Crater Distributions of Uranus's Mid-sized Satellites and Implications for Outer Solar System Bombardment

Abstract Outer solar system impact bombardment is largely unconstrained. Although recent data from the Jupiter, Saturn, and Pluto systems have produced new constraints, analysis is incomplete without inclusion of the Uranus system. We reanalyze Uranus system crater populations with recent improvements in processing of Voyager 2 imaging. No consensus in crater populations on mid-sized Uranian satellites, Miranda, Ariel, Umbriel, Titania, and Oberon, was resolved during the Voyager era. For satellites with available data, we find variability in crater size–frequency distributions (SFDs) for diameters (D) < 10 km. Most terrains on Miranda show a shallower slope (ratio of smaller to larger craters is smaller), while Inverness Corona on Miranda and Ariel's terrains show a steeper slope (ratio increases). For D > 10 km, satellites with available data show a steeper slope. Shallower-sloped SFDs for D < 10 km and steeper slopes for D > 10 km agree with Pluto system data—a proxy for the heliocentric impactor population originating from the Kuiper Belt—implying these SFDs represent heliocentric bombardment in the Uranus system. The shallow-sloped population for smaller diameters is also observed on Jovian satellites, but not on mid-sized, heavily cratered Saturnian satellites or Triton (Neptune), which have steeper slopes. This implies the heliocentric impactor population originating from the Kuiper Belt reaches throughout the outer solar system, but that the Saturnian, Neptunian, and maybe Uranian systems also might have their own planet-specific impactors. Finally, we find Ariel appears overall younger than the other Uranian satellites, supporting relatively recent geologic activity.