Together with most of LPI's summer intern students, we return to LPI after a futile attempt to witness the impact of fragment B at the local observatory at Brazos Bend State Park. The impact of this second of 21 broken comet fragments to hit Jupiter was supposed to be visible from North America. Instead, we eagerly await the first images of the first impact earlier that afternoon from the Hubble Space Telescope. They are quickly available on the Internet via the World Wide Web and are of stunning quality (proving in one evening the worth of HST to science, and the worth of the Web for that matter). One image sequence shows a plume of hot gas rising hundreds of kilometers above Jupiter's horizon. Another image shows a prominent dark spot on Jupiter's cloudy surface, surrounded by a semicircular dark ring as large as the Earth. Clearly the first fragment, believed to be one of the smaller ones, had had a dramatic impact on Jupiter's appearance. We could only wonder what the other larger fragments might do to the giant planet.
HST images obtained 1 1/2 hours after the impact of the largest fragment, G, of Shoemaker-Levy 9. The spot is larger than Earth even after this short time.
When astronomers on seven continents aimed every available telescope and instrument at Jupiter during the week of July 16, there were many predictions, ranging from "The Big Fizzle" to major impacts, to the disruptions of personal horoscopes and other catastrophes (Ice Ages) here on Earth. Although the magnitude of the damage done to Jupiter came as a pleasant surprise, equally surprising to scientists was that the event had actually produced visible results. After the failure of comet Kohoutek in 1974 and the great Perseid meteor storm of 1993 to materialize as predicted, most astronomers were extremely reluctant to predict anything (at least in public) concerning what might be seen.
HST images of Jupiter obtained on July 17, 1994. Visible in the southern hemisphere are 3 spots, formed by the impacts of (from left to right) the C, A, and E fragments. Also visible in the ultraviolet image are Jupiter's polar aurorae.
The frantic efforts of astronomers to observe the events in all portions of the spectrum was simple to understand: no one could predict with certainty what would happen, we have never witnessed a celestial collision before (LPI Bulletin, November, 1993). In general, many of the events did behave as expected (although a few fragments did produce no visible effects and "fizzled"). Numerical simulations suggested that the fragments, striking at 60 km per second, would penetrate to near or below the visible ammonia cloud decks and explode. The resulting plume of hot gas would expand back up the column bored out by the fragment and expand upward, somewhat like a nuclear fireball. The plume would then collapse, splash back down, and spread out laterally in the upper atmosphere after reaching its maximum height. The unanswered questions were: How big were the fragments, how deep would they penetrate before breaking up, and what effect would the impacts have on the atmosphere?
Dramatic plume eruptions, seen rising above the planet's limb, were imaged by HST for several of the impacts. The plumes were also very prominent in thermal infrared wavelengths (e.g., 10 microns) and persisted for at least 30 minutes before gradually fading. The impact sites were also very bright in near- infrared wavelengths (e.g., at 2.3 microns and in other methane bands) and remained so for several weeks. Methane absorbs light at these wavelengths and the persistence of these spots demonstrates that the material in these plumes rose to high altitudes above the visible cloud decks and remained in the stratosphere for some time.
Most atmospheric scientists adventurous enough to hazard predictions expected bright clouds rather than dark spots. They expected bright water or ammonia ice to condense as the fireball cooled. Instead, the condensed material was dark at visible and ultraviolet wavelengths, and the spots were described as the most prominent features visible on Jupiter's surface since Galileo first examined the planet with a telescope in 1610. The dark spots began spreading and twisting in the winds within a week after formation. The evolution of these dark spots will help us map the winds and circulation in the Jovian stratosphere, a region about which we currently have little information. The dark spots, visible in telescopes as small as 6 inches for a week or so after the events, were observed by amateur astronomers who continue to monitor their evolution.
Hubble observed the plume of hot gas from the impact of the A fragment on July 16. During a span of 12 minutes, the plume is seen rising into the sunlight and then collapsing. The dark zone between plume and planet is the shadow of Jupiter.
The composition of the dark material in the spots is still poorly understood but could help us to understand Jovian atmospheric chemistry and the composition of the comet. Some molecules suspected or known to be present on Jupiter were observed after the impacts. These include ammonia, hydrogen cyanide, carbon monoxide, methane, ethane, acetylene, and hydrogen sulfide. Each impact site had its own spectral signature and time evolution, suggesting that the comet fragments were different sizes and/or were of variable composition. Surprising was the detection of diatomic sulfur (S2), carbon disulfide (CS2) and carbon monosulfide (CS), all of which have difficulty forming in the presence of large quantities of oxygen and hydrogen. Most surprising was the apparent nondetection of water and other oxygen species for all but the largest of the impacts. If our current understanding of shock chemistry is correct, then the comet was apparently water poor (or it may have been an asteroid), and it did not penetrate to the level of the water clouds near 3 bars pressure. Metals were also observed in the G cloud several days after it formed, indicating the fragments contained a lot of rocky material. Only the Galileo spacecraft had a direct view of the impacts. It observed bright flashes lasting between 10 and 30 seconds during the K impact and the final impact, W.
The Galileo spacecraft obtained the only direct views of the impact. These 4 views taken over a space of 7.5 seconds show the impace of the last major fragment, W, on July 22.
(Drs. Schenk and Moses are Staff Scientists at LPI.)
Near infrared (2.2 microns) image of Jupiter on July 21. Numerous high altitude spots persist even several days after impact, forming a "necklace" of spots about Jupiter's South Pole.