iceshell

An ice shell, or iceshell, is a term used in planetary science to describe the outer solid crust of a body whose surface is dominated by water ice. On icy moons and dwarf planets, the ice shell forms the visible exterior, separating a possibly liquid interior from space. Thickness varies widely; estimates for Europa range from a few kilometers to tens of kilometers, depending on temperature, composition, and tidal heating. Other bodies show even thinner or thicker shells.

Formation and properties: Ice shells form as surface oceans freeze and as water ice is deposited from the interior. The ice is typically crystalline and contains impurities such as salts that affect its melting point and opacity. The mechanical behavior of the shell depends on temperature: the uppermost layer is brittle, while the shell can become ductile at greater depths or under higher temperatures.

Geophysics: Tidal forces from a parent planet can heat the interior, maintaining a subsurface ocean beneath the shell. Heat transport through conduction and convection in the ice shell drives deformation, creating features such as ridges, fractures, and chaos terrains. Evidence from missions studying objects like Enceladus and Europa suggests exchange between ocean and surface, including plumes and surface features shaped by upwelling ice.

Scientific importance: The ice shell is central to understanding the body's thermal evolution, potential habitability, and the prospect of liquid water beneath. Space missions aim to characterize shell thickness, composition, and dynamics, contributing to broader knowledge of icy worlds and their possible oceans.