WHEN the 19 fragments of Comet Shoemaker-Levy 9 met their violent end on Jupiter last July, it was the celestial drama of the century. Since then, astronomers have been wading their way through vast amounts of data in a bid to reconstruct the details of the collision. At last, a real picture is beginning to emerge.
Fortunately, they knew about the event well in advance and had prepared an arsenal of telescopes and instruments to witness it ("Live crash from Jupiter", New Scientist, 5 March 1994). But the predictions were cautious. Because the fragments would strike the far side of Jupiter, many astronomers were worried that the impacts would be invisible from the Earth. Also, no one knew whether the collisions would have any lingering effect on Jupiter's cloudy surface.
They need not have worried. The collisions occurred exactly when predicted, but there was more to see than they could ever have hoped. The damage was not subtle - even amateur astronomers using the simplest telescopes were able to watch the aftermath of the explosions. Making sense of this remarkable astronomical event has not been easy, mainly because there is so much evidence to sift through. During Comet Crash Week, scientists took more than a million pictures of Jupiter and recorded a similar number of spectra with wavelengths collectively spanning the whole electromagnetic spectrum. Now, just over five months later, most observers have analysed at least some of their data and the Galileo spacecraft, which had the only direct view of the impacts, has transmitted back to Earth most of its precious tape-recorded data. Galileo's transmission rate has been painfully slow because of the failure of the spacecraft's main antenna to open properly in April 1991.
The first surprise for the astronomers was how much they managed to see, even though the comet collided on Jupiter's dark side. It turned out that the impact site could not have been better for Earth-based observers. For one thing, fragments A, E, G and W created plumes shaped like ice-cream cones which were projected against the black sky above Jupiter's horizon, and were captured superbly on camera by the Hubble Space Telescope. These side views of the plume profiles provided a much better insight into plume eruption physics than looking directly down on them would have done, when they might have been lost in the glare of Jupiter's sunlit clouds. The Hubble science team, led by Heidi Hammel of Massachusetts Institute of Technology, found that all four plumes soared to about 3000 kilometres above Jupiter's ammonia cloud deck.
The angle of view
Even more fortunately, the fragments hit just behind the "morning" edge, which was in shadow, rather than the "evening" edge, which was bright. So, for example, Hubble managed to pick up light from the glowing plumes that soared up following impacts E and G because they were brightly lit against Jupiter's shadow
Jupiter spins rapidly - a Jovian day is less than ten hours long - so it took just 10 minutes for the impact sites to rotate into view and less than 10 more minutes for them to rotate into sunlight. High-altitude plumes caught the sunlight even sooner. And as plume debris rained back down into Jupiter's stratosphere, the planet's rotation towards the Earth kept pace, so both Hubble and ground-based observers were able to follow the plume decay.
Also, the Sun rose as the last plume debris re-entered Jupiter's stratosphere creating huge, black "bruises". For the next four hours astronomers photographed the evolution of each impact site, including waves that spread from the point of impact at 450 metres per second, before fading away about three hours later. The speed at which these waves travelled should help to reveal how they were formed, and may even provide clues about the depth to which the fragments penetrated.
If S-L 9 had instead hit just beyond Jupiter's evening edge, everything would have been hidden, even from Galileo's perspective. No shadowed region would have been visible from Earth. The highest plumes might have just appeared above Jupiter's edge before rotation quickly carried them below the horizon. We would have missed plume decay as well as the first few hours of debris spot evolution. By the time the sites had rotated into view five hours later, the propagating waves would have faded, the initial chemical evolution of Jupiter's perturbed atmosphere would be over, and remnant warmth from the millions of megatons of released energy would have dissipated.
The events surrounding one of the brightest and best observed fragments - fragment G - illustrate some of the important things that we have learned to date, and some of the questions that remain unanswered.
Observers at two telescopes at Siding Spring in Australia saw the first signs of the G impact at 7:33:00 on 18 July.
It appeared - in super-sensitive imaging at a wavelength of 2 micrometres - as a faint glow on Jupiter's morning edge. But if the reported times are accurate, the Australian observers must have seen something before the main G fragment hit, perhaps a small, precursor impact. The chief fragment itself was detected 30 seconds later by three instruments on the Galileo spacecraft. The impact flash changed in a few seconds through spectral hues from ultra-violet into the infrared. As something cools from a searing 10 000 K, its colour changes from blue-white, to yellow, to red-hot, to infrared invisibility, and that must have been what Galileo saw.
