Icy moons like Ganymede and Callisto often get overshadowed by their flashier rocky counterparts. But these worlds, cloaked in thick shells of water ice, hold deep clues about planetary formation, subsurface oceans, and even the conditions for life beyond Earth.
Beneath their frozen surfaces lie complex internal structures that challenge our assumptions about what icy worlds can be—and what they might become. Understanding these structures isn’t just about curiosity; it’s about building the frameworks we’ll use to model planetary systems across the solar system and assess their potential for habitability.
Craters as Probes
One of the most powerful tools in this exploration isn’t a drill—it’s a crater. Impact features serve as natural experiments, allowing scientists to infer the thickness and behavior of the ice shell. In a recent study I co-authored, Bjonnes et al. (2022), we analyzed impact craters across Ganymede and Callisto to estimate how their ice shells respond to stress and depth.
The findings suggest that Ganymede’s outer shell is stronger and thinner than previously thought, potentially overlaying a more ductile, convecting interior. Callisto, in contrast, may possess a thicker and more brittle shell, hinting at a different thermal and geologic history. These variations tell us that even among similar-sized moons with similar surface compositions, internal dynamics can diverge dramatically.
Layers and Oceans
Understanding the mechanical layering of these moons isn’t just about mapping the crust—it’s essential for modeling how heat moves, how potential subsurface oceans are maintained, and how surface features evolve. In Ganymede’s case, the structure supports long-term hypotheses of an active, salty ocean beneath the ice, which might interface with a rocky mantle, creating potential for chemical exchange—a key prerequisite for life.
These internal variations aren’t just theoretical. They shape everything from surface cracking patterns to how a future lander might drill or anchor. They even affect magnetic field interactions with Jupiter, which modulate radiation environments and surface chemistry.
Strategic Implications
Why does this matter beyond the outer solar system? Because Ganymede and Callisto serve as analogs for exoplanets and icy bodies beyond our observational reach. When we refine our models on these moons, we’re also improving our capacity to predict and interpret distant systems.
Moreover, understanding how layered structures form and evolve under ice can inform mission design for Europa Clipper, JUICE, and potential future missions to Enceladus and Titan. The engineering and scientific lessons extend from orbital dynamics to surface operations.
🔍 Further Reading
NASA – Europa Clipper Mission Overview
