Any body moving through an atmosphere is slowed by atmospheric drag, by having to push the molecules of that atmosphere out of the way. The kinetic energy lost by the body is given to the air molecules. They move a bit faster (become hotter) and in turn heat the moving body by conduction. This frictional process turns energy of mass motion (kinetic energy) into thermal energy (molecular motion). The drag increases roughly as the square of the velocity. In any medium a velocity is finally reached at which the atmospheric molecules can no longer move out of the way fast enough and they begin to pile up in front of the moving body. This is the speed of sound (Mach 1 -- 331.7 m/s or 741 mph in air on Earth at sea level). A discontinuity in velocity and pressure is created which is called a shock wave. Comet Shoemaker-Levy 9 will enter Jupiter's atmosphere at about 60 km/s, which would be about 180 times the speed of sound on Earth (Mach 180!) and is about 50 times the speed of sound even in Jupiter's very light, largely hydrogen atmosphere.
At high supersonic velocities (much greater than Mach 1) enough energy is transferred to an intruding body that it becomes incandescent and molecular bonds begin to break. The surface of the solid body becomes a liquid and then a gas. The gas atoms begin to lose electrons and become ions. This mixture of ions and electrons is called a plasma. The plasma absorbs radio waves and is responsible for the communication blackouts that occur when a spacecraft such as the Space Shuttle reenters Earth's atmosphere. The atmospheric molecules are also dissociated and ionized and contribute to the plasma. At higher temperatures, energy transfer by radiation becomes more important than conduction. Ultimately the temperatures of the plasma and the surface of the intruding body are determined largely by the radiation balance. The temperature may rise to 50,000 K (90,000 deg F) or more for very large bodies such as the fragments of Shoemaker-Levy 9 entering Jupiter's atmosphere at 60 km/s. The loss of material as gas from the impacting body is called thermal ablation. The early manned spacecraft (Mercury, Gemini, and Apollo) had "ablative heat shields" made of a material having low heat conductivity (through to the spacecraft) and a high vaporization temperature (strong molecular bonds). As this material was lost, as designed, it carried away much of the orbital energy of the spacecraft reentering Earth's atmosphere.
There are other forms of ablation besides thermal ablation, the most important being loss of solid material in pieces. In a comet, fragile to begin with and further weakened and/or fractured by thermal shock and by melting, such spallation of chips or chunks of material has to be expected. Turbulence in the flow of material streaming from the front of the shock wave can be expected to strip anything that is loose away from the comet and send it streaming back into the wake. The effect of increasing temperature, pressure, and vibration on an intrinsically weak body is to crush it and cause it to flatten and spread. Meanwhile the atmosphere is also increasing in density as the comet penetrates to lower altitudes. All of these processes occur at an ever increasing rate (mostly exponentially).
On Earth a sizable iron meteoroid or even some relatively low velocity stony meteoroids can survive all of this and impact the surface, where we collect them for study and exhibition. (Small bodies traveling in space are called meteoroids. The visible phenomena which occur as a meteoroid enters the atmosphere is called a meteor. Surviving solid fragments are called meteorites. There is no sharp size distinction between meteoroids and asteroids. Normally, if the body has been detected telescopically before entering the atmosphere, it has been called an asteroid.) Many meteoroids suffer what is called a "terminal explosion" when crushed while still many kilometers above the ground. This is what happened in Tunguska, Siberia, in 1908. There a body with a mass of some 109 kg (2.2 billion lb.) and probably 90 to 190 m in diameter entered Earth's atmosphere at a low angle with a velocity of less than 15 km/s. It exploded at an altitude of perhaps 5- 10 km. This explosion, equivalent to 10-20 megatons of TNT, combined with the shock wave generated by the body's passage through the atmosphere immediately before disruption, leveled some 2,200 km^2 of Siberian forest. The Tunguska body had a tensile strength of some 2x10^8 dynes/cm^2, more than 100,000 times the strength of Shoemaker-Levy 9, but no surviving solid fragments of it (meteorites) have ever been found. The fragile Shoemaker-Levy 9 fragments entering an atmosphere of virtually infinite depth at a much higher velocity will suffer almost immediate destruction. The only real question is whether each fragment may break into several pieces immediately after entry, and therefore exhibit multiple smaller explosions, or whether it will survive long enough to be crushed, flattened, and obliterated in one grand explosion and terminal fireball.
