Those astronomers who watched the Jupiter extravaganza emerged almost as worn and battered as the planet by the time the final fragment of Comet Shoemaker-Levy 9 exploded into smithereens last Friday. During the week-long spectacle, telescopes around the world recorded thousands of pictures at every possible wavelength - visible, infrared and ultraviolet. The cataclysmic images included fireballs 2000 kilometres across rising above the rim of Jupiter. They were enough to frighten the US Congress into taking the possibility of asteroids and comets heading towards Earth a bit more seriously.
While the fireworks were expected, the persistence of dark scars long after the fragments of comet exploded was not. The composition of the debris thrown out by the explosions also puzzled astronomers. It included sulphur compounds and metals astronomers had not anticipated, but not the water they had expected. Other surprises are just beginning to emerge, including the discovery that some old drawings of Jupiter show spots which resemble last week's impact scars.
The flood of data from the observations slowed traffic on the Internet and threatened to overwhelm the astronomers coordinating information for NASA. 'We've only shown you a tiny fraction of the images we have,' said Heidi Hammel of the Massachusetts Institute of Technology at NASA's final press briefing. 'It will take quite a while to put them together.'
The dark spot left by the first impact, of fragment A, remained clearly visible after a week, although the Jovian winds were smearing it out. According to Hammel, the spot has developed 'a little curlicue tail', where the comet's debris seems to be interacting with a white spot, a known feature in the Jovian atmosphere. She also reports other changes in the dark spot: 'What used to be a tiny streak with a black-eye smudge underneath is starting to evolve.' Even more complex changes were evident at the larger spot created by the giant impact of fragment G.
The dark areas which mark the spots where the fragments hit Jupiter 'are not holes or waves', says Robert West of the Jet Propulsion Laboratory in Pasadena. They are 'mostly dark particles and ultraviolet-absorbing gases' lingering high in the Jovian atmosphere, he says. The explosions as each fragment burst through Jupiter's clouds thrust cometary dust and other hot material high into the atmosphere. The way the debris changes over the coming weeks will give astronomers their first clues to the nature of the winds in the upper Jovian atmosphere.
One major question is how long the spots will last. 'The largest features are hanging in there pretty well,' says Hammel. Some astronomers think the spots may last a year or more. 'When stuff gets into stable layers (of the atmosphere) it will stay there a long time,' says Melissa McGrath of the Space Telescope Science Institute. She cites the eruption of Mount Pinatubo as an Earthly example. The dust from that explosion stayed in the atmosphere for several years, influencing global climate. However, Eugene Shoemaker of the Lowell Observatory in New Mexico, one of the comet's discoverers, points out that 'all the features we see on Jupiter are transitory. Even the great red spot must have a finite lifetime'.
Astronomers did not always find what they expected in the impact zones. 'We thought we would see hydrocarbons, which are normally present in the Jovian atmosphere,' says McGrath. Instead, at the early stages detectors picked up emissions at wavelengths characteristic of sulphur compounds. As the spots grew darker, emission 'spikes' characteristic of magnesium, silicon and possibly iron appeared. McGrath believes these come from comet debris.
The biggest puzzle is the scarcity of water and other oxygen compounds, expected to be present both in the comet and on Jupiter. The outer clouds of Jupiter's atmosphere are made of ammonia ice. Astronomers had expected to find clouds of ammonium hydrosulphide beneath the ammonia, with clouds of water ice below that, 50 to 100 kilometres below the ammonia clouds. If the comet fragments had penetrated as deeply as anticipated, their explosions should have hurled water vapour into the upper atmosphere. Yet observers failed to detect any convincing sign of it.
The absence of water is one reason researchers doubt that the fragments of Shoemaker-Levy 9 penetrated deep into the Jovian atmosphere. A second reason to suspect that the fragments exploded high up comes from Andrew Ingersoll of the California Institute of Technology. He detected low-frequency sound waves from the impact. A strong wave spread from the site of the G impact at 800 metres per second, the speed of sound in the Jovian stratosphere. He detected only a much weaker wave deeper in the atmosphere, where the speed of sound is slower. The much stronger wave in the stratosphere 'suggests the comet lost most of its energy up higher, rather than deeper', says Ingersoll.
Large objects entering the Earth's atmosphere also explode high up. The largest impact in historic times devastated 2000 square kilometres of Siberia when a 30-metre object exploded about 10 kilometres above the Tunguska region.
No single instrument recorded the week-long barrage. Exhausted staff coordinating the observations for NASA could not keep up with reports from around the world, and at the end of the week no one had a master list of impacts.
Observers agree that the impacts varied in their effects, with some differences caused by the fragment's position before impact. The 21 pieces of the comet were strung out in a line, generally known as the 'string of pearls'. But while some pieces lay along the 'main line' of the shattered comet, others were slightly off-line. The fragments on the main trajectory produced much brighter impacts than those off the line. Impacts of three off-line fragments, T, U and V, may not have been seen at all. Dan Green of the Harvard-Smithsonian Center for Astrophysics suggests the off-line fragments hit Jupiter with less energy because they were 'much more fluffy and not strongly held together'.
While T, U and V may have left no trace, astronomers did detect the impact of fragment M, which vanished from view months ago. That pleased Shoemaker, who had suspected that a solid body had remained but had 'just turned off' the release of gas and dust that produced the bright 'pearls' visible from Earth.
One event did disappoint - the 'triple whammy' of fragments Q, R and S. The pieces hit almost the same spot on successive rotations of Jupiter, which turns every 10 hours, but the impact zones did not overlap. Fragment R broke up into at least two pieces before slamming into Jupiter.
The fireworks may have ended, but observations will continue. David Levy, the amateur astronomer who discovered the comet with Eugene and Carolyn Shoemaker, urged amateurs to continue watching Jupiter as it fades into the evening twilight. Many amateur telescopes show the spots clearly, and frequent observations are needed to track their evolution.
Astronomers will also be searching old records for signs of previously unrecognised impacts on Jupiter. Brian Marsden of the Harvard-Smithsonian Center thinks comets may hit Jupiter more often than the once in a thousand years estimated by Eugene Shoemaker. Some people suspected old drawings might show something, says Marsden, 'but they weren't quite sure what to look for'. The dark spots that appeared on Jupiter last week jogged memories.
Thomas Hockey of Northern Iowa University recalled from his past research that several spots were reported from 1690 to 1872 by observers including William Herschel and Giovanni Cassini. Marsden quickly uncovered other observations in publications of the British Astronomical Association. In 1927, a T. E. R. Phillips drew five dots in a row on Jupiter on 28 July, and ten on 2 August. Marsden then pulled another volume from his shelf and found a 1948 drawing showing 'two rather nice black spots' on Jupiter.
No one has found records of anything as spectacular as the Shoemaker-Levy impacts, and both Marsden and Hockey warn that there could be other explanations for dark spots. Drawings can include mistakes and visual distortions, such as the nonexistent Martian 'canals'. However, drawings can record features that are only visible during the clearest instants. These fleeting sightings would not show in long-exposure photographs taken on old, insensitive film.
The US Congress needed no further evidence that apocalyptic collisions are more common than Shoemaker estimates. As pieces of comet pelted Jupiter last week, the House Committee on Science, Space, and Technology decided that NASA should be required 'to catalogue and track any major comets or asteroids that may cross the orbit of the Earth'.
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14:09 04 July 2008
