Io
Jupiter’s innermost Galilean moon and the most volcanically active body in the solar system. Unique among the active moons as a rocky volcanic world rather than an icy ocean world; not an ocean world candidate but the canonical case study for tidal-heating-driven activity.
Key features
- Plumes reaching hundreds of kilometres in altitude, driven by tidal-heating from orbital resonance with Europa and Ganymede.
- Sulfur-rich silicate volcanism produces an SO₂-dominated tenuous atmosphere.
- The 2026 NCCR PlanetS Legacy Book review (src-active-moons-review-2026-04) groups Io with Europa, enceladus, and triton as the four active solar-system moons whose outgassing signatures inform exomoon detection strategies.
Astrobiology relevance
Io itself is hostile to life as we know it (no liquid water, intense radiation), but it is foundational to two threads in this vault: (1) tidal-heating physics that keep Europa and enceladus habitable, and (2) Io-like volcanic outgassing as a potentially detectable exomoon signature from ground-based telescopes.