A primordial heavy water reservoir in the outer solar system

The oxygen (O) isotopic composition of outer solar system ices is believed to be enriched in heavy O isotopes owing to photochemical processes, but evidence for this O reservoir is exceedingly rare in meteorites. Hydrated interplanetary dust particles (IDPs) collected in Earth’s stratosphere that display high densities of solar energetic particle tracks originate from Edgeworth-Kuiper belt objects. Their mineralogy is dominated by hydrated minerals that formed by alteration of anhydrous precursor minerals at low temperatures through reactions with water (H 2 O)–carbon dioxide fluids. The O isotopic compositions of track-rich hydrated IDPs implicate 17,18 O-rich ices as the source of H 2 O involved in their alteration, providing compelling evidence for the presence of a major heavy water reservoir beyond the orbits of the gas giant planets that was largely isolated from inner solar system materials.