Why? Well, it appears that this black smudge on Jupiter was ... uh ... well ... NASA's fault! The decision had been made a while back that in order to not risk having the Galileo spacecraft floating around up there (and possibly eventually crashing into Europa and creating "biological contamination" of that's moons vast sub-ice liquid water ocean) they would steer it into Jupiter's atmosphere while they still had control and fuel to do so. What they seemingly did NOT figure on was that the plutonium pellets that were the power source for Galileo, could, under the vast pressures deep within Jupiter, implode and produce a major nuclear event.
Anyway, go read the piece about all the details. If Hoagland is right (and the figures he trots out are quite convincing) the black smudge on Jupiter is the result of chemical breakdown of Methane, releasing pure carbon atoms, as the nuclear fire-ball shot up through the Jovian atmosphere. Ooops ... sorry Jupiter folks ... didn't mean to nuke your planet or nothing ...
