Europa is a medium-sized icy moon of Jupiter, with a differentiated interior—an icy shell over a global, salty ocean, above a rocky (silicate) mantle and likely an iron-rich core.
Closeup views of Europa obtained on 26 September 1998; images clockwise from upper left show locations from north to south as indicated at lower left - wikimedia 
Its young, bright surface is crisscrossed by dark linear features (“lineae”), bands, and double ridges, with expansive fields of jumbled “chaos terrain” where blocks of crust have broken, rotated, and refrozen. Tidal flexing by Jupiter continually stresses the shell, driving fractures, heat flow, and resurfacing at geologically recent timescales.
Geophysical and geological clues point to an active world: Galileo magnetometer data indicate a conductive layer consistent with a global ocean; surface chemistry shows radiolysis products (e.g., hydrogen peroxide/oxygen) made by charged-particle bombardment; and imaging reveals ridge-building and plate-like motions that may recycle ice.
Model of Europa's possible interior structure, with a thin ice crust and a subsurface ocean atop a rocky mantle and metallic core - wikimedia 
Occasional plume candidates have been reported over the same region, and more recent work finds persistent water vapour in one hemisphere—signs of ongoing exchange between the surface, atmosphere, and subsurface.
# Habitability There is **no direct evidence of life** on Europa, but several habitability factors are favorable. A salty ocean likely contacts a rocky seafloor, enabling water–rock interactions and potential hydrothermal chemistry. Oxidants made at the surface by radiation could be transported downward through fractures, providing chemical energy to an ocean biosphere.
Open questions include ocean depth and salinity, ice-shell thickness and permeability, whether plume activity is sustained, and how efficiently surface oxidants reach the ocean — key targets for Europa Clipper’s flybys (and complementary JUICE to Jupiter data).
# Sources
- Galileo magnetometer: induction signature of a **global salty ocean** — science.org 
- **Europa geology**: ridges, bands, chaos terrain (NASA overview) — science.nasa.gov
- **Hydrogen peroxide** detected on the surface (radiolysis product) — science.nasa.gov
- **Plume candidates** from Hubble transit imaging — arxiv.org ; NASA release — nasa.gov
- **Persistent water vapour** (one hemisphere) — esahubble.org ; NASA explainer — science.nasa.gov