The first four moons of Neptune, Naiad, Thalassa, Despina, and Galatea, are so close to Neptune that they orbit within its ring system. Little is known about them.

The next one out, Larissa, was actually discovered in 1981, when it blocked a star. This was attributed to the ring arcs, but later was found to be the moon, being re-discovered by Voyager 2 spacecraft in 1989.

Proteus is the second-largest moon in orbit around Neptune. It is so close to the planet that Earth-bound telescopes cannot see it.

From July to September 1989, Voyager 2 spacecraft discovered six new Neptunian moons. Of these, the irregularly shaped Proteus is notable for being as large as a body of its density can be , without being pulled into a spherical shape by its own gravity.

Although the second-most massive Neptunian moon, it is only one-quarter of one percent the mass of Triton.

Neptune’s innermost four moons—Naiad, Thalassa, Despina and Galatea—orbit close enough to be within Neptune’s rings. Five new irregular moons discovered between 2002 and 2003 were announced in 2004. As Neptune was the Roman god of the sea, the planet’s moons have been named after lesser sea gods.

Triton is next (right), and is one of the strangest moons in the solar system. First, it is one of only three moons in the solar system that has an atmosphere (Jupiter’s Io and Saturn’s Titan are the other two). It is thicker than Io’s, yet much thinner than Titan’s. Its pressure is 1/100,000 of the Earth’s.

Triton did not have an official name until the twentieth century. The name "Triton” was suggested by Camille Flammarion in his 1880 book Astronomie Populaire, but it did not come into common use until at least the 1930s. Until this time it was usually simply known as "the satellite of Neptune”.

Second, Triton has a retrograde orbit, which means that it orbits the opposite way the planet spins. This is said to be a very strong indication that Triton may have been captured according to scientists. This in itself is not strange; both of Mars’ moons were captured.

What is said to be strange is that Triton is two-thirds the size of our moon. When two bodies have a close encounter, one does not automatically capture the other, especially if it is so big. One theory is that Triton must have actually hit Neptune, bounced off the atmosphere, and gone into orbit because it lost all of its momentum.

Another way this could have happened is that Triton collided with one of Neptune’s moons, smashed it to bits (possibly creating the rings), and lost so much momentum that it couldn’t escape Neptune’s gravity.

Triton’s icy surface reflects so much of what little sunlight reaches it that the moon is one of the coldest objects in the solar system, about -240 C (-400 F).

Third, it is only 38 °C (100 °F) above absolute zero (the temperature at which all matter comes to rest). In such frigid a climate, scientists did not expect to find active geysers (springs). But, they did. They spew out a gaseous form of nitrogen, which is what creates its atmosphere.

The eighth moon, Nereid, has a highly elliptical orbit that causes it to swing around Neptune at various distances. When closest, it is 1,342,530 km (834,210 miles) from the planet. At the farthest distance, it is 9,667,120 km (6,006,870 miles) from Neptune.

The last five moons were discovered in the first few weeks of and throughout 2003. They have not yet been given official names by the International Astronomical Union.

Very little is yet known about them. Neptune has 13 known moons. They are composed of rock and ice. Voyager 2 space craft discovered 6 of the moons not observable from Earth. Very recently, three more moons, S/2002 N1-3, were discovered from a ground-based telescope in 2002.

Seven of these icy bodies are small and irregularly shaped. But one, Triton, is quite large and has a diameter of 2,705 km. Unlike other dark moons, it reflects around 80% of the sunlight it receives. Discovered in 1846 by William Lassell, it is one of the most intriguing worlds in the Solar System.
 
Neptune’s Moon Triton

Triton is said to be having a lot of geological activity; Voyager 2 space craft witnessed hot springs erupting nitrogen gas up to 8 km above the surface. The springs are thought to be the result of a sort of Greenhouse Effect.

A layer of cloudy nitrogen ice coats the surface trapping sunlight and allowing nitrogen at the base to melt. If this liquid reaches the surface, it explodes upwards as a spring in the low-pressure environment.

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