Galileo scanned Jupiter every 5 seconds during the G impact. At 7:33:32, the ultraviolet spectrometer detected a flash about 20 per cent of the brightness of Jupiter, which lasted only a few seconds and was gone. The photopolarimeter radiometer (PPR) also recorded the modest beginnings of an event at 7:33:32 which then brightened and lasted for more than half a minute. Comparison of the UVS and PPR data at 7:33:32 shows that the detected radiation came from a source just 7 kilometres across at a temperature of 8000 K. This was almost certainly the impact's "bolide" phase, where the fragment flashed like a brilliant meteor as it streamed down through Jupiter's atmosphere. PPR data on the H and L impacts, as well as Galileo camera images of K, N and W, also show they suddenly became brighter, going from nothing to maximum luminosity in just 2 or 3 seconds, and this is exactly the time it would take the fragments, which were travelling at 60 kilometres per second, to plunge down onto Jupiter's cloud tops.
The near infrared mapping spectrometer (NIMS) on Galileo cannot detect extremely hot phenomena, but is tuned to detect heat at temperatures of hundreds to a few thousand Kelvin by measuring at several longer wavelengths in the so-called thermal infrared. NIMS saw nothing at 7:33:32, but 5 seconds later it detected a rising signal that grew and decayed over about a minute and a half. NIMS spectra show that by 7:33:37 (5 seconds after impact), the luminous source had already cooled to about 6000 K (the surface temperature of the Sun). It was still low in the atmosphere, just above the ammonia clouds. But over the next minute and a half, NIMS measured the source expanding to 75 kilometres in size as it rose toward the top of Jupiter's atmosphere and cooled to just 450 K.
NIMS team leader Robert Carlson of the Jet Propulsion Laboratory in California comments: "Very simply, it looks to us like an expanding, cooling 'bubble' of hot gas." In other words, this was the "fireball" phase, in which a huge volume of Jupiter's atmosphere expanded and was expelled to form the beginnings of a plume.
A few months before the crash, Kevin Zahnle of NASA Ames Research Center in Moffett Field, California and Mordecai Mac Low of the University of Chicago predicted that there would be a 10 or 20-second delay between the fiery fragments disappearing beneath the clouds (the bolide phase) and the fireballs erupting back into view. But Mark Boslough and David Crawford, from Sandia National Laboratories, Albuquerque, used numerical simulations on the world's fastest computer, Sandia's 1840-processor Paragon, to predict that the upper part of the fiery bolide train would explode immediately, and would itself form the top of the fireball. Boslough and Crawford were proved right - there was no gap between the bolide phase and the subsequent fireball for impact G and the other well-observed impacts. So it seems that most of the observable activity was high in the atmosphere; whatever happened lower down was much harder to see.
At 7:33:46, Hubble's camera shutter closed after a 30-second exposure through its methane filter. The picture shows a bright glow above the shadowed edge of Jupiter, well below the level where it could be sunlit. At first, it seemed to be the radiant fireball, but scientists quickly realised that this could not be the case. If Galileo's recorded impact time of 7:33:32 is correct, the fireball would have to have rocketed upwards at an unbelievable velocity of 40 kilometres per second to reach Hubble's line of sight so soon. Hubble's open shutter must somehow have caught the bolide entry and/or fireball by reflected light, perhaps from high-altitude debris lofted by an earlier precursor event. Such hints of precursor events could mean that the G fragment was actually a collection of smaller comet fragments.
The South Pole Infrared Explorer (SPIREX) caught its first glimpse of G half a minute after impact. Although the actual explosion was already over, the show from the Earth was just beginning. Less than a minute later, looking through fog and 98 per cent humidity, it was the turn of NASA's Infrared Telescope Facility atop 13 000 foot Mauna Kea in Hawaii. Between 7:35:16 and 7:35:32, Hubble took a picture that shows the tip of the plume reaching sunlight, nearly 2000 kilometres above Jupiter's cloud deck, implying a vertical velocity of 15 kilometres per second. By 7:35:47, the Australian observers measured an infrared signal sixty times stronger than their first detection 3 minutes earlier.
