Investigating the Spatial Distribution of Surface Composition on Miranda

Various physical and chemical processes modify the surfaces of icy satellites orbiting giant planets like Uranus, including excavation and overturn of the regolith by impacts, interactions with the host planet’s magnetosphere, and mantling by circumplanetary dust populations. The strength of these processes varies with orbital distance from the planet and with longitude on the satellite, forming asymmetries in the surface composition between the leading and trailing hemispheres of tidally locked moons. In the Uranian system, the icy classical satellites show hemispherical asymmetries in the strength of their water ice absorption bands. Miranda, the smallest and innermost classical satellite of Uranus, should display the strongest asymmetry in water ice abundance, but previous observations are consistent with no hemispherical asymmetries in this moon’s composition (Cartwright et al. 2018, Icarus, 314; Cartwright et al. 2020, Icarus, 338, 113513). Additionally, spectral evidence for ammonia (NH₃)-bearing species on the Uranian moons is intriguing, as ammonia is an antifreeze compound, significantly lowering the freezing point of water. With a sufficient heat source, this can enable endogenic geologic activity, including cryovolcanism, on icy bodies in the outer Solar System. The 2.2 micron absorption feature commonly attributed to NH₃-bearing species has been detected on both Miranda (Bauer et al. 2002, Icarus, 158, 178) and Ariel (Cartwright et al. 2020, ApJL, 898), the two Uranian moons showing the strongest evidence of past geological activity. However, NH₃ compounds exposed on the surface of Miranda and Ariel should be dissociated by irradiation on geologically short timescales. Consequently, spectral hints of the 2.2 micron NH₃ band on these Uranian moons could imply that this constituent was exposed or emplaced in the geologically-recent past, providing a crucial avenue for investigating possible endogenic activity on the Uranian satellites.

We will present new ground-based near-infrared spectra of Miranda obtained with the TripleSpec near-infrared spectrograph on the ARC 3.5m telescope at Apache Point Observatory in fall 2019. We will report the relative abundances of water ice via band area measurements, as well as discuss evidence for the 2.2 micron NH₃ feature on Miranda. These observations are part of an ongoing project to investigate the composition and possible presence of volatile species on Miranda, with the intent of gaining a better understanding of the surface evolution of this moon, as well as other small icy satellites.