Deuterium / Deuterium-to-Hydrogen (D/H) Ratio
Deuterium is a stable isotope of hydrogen with one proton and one neutron; water made with it (“heavy water,” HDO or D₂O) is measurably heavier than ordinary H₂O. The deuterium-to-hydrogen (D/H) ratio of an icy body is a chemical fingerprint of the temperature and radiation environment in which it formed — colder, lower-radiation regions favor higher deuterium enrichment.
Significance for interstellar objects
Per src-3i-atlas-heavy-water-2026-05, the interstellar comet 3i-atlas shows a D/H ratio roughly 30× higher than solar-system comets and ~40× higher than Earth’s oceans — the highest of any known planetary system or comet. The measurement, made via MDM Observatory gas detections plus alma submillimeter spectroscopy, was the first deuterated-water analysis of an interstellar object and implies 3I/ATLAS formed in a far colder birthplace than the solar nebula.
The D/H ratio is also central to the long-standing question of whether Earth’s water was delivered by comets: comparing cometary D/H against Earth’s ocean value tests that hypothesis. An interstellar comet with a wildly different ratio underscores how formation environments vary between planetary systems.