Triton and the Other Moons of Neptune

Neptune has two moons that can easily be seen from Earth with the aid of a telescope. One of those moons is one of the few moons in our Solar System with an atmosphere, Triton. The other moon Nereid is smaller, but still visible in a large enough telescope. Proteus is actually larger than Nereid, but not as easily seen as it does not reflect as much light.

Triton with Neptune in the Background

Image Credit:

NASA/Voyager



 

Triton is the largest moon orbiting Neptune, though its size is only 78% that of the Moon. It is one of the few moons (and the largest of all solar system moons) to have a retrograde orbit around its planet. It is believed that some interaction early in the formation of the Neptunian system caused it to reverse its orbit around Neptune. It also may be a Kuiper Belt object, which we will discuss later. Despite having a retrograde orbit, however, it does have a very circular orbit. It is very cold and because it is cold, it has an atmosphere. Our Moon, despite being larger, does not have any atmosphere as it is much closer to the Sun than Neptune. The atmosphere is dominated by molecular nitrogen and methane, but is 10 million times less dense than the Earth's atmosphere. Because it is so cold, there are no clouds in Triton's atmosphere and is transparent, allowing us to see the surface. Triton has an icy surface of nitrogen ice, methane ice, carbon monoxide ice, and carbon dioxide ice. It was found by William Lassell in 1846, just after the discovery of Neptune itself.

Triton has few craters, which tells us that its surface is relatively young, though it does have dark spots on the surface. Those dark regions are from organic molecules, created when methane is exposed to light. And because of the methane on its surface, there are regions near its southern pole where methane geysers can erupt through cracks in the surface. Around these cracks, dark smudges are found which are created in the same way as the dark spots on Triton.

Fuzzy Image of Nereid seen from Voyager 2

Image Credit:

NASA/Voyager

Nereid is unique as it has a very eccentric orbit, which may mean that it is a captured comet, based on its composition.. However, it does orbit prograde, and despite being smaller than Proteus, was actually the second moon discovered around Neptune in 1949 by Gerard Kuiper.

Proteus

Image Credit:

NASA

 

Proteus is the second largest moon but was not discovered until Voyager 2 spotted it in 1989, which is odd considering Nereid was discovered by an Earth-based telescope