Close up of Triton, from the Voyager 2 flyby in 1989. [NASA/JLP Space Australia]

Neptune, the planet named for the Roman sea god, was discovered in 1846. Just 17 days later, British astronomer William Lassell found a moon. It wasn't named, just called Neptune's moon.

It took over a hundred years to find a second moon.
Lassell had discovered Neptune's large moon which is some 2700 km across. It's a bit bigger than Pluto. The moon Gerard Kuiper discovered is only 360 km across. Kuiper named the moon Nereid after the mythical sea nymphs, the Nereids. And that meant the other moon also needed a name. It was called Triton, after a son of Neptune.

Triton is about 2/3 the size of our Moon, and at a distance from Neptune that's similar to the distance from the Earth to the Moon.
Our Moon takes about 28 days to orbit, but Triton takes less than six days. There wouldn't be a Triton if it moved as slowly as the Moon. Neptune's strong gravity would have broken it apart long ago.

Triton has a retrograde orbit.
A moon in a retrograde orbit moves in the opposite direction to the spin of its planet. Some small captured moons are in retrograde orbits, but Triton is the only large moon that is.

The early Solar System had lots of big rocks slamming into other bodies. They left behind impact craters.
If a planet or moon is covered in impact craters, we see an old surface. Since Triton has few impact craters, it means that the old craters were smoothed over.

Volcanic eruptions have smoothed the surface of Triton.
Triton's volcanoes don't shoot out hot lava. They are cryovolcanoes – cold volcanoes that erupt liquids and gases. The Voyager 2 space probe found evidence of cryovolcanoes on Triton. The Cassini probe saw active eruptions on Saturn's moon Enceladus.

A unique feature of Triton is is the cantaloupe terrain.
It got its name because it looks a bit like the rind of a cantaloupe melon.

Probably both Triton and Pluto were born in the Kuiper Belt.
They're almost the same size and density, the same reddish color, have polar icecaps and other similarities. Triton might have become a dwarf planet if Neptune's gravity hadn't captured it.

Triton has an atmosphere, but it's extremely thin.
The thin atmospheres of both Triton and Pluto are mainly nitrogen. Earth's atmosphere is nearly 80% nitrogen, but about 70,000 times thicker than Triton's.

Triton is colder than Pluto.
Neptune is about 4.5 billion km (2.8 billion mi) from the Sun. And on average, Pluto is 5.9 billion km (3.7 billion miles). But Triton is colder than Pluto. Its thin atmosphere retain very little of the Sun's radiation. Its bright icy surface reflects most of the sunlight it receives.

Nasa is considering a mission to Neptune and Triton.
It's likely that Triton has a liquid ocean under its surface. And if there is water, there might be life.