchers analyzing troilite grains from a specimen collected during Apollo 17 discovered sulfur isotope ratios that contradict decades of planetary science orthodoxy. The findings suggest the Moon’s early history involved far more complex chemical processes than the prevailing Giant Impact Hypothesis predicts.
The sample, collected on December 11, 1972, at Taurus-Littrow by astronauts Gene Cernan and Harrison Schmitt, was sealed in a helium chamber on the lunar surface and remained untouched for over fifty years. NASA’s Apollo Next Generation Sample Analysis program gained authorization to examine the specimen in 2022, deploying secondary ion mass spectrometry—technology unavailable during the Apollo era—to measure sulfur isotope compositions with extraordinary precision.
The Anomalous Discovery
James Dottin, an assistant professor at Brown University leading the research team, described his reaction when results contradicted all expectations. The troilite displayed sulfur-33 depletion patterns ranging from −2.8 to −0.1 in Δ³³S values, with δ³⁴S measurements spanning −4.1 to +1.5. These isotopic signatures had never been detected in any lunar sample before.
Isotopes function as nature’s fingerprints, revealing where materials originated and how they formed. Sulfur possesses four stable isotopes, and scientists measure their ratios to understand planetary assembly and ancient chemical environments. The exotic signature forced Dottin’s team to immediately rerun measurements to verify accuracy.
Challenging the Giant Impact Model
The Giant Impact Hypothesis has dominated planetary science since the 1970s, proposing that a Mars-sized protoplanet named Theia collided with proto-Earth approximately 4.5 billion years ago. This catastrophic collision melted both bodies, with debris eventually coalescing to form the Moon. The theory successfully explains why Earth and Moon share similar oxygen isotope ratios—they underwent mixing during formation.
Researchers assumed sulfur isotopes would tell the same story of complete material integration. Instead, the newly discovered sulfur anomaly indicates divergent isotopic signatures between the two bodies, suggesting the Moon’s formation involved more complex processes than simple coalescence of impact debris.
Competing Explanations
One compelling hypothesis proposes the exotic sulfur formed during the Moon’s violent infancy when a magma ocean covered the satellite’s entire surface. This global ocean, potentially hundreds of kilometers deep, cooled and crystallized over tens to hundreds of millions of years. During this period, sulfur likely evaporated from the molten surface into the Moon’s primordial atmosphere through volcanic outgassing. Ultraviolet radiation from the unshielded Sun would have photochemically altered sulfur, creating the observed depletion pattern through mass-independent fractionation processes.
A critical puzzle emerges if photochemical alteration created the sulfur signature: how did surface-altered material reach the lunar mantle? Earth’s plate tectonics constantly recycles surface material into the planet’s interior, but the Moon lacks plate tectonics and possesses no known mechanism for transporting surface material into the mantle. Researchers must therefore posit an unknown exchange mechanism operated on the early Moon—potentially involving planetary-scale volcanic processes or crustal disruption.
A second hypothesis proposes the exotic sulfur originated from Theia itself. If the giant impact collision didn’t achieve complete material mixing, pockets of Theia’s composition could have survived relatively intact within the lunar mantle. The distinctive sulfur signature might represent the first-ever direct chemical fingerprint of the impactor, challenging assumptions of complete homogenization during Moon formation.
Technological Breakthrough and Future Investigation
The breakthrough relied entirely on secondary ion mass spectrometry, capable of measuring isotopic ratios at microscale resolution with unprecedented precision. The Apollo program demonstrated remarkable foresight by preserving pristine samples sealed in vacuum containers, anticipating that future technological advances would enable revolutionary investigations.
Resolving whether photochemical alteration or Theia-derived material best explains the sulfur signature requires examining additional extraterrestrial samples from diverse sources. Research teams worldwide are now analyzing their own Apollo sample collections for similar sulfur isotope anomalies. NASA’s Artemis program aims to collect samples from previously unexplored lunar locations, potentially distinguishing between competing hypotheses through comparative analysis of material from geologically distinct regions.