The Longitudinal Distribution of Water Ice on Miranda

Various processes modify the surfaces of icy satellites orbiting giant planets, including excavation and overturn of their regoliths by impacts, interactions with the host planet’s magnetosphere, and mantling by circumplanetary dust populations. In the Uranian system, the strength of these processes likely varies with distance from Uranus and with sub-observer longitude, possibly explaining the observed asymmetries in water ice absorption bands on the leading and trailing hemispheres of its four largest “classical” moons Ariel, Umbriel, Titania, and Oberon. In contrast, Miranda, the smallest and innermost classical moon, does not appear to display a leading/trailing asymmetry in the strength of its water ice bands. This raises the question: what processes are controlling the surface composition of Miranda?

To further investigate Miranda’s surface composition, we carried out an observing campaign in 2019-2020 to collect near-infrared (NIR) spectra of its surface with the ARC 3.5m telescope and the TripleSpec instrument. This campaign achieved comprehensive longitudinal coverage of Miranda’s northern hemisphere, collecting a total of 20 new NIR spectra, supplemented by two new NIR spectra acquired with GNIRS on the Gemini North 8.1m telescope. This effort has brought the longitudinal coverage of NIR spectra of Miranda up to par with that of the other classical Uranian satellites.

Our results indicate that the ratio of integrated water ice absorption band areas and depths between the leading and trailing hemispheres of Miranda is significantly lower than that on the other Uranian satellites, suggesting minimal asymmetries in surface composition between these two hemispheres. However, we instead found a statistically significant asymmetry between the anti-Uranus and sub-Uranus quadrants of Miranda’s surface. We will present the results of our observing campaign, focusing on the longitudinal distribution of water ice on Miranda. Furthermore, we will discuss the implications of these results in the context of Miranda, the Uranian system, and the evolution of other small icy satellites elsewhere in the Solar System. This work was supported by NASA FINESST grant NSSC20K1378.