Distributions of H 2O and CO 2 ices on Ariel, Umbriel, Titania, and Oberon from IRTF/SpeX observations
We present 0.8-2.4 μm spectral observations of uranian satellites, obtained at IRTF/SpeX on 17 nights during 2001-2005. The spectra reveal for the first time the presence of CO 2 ice on the surfaces of Umbriel and Titania, by means of 3 narrow absorption bands near 2 μm. Several additional, weaker CO 2 ice absorptions have also been detected. No CO 2 absorption is seen in Oberon spectra, and the strengths of the CO 2 ice bands decline with planetocentric distance from Ariel through Titania. We use the CO 2 absorptions to map the longitudinal distribution of CO 2 ice on Ariel, Umbriel, and Titania, showing that it is most abundant on their trailing hemispheres. We also examine H 2O ice absorptions in the spectra, finding deeper H 2O bands on the leading hemispheres of Ariel, Umbriel, and Titania, but the opposite pattern on Oberon. Potential mechanisms to produce the observed longitudinal and planetocentric distributions of the two ices are considered.