Backward Integration of a SL9 Type Comet
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comet orbit Shoemaker-Levy 9 SL9↗
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Comets: What Do We Know and Where Do We Go From Here?
Understanding of comets has made a quantum advance since 1980 due to the Earth-orbiting observatories, to fly-bys of Halley and Giacobini-Zinner, to better ground-based instruments, and to the Shoemaker-Levy 9 impact with Jupiter. The present understanding and future plans are outlined.
comets Shoemaker-Levy↗
New Observations at the Slate Islands Impact Structure, Lake Superior
Slate Islands, a group of 2 large and several small islands, is located in northern Lake Superior, approximately 10 km south of Terrace Bay. Shatter cones, breccias and shock metamorphic features provide evidence that the Slate Islands Structure was formed as a result of asteroid or comet impact. Most of the island group is believed to represent the central uplift of a complex impact crater. The structure possibly has a diameter of about 32 km. For Sage (1978, 1991) shock metamorphic features, shatter cones and pervasive rock brecciation are the results of diatreme activity. The present investigations represent the second year of a co-operative study of the Lunar and Planetary Institute, Houston, Texas and the Field Services Section (Northwest) of the Ontario Geological Survey. The objective of this investigation is to come to a better understanding of the formation of mid-size impact structures on Earth and the planets of the solar system. Impact processes played a fundamental role in the formation of the planets and the evolution of life on Earth. Meteorite and comet impacts are not a phenomenon of the past. Last year, more than 20 pieces of the Shoemaker-Levy 9 impacted on Jupiter and the Tunguska comet impacted in Siberia in the early years of this century. The study of impact processes is a relatively young part of geoscience and much is still to be learnt by detailed field and laboratory investigations. The State Islands Structure has been selected for the present detailed investigations because of the excellent shoreline outcrops of rock units related to the impact. The structure is a complex impact crater that has been eroded so that important lithological and structural elements are exposed. We know of no other mid-size terrestrial impact structure with equal or better exposures. In this publication we present preliminary results of our 1994 and 1995 field and laboratory investigations. We have tentatively identified a few impact melt and a considerable number of suevite occurrences.
Dressler, B. O.↗
Shoemaker-Levy 9 and Plume-Forming Collisions on Earth
Computational models for the July, 1994 collision of comet Shoemaker Levy 9 with Jupiter have provided a framework for interpreting the observational data. Imaging, photometry, and spectroscopy data from ground-based, Hubble Space Telescope, and Galileo spacecraft instruments are consistent with phenomena that were dominated by the generation of incandescent fireballs that were ballistically ejected to high altitudes, where they formed plumes that subsequently collapsed over large areas of Jupiter's atmosphere. Applications of similar computational models to collisions into Earth's atmosphere show that a very similar sequence of events should take place for NEO impacts with energies as low as 3 megatons, recurring on 100 year timescales or less. This result suggests that the 1908 Tunguska event was a plume-forming atmospheric explosion, and that some of the phenomena associated with it might be related to the ejection and collapse of a high plume. Hazards associated with plume growth and collapse should be included in the evaluation of the impact threat to Earth, and opportunities should be sought for observational validation of atmospheric impact models by exploiting data already being collected from the natural flux of multikiloton to megaton sized objects that constantly enter Earth's atmosphere on annual to decadal timescales.
Boslough, Mark B. E.↗
Temperature, Size and Energy of the Shoemaker-Levy 9 G-Impact Fireball
The fortunate position of the Galileo spacecraft provided us with a unique opportunity to directly observe the Shoemaker-Levy 9 impacts as they occurred on the far side of Jupiter, and we present observations of the G fireball obtained by the Near Infrared Mapping Spectrometer (NIMS).
Comets↗
Nonlinear Propagating Features in the Stratosphere of Jupiter Generated by the Impact of SL-9
Among the more intriguing features observed by HST after the impacts of the fragments of Shoemaker-Levy 9 were rings, encircling several of the impact sites, that expanded radially at constant velocities of order 450-500 m/s. One credible suggestion has been that the rings are stratospheric manifestations of linear gravity waves seated in the troposphere (Ingersoll and Kanamori, 1995). In this scenario the waves are made visible by a condensible material of unknown composition. Based on simulations of the comet impact by Zahnle and Mac Low (1995), we suggest an alternative hypothesis, namely that the rings are nonlinear stratospheric gravity waves which break and entrain impact debris material in the breaking wave front. The computations produce just such a feature, with the nonlinear breaking wave front in the stratosphere propagating radially away from the impact site with the correct speed. The computed ring of material is located in an altitude region where the static stability profile has a local maximum and therefore forms a waveguide. Because entrained impact debris forms the ring feature in the model, a condensate of some unknown composition is not required in order to view the wave front. However, there is one severe problem with the computed nonlinear wave: in contrast to a linear wave, it slows down. The computed feature stops within 1000 seconds, while the observed feature lasted at least 10,000 seconds. Since the nonlinear breaking wave front is a natural product of our impact simulations, we are now investigating the conditions that will enable the breaking wave front to propagate further, in accord with the observations. The results of these computations will be presented.
Young, R. E.↗