David A. Rothery Teach Yourself Planets, Chapter 9, pp. 107-39, Hodder Education, 2000, 2003.
Copyright © David Rothery
A remarkable opportunity to study Jupiter's atmosphere presented itself with the discovery in March 1993 of comet Shoemaker Levy 9 on a collision course with the planet. The comet had evidently been captured into orbit about Jupiter some decades previously, but this orbit was unstable. It had made an unnoticed close pass by Jupiter in July 1992, only about 21,000 km above the cloud tops, which had tidally disrupted it into a string of about 20 fragments (showing that this comet, like many asteroids, was a loosely bound rubble pile). When discovered these fragments were heading away from Jupiter prior to falling back for a series of 60 km per second impacts over the period 16-22 July 1994.
The impact coordinates were on Jupiter's night-side just out of direct view from the Earth, but fortunately in a position where Jupiter's rapid rotation would bring the site of each impact into sunlight and direct view from the Earth about 25 minutes later. Even more fortunately, the Galileo spacecraft, still 240 million km and 17 months away from arrival but with a significantly different vantage point, had a direct view of the impact site. Some images of the impacts are shown in Figure 9.7.
The Hubble Space Telescope images in this figure show a spot to the left of the most recent impact, which marks the site of an impact that happened 19 hours 43 minutes (almost two rotations) previously. These impact scars were remarkably persistent, some remaining visible for many months before shearing in the atmosphere completely disrupted them. The nature of the dark material, which was probably ejected from some 30 km below the cloud tops remains uncertain, even the professional literature referring to it simply as 'dark stuff'.
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