Near-Infrared Spectra of Icy Outer Solar System Surfaces: Remote Determination of H2O Ice Temperatures

We present new 1.20 to 2.35 μm spectra of satellites of Jupiter, Saturn, and Uranus, and the rings of Saturn, obtained in 1995 and 1998 at Lowell Observatory. For most of the target objects, our data provide considerable improvement in spectral resolution and signal-to-noise over previously published data. Absorption bands with shapes characteristic of low-temperature, hexagonal crystalline H2O ice dominate the spectra of most of our targets in this wavelength range. We make use of newly published temperature-dependent wavelengths and relative strengths of H2O absorption bands to infer ice temperatures from our spectra. These ice temperatures are distinct from temperatures determined from thermal emission measurements or simulations of radiative balances. Unlike those methods, which average over all terrains including ice-free regions, our temperature-sensing method is only sensitive to the ice component. Our method offers a new constraint which, combined with other observations, can lead to better understanding of thermal properties and textures of remote, icy surfaces. Ice temperatures are generally lower than thermal emission brightness temperatures, indicative of the effects of thermal inertia and segregation between ice and warmer, darker materials. We also present the results of experiments to investigate possible changes of water ice temperature over time, including observations of Titania at two epochs, and of Ganymede and saturnian ring particles following emergence from the eclipse shadows of their primary planets. Finally, we discuss limitations of our temperature measurement method which can result from the presence of H2O in phases other than hexagonal ice-Ih, such as amorphous ice, hydrated mineral phases, or radiation-damaged crystalline ice. Our spectra of Europa and Enceladus exhibit peculiar spectral features which may result from effects such as these.