Evidence for ammonia-bearing species on the Uranian satellite Ariel supports recent geologic activity

Abstract:

The Voyager 2 encounter with the Uranian system revealed that the surface of Ariel exhibits many geologic landforms with unusual morphologies. The morphologies and estimated flow rheologies of these features are consistent with emplacement of ammonia (NH₃) rich cryolavas on Ariel. NH₃ is an efficient antifreeze agent when mixed with liquid H₂O, allowing icy bodies to retain subsurface salty oceans for longer periods of time compared to “pure” H₂O oceans. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, the composition of these geologic features is unknown. Recent ground-based telescope observations have detected spectral hints of a 2.2 μm band on Ariel, which has been attributed to NH₃-bearing species on other icy bodies, including Pluto and its moons Charon, Nix, and Hydra. However, the spatial distribution and spectral signature of the 2.2-μm band on Ariel have not been previously investigated, limiting our ability to interpret its origin.

We analyzed 32 ground-based telescope spectra of Ariel, spanning a wide range of sub-observer longitudes and latitudes. We measured the area and depth of the 2.2-μm band, finding that ten spectra display prominent 2.2-μm features (> 2σ measurements). We found no discernable spatial trends in the distribution of the 2.2-μm band, unlike the distribution of CO₂ ice and H₂O ice, which display significant hemispherical asymmetries on Ariel. Furthermore, we compared the Ariel spectra displaying the strongest 2.2-μm bands to laboratory spectra of different NH₃-bearing species, finding that NH₃-H₂O solutions and NH₃-hydrates provide the best match to its spectral signature. We also found evidence for a 2.24-μm band in four spectra, with a spectral signature most consistent with NH₃ ice.

The possible presence of NH₃ is intriguing given that this constituent should be removed by energetic electrons (~1 MeV) over geologically short timescales. Consequently, the hints of NH₃ detected on Ariel might result from geologic activity in the fairly recent past (< 2 Ga, based on regional crater density estimates), perhaps including emplacement of NH₃-rich cryolavas. Similar connections have been made between the presence of NH₃ and possible cryovolcanic features on Pluto, suggesting that NH₃ might be a useful tracer of endogenic activity on some icy bodies.