An absorbing species likely to be ammonia or an ammoniated compound (e.g., a hydrate, salt, or mineral) detected in the near-infrared reflectance spectra of some regions on Pluto is associated with geologic structures that appear consistent with effusive cryovolcanic activity (1-3). West of Sputnik Planitia, water ice carrying a red-brown pigment is prominently exposed in and around Virgil Fossae (1,2) and in Uncama Fossa and adjacent Hardie crater (3) in patterns suggestive of the effusion of a fluid that has filled some topography by flow and may have covered adjacent regions in episodes of fountaining. It has been proposed that a cryofluid consisting of water, a pigment that may consist of complex organic material, and an ammoniated substance has been ejected from a subsurface reservoir of unknown depth, volume, and lateral extent (4). Attention was drawn to the two regions cited above by their strong H2O ice absorption bands detected in the New Horizons LEISA maps of the encounter hemisphere of Pluto and their strong coloration, leading to the subsequent detection of the associated ammonia absorption band near 2.2 μm. Other regions where the H2O ice absorption is strong are not strongly colored, notably Kiladze crater (lat 28.4°, long 212.9°), situated east of Sputnik Planitia. Using a statistical clustering technique (1) in LEISA spectra of Kiladze and surroundings, we have detected the signature of the ammoniated material, although it is less prominent than in Virgil Fossae and Uncama Fossa. Competing absorption bands of CH4 complicate extraction of the ammonia signal, but it is present at a high level of confidence. The geologic structure of Kiladze crater and surroundings is not strongly suggestive of cryovolcanic activity, and the presence of H2O ice with ammoniated material, but without the strong color, challenges the cryovolcanic interpretation of the other exposures. The wide geographic separation of Kiladze from the features west of Sputnik Planitia could, however, indicate differences in the composition of putative subsurface source reservoirs.
1. Dalle Ore, C. M. et al. 2019 Sci. Adv. 5, 29 May.
2. Cruikshank, D. P. et al. 2019 Icarus 330, 155.
3. Cruikshank, D. P. et al. 2021 Icarus 356 (113786)
4. Cruikshank, D. P. et al. 2019 Astrobiology 19 (7)