Probing Microporous ASW with Near-infrared Spectroscopy: Implications for JWST's NIRSpec

The presence of porous amorphous solid water (ASW) in interstellar ice mantles has long been suspected but, to date, has not been identified. With the upcoming launch of the James Webb Space Telescope (JWST), future detections of porous ASW seem promising. To this end, we investigated the near-infrared spectra of ASW to confirm that two features that have been seen sporadically in literature and have either not been identified or only tentatively identified were indeed due to dangling bond (DB) absorption bands. These bands may provide an additional way to identify porous ASW with remote sensing spectroscopy. Our systematic study tested the thermal stability of these features and their spectral shifts during gas adsorption. We correlated these changes to what we observed in the well-studied fundamental DB absorptions at 3720 and 3696 cm-1. We find excellent agreement between the behavior of the fundamental DB absorption bands with those centered at 5326 and 7235 cm-1, as well as a shoulder located at 7285 cm-1, confirming that all these features are due to DBs present in porous ASW. We estimate that these weaker DB absorptions should be identifiable after 4-5 hr of observation time with JWST's NIRSpec instrument. These new features may not only prove useful in future observational campaigns in search of porous ASW but also may serve as a powerful way to indirectly detect weakly absorbing species that challenge the ability of remote sensing spectroscopy.