At the edge of our Solar System is a Belt of small ice-worlds – the well-known Pluto and Charon, the unofficial Tenth Planet, Eris, and several other largish objects that the media doesn’t say much about. One of the latter is 2003 EL61, which is yet to get a real name, as the discovery code-name makes clear. It’s roughly Pluto-sized, but flattened into a football shape due to a very high rotation rate. It’s also much denser than its fellow ice-worlds, which led its discoverers to conclude it had formed in a violent collision with another ice-world. Supporting that theory are small objects in the Belt that have the same spectroscopic colour as 2003 EL61. In the Main Belt of asteroids that ALWAYS means the objects are related – for example, we have pieces of the asteroid Vesta that came to Earth as meteorites so we could check.
Computer simulations of the big smash-up support the idea, but there seemed to be no way of dating just when it occurred – bar one. Ice mixed with methane changes colour over millions of years as the methane is broken down by cosmic-rays and its break-down products recombine as “tholins” – pinkish organic material. Older ice-worlds in the Belt show these colours, indicating their icy crust once contained methane and now is diluted with a layer of tholins.
BUT 2003 EL61, and the orbitting bits of it, don’t show those tholin colours….
The Youthful Appearance of the 2003 EL61 Collisional Family
…which basically concludes that the whole group of parent body and fragments either have an anomalously low level of methane or their collision occurred less than 100 million years ago. Either conclusion is quite startling, as ice-worlds everywhere else in the Belt seem to have methane. Either the smash-up was so violent and hot that all the methane was driven off – but why didn’t the water explode as a great cloud of steam? Or the methane is there and the smash-up occurred, at the earliest, in the days of the dinosaurs. In which case a rogue dwarf-planet careened through the Belt in what is “recent time” for big planetary collisions.
