Voyager 2 photomosaic of Triton. Image source: [1]
At 4,500 million kilometers from Earth, we find Triton that has very peculiar characteristics. It bears the name of the sea god, son of Poseidon in Greek mythology. Triton was discovered October 10, 1846 by British astronomer William Lassell, 17 days after the discovery of Neptune. It is the only known large satellite with retrograde orbit, rotating in the opposite direction to the rotation of its planet, something really unusual.
Triton has an atmosphere, although it is extremely thin, composed mainly of nitrogen with a small fraction of methane. A fine mist extends to about 5-10 km. The temperature on the surface of Triton is -235 ° C, as cold as Pluto. At this temperature methane, nitrogen and carbon dioxide are in the form of solid ice. There are very few craters visible; The surface is relatively young. Nearly the entire southern hemisphere is covered by an "ice cap" of nitrogen and frozen methane.
With a size 4.7 times lower than the Earth is the seventh largest satellite in the Solar System. Almost everything we know about this moon is due to the only probe that has visited it to date, the Voyager 2 spacecraft in 1989.
More details here: Triton
The orbit of Triton (red) is opposite in direction and tilted -23 ° compared to a typical moon's orbit (green) in the plane of Neptune's equator. Image source: [2]
Artist's impression of Triton, showing its tenuous atmosphere just over the limb. Image source: [3]
NASA Jet Propulsion Laboratory