Neptune has at least eight moons. Two of the moons, Triton and Neried are fairly large while the other six (Naiad, Thalassa, Despina, Galatea, Larissa and Proteus) are small, dark worlds we know little about. These small worlds orbit among the rings, sometime interacting with them and changing their appearance.

     Nereid is 1068 km in diameter and orbits Neptune in a large elliptical orbit. It takes 360 days for Neried to make one trip around Neptune.

    Triton is, by far, the strangest moon in the entire solar system. It is the largest of Neptune's moons with a diameter of 8495 km, about 78 percent the size of our moon. Triton orbits Neptune with a period of 5.8 days, but in retrograde motion, the opposite direction to most other moons! The retrograde rotation hints that some time in the past, the system was disturbed, the result being the strange orbit.

 Triton is a very cold world with a temperature of 37 K, so cold, that it is able to maintain an atmosphere of nitrogen and methane. This atmosphere though, is only a fraction as dense as Earth's atmosphere.

     A likely model for Triton is a rocky interior and an icy crust. The northern pole of Triton that has been turned away from the Sun for the last 30 years showed deposits of nitrogen frost which appear to be vaporizing and refreezing.

     It would also appear that Triton has, and may still be, an active moon. There are few craters on the surface, but there are long faults that appear to have formed when the icy crust broke. This suggests that whatever impact broke the crust was covered up. There are also large basins that appear to have been repeatedly refilled from an inflow of slushy material that probably came from under the crust.

  The most interesting evidence of activity on Triton are dark smudges seen near the Sun exposed northern pole. These are deposits produced when sunlight warms liquid nitrogen in Triton's crust causing it to erupt through vents in a nitrogen volcano! Methane in the gas undergoes chemical reactions, turning it into dark deposits that appear as the smudges we see.

     It appears that the volcanic eruptions are about 1 million years old, but that is recent enough that they may still be active. The energy source for the volcanos may be radioactive decay in what rocky material Triton has. Further studies of Triton would be fascinating indeed!