Some are calling the black mark near Jupiter's south pole a crater, while others say it is the dust cloud left after the impact explosion. What most of them agree upon, however, is that it might have been caused by "a 1 km-diameter comet traveling at 135,000 mph (60 km per second)." (source)
It should go without saying that we are very lucky. This object came effectively out of nowhere, hitting unexpectedly; had it come our way instead, I can't imagine any level of preparedness, short of a deeply-buried bunker and several years supply of food, being enough to save any of us. It would, in fact, be a monumental effort just to try to make sure some few survived to ensure continuity of our very species. But, we were lucky, and we have the gas giant to thank.
Apparently, poor Jupiter is like our big brother on the playground taking hits for us and it's nothing new. "That’s Jupiter doing its cosmic job, astronomers like to say. Better it than us. Part of what makes the Earth such a nice place to live, the story goes, is that Jupiter’s overbearing gravity acts as a gravitational shield deflecting incoming space junk, mainly comets, away from the inner solar system where it could do for us what an asteroid apparently did for the dinosaurs 65 million years ago. Indeed, astronomers look for similar configurations — a giant outer planet with room for smaller planets in closer to the home stars — in other planetary systems as an indication of their hospitableness to life." (source)