Are ammonina-bearing species present on mid-sized Saturnian moons?

The surfaces of the mid-sized Saturnian moons Tethys, Dione, Rhea, and Iapetus are predominantly composed of H₂O ice. Dark and spectrally red material that likely originated on the retrograde irregular satellite Phoebe has mantled much of Iapetus’ leading hemisphere, which is dramatically darker (Vmag ~12.8) than its ice-rich trailing hemisphere (Vmag ~11.3). Lower albedo regions are also present on the trailing hemispheres of Tethys, Dione, and Rhea, likely resulting from charged particle bombardment of H₂O ice and other species on their surfaces. Along with H₂O ice, some near-infrared (NIR) spectra collected previously with ground-based telescopes hint at the presence of NH₃-bearing species on the surfaces of Tethys and Rhea via the detection of weak spectral features near 2.2 μm. Some NIR spectra of Dione and Iapetus, collected by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS), show a 2.97-μm band, which might also result from NH₃-bearing species. However, VIMS did not definitively detect a 2.2-μm band on any of Saturn’s moons, possibly because VIMS may not have had sufficient resolving power to detect this feature. Furthermore, the 2.97-μm band detected by VIMS falls on a spectral channel that is adjacent to a spectral sorting filter, confounding prior attempts to confirm the presence of this feature. The possible presence and nature of NH₃-bearing species on the mid-sized Saturnian moons has therefore been difficult to assess using VIMS data. To determine whether NH₃-bearing species are present on these moons, we collected new ground-based spectra using the SpeX spectrograph (~0.7 – 2.5 μm) on NASA’s Infrared Telescope Facility (IRTF) in 2020 and 2021. In total, we have collected six new spectra of Tethys, five of Dione, six of Rhea, and ten of Iapetus, spanning both their leading and trailing hemispheres. Many of these 27 SpeX spectra show hints of a weak 2.2-μm band on all four satellites, supporting prior detection of this feature (example spectra shown in the figure). We will present the 27 new SpeX spectra and the results of our ongoing analysis of the 2.2-μm band.