Over the next 8 minutes, the G plume brightened a hundred times more as seen from Australia, due to the expansion detected by Hubble and the fiery debris beginning to cascade back down onto Jupiter. At its peak, the plume towered 3200 kilometres above Jupiter's clouds.
Observing from the South Pole, Hien Nguyen exclaimed: "My God, it was bright!" Soon afterwards, Peter McGregor at the Australian National University's telescope at Siding Spring took the most memorable picture of Comet Crash Week. It looks like a brilliant, star-like "explosion" on the edge of a faintly visible Jupiter, but is in fact the sunlit plume at its maximum extent and the glowing zone caused by the plume falling back onto Jupiter.
From Earth, fragment G's thermal radiation reached its maximum value about 19 minutes after impact, according to a team of observers flying out of Melbourne, Australia, on NASA's Kuiper Airborne Observatory. By then, the impact site had rotated into full view from the Earth and the debris was raining back down at more than 10 kilometres per second, elevating temperatures across a vast region of Jupiter's stratosphere. Over the next few hours, the impact apron gradually cooled.
Meanwhile, J. Watanabe and other astronomers at the Okayama Astrophysical Observatory in Japan reported that a new dark spot, larger than Jupiter's famous Great Red Spot and twice the size of the Earth, was rotating across the planet. The next time the G site rotated around, French observers detected - for the first time ever on Jupiter - the spectroscopic emission signature of hot carbon monoxide.
This could explain one of the biggest post-crash puzzles - what happened to the water? Comets contain large amounts of water ice. Also, before the crash everyone expected the fragments to penetrate beneath the hydrosulphide clouds on Jupiter to water-rich regions, from where the fireball would carry the water up into the visible stratosphere. So the question was not whether water would be observed, but what fraction would come from the comet and what fraction from Jupiter. Initial reports that no one had detected any water led to speculation, mainly in the news media, that Shoemaker-Levy 9 was really an asteroid, not a comet - a pointless exercise in nomenclature, since the interiors of many asteroids are thought to be as icy as comets.
Later, in a circular published by the International Astronomical Union, Gordon Bjoraker from NASA's Goddard Spaceflight Center in Maryland reported that the Kuiper Airborne Observatory had caught a brief glimpse of the spectral signature of hot water in impacts G and K before the temperatures cooled and the signatures (but not necessarily the water itself) disappeared. Water is highly reactive and may have been rapidly consumed. One possibility is that the oxygen from the water was converted into carbon monoxide, as detected for fragments G and K. The jury is still out on whether the water came from the comet or from Jupiter.
Another puzzle is that at first sight all the impacts seemed remarkably similar, in spite of the varying sizes of the different fragments. One of the most contentious issues before the comet crash concerned the sizes of the comet fragments. If they were 3 or 4 kilometres across, the original comet would have been the size of the huge projectile that struck the Earth 65 million years ago, possibly causing the extinction of the dinosaurs, and the like of which has fortunately not been seen since. But if they were only a few hundred metres across the situation is much more worrying. In that case, havoc such as that wrought on Jupiter could be caused by the smaller comets that strike the Earth much more often - every million years or so. The tiny N fragment's bolide brightness was more than half that of superfragment K. And the four plumes photographed by Hubble reached the same altitude, including giant G and wimpy W.
We may never be able to calculate fragment masses from the bolide and plume phenomena. After all, the total luminous energy measured by Galileo was less than 0.1 per cent of the total energy that a 1-kilometre solid fragment should have released upon impact with Jupiter. Since nobody imagines that all this havoc was wrought by fragments less than 100 metres in size, most of the energy must have been released invisibly. Perhaps all of the fragments had a similar effect on Jupiter's upper atmosphere as they streaked through, while vast differences were muffled deep within the planet's gaseous interior, and thus hidden from our view. It is as if we tried to determine, just from measuring how much water squirts from a hose, whether its source is a cistern or the whole city water supply.
Attempts to understand the effects of the comet on the chemistry of Jupiter's atmosphere have yet to produce clear results. Usually astronomers struggle to detect weak spectral signatures, but this time they were overwhelmed with information. The signatures varied with time as the impact sites evolved, and they also differed greatly from one impact to the next. Some stratospheric contaminants were clearly derived from the comet (see The astronomical detective's toolkit), while others were so abundant that they must have been dredged up from Jupiter's atmospheric depths. For example, it is inconceivable that the single G fragment could have brought in the 100 million tons of molecular sulphur (S2) - not to mention the sulphur contained in detected CS2 and H2S - that was measured several hours after the impact of fragment G by Hubble's Faint Object Spectrograph. Before S-L 9 no one had seen sulphur in any form in Jupiter's atmosphere, although a layer of ammonium hydrosulphide clouds had long been thought to exist beneath the visible ammonia clouds. It took S-L 9 to stir the pot.
