<Tr> <Th> Composition by volume </Th> <Td> nitrogen; methane traces . </Td> </Tr> <P> Triton is the largest natural satellite of the planet Neptune, and the first Neptunian moon to be discovered . It was discovered on October 10, 1846, by English astronomer William Lassell . It is the only large moon in the Solar System with a retrograde orbit, an orbit in the opposite direction to its planet's rotation . At 2,710 kilometres (1,680 mi) in diameter, it is the seventh - largest moon in the Solar System . Because of its retrograde orbit and composition similar to Pluto's, Triton is thought to have been a dwarf planet captured from the Kuiper belt . Triton has a surface of mostly frozen nitrogen, a mostly water - ice crust, an icy mantle and a substantial core of rock and metal . The core makes up two - thirds of its total mass . Triton has a mean density of 2.061 g / cm and is composed of approximately 15--35% water ice . </P> <P> Triton is one of the few moons in the Solar System known to be geologically active (the others being Jupiter's Io and Europa, and Saturn's Enceladus and Titan). As a consequence, its surface is relatively young with few obvious impact craters, and a complex geological history revealed in intricate cryovolcanic and tectonic terrains . Part of its surface has geysers erupting sublimated nitrogen gas, contributing to a tenuous nitrogen atmosphere less than 1 / 70,000 the pressure of Earth's atmosphere at sea level . It is the second - largest planetary moon in relation to its primary, after Earth's moon . </P> <P> Triton was discovered by British astronomer William Lassell on October 10, 1846, just 17 days after the discovery of Neptune . </P>

What do the moons titan & triton have in common
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