3I/ATLAS water D/H ratio exceeds 40 times Earth's ocean value
Updated
Updated · Nature.com · Apr 23
3I/ATLAS water D/H ratio exceeds 40 times Earth's ocean value
3 articles · Updated · Nature.com · Apr 23
ALMA observations reveal the interstellar comet 3I/ATLAS has a water deuterium-to-hydrogen (D/H) ratio greater than 6.6 × 10⁻³, far surpassing Solar System comet values.
This extreme deuterium enrichment indicates 3I/ATLAS's water formed under much colder, less irradiated conditions than those in our Solar System, suggesting a markedly different planetary system origin.
The findings highlight the diversity of water formation environments in the galaxy and provide rare insight into the chemical evolution of planetary systems beyond our own.
This alien comet's water is 40 times heavier than Earth's. What does this reveal about its home star system?
Could life evolve in oceans made from the 'heavy water' found in this interstellar visitor?
One alien comet is D/H-rich, another CO-rich. What creates this diversity among planetary building blocks?
Scientists measured 'heavy water' but not normal water in the comet. How is that possible?
If our solar system's recipe for planets isn't universal, what other kinds of worlds could exist out there?
3I/ATLAS Exhibits 30x Solar System Deuterium Levels, Challenging Planetary Formation Models
Overview
In April 2026, researchers led by Salazar Manzano and Paneque-Carreño announced that the interstellar comet 3I/ATLAS exhibits the highest deuterium-to-hydrogen ratio ever recorded—over 30 times greater than Solar System comets and 40 times that of Earth's oceans. This extreme enrichment reveals the comet formed in an ultra-cold, low-radiation environment more than 3 billion years ago, preserving unique chemical signatures through ion-molecule reactions in icy dust grains. ALMA's advanced observations during the comet's perihelion enabled these findings, which challenge existing models by showing planetary systems can form under vastly different conditions, expanding our understanding of galactic chemical diversity and planetary formation.