Suppressed 1.5 micrometer water-ice absorption in comet 17P/Holmes

Determine the physical mechanism responsible for the absence of the 1.5 micrometer water-ice absorption band in the spectrum of comet 17P/Holmes, including whether refractory-rich water-ice-bearing aggregates or particle-size distributions extending from submicron to micron scales can explain the observation.

Background

The paper identifies 17P/Holmes as an intermediate case between classical water-ice-rich cometary comae and the refractory-rich aggregate interpretation proposed for 3I. Its spectrum shows a blue near-infrared continuum together with 2.0 and 3.0 micrometer water-ice absorptions, but no detectable 1.5 micrometer band.

Previous work interpreted the spectrum using pure micron-sized water-ice grains physically separated from refractory dust, yet the missing 1.5 micrometer band was not explained. The authors suggest that refractory-rich aggregates and broad fine-particle-size distributions could provide an alternative explanation, but state that dedicated modeling is needed to test it.

References

Yang et al.\ interpreted these observations in terms of pure micron-sized water-ice grains physically separated from refractory dust, similar to the Hartley~2-like case, and identified the absence of the 1.5~\textmu{}m band as an unresolved problem.

— JWST Reveals Refractory-Rich Water Ice in Interstellar Comet 3I/ATLAS: Evidence for a Continuum of Grain Properties across Protoplanetary Disks  (2609.19286 - Protopapa et al., 16 Sep 2026) in Section 3.1, paragraph discussing comet 17P/Holmes