Signatures of an exotic element discovered when retesting Apollo moon samples

Apollo 17 brought home more than dust and stones. A new study reports that some lunar rocks carry an unusual sulfur signature that does not match Earth’s.

That mismatch lives in tiny mineral grains formed by ancient volcanism, the process where molten rock erupted and cooled.

The pattern points to a source in the Moon’s interior that did not fully mix with Earth material.

Sulfur helps determine Moon’s age

An isotope, an atom of the same element with a different number of neutrons, acts like a tracer that helps scientists figure out where materials came from and what happened to them along the way.

Geochemists compare the isotopic fingerprint, the unique ratio of isotopes that identifies where a substance came from, to connect samples to their sources. If two places share the same pattern, they likely share a history.

James W. Dottin III, an assistant professor at Brown University, led the work that brought this story together.

His team focused on sulfur-33, a stable form of sulfur with 33 particles in its nucleus, that sometimes behaves differently than expected.

How Apollo sample was stored

Apollo 17 astronauts hammered two connected aluminum tubes about 1.5 by 14 inches into the soil at Taurus Littrow, a valley on the Moon’s surface, in December 1972.

They then sealed one tube under vacuum on the Moon before returning it to Earth. The sealed core became a time capsule for future technicians to analyze with better technology.

The Apollo Next Generation Sample Analysis (ANGSA), NASA’s program that reexamines old Apollo samples for modern study, opened the sealed core in 2022.

They then coordinated precision analyses to take advantage of modern instruments with cutting-edge technology. That effort aims to wring new science from carefully preserved material.

The lower section, cataloged as 73001, came from below about 8.7 inches and was stored in a core sample vacuum container, a sealed metal cylinder used to preserve lunar soil without air contamination. The upper section, 73002, returned unsealed and was studied earlier.

What the measurements show

The team used secondary ion mass spectrometry, a technique that bombards a sample with ions to measure its atomic makeup, to measure sulfur isotopes in volcanic grains, tiny bits of rock formed when lava cooled on the Moon’s surface.

They found sulfur that is strongly depleted in sulfur-33 compared with Earth.

Scientists measured two sulfur signals in these rocks. The first, δ34S, is a score that compares how much sulfur-34 the rock has to a standard; it ranges from about −4.1 to +1.5.

The second, Δ33S, shows unusual behavior of sulfur-33; it ranges from about −2.8 to −0.1. These numbers are way outside what we usually see in Earth rocks.

These numbers come with a clear trend.

“All Δ33S and δ34S data are positively correlated,” wrote Dottin, the assistant professor at Brown University and lead author, adding another key point with a narrow focus.

“One of which is associated with photochemically processed sulfur from a gaseous environment,” stated Dottin, the assistant professor at Brown University and lead author.

How sulfur formed on the moon

One explanation starts at the surface long ago.

Mass independent fractionation, a process where isotopes separate through light-driven chemistry rather than by weight, can arise when UV light drives photochemistry in a thin or optically thin atmosphere.

That process leaves a distinct Δ33S footprint.

If that altered sulfur later sank or was otherwise delivered into the lunar mantle, the dense layer beneath the Moon’s crust, then ancient surface material must have been exchanged with the interior.

It suggests that early lunar processes could move material downward without plate tectonics, Earth’s system of shifting crustal plates.

Another explanation for this exotic sulfur reaches back to the Moon’s birth.

The leading idea is that a Mars-sized object called Theia, a protoplanet that likely struck the early Earth, created debris from which the Moon grew.

If Theia’s sulfur signature differed from Earth’s, part of that distinct signal may still reside in the Moon’s interior.

The unusual sulfur-33 depletion would then chronicle imperfect mixing efficiency, or how completely materials from different sources blended during or after the impact.

Data changes lunar’s chemistry

Scientists have often pointed to similar oxygen isotope patterns as evidence that Earth and Moon share a common reservoir. This sulfur story breaks that simple link by flagging a component that does not match Earth.

A distinct mantle source means the lunar interior chemistry kept more than one thread of history. That invites new tests of how efficiently materials blended during the impact and the Moon’s earliest days.

The core came from a landslide deposit, material that slid down a slope and settled in layers, near Lara Crater in the Taurus Littrow valley.

The sealed tube preserved fragile minerals and volatile elements, materials that can easily vaporize, such as sulfur or water, which can lose information when exposed to air.

The drive tube extended roughly 24 inches into the soil, capturing layers that formed at different times.

Because the seal held, the sample avoided contamination and kept fine-scale textures that matter for isotope work.

Moon samples, sulfur, and future testing

Secondary ion mass spectrometry turns a tiny spot into a rich dataset. In these grains, this approach lets the team compare sulfur and other elements in place and tie the chemistry to rock textures. This is important because context reduces ambiguity.

Seeing sulfur in volcanic grains suggests the isotopic signal was present during eruption rather than added later by alteration.

Future comparisons will tighten the picture. Mars rocks and other lunar samples can test whether similar Δ33S signals mark early surface chemistry or impact leftovers.

ANGSA continues to make high-value Apollo samples available to researchers who can use sensitive methods to preserve and extend the science return.

More work on sulfur from multiple worlds will help map how the solar nebula, the ancient cloud of gas and dust that formed the Sun and planets, assembled and evolved.

The study is published in the Journal of Geophysical Research: Planets.

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