Triton
Neptune’s largest moon and the only major moon in the solar system with a retrograde orbit, strongly suggesting it is a captured Kuiper Belt object. Cryovolcanic plumes were observed by Voyager 2 during its 1989 flyby, and a possible subsurface ocean has been inferred from internal-heating models.
Key features
- Retrograde orbit consistent with capture origin.
- Voyager 2 (1989) imaged ~8 km tall dark plumes erupting from the south polar cap; nitrogen-driven cryovolcanism is the leading interpretation.
- Possible subsurface liquid-water ocean maintained by tidal-heating and radiogenic heating.
- 2026 NCCR PlanetS Legacy Book review (src-active-moons-review-2026-04) flags Triton as morphologically and geochemically distinct from the other active moons — “different in many ways” — and as raising “unexplored questions.”