The Jupiter Impact
Posted by Emily Lakdawalla
13-08-2009 14:38 CDT
I've received a half-dozen emails asking why there hasn't been anything on the blog yet about the Jupiter impact. The answer: there's only one of me! But now I'm finally getting to the story, nearly a month after it happened. There is one advantage to being so late to the party: I can tell you about what's happened since the discovery.
On July 19, 2009, fifteen years almost to the day after comet Shoemaker-Levy 9 struck Jupiter, an amateur astronomer named Anthony Wesley pointed his home telescope (located in Murrumbateman, New South Wales, Australia) at Jupiter. Wesley's setup looks serious but I'm sure there are plenty of other amateurs out there with scopes just as spiffy; Wesley was in the right place on Earth at the right time to be the first one to the discovery.Jupiter is a popular target just now for amateur astronomers, for two reasons: opposition is tomorrow (meaning it's relatively close to Earth and a brilliant target high in the sky at night), and its equinox was on June 22 (which means its moons cast shadows on the planet, a fun thing to try to observe). Wesley been observing Jupiter since 11 pm local time (1300 UTC) and was debating quitting for the night shortly after midnight because the "seeing" (astronomer-ese for the quality of sky conditions for observing) had deteriorated; instead, he took a half-hour break. I'll continue the story in Wesley's own words:
When I came back to the scope at about 12:40 am [14:40 UTC] I noticed a dark spot rotating into view in Jupiter's south polar region [and] started to get curious. When first seen close to the limb (and in poor conditions) it was only a vaguely dark spot, I thought [it] likely to be just a normal dark polar storm. However as it rotated further into view, and the conditions improved I suddenly realised that it wasn't just dark, it was black in all channels, meaning it was truly a black spot.Wesley's 14.5-inch scope produced the lovely image below.A photo posted to spaceweather.com by David Kolb dating from 00:22 on July 18 shows no impact mark in the same location, confirming that it was a new feature.
My next thought was that it must be either a dark moon (like Callisto) or a moon shadow, but it was in the wrong place and the wrong size. Also I'd noticed it was moving too slow to be a moon or shadow. As far as I could see it was rotating in sync with a nearby white oval storm that I was very familiar with - this could only mean that the back feature was at the cloud level and not a projected shadow from a moon. I started to get excited.
It took another 15 minutes to really believe that I was seeing something new - I'd imaged that exact region only 2 days earlier and checking back to that image showed no sign of any anomalous black spot.
Now I was caught between a rock and a hard place - I wanted to keep imaging but also I was aware of the importance of alerting others to this possible new event. Could it actually be an impact mark on Jupiter? I had no real idea, and the odds on that happening were so small as to be laughable, but I was really struggling to see any other possibility given the location of the mark. If it really was an impact mark then I had to start telling people, and quickly. In the end I imaged for another 30 minutes only because the conditions were slowly improving and each capture was giving a slightly better image than the last.
Eventually I stopped imaging and went up to the house to start emailing people, with this image above processed as quick and dirty as possible just to have something to show.
A corroborating observation of the new feature came 10 hours later from amateur astronomer António Cidadão, who photographed Jupiter in the near-infrared (specifically, at 889 nm, a "methane absorption band") and found it to appear bright, meaning that it was located high in the atmosphere. You can see a small version of his image here; I wasn't able to locate a higher-resolution version.
Another 10 hours later -- nearly a full Earth's rotation -- Jupiter came into view from Hawaii, where professional astronomer Glenn Orton just happened to be observing Jupiter using the Infrared Telescope Facility on Mauna Kea. "We were extremely lucky to be seeing Jupiter at exactly the right time, the right hour, the right side of Jupiter to witness the event. We couldn't have planned it better," Orton said in a JPL press release. Their image was taken at a wavelength longer than Cidadão's, 1.65 microns. (1.65 microns is also known as "H band" and is the wavelength of a "window" through Earth's atmosphere, where water vapor is not strongly absorbing.) According to the release, the wavelength is "sensitive to sunlight reflected from high in Jupiter's atmosphere." In other words, the bright spot indicates the presence of a new something, loacted in the uppermost atmosphere. In this image, south is at the bottom.The JPL release also states that they observed Jupiter in mid-infrared wavelengths and detected "a warming of the upper troposphere with possible extra emission from ammonia gas detected at mid-infrared wavelengths." The release goes to trouble to give props to a telescope operator: "The observations were made possible in large measure by the extraordinary efforts of the Infrared Telescope Facility staff, including telescope operator William Golisch, who adroitly moved three instruments in and out of the field during the short time the scar was visible on the planet, providing the wide wavelength coverage."
