  Water ice has a strong spectral feature at a wavelength of approximately
$3~\mu$m, which plays a vital role in our understanding of the icy universe. In
this study, we investigate the scattering polarization of this water-ice
feature. The linear polarization degree of light scattered by $\mu$m-sized icy
grains is known to be enhanced at the ice band; however, the dependence of this
polarization enhancement on various grain properties is unclear. We find that
the enhanced polarization at the ice band is sensitive to the presence of
$\mu$m-sized grains as well as their ice abundance. We demonstrate that this
enhancement is caused by the high absorbency of the water-ice feature, which
attenuates internal scattering and renders the surface reflection dominant over
internal scattering. Additionally, we compare our models with polarimetric
observations of the low-mass protostar L1551 IRS 5. Our results show that
scattering by a maximum grain radius of a few microns with a low water-ice
abundance is consistent with observations. Thus, scattering polarization of the
water-ice feature is a useful tool for characterizing ice properties in various
astronomical environments.

Astrophysics of Galaxies;Earth and Planetary Astrophysics;Solar and Stellar Astrophysics