The Double Planet

Next week the New Horizons spacecraft falls through (or “flies by”) the Pluto-Charon binary system. This week New Horizons photos reveal dramatic differences between Pluto and Charon, despite their presumed common origin in an interplanetary collision. (By the way, some astronomers — and apparently the New Horizons science team — pronounce “Charon” more like “Charlene”, the name of the wife of Charon’s discoverer, and less like “Karen”).

We already know spectroscopically that water H₂O, methane CH₄, nitrogen N₂, and carbon monoxide CO ices cover Pluto’s surface (and form its tenuous and dynamic atmosphere). Ultraviolet or UV radiation probably converts nitrogen and methane into reddish, organic tholins C_xH_yN_z. But what causes the patterns of bright ices and dark orangish tholins? By contrast, Charon’s darker surface appears to contain water crystals and ammonia hydrates (NH₃)₂H₂O. Like Triton, the largest moon of Neptune, both Pluto and Charon may have active geysers, which may contribute to the distribution of their surface materials. Stay tuned!