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Saturday, 22 August 2009

Jupiter's Impact: Gone in 30 Days

It's been a month since July 19th — the night Anthony Wesley, a software engineer by day and a dedicated amateur astronomer by night, discovered something amiss on Jupiter.

Jupiter impact on August 19, 2009
Taken on July 19, 2009, Anthony Wesley's image of Jupiter shows a dark marking strikingly similar to the ones left when Comet Shoemaker-Levy 9 crashed into Jupiter in 1994. South is up.
Anthony Wesley
From his home observatory just outside Murrumbateman in New South Wales, Australia, Wesley identified a dark spot near the planet's south pole using a custom 14½-inch f/5 reflector and a high-end camera and filters — not the usual gear found in stargazers' backyards, but by no means rare among serious observers.

A quick check ruled out a transiting Galilean moon or its shadow as the cause, and the spot was absent in images he'd taken two days earlier. That's when the importance of what he'd found began to sink in. His spot looked just like the ones created exactly 15 years earlier after fragments from Comet Shoemaker-Levy 9 had slammed into the giant planet. "If it really was an impact mark," he notes in an online report, "then I had to start telling people, and quickly."

According to John Rogers, director of the British Astronomical Association's Jupiter Section, no spot was seen during the planet's previous rotation some 10 hours earlier, so the impact must have occurred on Jupiter's far (dark) side some time between 7:40 and 14:00 UT on July 19th.

Jupiter impact seen by Keck II
On July 20, 2009, one day after Jupiter suffered an impact, the Keck II telescope recorded this detailed infrared view. Click on the image for a larger view.
P. Kala / M. Fitzgerald / F. Marchis / J. Graham
As news of Jupiter's new feature spread over the next hours and days, other amateur astronomers and major observatories swung into action. Glenn Orton, a planetary scientist at the Jet Propulsion Laboratory, was among the first to get word. Luckily, the next night Orton and his team had an observing run scheduled with NASA's Infrared Telescope Facility atop Mauna Kea. The IRTF images showed the spot as very bright, meaning it was quite high up in Jupiter's atmosphere. As Orton explains, "For me, this totally clinched the case that this was an impact." Team member Leigh Fletcher also has a blog detailing the IRTF run.

Observers at other major observatories abandoned their scheduled targets to slew over to Jupiter. Paul Kalas (UC Berkeley) and Michael Fitzgerald (UCLA) had time on the Keck II telescope, also on Mauna Kea. With help from Kalas's Berkeley colleague Franck Marchis, they tracked down the spot with an infrared camera that revealed a double-lobed center surrounded by a darker halo. Marchis describes the tense excitement in his online blog.

Nearby, the Gemini North telescope was recording the spot in the deep infrared. "At these wavelengths we receive thermal radiation (heat) from the planet's upper atmosphere," explains observer Imke de Pater (UC Berkeley) in a Gemini press release. "The impact site is clearly much warmer than its surroundings, as shown by our image taken at an infrared wavelength of 18 microns."

Hubble's new Wide Field Camera 3 was rushed into service to take this closeup view of Jupiter's impact mark on July 23, 2009. Click here for a larger view.
NASA / ESA / Heidi Hammel / Jupiter Impact Team
NASA managers even interrupted their checkout of the recently refurbished Hubble Space Telescope and quickly assembled a team of veteran observers led by Heidi Hammel (Space Science Institute). On July 23rd they used the orbiting observatory and its newly installed Wide Field Camera 3 to record amazing close-ups of the dark splotch, which by then had already begun to smear out in the east-west tug of war created by Jupiter's zonal winds. The team took more HST images later on, but those have yet not been made public.

The spot's similarity to the atmospheric stains created during the "Great Comet Crash" of 1994 is striking.

1994 Jupiter impact seen by Hubble
After Comet Shoemaker-Levy 9's remains struck Jupiter in July 1994, the Hubble Space Telescope captured this double hit: a large multiring spot caused by the G fragment and a small dark spot from the D fragment.
H. Hammel / MIT / NASA
Back then, no one had ever observed a planetary impact, so the resulting Earth-size stains in Jupiter's atmosphere caught astronomers by surprise. Over six days, they looked on in amazement as 20 fragments from Comet Shoemaker-Levy 9 slammed into Jupiter at 40 miles (60 km) per second. Those pieces came from a precursor about 1 mile across before Jupiter broke it apart during a close passage two years earlier.

This time, at least, they knew what they'd found. July's impact likely involved a single interloper no more than few hundred yards across, likewise creating a superficial powder burn with a black core (where the impactor exploded below Jupiter's uppermost cloud layer) and a broader, fainter, asymmetric halo (fallout from the towering plume of debris).

We'll likely never know whether the colliding object was a small asteroid or a comet — it was far too small to be picked up telescopically prior to its strike. "The problem," laments de Pater, "is to unravel stuff dredged up from Jupiter's deeper atmosphere (like ammonia) from that of the impactor." Another snag involves disentangling the spectral signature from any water deposited by a comet in Jupiter's atmosphere and that from the ever-present water in Earth's atmosphere.

Jupiter impact on August 19, 2009
The impact scar on Jupiter has spread sideways and broken up into faded, feathery bits. Yellow ticks mark its approximate ends in images taken 75 minutes apart on August 17, 2009, when the System II central-meridian longitude was 182° (top) and 227°.
Christopher Go
Meanwhile, the last vestiges of Jupiter's overnight sensation have nearly faded from view. According to the BAA's Rogers, the impact site was still nearly black as of August 1st but had become fainter in the days thereafter as it stretched out.

More recent images taken by Christopher Go, seen here, show that evidence of the collision is nearly impossible to pick out. He writes, "The Wesley impact remnant has faded a lot. It is difficult to see now even in almost perfect conditions."

I'm sure to get the full story when I attend a meeting of planetary scientists in early October. Until then, dynamicists are already chewing on the fact that Jupiter has endured two well-observed collisions within a span of just 15 years.

So have any other Jovian impacts been seen telescopically? You'll find the answer in the November issue of Sky & Telescope, which is being printed right now!

Posted by Kelly Beatty, August 21, 2009
Good Clear Skies
Colin James Watling
Astronomer and head of the Comet section for LYRA (Lowestoft and Great Yarmouth Regional Astronomers) also head of K.A.G (Kessingland Astronomy Group) and Navigator of the Stars (Fieldwork)

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