Scientists have differed in their computations of the depths to which fragments of given mass will penetrate Jupiter's atmosphere before being completely destroyed. If a "terminal explosion" occurs above the clouds, which are thought to lie at a pressure level of about 0.5 bar or roughly 0.5 Earth atmosphere (see Section 5), then the explosion will be very bright and easily observable by means of light reflected from Jupiter's satellites. Using ablation coefficients derived from observation of many terrestrial fireballs, Sekanina predicts that the explosions indeed will occur above the clouds. Mordecai-Mark Mac Low and Kevin Zahnle have made calculations using an astrophysical hydrodynamic code (ZEUS) on a supercomputer. They assume a fluid body as a reasonable approximation to a comet, since comets have so little strength, and they predict that the terminal explosions will occur near the 10-bar level, well below the clouds. Others have suggested still deeper penetration, but most calculations indicate that survival to extreme depths is most unlikely. The central questions then appear to be whether terrestrial experience with lesser events can be extrapolated to events of such magnitude and whether all the essential physics has been included in the supercomputer calculations. We can only wait and observe what really happens, letting nature teach us which predictions were correct.
O.K. So an explosion occurs at some depth. What does that do? What happens next? Sekanina calculates that about 93% of the mass of a 10^13-kg fragment remains one second before the terminal explosion and the velocity is still almost 60 km/s. During that last second the energy of perhaps 10,000 100-megaton bombs is released. Much of the cometary material will be heated to many tens of thousands of degrees, vaporized, and ionized along with a substantial amount of Jupiter's surrounding atmosphere. The resulting fireball should balloon upward, even fountaining clear out of the atmosphere, before falling back and spreading out into Jupiter's atmosphere, imitating in a non-nuclear fashion some of the atmospheric hydrogen bomb tests of the 1950s. Once again, the total energy release here will be many thousands of times that of any hydrogen bomb ever tested, but the energy will be deposited initially into a much greater volume of Jupiter's atmosphere, so the energy density will not be so high as in a bomb, and, of course, there will be no gamma rays or neutrons (nuclear radiation or particles) flying about. The energy of these impacts will be beyond any prior experience. The details of what actually occurs will be determined by the observations in July 1994, if the observations are successful.
If differential gravitation (tidal forces) should further fragment a piece of the comet, say an hour or two before impact, the pieces can be expected to hit within a second of each other. In one second a point at 44 deg. latitude on Jupiter will rotate 9 km (5.6 mi.), however, so the pieces would enter the atmosphere some distance apart. Smaller pieces will explode at higher altitudes but not so spectacularly. If smaller pieces do explode above the clouds, they may be more "visible" than larger pieces exploding below the clouds. It is also possible that implanting somewhat less energy density over a wider volume of atmosphere might create a more visible change in Jupiter's atmosphere. Sekanina notes that pieces smaller than about 1.3-km mean radius should not be further fragmented by tidal forces unless they were already weakened by earlier events.
One of the more difficult questions to answer is just how bright these events will be. Terrestrial fireballs have typically exhibited perhaps 1% luminous efficiency. In other words about 1% of the total kinetic energy has been converted to visible light. The greater magnitude of the Jupiter impacts may result in more energy appearing as light, but let's assume the 1% efficiency. Then Sekanina calculates that a 10^13-kg fragment, a reasonable value for the largest piece, will reach an apparent visual magnitude of -10 during the terminal explosion. This is 1,000 times Jupiter's normal brilliance and only 10 times fainter than the full moon! Sekanina, of course, calculates that the explosions will occur above the clouds. And, remember that, unfortunately, these impacts will occur on Jupiter's back side as seen from Earth. There will be no immediate visible effect on the appearance of Jupiter. The light of the explosion may be seen reflected from the Galilean satellites of Jupiter, if they are properly placed at the times of impacts. Ganymede, for example, might brighten as much as six times, while Io could brighten to 35 times its normal brilliance for a second before fading slowly, if the explosions occur above the clouds. This would certainly be visible in an amateur telescope and could conceivably be visible to the naked eye at a dark mountain site as a tiny flash next to Jupiter at the location of the normally invisible satellite. Emphasis on "tiny"! The brightness of explosions occurring below the clouds will be attenuated by a factor of at least 10,000, making them most difficult to observe. In the best of cases, these events will be spectacles for the mind to imagine and big telescopes to observe, not a free fireworks display.
The most recent predictions are that at least some of the impacts will occur very close to the planetary limb, the edge of the planet's disk as seen from Earth. That edge still has 11 degrees to rotate before it comes into sunlight. This means that the tops of some of the plumes associated with the rising fireballs may be just visible, although with a maximum predicted height of 3,000 km (0.8 arc second as projected on the sky) they will be just "peeking" over the limb. The newly repaired Hubble Space Telescope (HST), with its high resolution and low scattered light, may offer the best chance to see such plumes. By the time they reach their maximum altitude the plumes will be transparent (optically thin) and not nearly so bright as they were near the clouds. Some means of blocking out the bright light from Jupiter itself may be required in order to observe anything. A number of observers plan to look for evidence of plumes and to attempt to measure their size and brightness.
