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Published near-infrared spectra of the four largest classical Uraniansatellites display the presence of discrete deposits of CO$_2$ ice, along withsubtle absorption features around 2.2 $\mu$m. The two innermost satellites,Miranda and Ariel, also possess surfaces heavily modified by past endogenicactivity. Previous observations of the smallest satellite, Miranda, have notdetected the presence of CO$_2$ ice, and a report of an absorption feature at2.2 $\mu$m has not been confirmed. An absorption feature at 2.2 $\mu$m couldresult from exposed or emplaced NH$_3$- or NH$_4$-bearing species, which have alimited lifetime on Miranda's surface, and therefore may imply that Miranda'sinternal activity was relatively recent. In this work, we analyzednear-infrared spectra of Miranda to determine whether CO$_2$ ice and the2.2-$\mu$m feature are present. We measured the band area and depth of theCO$_2$ ice triplet (1.966, 2.012, and 2.070 $\mu$m), a weak 2.13-$\mu$m bandattributed to CO$_2$ ice mixed with H$_2$O ice, and the 2.2-$\mu$m band. Weconfirmed a prior detection of a 2.2-$\mu$m band on Miranda, but we found noevidence for CO$_2$ ice, either as discrete deposits or mixed with H$_2$O ice.We compared a high signal-to-noise spectrum of Miranda to synthetic andlaboratory spectra of various candidate compounds to shed light on what speciesmay be responsible for the 2.2-$\mu$m band. We conclude that the 2.2-$\mu$mabsorption is best matched by a combination of NH$_3$ ice with NH$_3$-hydratesor NH$_3$-H$_2$O mixtures. NH$_4$-bearing salts like NH$_4$Cl are alsopromising candidates that warrant further investigation.

Are NH$_3$ and CO$_2$ ice present on Miranda?