A New Look at the Surface Composition of Miranda

The smallest and innermost of the five major moons of Uranus, Miranda is an enigmatic icy body with a surface heavily modified by past geological activity. The surface compositions of the Uranian satellites can be studied with near-infrared spectroscopy of reflected sunlight. I acquired numerous new, high quality near-infrared spectra of Miranda with the 3.5-meter telescope at Apache Point Observatory and with the 8.1-meter Gemini North telescope. I also included previously published and unpublished telescopic spectra of Miranda from other authors.I used this new comprehensive dataset to measure the strength of the near-infrared water ice absorption bands and how they vary with longitude across the surface of Miranda. I found that contrary to the other Uranian satellites, Miranda does not show a leading/trailing hemispherical asymmetry in the strength of the water ice absorption bands. I instead found an unusual asymmetry in the strength of the water ice bands between the anti-Uranus and sub-Uranus hemispheres, which is not seen on the other Uranian satellites. While there are several possibilities for this signature, I suggest that it may be due to a reorientation of Miranda's polar axis (true polar wander) at some time in the geologic past.I also used the same dataset to investigate spectral features from compounds other than water ice. While the other Uranian moons show spectral features from carbon dioxide ice, I found no evidence for discrete carbon dioxide ice deposits on Miranda. I did find robust evidence for an absorption band at 2.2 micrometers, which could be associated with ammonia-bearing compounds. Ammonia is cosmically abundant and an efficient antifreeze, and could potentially enable internal activity on icy worlds. However, ammonia is dissociated by radiation on short geological timescales; a detection of ammonia on the surface of Miranda implies relatively recent emplacement or exposure. I concluded that the 2.2-micrometer band on Miranda is best matched by a combination of ammonia ice with ammonia hydrates or ammonia-water mixtures. Ammonium-bearing salts like ammonium chloride are also promising candidates that warrant future investigation.