Red material on the large moons of Uranus: Dust from irregular satellites?

TL;DR

This study investigates the origin of red material on Uranus's large moons, proposing that dust from irregular satellites causes observed surface reddening and compositional trends, supported by new spectral data and modeling.

Contribution

The paper presents new spectral observations of Uranus's moons' northern hemispheres and supports the hypothesis that irregular satellite dust causes surface reddening and compositional variations.

Findings

Red material is predominantly on southern hemispheres, but trends are consistent across hemispheres.

Spectral data suggest complex organics and amorphous pyroxene as reddening agents.

New spectra in 2.9-4.1 microns range provide novel insights into moon surface composition.

Abstract

The large and tidally-locked classical moons of Uranus display longitudinal and planetocentric trends in their surface compositions. Spectrally red material has been detected primarily on the leading hemispheres of the outer moons, Titania and Oberon. Furthermore, detected H2O ice bands are stronger on the leading hemispheres of the classical satellites, and the leading/trailing asymmetry in H2O ice band strengths decreases with distance from Uranus. We hypothesize that the observed distribution of red material and trends in H2O ice band strengths results from infalling dust from Uranian irregular satellites. These dust particles migrate inward on slowly decaying orbits, eventually reaching the classical satellite zone, where they collide primarily with the outer moons. The latitudinal distribution of dust swept up by these moons should be fairly even across their southern and northern hemispheres. However, red material has only been detected over the southern hemispheres of these moons (subsolar latitude 81 S). Consequently, to test whether irregular satellite dust impacts drive the observed enhancement in reddening, we have gathered new ground-based data of the now observable northern hemispheres of these moons (sub-observer latitudes, 17 to 35 N). Our results and analyses indicate that longitudinal and planetocentric trends in reddening and H2O ice band strengths are broadly consistent across both southern and northern latitudes of these moons, thereby supporting our hypothesis. Utilizing a suite of numerical best fit models, we investigate the composition of the reddening agent detected on these moons, finding that both complex organics and amorphous pyroxene match the spectral slopes of our data. We also present spectra that span 2.9 to 4.1 microns, a previously unexplored wavelength range in terms of spectroscopy for the Uranian moons.