Longitudinal Variation of H$_2$O Ice Absorption on Miranda

TL;DR

This study investigates the longitudinal variation of water ice absorption on Miranda, finding no leading/trailing asymmetry but revealing an unexpected anti-Uranus pattern, indicating complex surface processes.

Contribution

It provides the first extensive spectral analysis of Miranda's H₂O ice distribution, uncovering unique asymmetry patterns not observed on other Uranian satellites.

Findings

No significant leading/trailing asymmetry in H₂O ice absorption on Miranda.

Evidence of an anti-Uranus/sub-Uranus asymmetry in ice band strength.

Additional processes likely influence Miranda's surface ice distribution.

Abstract

Many tidally locked icy satellites in the outer Solar System show leading/trailing hemispherical asymmetries in the strength of near-infrared (NIR) H$_2$O ice absorption bands, in which the absorption bands are stronger on the leading hemisphere. This is often attributed to a combination of magnetospheric irradiation effects and impact gardening, which can modify grain size, expose fresh ice, and produce dark contaminating compounds that reduce the strength of absorption features. Previous research identified this leading/trailing asymmetry on the four largest classical Uranian satellites but did not find a clear leading/trailing asymmetry on Miranda, the smallest and innermost classical moon. We undertook an extensive observational campaign to investigate variations of the NIR spectral signature of H$_2$O ice with longitude on Miranda's northern hemisphere. We acquired 22 new spectra with the TripleSpec spectrograph on the ARC 3.5m telescope and 4 new spectra with GNIRS on Gemini North. Our analysis also includes 3 unpublished and 7 previously published spectra taken with SpeX on the 3m IRTF. We confirm that Miranda has no substantial leading/trailing hemispherical asymmetry in the strength of its H$_2$O ice absorption features. We additionally find evidence for an anti-Uranus/sub-Uranus asymmetry in the strength of the 1.5-$\mu$m H$_2$O ice band that is not seen on the other Uranian satellites, suggesting that additional endogenic or exogenic processes influence the longitudinal distribution of H$_2$O ice band strengths on Miranda.