The next astronomer who already had time on the right kind of major telescope at the right time of night appears to have been Paul Kalas, who (along with Michael Fitzgerald) was lucky enough be at the Keck II telescope. They were planning to study a Jupiter-sized planet orbiting Fomalhaut, but shifted gears quickly when they read about the impact on the blog of Franck Marchis, another astronomer. They collaborated with Marchis to develop the best imaging plan. Marchis has a lengthy post on what developed, including more images. Among other things, Marchis says they were able to repeat the observations 4 hours later, which allowed them to determine that "there is no additional impact features on Jupiter. So whatever hit Jupiter this time was not a disrupted with multiple fragment comet like Shoemaker-Levy 9."The next major telescope to observe the impact site was Gemini North, in Hawaii, on July 22. This was just about exactly three days after Wesley's initial observation. Unlike previous observations, this one was taken in thermal infrared wavelengths, and even three days after the impact the area was warmly glowing. In the release, astronomer Imke de Pater says it took "a heroic effort to get these data. We were on the telescope observing within 24 hours of contacting the observatory." The release also says that "By using the full set of Gemini images taken over a range of wavelengths from 8 to 18 microns, the team will be able to disentangle the effects of temperature, ammonia abundance, and upper atmospheric aerosol content. Comparing these Gemini observations with past and future images will permit the team to study the evolution of features as Jupiter's strong winds disperse them." The next big observatory to the punch was Hubble. This is pretty incredible, considering that the Servicing Mission was only completed in May; and the image was taken with one of the new instruments, the Wide Field Camera 3. They haven't even had time to fully calibrate it -- they had to interrupt the checkout process to get this shot. This wasn't the first image taken with WFC3, but it was the first to be released to the public. It was taken on July 23 at 19:00 UT, about five days after Wesley's discovery, and is the first one I've posted here since Wesley's original that shows the impact site in natural color (that is, approximately as the human eye would see it). In the explanation accompanying the images on the Hubble website, Amy Simon-Miller explains "that the details seen in the Hubble view shows a lumpiness to the debris plume caused by turbulence in Jupiter's atmosphere. The spot is presently twice the length of the United States." Also, "the diameter of the object that slammed into Jupiter was at least the size of several football fields," by which I am guessing they mean in the neighborhood of 300 to 500 meters in diameter. Both the Gemini and Hubble releases stated that the appearance of the impact mark bore striking similarities to the appearance of the Shoemaker-Levy 9 impact (except that there is only one mark this time around, whereas SL9 had broken into numerous pieces that made separate marks). To review what SL9 looked like, you can check out this helpful website from the University of Central Florida (scroll down to the bottom).
That's the last of the major telescope facilities to have issued releases. Some images from the Very Large Telescope in Chile have shown up at Franck Marchis' blog, though.
All this time, of course, there have been plenty of amateurs pointing their scopes at Jupiter (they now have three good reasons to do so!) The best place I know to check for collected amateur images is spaceweather.com. Here's a shot from discoverer Anthony Wesley dated July 24:A few hours later, a Spanish amateur, Juan Miguel González Polo, captured this lovely animation: Here's a very pretty one by David Kolb of Lawrence, Kansas, taken on July 26. Franck Marchis also posted on his blog another near-infrared image, this one from the Telescopio Nazional Galileo, a 3.5-meter instrument in the Canary Islands.
By the 28th, amateurs were reporting the mark to have elongated, developing two distinct lobes. Here's a pretty picture taken by Brazilian Fabio Carvalho on the 29th:The impact mark has continued to spread, lighten, and change shape. By August 1, amateurs were describing its shape as having a "hole" or perhaps as a "curlicue," and appeared to be fainter than before. By August 7, it had split into three distinct clouds. Many of the best amateur images were collected by German astronomer Hans Joerg Mettig and converted into polar projections; then Theo Ramakers assembled the polar views together into the movie below, which was posted on spaceweather.com on August 8. It didn't say what the spacing between images was but I am guessing it is approximately one day per frame, keeping in mind that the images were taken from spots all over the world at varying times of night so wouldn't be precisely one day apart. I think it's appropriate to close this too-lengthy entry with some words from Franck Marchis, in response to a common question: how often should this kind of event happen?
This is a very difficult question, and I am not sure I can provide a response. It is evident that amateur astronomers are equipped with state-to-art cameras which should allow them to detect these events more often. Because the planetary science community does not have a survey telescopes dedicated to observe the planets (unfortunately), we entirely rely on the contribution of amateur astronomers to detect these impacts. This is also true for detecting supernovae, recording light curves of asteroids and astrometric positions of near Earth asteroids. It is quite possible that over the last 15 years we have missed most of them. The future will tell us the frequency of these events, we need to keep the amateur astronomy [community] around the world motivated to make sure that these Giant planets are often observed, not only for the science, but also for their beauty and for inspiring the young generation.
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