Cometary conundrum

Fresh data on the collision of shards of Shoemaker-Levy 9 with Jupiter last year has divided scientists on the size of the comet fragments

Published: Tuesday 31 January 1995

Dimension dilemma: how big wer (Credit: Nehru Planetorium)AS scientists unravel more data on the crash of the comet Shoemaker-Levy 9 with Jupiter, consensus among experts on the basic aspects of the impact seems to be fast disappearing. New evidence suggests that the size of the cometary fragments, for instance, might have been much smaller than believed.

Most researchers who witnessed the celestial event were entranced by the brilliance of the impact flashes, and the sight of debris plumes soaring thousands of kilometres above the planet. On the assumption that the larger the fragments, the bigger would be the fireballs, they concluded that all the fireworks they observed were caused by the collision of impactors several kilometres in diameter. However, new evidence reveals that most of the comet's fragments were less than half a kilometre in diameter.

Argues Clark Chapman of the Planetary Sciences Institute in Tucson, Arizona, "The impact events occurred on the night side of Jupiter, making a direct observation from Earth impossible." Based on the data just returned from the spacecraft Galileo which is en route to Jupiter and which had the direct view of the impact events, Chapman further explains that the fireballs created as 3 of the 21 fragments of Shoemaker-Levy ploughed into Jupiter were considerably less brilliant than first thought. So, while looking for the fireballs to rise from the back of the planet, Earth-based scientists must have mistaken the heat produced when the impact debris fell back onto the upper Jovian atmosphere as the original fireball flash (Science, Vol 266, No 5187).

Scientists had also first expected that shock cloudwaves and globe girdling seismic vibrations would be produced when the fragments of Shoemaker-Levy struck Jupiter. These vibrations were to reveal themselves above the clouds as concentric rings of subtle temperature change. But Benoit Mosser of the Astrophysics Institute of Paris, who examined 4 Shoemaker-Levy impacts, including the 2 that left sizeable debris clouds, and searched for the temperature change at infrared wavelengths, found nothing. Mosser argues that "the failure to detect seismic waves which are usually generated after the impact could mean that the fragments had a diameter less than 0.5 km, if they had the density of ice".

Further doubts on the magnitude of the fragments comes from the observations of Heidi Hammel of the Massachusetts Institute of Technology, Boston. Hammel reports that all the impact plumes appeared nearly at the same height of about 3,300 km, regardless of the size of impactors that generated them. This confounds the impact modellers who have tried to use the heights of the plumes to estimate the diameter of the comet fragments. This finding also indicates that plume height may not be a meaningful discriminator for predicting the diameter of the impactor.

"Much of the confusion," says Paul Weissman of the Jet Propulsion Laboratory in Pasadena, California, "is attributed to the failure to see the cometary fragments which were cloaked in dust, just before the actual collisions. The morphology of the plumes or debris clouds may provide a clue to how large the impactors were and how deep they penetrated into the Jovian atmosphere."

Scientists monitoring the impact-events claim to have observed large amounts of sulphur compounds, carbon monoxide and water in the plumes. The amount of sulphur was so great that it must have had a Jovian source, presumably the clouds of ammonium hydrosulphide present in the planet's atmosphere. But the sources of carbon, oxygen and hydrogen were not clear. Most theorists also agree that the fragments seem to have deposited a large amount of energy high in atmosphere, above Jupiter's cloud decks (Nature, Vol 372, No 6505).

Kevin Zahnle of the NASA Ames Research Centre and Mordecai Mark Mac Low of the University of Chicago, using a 2-dimensional simulation, have argued that "the comet deposited its energy high up in the Jovian atmosphere because it was small, only 0.5-1.0 km in diameter". Their model predicts that the comet must have penetrated the ammonia and ammonium sulphide clouds in Jupiter, but could not reach the water clouds. This models explains the presence of large amounts of sulphur but lack of Jovian water, as observed in the ejected plumes. The water observed in the plumes must have come from the cometary fragments.

However, modellers Mark Boslough and David Crawford of the Sandia National Laboratories, using a 3-dimensional simulation, have speculated that "the impactor had to be large, up to 3 km in diameter, so that its large cross-section would result in substantial energy release at such high altitude". However, their model fails to explain why large amounts of sulphur were seen in the debris clouds by the Hubble Space Telescope.

The key to answering many of the questions may be new data from the spacecraft Galileo, which had the only direct view of the cometary impacts on the nightside of the giant planet. But because of damaged high-gain antennae, data from Galileo has trickled back to Earth at a mere 10 bits per second.

Since both sides have their arguments, none of which are conclusive --- and since it is impossible to re-enact this celestial event --- it seems that an early consensus, among scientists, on the size of the cometary fragments and the depth of their penetration in the Jovian atmosphere, is likely to remain elusive.

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