Icy worlds: Moons and Dwarf Planets

TL;DR

This paper assesses the state of knowledge on icy bodies in the outer Solar System, focusing on surfaces, interiors, and atmospheres and the evidence for subsurface oceans, cryovolcanism, and tenuous atmospheres. It integrates three decades of data from missions such as Voyager, Galileo, Cassini, New Horizons, Juno, and JWST with gravity, magnetic, and geological constraints, outlining where interior architectures are well established and where degeneracies persist. The work highlights key results for Europa, Ganymede, Callisto, Enceladus, Titan, Triton, and dwarf planets, and maps near-term and future missions (JUICE, Europa Clipper, Dragonfly, Uranus Orbiter and Probe) as critical to resolving remaining questions about habitability and planetary evolution. It also identifies gaps in chemistry, surface history, and interior–orbital coupling, advocating for continued laboratory experiments and advanced modeling to interpret forthcoming data.

Abstract

In the outer solar system beyond Jupiter, water ice is a dominant component of planetary bodies, and most solid objects in this region are classified as icy bodies. Icy bodies display a remarkable diversity of geological, geophysical, and atmospheric processes, which differ fundamentally from those of the rocky terrestrial planets. Evidence from past and ongoing spacecraft missions has revealed subsurface oceans, cryovolcanic activity, and tenuous but persistent atmospheres, showing that icy bodies are active and evolving worlds. At the same time, major questions remain unresolved, including the chemical properties of icy materials, the geological histories of their surfaces, and the coupling between internal evolution and orbital dynamics. Current knowledge of the surfaces, interiors, and atmospheres of the principal icy bodies is built on spacecraft measurements, telescopic observations, laboratory experiments, and theoretical modeling. Recent contributions from Juno, JWST, and stellar occultation studies have added valuable constraints on atmospheric composition, interior structure, and surface activity. Looking ahead, missions such as JUICE, Europa Clipper, Dragonfly, and the Uranus Orbiter and Probe are expected to deliver substantial progress in the study of icy bodies. Their findings, combined with continued Earth- and space-based observations and laboratory studies, will be critical for assessing the potential habitability of these environments and for placing them within a broader framework of planetary system formation and evolution.

Figures (9)

• Figure 1: Major icy bodies in the Solar System, with the Earth's moon to scale. The images are shown at the proper relative size. From NASA's Planetary Photojournal (https://photojournal.jpl.nasa.gov/), Earth's Moon (PIA00302), Europa (PIA00502), Ganymede (PIA00716), Callisto (PIA03456), Mimas (PIA06258), Enceladus (PIA06254), Tethys (PIA07738), Dione (PIA08256), Rhea (PIA08256), Titan (PIA08256), Hyperion (PIA07740), Iapetus (PIA08384), Phoebe (PIA06064), Miranda (PIA18185), Ariel (PIA01534), Umbriel (PIA00040), Titania (PIA01979), Oberon (PIA00034), Triton (PIA00317), Pluto (PIA19952).
• Figure 2: Radius-density relation of the icy bodies, Io and the terrestrial Moon.
• Figure 3: Inferred internal structures for selected icy bodies. Note that Ganymede, Callisto, and Titan are shown at their correct relative sizes, while Europa, Pluto and Enceladus are plotted 1.5, 2, and 10 times their actual sizes, respectively. Also see Figure 1 and Table 1 for a size comparison of each object.
• Figure 4: Four types of surface features on Europa. (a) double ridge feature (PIA00589), (b) Conamara chaos (PIA00591), (c) Thera (left) and Thrace (right) macula showing dark, reddish disrupted regions (PIA02099), and (d) Large ringed feature, Tyre (PIA01661).
• Figure 5: (Left) Grooved terrain on Ganymede. A mosaic of Galileo spacecraft high-resolution images of the Uruk Sulcus region is shown within the context of an image of the re-gion taken by Voyager 2 (PIA00281). (Right) Cratered terrain on Ganymede. Ancient impact craters shown in the image taken by Galileo spacecraft (PIA00279).