Changes in Jupiters Surface within Human History Comet Shoemaker Levy

A major event in the histories of both Jupiter and mankind occurred in July 1994, when the comet Shoemaker-Levy collided with Jupiter and was witnessed by both ground-based observatories and the then-nearby Galileo spacecraft. The comet was first discovered in March 1993 by the immensely influential husband-and-wife scientific team of Eugene and Carolyn Shoemaker and their colleague David Levy, using the 1.3-foot (0.4-m) Schmidt telescope on Mount Palomar in California. The comet had already broken into a chain of about 20 fragments, as shown in the image from the Hubble Space Telescope. As its orbit was better defined, it was identified as a Jupiter-family planet whose orbit had been disrupted by tidal forces from Jupiter in the previous July, and that it would in fact crash into the planet the following July. From modeling efforts of the Jupiter system, it has been estimated that a giant impact of this type should only happen once every few hundred years, and in every way its observation was a great opportunity for the advancement of the understanding of giant impacts and the Jupiter system (it is possible that such an event had been seen by Giovanni Cassini in 1690 at the Paris Observatory, when he reported and made drawings remarkably similar to the images of the Shoemaker-Levy impacts).

Observers prepared for the impact, taking images and making measurements at wavelengths from the infrared to the radio, from at least 30 professional ground-based observatories, and from Galileo, the Hubble Space Telescope, the International Ultraviolet Explorer, and the X-ray satellite ROSAT. There was a lot of disagreement about what to expect from the impacts. Some scientists predicted that the impacts would be virtually invisible from Earth because they would produce so little energy. Others predicted that the icy comet fragments would burn up like meteors in Jupiter's atmosphere, leaving nothing to make an impact. As it turned out, the impacts were far more exciting than most people had dared hope: The visible light flash from each impact was so bright that it could be seen clearly even with a tiny backyard two-inch refracting telescope at a magnification of 75 times.

The various fragments entered Jupiter's atmosphere between July 16 and 22, 1994, at about 44 degrees south latitude.The first impacts struck the surface just behind the edge of the disk of Jupiter, on the night side of the planet out of sight from Earth, but they rotated into view after about 10 minutes. By great good timing and fortune, the Galileo spacecraft did have a direct view of the impacts.Though Earth-based observers could not see the impacts themselves, they could see flashes of light reflect off Jupiter's inner moons.

The moment of impact of each fragment is badly recorded, and the moment of impact appears to be unimpressive in terms of visible light emissions. About a minute after each impact, though, came the more dramatic explosion phase: A brilliant visible-light flash was created by the explosion of the fragment when it reached high enough temperatures and pressures, and by the rapid rise of the resulting fireball.Two large concentric waves radiated out through the atmosphere from each impact site, moving at 980 to 1,600 feet per second (300 to 500 m/sec), in a mechanism similar to earthquake waves. Starting about six minutes after impact, the splash phase began, when ejected material began raining back into the atmosphere, causing significant heating.The ejecta appears to have been thrown as high as 1,875 miles (3,000 km), and its raining back took about 15 minutes.

The four images below of Jupiter and the bright night-side impact of fragment W of the comet were taken by the Galileo spacecraft.The spacecraft was 148 million miles (238 million km) from Jupiter at the time, about 40 degrees from Earth's line of sight to Jupiter, permitting this direct view. The images were taken at intervals of two and one-third seconds, using the green filter in visible light. The first image shows no impact. In the next three images, a point of light appears,

Galileo was positioned to photograph the comet Shoemaker-Levy fragments as they impacted Jupiter. The impact occurs in a shadowed region of Jupiter in these images, but their energy output saturated the camera's photo elements. (NASA/jPL/Ga/i/eo)

brightens so much as to saturate its picture element, and then fades again, seven seconds after the first picture. The location is approximately 44 degrees south as predicted; dark spots visible in the figure on page 53, to the right, are from previous impacts.

Based on modeling of the radiation emissions of the phases of impact, it has been estimated that the fragments varied in size from about 245 to 985 feet (75 to 300 m) in radius (note that these are much, much smaller than the near-Earth asteroids that are being tracked for possible Earth impact). Each observed impact was given a letter designation starting with A, and there seem to have been 21 impacts. The largest fragment, G, struck Jupiter with an estimated energy equivalent to 6,000,000 megatons of TNT (about 600 times the world's estimated total arsenal). It is not known how deeply the fragments penetrated, though some appeared to explode above the cloud layer, and some below.

The temperatures of the fragments' fireballs reached 18,000°F (10,000°C) immediately, and decreased to about 3,600°F (2,000°C) within 15 seconds.The diameters of the fireballs began at about 10 miles (15 km), and grew to 60 miles (100 km) after 40 seconds.Violent eddies began in the atmosphere around the impact sites. Despite these huge temperatures and sizes, the splash phase, when ejecta reentered the atmosphere, was by far the most consequential for the planet: The atmosphere was significantly heated, and new molecules (H, C, S, O, and N compounds) were formed in the upper atmosphere because of the kinetic heating of the falling ejecta. The atmosphere in the region of the ejecta was heated by hundreds of degrees, and cooling to normal temperatures took a day or two.

Some of the new molecular species caused by the ejecta were still detectable years after the impacts. Each impact site was marked by a brown spot, probably a layer of condensed matter from the fireball and plume, and most were still visible after five months, even in Jupiter's active atmosphere. These effects mirror what scientists have suspected would happen on Earth in the case of a giant impact:The heating of the atmosphere from ejecta would trigger worldwide forest fires, and changing the composition of the atmosphere and injecting dust and ash into it would block solar radiation, perhaps for years. The impacts were such a huge event in planetary science that within two years, over 100 scientific papers about the event had been published.

Since the beginning of space observation through telescopes, in 1608, Jupiter has been a great subject of study. Its immense size and brightness and relative nearness compared to the other gas giant planets have all made it a desirable viewing target. With the development of better and better observational tools, Jupiter has simply become more interesting and informative: Its wildly powerful storms and ever-changing weather patterns are relatively easy to watch and form a kind of window into Jupiter's internal processes and temperature structure. Jupiter's huge gravity field attracted comet Shoemaker-Levy, and its multiple impacts have formed a seemingly infinite source of data for the scientists who study impacts. Scientific papers on that topic are still being published, and Jupiter will no doubt stay a main focus of planetary study.

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