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 CO _2 ice, along with subtle absorption features around 2.2 μ m. 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 CO _2 ice, and a report of an absorption feature at 2.2 μ m has not been confirmed. An absorption feature at 2.2 μ m could result from exposed or emplaced NH _3 - or NH _4 -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 CO _2 ice and the 2.2 μ m feature are present. We measured the band area and depth of the CO _2 ice triplet (1.966, 2.012, and 2.070 μ m), a weak 2.13 μ m band attributed to CO _2 ice mixed with H _2 O ice, and the 2.2 μ m band. We confirmed a prior detection of a 2.2 μ m band on Miranda, but we found no evidence for CO _2 ice, either as discrete deposits or mixed with H _2 O 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 μ m band. We conclude that the 2.2 μ m absorption is best matched by a combination of NH _3 ice with NH _3 hydrates or NH _3 –H _2 O mixtures. NH _4 -bearing salts like NH _4 Cl are also promising candidates that warrant further investigation.

Keywords

• Planetary surfaces
• Surface composition
• Surface ices
• Surface processes
• Uranian satellites