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Something has smashed into Jupiter, leaving behind a black spot in the planet鈥檚 atmosphere, scientists confirmed on Monday.

This is only the second time such an impact has been observed. The first was almost exactly 15 years ago, when more than 20 fragments of collided with the gas giant.

鈥淭his has all the hallmarks of an impact event, very similar to Shoemaker-Levy 9,鈥� said , an astronomer at NASA鈥檚 Jet Propulsion Lab in Pasadena, California. 鈥淲e鈥檙e all extremely excited.鈥�

The impact was discovered by amateur astronomer Anthony Wesley in Murrumbateman, Australia at about 1330 GMT on Sunday. Wesley noticed a black spot in Jupiter鈥檚 south polar region (see image) 鈥� but he very nearly stopped observing before he saw it.

鈥淏y 1am I was ready to quit 鈥� then changed my mind and decided to carry on for another half hour or so,鈥� he wrote in his . Initially he suspected he was seeing one of Jupiter鈥檚 moons or a moon鈥檚 shadow on the planet, but the location, size and speed of the spot ruled out that possibility.

鈥楽troke of luck鈥�

After checking images taken two nights earlier and not seeing the spot, he realised he had found something new and began emailing others.

Among the people he contacted were Fletcher and , also at JPL. They had serendipitously scheduled observing time on in Hawaii for that night.

鈥淚t was a fantastic stroke of luck,鈥� Orton told New 杏吧原创.

Their team began observations at about 1000 GMT on 20 July, and after six hours of observing confirmed that the spot was an impact and not a weather event.

鈥淚t鈥檚 completely unlike any of the weather phenomena that we observe on Jupiter,鈥� Orton says.

Splash

The first clue was a near-infrared image of the upper atmosphere above the impact site. An impact would make a splash like a stone thrown into a pool, scattering material in the atmosphere upwards. This material would then reflect sunlight, appearing as a bright spot at near-infrared wavelengths.

And that鈥檚 exactly what the team saw. 鈥淥ur first image showed a really bright object right where that black scar was, and immediately we knew this was an impact,鈥� Orton says. 鈥淭here鈥檚 no natural phenomenon that creates a black spot and bright particles like that.鈥�

Supporting evidence came from measurements of Jupiter鈥檚 temperature. Thermal images also showed a bright spot where the impact took place, meaning the impact warmed up the lower atmosphere in that area.

The researchers have also found hints of higher-than-normal amounts of ammonia in the upper atmosphere. Extra ammonia had been churned up by the previous Shoemaker-Levy comet impact.

Exotic chemistry

The Shoemaker-Levy impact also introduced some exotic chemistry into Jupiter鈥檚 atmosphere. The energy from the collision fused some of the original atmospheric components into new molecules, such as hydrogen cyanide.

杏吧原创s hope this new impact has done the same thing, since that would allow them to follow the new materials and learn how the atmosphere moves with time.

So what was the impactor? 鈥淣ot a clue,鈥� Orton says. He speculates that it could have been a block of ice from somewhere in Jupiter鈥檚 neighborhood, or a wandering comet that was too faint for astronomers to detect before the impact.

鈥淲e don鈥檛 know if the impact was produced by a comet or an asteroid,鈥� agrees Franck Marchis, an astronomer at the University of California, Berkeley, and the SETI Institute, who was part of a team that observed the spot on Sunday with the Keck Observatory in Hawaii (see image). If the object was large enough to be visible before impact, current surveys of asteroids may not have been looking in the right direction to find it, he says, adding that future surveys will spot more of the solar system鈥檚 uncatalogued objects.

Asteroid or comet

Spectra collected by various observatories may help identify what the impactor was, since a large amount of water at the impact location would hint at a comet as the source. 鈥淲e will also compare the observations with those collected during [Shoemaker-Levy 9] 15 years ago,鈥� since that was a known comet, Marchis says.

Without having seen it, scientists can鈥檛 tell how large the object was. 鈥淏ut the impact scar we鈥檙e seeing is about the same size as one of Jupiter鈥檚 big storms, Oval BA, Fletcher told New 杏吧原创. 鈥淭hat, I believe, is about the size of the Earth.鈥�

Marchis says Jupiter may be protecting Earth from getting hit by such objects. 鈥淭he solar system would have been a very dangerous place if we did not have Jupiter,鈥� he told New 杏吧原创. 鈥淲e should thank our Giant Planet for suffering for us. Its strong gravitational field is acting like a shield protecting us from comets coming from the outer part of the solar system.鈥