Breaking Up Is So Very Hard To Do
8 February, 2008
Far from Earth, orbiting the furthest major planet from the Sun , there is a very cold, very odd moon. The planet Neptune was discovered in 1846 and the moon Triton was discovered only two weeks later. For a long time it was known just as Neptune’s satellite, it would take over a hundred years for the next moon to be discovered orbiting this distant world. Triton is a big moon, the seventh largest in the solar system, roughly the same size as Earth’s moon. But unlike the other large moons Triton goes the wrong way around its parent planet.
Most moons seem quite happy to trundle around their planet in the same direction as their planet spins. This is thought to be because the moons formed at the same time as the planet, and Triton’s orbit around Neptune indicates that it is an intruder into the Neptunian system. Evidence of this has been seen with the Voyager space probe and the Hubble telescope. The front part of Triton, the leading part as it sweeps through its orbit, is heavily cratered. When Triton first arrived at Neptune it seems to have swept up much of the orbiting debris, and since it was going in the opposite direction the collisions were that much nastier. These head on collisions have caused one half of Triton to bear the marks of its entry into the space around Neptune.
Triton has other interesting characteristics too. It has a faint atmosphere. It has cryovulcanism, nitrogen geysers, rift valleys and a large polar cap. Since Triton was only visited once during a brief flyby, we only limited close up information on it. But one thing we do know is the eventual fate of this backwards moon. Unlike our moon which is slowly moving further from the Earth, Triton is inexorably being pulled towards Neptune. Its retrograde motion has sealed its fate. It will take a few billion years, but Triton will eventually get too close to Neptune. When it does the moon will be subject to large gravitational forces that will overcome its own and the moon will begin to break up. The tidal forces will shatter Triton into an immense number of smaller and smaller particles until it will likely become a ring of moonlets. And unlike the tiny rings that circle Neptune just now, this ring will be spectacular. Triton is made of bright ice, and this reflective substance will be much more visible. The sheer size of Triton will also mean that the rings will be suitably impressive, possibly larger than those of Saturn.
Triton has barged its way into orbit around Neptune. It has suffered collisions and will eventually be ripped apart. But in it’s death it will at least look fantastic.