There is so much information about the collisions that we should eventually learn the answers to these questions. Meanwhile, it is worth noting that for each impact, the release of explosive energy into Jupiter's atmosphere was all over within the first few seconds. Even the fireballs had cooled below Galileo's NIMS detection limit while they were still hidden from Earth behind Jupiter's horizon. What stunned observers on Earth over the ensuing 20 minutes was the aftermath - towering plumes of gas and the firestorm of debris dramatically plunging back into Jupiter's atmosphere, carried into direct view by the planet's rotation. The resulting black spots, some over 20 000 kilometres across, provided every Jupiter-watcher on Earth with a chilling warning of what a small comet or asteroid could do to the atmosphere of our own planet.
The astronomical detective's toolkit
THE terrific heat generated by the Shoemaker-Levy 9 impacts enabled astronomers to use powerful new observational tools to "see" the events. At optical wavelengths even the brightest impacts and plumes were over-whelmed by the glare from Jupiter's reflected sunlight, and were virtually invisible to Earth-based observatories. But at thermal infrared wavelengths (from 3 to 50 micrometres), Jupiter is normally faint because it receives hardly any warmth from the distant Sun and its internally generated heat is minimal. So "room-temperature" events, not to mention fireball phenomena at thousands of degrees, would be prominent by comparison. Sensitive infrared cameras have been developed only during the past decade.
Many of the beautiful photographs published during Comet Crash Week show the "light" of thermal radiation from the impacts' residual warmth and the firestorm of debris re-entering the atmosphere.
The most spectacular pictures used another technique in the infrared astronomer's toolkit - methane absorption band filters. Such filters admit only a small range of wavelengths, near 2 micrometres, centred on absorption bands in the spectrum of methane. Though methane is only a trace constituent (0.2 per cent) of Jupiter's atmosphere, it absorbs sunlight so effectively that Jupiter is virtually black at methane-band wavelengths, even at long exposures. Thus, any high-altitude, sunlight material above the methane (like ejecta arching through space), look brilliant against the muted backdrop of Jupiter. If such features are also hotter than several hundred degrees K, their 2 micrometre thermal radiation further enhances their brightness. Heat from ejecta plummeting back into the top of Jupiter's stratosphere 15 to 20 minutes after the impacts explains some of the longer-lasting features on the 2-micrometre filter pictures.
For days and weeks after the impacts, infrared images of Jupiter continued to show bright regions at the same places that visible images showed enormous, black "bruises" on Jupiter. They were black because they contained vast numbers of aerosol particles created in the explosions, perhaps from organic material in the cometary fragments, or from high-temperature processing of large volumes of Jupiter's atmosphere. Depending on wavelength, the infrared images recorded local stratospheric warmth left over from the impacts or sunlight reflected by the new, very-high-altitude aerosol layers, undimmed by methane absorption.
Many tell-tale spectroscopic lines, used for chemical analysis, are activated only at hot temperatures. A number of chemical compounds never seen before in Jupiter's stratosphere suddenly, though briefly, radiated their spectroscopic signatures, thanks to the temporary heat in the aftermath of the explosions. Some of the new or enhanced spectral lines were due to normal constituents of Jupiter's atmosphere at abnormally high temperatures.
Radiation emitted from excited methane molecules during the first 15 minutes after several impacts showed temperatures near 1000 K. Other chemical species (such as sulphur compounds) were dredged up by the fireballs from beneath Jupiter's ammonia cloud decks, where they normally reside. Disintegrated cometary material also contributed to the wealth of spectral emission lines. Keith Noll at the Space Telescope Science Institute in Maryland and his Hubble team detected ionised metals including magnesium and iron, which are usually found in rocks, not atmospheres. Jupiter's own stratospheric chemistry also joined in, as chemical reactions fostered by high temperatures redressed the imbalances caused by the impacts. This rich chemical feast, revealed by a panoply of spectral lines from ultraviolet to radio wavelengths, will take years to decipher completely.
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