Fifteen years since the impact of Comet Shoemaker Levy-9 with Jupiter and the giant planet is sporting a possible new impact scar.

On 16 July 1994, Comet Shoemaker Levy-9 began a six day assault on the gas planet Jupiter (read our review here). Fifteen years later, on 19 July 2009, astronomers from around the world have reported a new jet black mark near Jupiter's south pole. Is this due to a surprise impact event or can it be explained by the weird and wonderful weather systems that Jupiter is famous for?

Reporting on the website http://jupiter.samba.org/, Anthony Wesley of New South Wales in Australia described the observations he made with his 14.5" newtonian telescope:

"I'd noticed a dark spot rotating into view in Jupiter's south polar region and was starting to get curious. When first seen close to the limb (and in poor conditions) it was only a vaguely dark spot, I thought likely to be just a normal dark polar storm. However as it rotated further into view, and the conditions also improved, I suddenly realised that it wasn't just dark, it was black in all channels, meaning it was truly a black spot."

Noting that the black spot was rotating in sync with a nearby white oval storm, Wesley realised the spot was located at cloud level and was not, therefore, one of Jupiter's many moons, or a shadow from a moon, for example.

"I started to get excited," he writes. "It took another 15 minutes to really believe that I was seeing something new - I'd imaged that exact region only two days earlier and checking back to that image showed no sign of any anomalous black spot."

Other amateurs' observations are reported on spaceweather.com.

Unlike the events of 1994, this new event was not accompanied by observations of a comet or asteroid in the Jovian neighbourhood. Weather systems change extremely rapidly on Jupiter so the jury is still out as to what caused the curious feature, but if it was an impact event it is certainly a chilling reminder that such events can occur with little warning in our Solar System today.

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