The interstellar comet 3I/ATLAS carries water with a deuterium-to-hydrogen ratio of 0.95% — far exceeding any known Solar System body. Its carbon isotope ratios in CO and CO2 similarly dwarf local values. These are not minor enrichments. They are signatures of formation at temperatures below 30 kelvin in a metal-poor environment, conditions consistent with the Galaxy's early epoch rather than its current chemical state.
Galactic chemical evolution models place the accretion of 3I/ATLAS roughly 10 to 12 billion years ago, following a burst of early star formation in a now-dissolved planetary system. The object has been wandering interstellar space since before the Sun existed — a preserved fragment of primordial ice chemistry, carrying the isotopic fingerprint of an era when heavy elements were scarce and molecular clouds were colder.
Every interstellar object is a core sample from a foreign planetary system, but 3I/ATLAS is a core sample from a foreign epoch. The isotopic ratios don't just identify where it formed — they identify when. Chemistry encodes time, and time encodes galactic history. A single dirty snowball, passing through our inner solar system for a few months, carries information about star formation rates billions of years before Earth coalesced. The Galaxy's past is not locked in distant light — it occasionally drifts close enough to measure directly.
(arXiv:2603.06911)