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 CxHyNz. 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!