Water D/H in 3I/ATLAS as a Probe of Formation Conditions in Another Planetary System

TL;DR

This study uses ALMA observations of interstellar comet 3I/ATLAS to measure its water D/H ratio, revealing it formed under colder, less irradiated conditions than Earth's oceans and Solar System comets.

Contribution

First measurement of water D/H ratio in extrasolar cometary material, providing insights into formation conditions in a different planetary system.

Findings

Water D/H in 3I/ATLAS exceeds Earth's ocean value by over 40 times.

Deuterium enrichment indicates formation in colder, less irradiated environments.

Supports the idea of diverse planetary system formation conditions.

Abstract

Water reservoirs in the Solar System exhibit a deuterium enrichment that links back to the physical environment at the time of stellar birth. Gas-phase and ice-grain deuterium enrichments occur through chemical processes that operate at low temperatures ($<$~30~K) pointing towards an origin in the prestellar molecular cloud or in the outer parts of the protoplanetary disk. However, not all stars are born in environments similar to our Sun, nor do their subsequent evolutionary histories follow the same path. These environmental differences can be traced by the water deuterium-to-hydrogen (D/H) ratio. Here we use ALMA observations of the interstellar comet 3I/ATLAS to constrain the water D/H ratio in extrasolar cometary material. With a water D/H value of [D/H]$_{\mathrm{H_2O}} > 6.6\times10^{-3}$, 3I/ATLAS shows a deuterium enrichment exceeding Earth's ocean value by more than a factor of $\gtrsim40$ and typical Solar System cometary values by more than a factor of $\gtrsim30$. The elevated deuterium enrichment points to water that formed under colder, less irradiated conditions and from less thermally processed material, consistent with an origin in a planetary system that formed under different physical and chemical conditions than our own.