Observational possibility of the "snow line" on the surface of circumstellar disks with the scattered light

We discuss how we obtain the spatial distribution of ice on the surface of the circumstellar disk around young stars. Ice in the disks plays a very important role in various issues, for instance, on the disk structure, on planet formation, on an isotopic anomaly in meteorites, and on the origin of the oceans on Earth. Therefore, spatially resolved observations of the condensation/sublimation front of ice, the so-called snow line is strongly required. Here, we propose a new method for obtaining a spatially resolved snow line on circumstellar disks by observing a 3 μm H₂O ice feature in the scattered light. Based on radiative transfer considerations, we show that the feature is clearly imprinted in the spectrum of the scattered light from both optically thick and thin circumstellar disks. We also show that the scattered light and the H₂O ice feature from protoplanetary disks are detectable and spatially resolvable with current instruments through an H ₂O narrowband filter around 3 μm. Finally, we present a diagnostics of disk dust properties on K - H₂O and K-L′ two-color diagram.