There interstellar comet 3I/ATLASdiscovered in July 2025 and already at the center of a media case due to the statements of Avi Loeb on its alleged alien origin, widely denied by the scientific community, continues to prove of enormous scientific interest even months after its passage through the internal Solar System. A new analysis conducted with the radio telescope ALMA reveals that the ices of this comet are composed of large quantities of semi-heavy wateri.e. containing deuterium: we are talking about a semi-heavy water content 30 times higher to comets in the Solar System. The discovery confirms observations made with the James Webb telescope (from which it appears that the comet could have up to 12 billion years) and tells of a formation environment for 3I/ATLAS that was very different from the one in which the Solar System formed.
What is semi-heavy water: 3I/ATLAS has up to 40 times more than terrestrial water
Let’s first understand what is meant by “semi-heavy water”. We all know the chemical formula of water: H2ORthat is, two atoms of hydrogen and one of oxygen. Hydrogen, in its most common form, is composed of an electron around a proton. However, there are other “forms” (in jargon isotopes) of hydrogen: here we are particularly interested in the deuterium (D), whose nucleus is not a single proton but a pair formed by a proton and a neutron. Now, if we take a very normal water molecule and replace one of the hydrogen atoms with a deuterium atom (HDO), what we get is a molecule of semi-heavy water (with two deuterium atoms we have instead heavy water).
Chemically speaking, semi-heavy water behaves almost exactly like conventional water. His presence is not strange: we also find it in small quantities in the Earth’s oceans and in “local” comets. We don’t find much of it because deuterium is rarer than “normal” hydrogen and they are needed very low temperatures (below –234 °C) to form a semi-heavy water molecule.
The study of the ALMA radio telescope
To be clear, water on Earth has 1-2 semi-heavy water molecules for every 10,000 water molecules. In the comets of the Solar System we find a little more: 2-5 atoms per 10,000. According to the analysis conducted with ALMA, comet 3I/ATLAS instead has 66 heavy water molecules per 10,000 water molecules. Never before had such an abundance of deuterium been observed in a comet.
This means that 3I/ATLAS was born in a much colder environment than the one in which the Sun was born. Perhaps “our” primordial cloud was probably heated by the radiation of massive stars that were nearby, while the cloud from which the star system from which 3I/ATLAS came was formed was not. All the data collected on this very interesting comet will therefore be very useful for understanding the formation of bodies in stellar systems subjected to different chemical and physical conditions very different from ours.
