Interstellar ices play a fundamental role in the physical and chemical evolution of molecular clouds and star-forming regions, yet their large-scale distribution and abundance remain challenging to map. In this work, I present the ice color excess method (ICE), which parametrizes the peak optical depth (τ3.0max) of the prominent 3 μm absorption feature, which is predominantly caused by the presence of solid H2O. The method builds on well-established near-infrared color excess techniques and uses widely available infrared broadband photometry. Through detailed evaluation of passband combinations and a comprehensive error analysis, I constructed the ICE color excess metric Λ(W1 − I1). This parameter emerges as the optimal choice that minimizes systematic errors while leveraging high-quality, widely available photometry from Spitzer and WISE data archives. To calibrate the method, I compiled from the literature a sample of stars located in the background of nearby molecular clouds for which spectroscopically measured optical depths are available. The empirical calibration yielded a remarkably tight correlation between τ3.0max and Λ(W1 − I1). This photometric technique opens a new avenue for tracing the icy component of the interstellar medium on Galactic scales, providing a powerful complement to spectroscopic surveys, and enables new insights into the environmental dependence of the formation and evolution of icy dust grains.
Stefan Meingast (2025) studied this question.