Water frost and ice: The near‐infrared spectral reflectance 0.65–2.5 μm

Abstract

The spectral reflectance of water frost and frost on ice as a function of temperature and grain size is presented with 1½% spectral resolution in the 0.65‐ to 2.5‐μm wavelength region. The well‐known 2.0‐, 1.65‐, and 1.5‐μm solid water absorption bands are precisely defined along with the little studied 1.25‐μm band and the previously unidentified (in reflectance) 1.04‐, 0.90‐, and 0.81‐μm absorption bands. The 1.5‐μm band complex is quantitatively analyzed using a nonlinear least squares algorithm to resolve the band into four Gaussian components as a function of grain size and temperature. It is found that the 1.65 μm component, which was thought to be a good temperature sensor, is highly grain size dependent and poorly suited to temperature sensing. Another Gaussian component appears to show a dependence of width on grain size while being independent of temperature. The relative apparent band depths are different for frost layers on ice than for thick layers of frost and may explain the apparent band depths seen in many planetary reflectance spectra.