Triton, the largest moon of Neptune (diameter of 2,700 kilometers: 1,680 miles) was discovered by William Lassell (a British astronomer) in 1846. The moon exhibits many interesting characteristics that make it unique to the other moons of Neptune. For one, Triton orbits Neptune in a retrograde direction, meaning that it moves in the opposite direction of the planet's rotation. Second, Triton has a density of 2.066 grams per cubic centimeter, while water has a density of 1.0 gram per cubic centimeter. Therefore Triton is composed more of rock than ice and is thus the counterpart to other Neptune moons which are composed mainly of ice.
When the Voyager spacecraft flew by Triton (passing at a distance of 39,800 kilometers at a speed of 27 kilometers a second), it discovered that the satellite has a thin atmosphere of 15 microbars (0.000015 times the surface pressure on Earth at sea level) made up of a few thin clouds several kilometers above the surface. The surface temperature of Triton is -235° C (-391° F), the coldest known temperature of all the solar system!
Triton has several interesting surface features as well. On the underlying area of the moon is the south polar ice cap, slightly pink in color (possibly due to the evaporation of nitrogenous material) and is also highly reflective. There are various plains of ice across the surface, possibly the result of water- ammonia eruptions. In addition, several areas of Triton's surface are truly unique (unlike anything found in the solar system), including strange icy patterns caused by unknown surface activity.
Triton 
