  We investigated whether ammonia-rich constituents are present on the surface
of the Uranian moon Ariel by analyzing 32 near-infrared reflectance spectra
collected over a wide range of sub-observer longitudes and latitudes. We
measured the band areas and depths of a 2.2-{\micron} feature in these spectra,
which has been attributed to ammonia-bearing species on other icy bodies. Ten
spectra display prominent 2.2-{\micron} features with band areas and depths >
2{\sigma}. We determined the longitudinal distribution of the 2.2-{\micron}
band, finding no statistically meaningful differences between Ariel's leading
and trailing hemispheres, indicating that this band is distributed across
Ariel's surface. We compared the band centers and shapes of the five Ariel
spectra displaying the strongest 2.2-{\micron} bands to laboratory spectra of
various ammonia-bearing and ammonium-bearing species, finding that the spectral
signatures of the Ariel spectra are best matched by ammonia-hydrates and flash
frozen ammonia-water solutions. Our analysis also revealed that four Ariel
spectra display 2.24-{\micron} bands (> 2{\sigma} band areas and depths), with
band centers and shapes that are best matched by ammonia ice. Because ammonia
should be efficiently removed over short timescales by ultraviolet photons,
cosmic rays, and charged particles trapped in Uranus' magnetosphere, the
possible presence of this constituent supports geologic activity in the recent
past, such as emplacement of ammonia-rich cryolavas and exposure of
ammonia-rich deposits by tectonism, impact events, and mass wasting.

Earth and Planetary Astrophysics