What an incredible few hours it's been for astronomers everywhere, as we witness a chance of a lifetime event: evidence of a space rock of some sort slamming into Jupiter. Images taken after the impact show the debris field and aftermath of a gigantic collision that occurred in the southern polar region of the enormous planet.
An extremely dedicated and meticulous team of amateur astronomers observe Jupiter's changing cloud patterns on a regular basis, and it came as an amazing surprise when Anthony Wesley, near Canberra, Australia, reported his Sunday-morning (July 19, 2009) observations (http://jupiter.samba.org/jupiter-impact.html) of a dark scar that bore all the hallmarks of the Shoemaker Levy 9 impacts at Jupiter in 1994. By an amazing coincidence, I was part of a team that had already been allocated time to observe Jupiter from the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Based on Anthony's discovery, we were crowded around our computers at 3 a.m. PDT (with Anthony observing with us remotely from Australia) as the first near- and mid-infrared images started to come in... it was such an exciting moment, seeing the high altitude particles that had been lofted by the impact (they appear bright in the infrared). Anthony celebrated with us, but then the real work began. We celebrated and then rolled up our sleeves and began an exciting night of observations.
With the assistance of William Golisch at the IRTF, Glenn Orton and I viewed the impacts in as many wavelengths and spectra as we possibly could, as Jupiter rotated and carried the impact scar out of Earth's view. We used these many views to show evidence for high temperatures at the impact location, and suggestions of ammonia and aerosols that had been carried high into the atmosphere. The observations were repeated again today, Tuesday morning, to track the shape and properties of the site. The scar is extremely large, almost as big as Earth and will continue to grow as Jupiter's atmospheric winds and jet streams redistribute the material, and then, like Shoemaker-Levy 9, it will begin to fade in the coming weeks and months. Based on comparisons to SL-9, the impactor was likely to be small despite the large aftermath, maybe a few hundreds of metres across. Not only will this tell us a lot about impacts in the outer solar system, and how they contribute to the nature of the planets and icy moons, but they'll also serve as a probe for the fundamental weather patterns in Jupiter's high atmosphere.
Amateur observers continue to flood the Internet with new images of the dark spot at approximately 60 degrees south on Jupiter, and so far it looks as though the impact took place sometime in the 24 hours preceding Anthony's discovery. The debris field now extends out to the west and northwest, with additional high-resolution images from the Keck telescope (Marchis, Wong, Kalas, Fitzgerald and Graham http://keckobservatory.org/index.php/news/jupiters_adds_a_feature/) showing the detailed morphology of the impact region. The hard work continues today, as an international team of planetary astronomers scrambles for time on some of the world's largest astronomical facilities.
Finally, it's a shame but perhaps not surprising that we didn't see the collision, or the impactor itself, given the great distance to Jupiter. Like throwing a rock in a pond, we're seeing and analyzing the splash that it's made, and we can't yet infer many details about the rock itself - the detailed shape of the impact site could help determine the trajectory and energy of the collision. But it certainly made quite a splash, and we hope to learn a lot about Jupiter from this event!
Anthony's discovery is truly astounding, as it united astronomers in looking again at the gas giant Jupiter. It's overwhelming and spectacularly exciting to watch this event unfolding before our eyes!
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