Triton is doomed. The largest moon of Neptune, only slightly smaller than our moon, will either self-destruct or crash into Neptune sometime in the next 100 million years. Most likely, a massive break-up will form a new layer of rings around Neptune.
Triton orbits in a direction opposite to the planet's rotation, in an orbital plane inclined at 23 degrees to Neptune's equator. So it's very likely that Triton originally formed elsewhere in the solar system, possibly collided with a satellite of Neptune, and was captured by Neptune's gravity.
Triton is the coldest moon in the solar system, with a surface temperature of only minus 236 degrees Celsius (minus 391 Fahrenheit)—in fact, only comets are colder.
At that temperature, ice has the structural strength of steel and can form mountains. But cold as it is, Triton has an atmosphere, though it's very thin and made mostly of nitrogen.
The surface of Triton shows very few large craters, indicating that it's still tectonically active, thanks to the tidal gravity of Neptune. An extensive network of ridges and valleys covers Triton's surface, caused by freezing and thawing cycles on Triton's icy nitrogen-methane surface.
And Triton has active volcanoes. Look closely at the Voyager image and you will see dark smudges. These are drifts of material that spread downwind of volcanoes. Most planetary geoscientists believe that dark areas of methane on the surface absorb just enough heat from the faraway sun to melt nitrogen ice below them.
In the low pressure of Triton's atmosphere, that nitrogen goes from solid almost directly to gas. And that gas has to go somewhere: it forms active geysers just 37 degrees Celsius (99 Fahrenheit) above absolute zero.
Triton is not alone in orbit around Neptune, but all the other moons are tiny.
Neptune has 13 known moons. Only Nereid and Triton were known prior to the Voyager 2 mission, which discovered six new moons. One of those six is bigger than Nereid but was hiding in an orbit too near to Neptune itself to be detected easily from Earth. All six are inside the orbit of Triton, at distances ranging from 48,000 to 118,000 kilometers (28,800 to 70,800 miles) from Neptune's center.
Most of these newly found moons are closer to Neptune than its rings are, but they could not have formed there. Neptune's tidal gravity would have kept them from coming together—as it does with the rings. All these moons must have been deflected from orbits farther out.
Perhaps the capture of Triton had something to do with this, but it'll take more detective work to know.
Neptune's outermost moon, S/2003 N1, is 38 kilometers (23 miles) in diameter and has the most eccentric orbit of any moon. It takes 26 years to orbit Neptune and is the farthest from a planet of any moon in our solar system.