Volatile Transport on Ariel and Implications for the Origin and Distribution of Carbon Dioxide on Uranian Moons

Abstract

The moons of Uranus have only been visited once by Voyager 2 during its 1986 flyby. Earth-based telescopic observations show a spectral signature of carbon dioxide ice on the Uranian moons Ariel, Umbriel, Titania, and Oberon, with a somewhat higher abundance on their trailing hemispheres. The inner major moon Ariel exhibits the strongest carbon dioxide ice absorption bands, which then decrease in strength with increasing orbital distance from Uranus, with the outer major moon Oberon exhibiting the weakest absorption bands. Previous work has suggested that these hemispherical and radial trends result from radiolytic production of carbon dioxide ice from interactions between the moons' surfaces and charged particles trapped in Uranus' magnetosphere. Here, we use volatile transport modeling to characterize a possible migration cycle of carbon dioxide on Ariel. We find that carbon dioxide is readily mobilized toward Ariel's equator, and that existing topography such as canyons are locations of favorable deposition for carbon dioxide ice. We predict the presence of carbon dioxide ice deposits on the floors of Ariel's canyons. Our work suggests two possible classes of sources of carbon dioxide: an active source, which may be consistent with either radiolytic production from Uranus' magnetosphere or outgassing from Ariel's interior, or an ancient source that produced CO₂ that still exists in stable canyon deposits. A future Uranus orbiter could determine which hypothesis is most likely, or if carbon dioxide could be found both in the form of ice deposits on the surface and in a global exosphere.