Ammonia discovered on Pluto's moom Charon

NASA reports that significant and unexpected amounts  of ammonia have been discovered on Pluto's moon Charon.  These substantial traces have been discovered in the ejecta of at a certain crater called Organa, while the ejecta of a nearby crater, Skywalker shows no similar traces.   Why one, and not the other?  It is a mystery, but did anyone think of asking these guys?