A Future Mission to Uranus: Exploration of Five Possible Ocean Worlds and a Bevy of Small Icy Moons

Exploration of the Uranian system is one of the top priorities recommended by the Planetary Science and Astrobiology Decadal Survey (2023 – 2032). Voyager 2, the only spacecraft to visit the Uranian system, collected spatially resolved images of Uranus’ largest ring moon Puck and its five large “classical” moons, Miranda, Ariel, Umbriel, Titania, and Oberon. The surfaces of the classical moons display evidence for endogenic resurfacing, in particular Miranda and Ariel. However, Voyager 2 only imaged the southern hemispheres of the classical moons during its flyby in 1986. A new mission to Uranus is critical for understanding the geology, surface chemistry, and internal structure of these moons, as well as Uranus’ thirteen ring moons and nine known irregular satellites.

An orbiter making close passes of Uranus’ classical moons could use visible and mid-infrared cameras and a near-infrared mapping spectrometer to characterize the geology and composition of young regions on their surfaces. These data could be used to determine whether geologic activity is ongoing. An orbiter equipped with a magnetometer could determine whether the classical moons contain salty liquid water layers, making them ocean worlds. A plasma spectrometer and energetic particle detector on an orbiter could determine the nature and extent of charged particle weathering on Uranus’ ring moons and classical moons. Furthermore, a dust analyzer could collect ring particles and material ejected from the surfaces of Uranus’ moons to determine their molecular compositions. A spacecraft, inbound to orbital insertion, could also make a close pass of an irregular satellite, representing an unparalleled opportunity to study one of these poorly understood objects.

Disk-integrated spectra collected with telescopes have determined that the surfaces of the classical moons are composed of H₂O ice, carbon-bearing species including CO₂ ice, and possibly nitrogen-bearing constituents like NH₃, which could be a tracer of geologically recent activity. Although suspected to be present, the nature of organics in the Uranian system is essentially unknown. A near-infrared mapping spectrometer on an orbiter could measure the composition and distribution of surface deposits rich in organics and N-bearing constituents to determine whether they originated in the interiors of Uranus’ moons and are therefore chemical indicators of their internal states. Furthermore, upcoming observations by the James Webb Space Telescope will provide new knowledge on the surface compositions of Uranus’ classical moons in preparation for a future mission.