Image from acquerra.com.au (I couldn't get permission, no contact info on website)
This has happened before. Jupiter's southern belt seems to disappear on a roughly 15-year cycle, with the last disappearance being in the early 1990s. Astronomers weren't really quite sure what was going on. Amateur astronomer Anthony Wesley from Australia was the first to catch a glimpse of the belt's disappearance as it rounded the sun.
"It is not known why the belt periodically disappears, but it may be that it sinks lower if it cools, and then the view of it is obscured by clouds pouring in over the top of it. The clouds on Jupiter are tens of thousands of kilometers deep. Wesley said the phenomenon could be linked to storm activity that preceded the change." (Jupiter has lost one of its cloud stripes - Physorg.com)
Wow. Apparently, astronomers were suspecting that Jupiter would be going through another fading cycle as it passed behind the sun. The exact date probably never will be discovered.
On June 3, Anthony Wesley was taking a real-time video of Jupiter and noticed a bright flash on the disc. Something had just collided with the gas giant! The event was quickly confirmed and telescopes all over the world pointed their eyes at our biggest neighbour. Hubble even took a peek:
Image credit: NASA, ESA, M. H. Wong (University of California, Berkeley, USA), H. B. Hammel (Space Science Institute, Boulder, Colorado, USA), A. A. Simon-Miller (Goddard Space Flight Center, Greenbelt, Maryland, USA) and the Jupiter Impact Science Team; source PhysOrg.com
This impact didn't even leave a mark on Jupiter! The inset shows the crash site, in past events usually a cloud of gas and dust gets kicked up, leaving a telltale "black eye" effect on the visible surface. This means that the impactor was probably a conglomerate, a loose pile of rubble held together only by its own gravity. The pieces of rock probably burned up in Jupiter's upper atmosphere, leaving no trace other than the flash of light observed from Earth.
The Hubble picture also gave astronomers an amazing chance to study the Jovian belt disappearance. It appears that a higher layer of ammonia ice is obscuring the normally orange band of ammonia mixed with phosphorous and sulphur.
""Weather forecast for Jupiter's Southern Equatorial Belt: cloudy with a chance of ammonia," ["says team member Heidi Hammel of the Space Science Institute in Boulder, Colorado, USA."]" (Hubble scrutinizes site of mysterious flash and missing cloud belt on Jupiter - PhysOrg.com)
The ammonia clouds are expected to clear out in a couple of months, starting with localized downdrafts on the border with the southern tropical zone (that row of dark spots in the image.)
So basically, Jupiter undergoes a massive weather pattern shift, and gets a prick in the side by an unknown entity. It's been a rough few months for the King of Worlds!