Volatile Ratios (CO2/H2O anomaly)

Preprint Chemical and Formation Analysis of Interstellar Comet 3I/ATL...

Spectroscopic observations yield a CO2/H2O ratio of 8.0 ± 0.5 at 3.3 AU. However, modeling of sublimation fronts and coma expansion predicts that this ratio could rise to ∼19 ± 2 near 2 AU, as the dust-to-gas coupling efficiency changes with heliocentric distance. Specifically, CO2 sublimation continues efficiently at 2 AU, while much of the H2O remains trapped in grains larger than ∼100 μm, which have sublimation timescales longer than the nucleus rotation period. Thus, the apparent CO2/H2O ratio is distance-and grain-size dependent, and its evolution must be considered when comparing to solar system comets.

At 3.3 AU (JWST epoch): water sublimation is inefficient — surface temperatures are too low to sustain large fluxes. CO2, with a lower sublimation temperature, dominates, giving ∼8:1.

As ATLAS moves inward toward 2 AU: small grains (∼1 μm) release their embedded H2O quickly, but larger grains (>100 μm) retain H2O ice, effectively ”delaying” its release. This skews the observed ratio toward even greater CO2 dominance (∼19:1).

Distance dependence: sublimation rates scale roughly as r−2 for absorbed flux, but the activation thresholds for CO2 vs H2O differ, so their relative contributions diverge with decreasing heliocentric distance. Implication: This ratio is not a fixed property, but an evolving diagnostic — higher than any measured in solar system comets, pointing to unusual formation conditions.

(PDF) Chemical and Formation Analysis of Interstellar Comet 3I/ATLAS: Evidence for AGB Star Origin. Available from: https://www.researchgate.net/publication/395405143_Chemical_and_Formation_Analysis_of_Interstellar_Comet_3IATLAS_Evidence_for_AGB_Star_Origin [accessed Sep 11 2025].