A primordial heavy water reservoir in the outer solar system.

Track-rich hydrated interplanetary dust particles show O isotopic compositions consistent with 17,18 O-rich ices, indicating a major heavy water reservoir beyond gas giant orbits.

• Why it matters: Outer solar system ices are expected to be enriched in heavy O from photochemistry, yet direct meteorite evidence for such an O reservoir is exceedingly rare.
• What they did: The study analyzed hydrated IDPs from Earth’s stratosphere whose mineralogy reflects low-temperature alteration by H2O–carbon dioxide fluids, using O isotopes from solar energetic particle track–rich particles.
• The result: The measured O isotopic signatures implicate 17,18 O-rich ices as the water source for alteration, supporting a largely isolated heavy water reservoir beyond the gas giant planets.

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