Probing the regoliths of the classical Uranian satellites using near-infrared telescope observations: CO2 ice deposits mantled by a veneer of tiny H2O ice grains?

In the late 1970s and early 1980s, near-infrared (NIR) ground-based observations (~1 - 2.5 µm) determined that the surfaces of the large and tidally-locked "classical" moons of Uranus are composed of H₂O ice, mixed with a dark, C-rich constituent. In the late 1990s and early 2000s, CO₂ ice was detected in NIR spectra of these moons, with spectral properties remarkably similar to the second and third order combination and overtone bands of CO₂ ice (between 1.9 to 2.1 µm and 1.57 to 1.61 µm, respectively). The presence of these relatively weak CO₂ bands indicates that the detected CO₂ ice is retained in thick deposits (~1 - 100 mm) on the surfaces of these moons. However, over longer NIR wavelengths (~3 - 5 µm), the spectral signature of CO₂ ice is mostly absent from these moons. Photon penetration depths into H₂O-rich particulate surfaces are a steep function of wavelength, with NIR photons between 1 and 2.5 µm penetrating deeper (~0.15 - 10 mm depths) than NIR photons between 3 and 5 µm (~0.001 - 0.05 mm depths). Thus, we hypothesize that the regoliths of the classical Uranian satellites are compositionally stratified, with thick deposits of CO₂ ice retained beneath a veneer of tiny H₂O ice grains.

To test this hypothesis, we have collected data using the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope (spanning ~3 to 5 µm). We compared these new IRAC data to an IRAC dataset collected previously, as well as longer NIR spectra, spanning 3 to 4 µm. Analysis of these longer NIR datasets indicates that the Uranian moons are relatively bright over the 3 to 5 µm range, with enhanced 3.6-µm H₂O ice peaks. Best fit spectral models of the IRAC datasets are primarily composed of tiny H₂O ice grains (≤ 2 µm diameters), with no trace of CO₂ ice. Thus, our analyses support compositional stratification of these moons' regoliths. We explore some of the processes that could generate layered media on the Uranian moons and discuss why the surfaces of icy Jovian and Saturnian moons lack similar veneers of tiny H₂O ice grains. Additionally, we discuss how next generation telescopes and future spacecraft missions would revolutionize our understanding of the classical Uranian moons and ice giant planetary systems.