The Icy Surface of Great Comet Hale-Bopp at 46 au as Revealed by JWST

Comets are small solar system objects that undergo periods of mass-loss (activity) due to insolation-driven sublimation.  When a comet is active, surface and near-surface material is ejected from the nucleus and made available for study in a rarefied environment.  Thus, cometary mass-loss provides a window into the bulk composition of cometary nuclei.  However, activity often precludes direct characterization of their surfaces.  Yet, spectra of cometary nuclei are invaluable, as they can be directly compared to trans-Neptunian objects, centaurs, etc., providing key insights into how environmental factors shape the observable properties of primitive bodies. To date, surface spectroscopy has only been achieved for a handful of comets, primarily Jupiter-family comets such as 67P/Churyumov–Gerasimenko and 9P/Tempel 1 (e.g., Sunshine et al. 2006; Barucci et al. 2016), the best of which were obtained through dedicated spacecraft missions.

With the advent of JWST, we now have the capability to directly observe the surfaces of many cometary nuclei at distances where their activities have stopped.  This even includes long-period comets, such as the subject of our presentation here: great comet C/1995 O1 (Hale-Bopp). Hale-Bopp is a long-period comet originating from the Oort Cloud, which passed perihelion in 1997, at a distance of 0.9 au from the Sun.  It was one of the brightest and best-observed comets of the 20th century, largely owing to the size of the nucleus.  The comet is notable for having a higher than typical CO/H₂O coma mixing ratio, even for Oort cloud comets (Harrington Pinto et al. 2023).   It remained active out to 27 au, likely driven by CO sublimation (Szabó et al. 2012; Kramer et al. 2014), beyond which only a point-source nucleus has been detected.

With Hale-Bopp’s activity apparently ceased, the spectroscopic study of a large CO-rich nucleus has been made possible.  We used JWST's NIRSpec and NIRCam instruments to characterize its nuclear surface at 46 au from the Sun.  Analysis of the NIRCam data show no evidence for activity.  The low resolution NIRSpec spectrum from 0.8 to 4 μm shows evidence for surface water ice through diagnostic features at 1.5, 2.0, and 3 μm (Mastrapa et al. 2009).  The clear detection of water ice is in stark contrast to the minimal (<1% by surface area) water ice coverage seen by spacecraft visits to Jupiter-family comets, but has more in common with recent JWST spectra of TNOs (Pinilla Alonso et al. 2025).  An apparent absorption feature at 3.4 to 3.5 μm suggests the presence of C–H bearing organics, but the low spectral resolution may preclude precise identifications.  The 4 to 5 μm spectrum lacks emission features from CO₂ and CO gases.  Despite the warm temperatures suffered by the comet in the inner-solar system, the nucleus surface is not desiccated, but remains enriched with water ice, likely due to continued mass-loss outside of the water sublimation zone.  We argue that the canonical 4% visual albedo may not be appropriate for distant observations of returning long-period comets.  These data offer a fascinating point of comparison to trans-Neptunian objects and centaurs, bodies with similar origins but markedly different dynamical and thermal histories.

Support for this work was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127.