Are NH3 and CO2 Ice Present on Miranda?

Published near-IR spectra of the four largest classical Uranian satellites display the presence of discrete deposits of CO₂ ice, along with subtle absorption features around 2.2 μm. The two innermost satellites, Miranda and Ariel, also possess surfaces heavily modified by past endogenic activity. Previous observations of the smallest satellite, Miranda, have not detected the presence of CO₂ ice, and a report of an absorption feature at 2.2 μm has not been confirmed. An absorption feature at 2.2 μm could result from exposed or emplaced NH₃- or NH₄-bearing species, which have a limited lifetime on Miranda's surface, and therefore may imply that Miranda's internal activity was relatively recent. In this work, we analyzed near-IR spectra of Miranda to determine whether CO₂ ice and the 2.2 μm feature are present. We measured the band area and depth of the CO₂ ice triplet (1.966, 2.012, and 2.070 μm), a weak 2.13 μm band attributed to CO₂ ice mixed with H₂O ice, and the 2.2 μm band. We confirmed a prior detection of a 2.2 μm band on Miranda, but we found no evidence for CO₂ ice, either as discrete deposits or mixed with H₂O ice. We compared a high signal-to-noise-ratio spectrum of Miranda to synthetic and laboratory spectra of various candidate compounds to shed light on what species may be responsible for the 2.2 μm band. We conclude that the 2.2 μm absorption is best matched by a combination of NH₃ ice with NH₃ hydrates or NH₃-H₂O mixtures. NH₄-bearing salts like NH₄Cl are also promising candidates that warrant further investigation.