= Solution
Io's volcanic structures and young resurfaced regions indicate strong ongoing geological activity, despite a low sunlight-set <equilibrium> <temperature>. <Tidal dissipation> forced by its orbital interactions supplies interior <energy>. Europa's fractured relatively lightly cratered ice indicates resurfacing and a mechanically active ice shell on geological timescales, with <tidal heating> a plausible contributor; surface appearance alone does not prove every fracture is currently opening. Ganymede has both older cratered regions and grooved younger terrain, indicating a more varied resurfacing history; primordial/radiogenic <heat> and past tides can matter. Callisto's heavily cratered surface suggests much less extensive recent resurfacing, without proving that all interior activity or subsurface liquid is absent.
The comparison is between surface ages and possible <energy> sources, not simply “warmer means active.” Surface irradiation is too cold to explain silicate volcanism, while internal <heat>, tidal <work> and material-dependent melting <Temperature>[temperatures] can sustain processes below a cold surface. The NASA images in the source are evidence to interpret; no exam illustration is reproduced.
Back to article page
