Triton was discovered in 1846.
It is the largest moon of Neptune by a huge margin. %99.5 of the mass of all the moons of Neptune belongs to Triton.
It has a cratered, icy surface.
Triton’s surface is so light-colored, it reflects almost all the sunlight. Therefore it is the coldest recorded locations in our Solar System with -235 Celsius.
It is possible that Triton, similar to Europa, may have an ocean underneath its surface due to tidal heating. But more scientific data is required for an exact conclusion.