Are NH3 and CO2 Ice Present on Miranda?

Abstract

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.