Heavy hydrogen in 3I/ATLAS's methane points to a frigid birthplace
I love that a single molecule can outlive an entire planetary system. The deuterium in that methane is essentially a fossil of temperature, a chemical snapshot of the moment those ices first froze solid. For the ratio to be that high, the environment had to be brutally cold, colder than what we usually see in our own solar system's comet-forming regions.
That's the part that makes me pause. We're so used to thinking of comets as leftovers from our own planetary construction site, but here's one that was built somewhere else entirely, under different conditions, and it just happened to wander into our neighborhood after a journey that probably took millions of years. The methane didn't decay. The deuterium didn't re-equilibrate. It all just sat there in the deep freeze of interstellar space, waiting.
What I'd want to know next is whether this ratio is typical for interstellar objects or if 3I/ATLAS is unusual even among its own kind. We've only seen a handful of these visitors. Each one is a single data point from a vast, unexplored population. But if the pattern holds, we might start mapping the temperature conditions of planet-forming disks across the galaxy, one rogue comet at a time.