A Half‑Century‑Old Sample Yields New Secrets

In the 1960s and ’70s, astronauts returned over 382 kg of lunar rock to Earth. Some of these samples were deliberately sealed and stored — waiting for future technologies to probe deeper than ever before.

Now, with modern mass‑spectrometry techniques, a long‑forgotten fragment from the Apollo 17 payload (drive tube 73001/2) has yielded a dramatic surprise. Embedded within this rock were grains of Troilite — a common iron‑sulfur compound in space — whose sulfur isotope composition defies precedent.

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Unexpected Isotopes: What the Sulfur Reveals

When scientists analyzed the sulfur isotopes within the troilite, they looked specifically at the ratio of Sulfur‑33 (³³S). Some grains showed slightly elevated ³³S — consistent with expectations for volcanic degassing on the Moon.

But others displayed a strong depletion in ³³S — an isotope ratio “very different from anything we find on Earth.”

This stark difference challenges previous assumptions that the Moon’s mantle and Earth’s share a uniform sulfur isotope composition.

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Two Possible Origins — And Both Are Earth‑Shaking

Researchers propose two scenarios that could create such unusual sulfur signatures:

• A primordial lunar atmosphere: When the newborn Moon was covered by a global “magma ocean,” sulfur may have evaporated from its surface. As sulfur escaped into a thin early lunar atmosphere under intense ultraviolet radiation, lighter isotopes like ³³S could have preferentially been lost — leaving behind a sulfur‑rich residue depleted in ³³S.
• Sourcing from a planetary collision: The leading theory for the Moon’s formation — the Giant‑impact hypothesis — posits that a Mars‑sized body (often called Theia) collided with early Earth, and debris coalesced into the Moon. It’s possible that some sulfur in the troilite comes from Theia itself, preserved in the Moon ever since.

Both scenarios carry profound implications: they hint at previously unknown processes in the Moon’s infancy — from atmospheric interactions to mixing of material from two planetary bodies.

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Why This Matters

• It challenges our models of lunar formation. If the sulfur did come from Theia, it suggests the Moon retains some primordial extrasolar material distinct from Earth.
• It suggests a dynamic early Moon. Evidence for a primitive atmosphere or volatile exchange — even for a short time — implies that the Moon’s early geology was more active and complex than assumed.
• It underscores the value of preserved samples. This discovery comes from a single Apollo‑era sample that sat untouched for decades — a reminder that “old” materials can harbor new science, especially as analytical tools improve.

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What Comes Next

To confirm which scenario is correct — atmospheric volatilization or Theia heritage — scientists will need more samples: ideally from different parts of the Moon, including its far side, and even from future sample‑return missions (lunar or asteroid).