Charged particle bombardment– a dominant surface modification process on the Uranian moons?

Abstract

The Uranus system hosts a diverse set of large moons, from the enigmatic Miranda with its striking coronae, to the outermost and second-largest moon Oberon with its dark and ancient surface. These moons mostly orbit within Uranus’ magnetosphere, which while relatively depleted in low energy-plasma, was found by Voyager 2 to possess surprisingly intense electron radiation belts. Prominent energetic electron absorptions associated with the Uranian moons were observed by Voyager 2, indicating that these particles interact significantly with the moons. Thus, the surfaces of the moons are continuously bombarded by high energy electrons, which are capable of breaking chemical bonds in surface material, leading to radiolytic chemistry that can alter surface composition. In addition, charged particle bombardment can alter the microstructure of surface ice as well as affect grain sizes by sputtering. Ground-based remote sensing observations have revealed planetocentric and hemispherical trends in several key spectral parameters, including the abundance of CO2 ice, whose concentration on the trailing hemispheres of the moon hint at a possible radiolytic origin. Furthermore, possible signatures of NH3 have been detected on the surfaces of several of the moons, including Ariel, where the sub-observer longitudinal distribution of this species appears to support spatial association with geologic features and terrains. It is known from laboratory experiments that NH3 is readily decomposed by charged particle radiation. If NH3 or NH3-related compounds are present on the moons, it therefore could be an indication of recent emplacement or exposure. Here, we will present possible signatures of charged particle surface modification on the Uranian moons and discuss implications for observations by future missions to the Uranus system.