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

Abstract

Published near-infrared 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 $\mu$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 $\mu$m has not been confirmed. An absorption feature at 2.2 $\mu$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-infrared spectra of Miranda to determine whether CO${}_2$ ice and the 2.2-$\mu$m feature are present. We measured the band area and depth of the CO${}_2$ ice triplet (1.966, 2.012, and 2.070 $\mu$m), a weak 2.13-$\mu$m band attributed to CO${}_2$ ice mixed with H${}_2$O ice, and the 2.2-$\mu$m band. We confirmed a prior detection of a 2.2-$\mu$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 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-$\mu$m band. We conclude that the 2.2-$\mu$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.

29 pages, 10 figures. Accepted for publication in Planetary Science Journal

References in corpus (8)

• Array Programming with NumPy
• Infrared Spectra and Optical Constants of Astronomical Ices: II. Ethane and Ethylene
• Water Ice on the Satellite of Kuiper Belt Object 2003 EL61
• Distribution of CO2 ice on the large moons of Uranus and evidence for compositional stratification of their near-surfaces
• Evidence for ammonia-bearing species on the Uranian satellite Ariel supports recent geologic activity
• Composition, Mineralogy, and Porosity of Multiple Asteroid Systems from Visible and Near-infrared Spectral Data
• Probing the regoliths of the classical Uranian satellites: Are their surfaces mantled by a layer of tiny H2O ice grains?
• Co-accretion + giant impact origin of the Uranus system: Tilting Impact