Do interstellar comets differ in composition from Solar System comets?
In plain words
2I/Borisov carried an unusually large share of carbon monoxide and 3I/ATLAS showed a coma dominated by carbon dioxide. Whether such differences reflect where they formed or billions of years of cosmic-ray exposure in space is unknown.
Precise statement
Compare volatile and metal abundance ratios ($\mathrm{CO}/\mathrm{H2O}, \mathrm{CO2}/\mathrm{H2O}, \mathrm{Ni}/\mathrm{Fe}$) of 2I/Borisov and 3I/ATLAS with Solar System comets, and determine whether the differences reflect formation location and stellar environment or processing during Gyr-long interstellar travel. The answer is a classification of composition differences by origin, with predictions testable on future objects.
What would settle it
Spectroscopic surveys of a sample of interstellar comets compared with laboratory and model predictions of cosmic-ray processing of ices.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
JWST spectra in 2025 showed a $\mathrm{CO_2}$-dominated coma of 3I/ATLAS ($\mathrm{CO2}/\mathrm{H2O} \sim 8$), VLT spectra detected nickel vapor with little or no iron, and its non-gravitational acceleration near perihelion is attributed to outgassing.
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