A New Look at the Surface Composition of Miranda

Abstract

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 I also included previously published and unpublished telescopic spectra of Miranda from other authors.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.