The mystery of the Plutonic color scheme


Pluto’s three satellites, Hydra, Nix and Charon, are all a similar shade of grey. In fact, Nix and Hydra have exactly the same colour to within our ability to measure it. Pluto, on the other hand, is a beautiful shade of red. How come?

The current thinking is that Charon, Hydra and Nix are a similar colour because they were all formed in the giant impact that created this satellite system.

But today, Alan Stern, former head of NASA’s Planetary Science’s division and principal investigator for the New Horizons mission to Pluto, puts forward an alternative hypothesis.

His idea is that the impact of debris from the Kuiper belt on these bodies could send enough surface material into orbit to coat the satellites nearby. Interesting idea.

Stern calculates that the ejecta velocities on Pluto and Charon would be too low to escape. However, the ejecta from Nix and Hydra could easily escape in enough quantity to cover one another to a depth of tens of metres and to cover Pluto and Charon to a depth of tens of centimetres.

The weather on Pluto generates regular frosts which cover this up as qucikly as it was laid down but no such mechanism operates on the other satellites.

So that might explain the differences and similarities in the Plutonic color scheme. Stern also predicts that if he is right, the colours and albedos of Nix, Hydra and Charon should change slowly as more material is ejected and deposited. So by keeping a sharp eye on them, he can gain further evidence for his theory.

What’s more, he says that this mechanism may be common in the solar sytem wherever small binary systems are found, such as in the asteroid and Kuiper belts. And where this happens, these bodies should have similar colours too.

All we have to do now is to look out for the flurry of papers pointing to evidence that he’s right.
Ref: Ejecta Exchange, Color Evolution in the Pluto System, and Implications for KBOs and Asteroids with Satellites

Comments are closed.