Interstellar Comet: Water 14 Times Heavier Than Earth's express
Comet 3I/ATLAS was spotted when it entered the Solar System from interstellar space. The James Webb Telescope split the light from its gas cloud into a rainbow, a spectrum, and in this pattern, the lines of rare molecules with deuterium (heavy hydrogen) appeared.
For 3I/ATLAS, this 'thermometer' is off the charts: for every 30 ordinary hydrogen atoms, there's one deuterium atom — in Earth's oceans, that ratio is hundreds of times poorer. This means the comet was born in the icy darkness of a distant protoplanetary disk — a region where planets are just beginning to coalesce from cosmic dust and gas. This alien world is far colder than the cloud that gave birth to our Sun and Earth.
Original abstract
as written by the authors arXiv:2603.20445
Interstellar objects are interlopers from other planetary systems, and their volatile compositions provide a glimpse into planet formation around their host star. We present near-infrared spectra of the coma of interstellar object 3I/ATLAS measured with the James Webb Space Telescope. Our results demonstrate an unexpectedly high $$\mathrm{D}/\mathrm{H} = (3.31\pm0.34)\%$$ for methane and represent an exceedingly rare detection of deuterated organic molecules in an interstellar object. This D/H ratio is a factor of $$14\pm2$$ higher than that measured in comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft, the only other comet for which CH$$_3$$D has been detected, yet the ratio of deuteration in methane compared with water is consistent for both comets within $$1.2\sigma$$. The D/H ratio in methane is observationally unconstrained in extrasolar sources to date, but the enriched ratio in 3I/ATLAS is most similar to those measured in other organic molecules toward primitive environments. The high D/H ratios of water and methane in 3I/ATLAS are a natural consequence of formation in a high D/H elemental ratio environment as a result of locally cold conditions in the protoplanetary disk and prior interstellar cloud. Thus, 3I/ATLAS formed in an environment very different from that in which our Sun and planets originated.
The deuterium abundance points to a comet birth temperature below –250°C — colder than the eternal shadows on the Moon.
Interstellar wanderers like 3I/ATLAS come to life in the pages of science fiction: for example, in Arthur C. Clarke's novel 'Rendezvous with Rama'.