The Moons of

Neptune has two large moons that are easily seen from Earth, Triton and Nereid. Voyager 2 discovered six additional moons. One of these is actually larger than Nereid, but could not be seen easily from Earth because it orbits close to Neptune.


The image on the right (Ref) shows a portion of the southern region of Triton, Neptune's largest moon, taken in 1989 by Voyager. Triton is comparable in size with our own moon, and has a thin atmosphere, mostly of nitrogen. The polar ice cap in this image is probably mostly nitrogen ice. Triton orbits Neptune with retrograde motion, which probably means that it is a captured object.

Voyager 2 found some of the most varied terrain in the Solar System, a thin atmosphere, and even evidence for ice volcanoes on Triton. The dark streaks seen in this image are material spread downwind from recent volcanic eruptions. Giant faults cross the surface of Triton. The favored mechanism for the volcanoes is that the Sun heats darkened methane ice on the surface, which heats underlying nitrogen ice that vents through the surface volcanoes (more info).

The New Moons of Neptune

The six newly-discovered moons orbit with direct motion nearly in the equatorial plane. Most are closer to Neptune than its rings. Because this lies inside the Roche limit, these moons could not have formed by accretion in their present location. They must have formed elsewhere before coming to their present orbits, though we are not certain where.