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.

Ariel's Elastic Thicknesses and Heat Fluxes

“…Ariel's surface can be separated into four broad geologic units: ancient, cratered plains that represent the oldest terrain on Ariel and are thought to be 1.3 (−0.6/+2.0) Ga (Kirchoff et al 2022), and three younger, "smooth materials" units that are subdivided based on their relative ages (Smith et al 1986;Jankowski & Squyres 1988; see Figure 2). The Pixie Group is mostly found within the "intermediate-age smooth materials" unit, and the Kachina Group is in the "cratered plains" unit.…”

“…The Pixie Group is mostly found within the "intermediate-age smooth materials" unit, and the Kachina Group is in the "cratered plains" unit. Arielʼs tectonized region near the south pole, including the area between the Kachina Group and Pixie Group, is estimated to be 0.8 (−0.5/+1.8) Ga based on impact crater densities (Kirchoff et al 2022). However, it is unknown what the difference in ages is between the Kachina Group and the Pixie Group.…”

“…These chasmata may have formed because of these events and were then subsequently modified to produce the observed flexure. Furthermore, investigation of impact crater distributions across Ariel's tectonized region in the area between Kachina Chasmata and the Pixie Group found a surface age of only 0.8 Ga (−0.5/+1.8 Ga) (Kirchoff et al 2022). The age of this younger tectonized region near the south pole may reflect the more recent 5:3 MMR between Ariel and Umbriel that is estimated to have occurred ∼1 Ga ago (e.g., Ćuk et al 2020).…”

Cupid Is Not Doomed Yet: On the Stability of the Inner Moons of Uranus

“…At the time of their study there were large uncertainties concerning the size distribution of heliocentric impactors in the outer Solar System. More recent results from the Pluto-Charon system established that the Zahnle et al (2003) "Case A" model impactor distribution largely matches the Kuiper Beltderived impactors (Singer et al 2019), while the steeper "Case B" size-distribution (seen at Neptune and Saturn) is likely to be planetocentric in origin (Kirchoff et al 2022). Crater size-distribution on Uranian moons generally match those on Charon (Kirchoff et al 2022), and are therefore likely to be heliocentric.…”

“…More recent results from the Pluto-Charon system established that the Zahnle et al (2003) "Case A" model impactor distribution largely matches the Kuiper Beltderived impactors (Singer et al 2019), while the steeper "Case B" size-distribution (seen at Neptune and Saturn) is likely to be planetocentric in origin (Kirchoff et al 2022). Crater size-distribution on Uranian moons generally match those on Charon (Kirchoff et al 2022), and are therefore likely to be heliocentric. We are specifically interested in a steady-state heliocentric impactor population here (as planetocentric bombardment is likely to be highly episodic), and we will assume it corresponds to the Zahnle et al (2003) "Case A" model population.…”