It also is difficult to predict the effects of the impacts on Jupiter's atmosphere. Robert West points out that a substantial amount of material will be deposited even in the stratosphere of Jupiter, the part of the atmosphere above the visible clouds where solar heating stabilizes the atmosphere against convection (vertical motion). Part of this material will come directly from small cometary grains, which vaporize during entry and recondense just as do meteoritic grains in the terrestrial atmosphere. Part will come from volatiles (ammonia, water, hydrogen sulfide, etc.) welling up from the deeper atmosphere as a part of the hot buoyant fireballs created at the time of the large impact events. Many millimeter-sized or larger pieces from the original breakup will also impact at various times for months and over the entire globe of Jupiter. There is relatively little mass in these smaller pieces, but it might be sufficient to create a haze in the stratosphere.
James Friedson notes that the fireball created by the terminal explosion will expand and balloon upward and perhaps spew vaporized comet material and Jupiter's entrained atmospheric gas to very high altitudes. The fireball may carry with it atmospheric gases that are normally to be found only far below Jupiter's visible clouds. Hence the impacts may give astronomers an opportunity to detect gases which have been hitherto hidden from view. As the gaseous fireball rises and expands it will cool, with some of the gases it contains condensing into liquid droplets or small solid particles. If a sufficiently large number of particles form, then the clouds they produce may be visible from Earth-based telescopes after the impact regions rotate onto the visible side of the planet. These clouds may provide the clearest indication of the impact locations after each event.
After the particles condense, they will grow in size by colliding and sticking together to form larger particles, eventually becoming sufficiently large to "rain" out of the visible part of the atmosphere. The length of time spent by the cloud particles at altitudes where they can be seen will depend principally on their average size; relatively large particles would be visible only for a few hours after an impact, while small particles could remain visible for several months. Unfortunately, it is very difficult to predict what the number and average size of the particles will be. A cloud of particles suspended in the atmosphere for many days may significantly affect the temperature in its vicinity by changing the amount of sunlight that is absorbed in the area. Such a temperature change could be observed from Earth by searching for changes in the level of Jupiter's emitted infrared light.
Glenn Orton notes that large regular fluctuations of atmospheric temperature and pressure will be created by the shock front of each entering fragment, somewhat analogous to the ripples created when a pebble is tossed into a pond, and will travel outward from the impact sites. These may be observable near layers of condensed clouds in the same way that regular cloud patterns are seen on the leeward side of mountains. Jupiter's atmosphere will be sequentially raised and lowered, creating a pattern of alternating cloudy areas where ammonia gas freezes into particles (the same way that water condenses into cloud droplets in our own atmosphere) and clear areas where the ice particles warm up and evaporate back into the gas phase. If such waves are detected, measurement of their wavelength and speed will allow scientists to determine certain important physical properties of Jupiter's deep atmospheric structure that are very difficult to measure in any other way.
Whether or not "wave" clouds appear, the ripples spreading from the impact sites will produce a wave structure in the temperature at a given level that may be observable in infrared images. In addition there should be compression waves, alternate compression and rarefaction in the atmospheric pressure, which could reflect from and refract within the deeper atmosphere, much as seismic waves reflect and refract due to density changes inside Earth. Orton suggests that these waves might be detected "breaking up" in the shallow atmosphere on the opposite side of the planet from the impacts. Others suggest the possibility of measuring the small temperature fluctuations wherever the waves surface, but this requires the ability to map fluctuations in Jupiter's visible atmosphere of a few millikelvin (a few thousandths of a degree). Detection of any of these waves will require a very fine infrared array detector (a thermal infrared camera).
Between the water and other condensable gases (volatiles) brought with the comet fragments and those exhumed by the rising fireballs, it is fairly certain that a cloud of condensed material will form at the location of the impacts themselves, at high altitudes where such gases seldom, if ever, exist in the usual course of things. It may be difficult to differentiate between the color or brightness of these condensates and any bright material below them in spectra at most visible wavelengths. However, at wavelengths where gaseous methane and hydrogen absorb sunlight, a distinction can easily be made between particles higher and lower in the atmosphere, because the higher particles will reflect sunlight better. Much of the light is absorbed before reaching the lower particles. Observing these clouds in gaseous absorption bands will then tell us how high they lie in the atmosphere, and observations over a period of time will indicate how fast high- altitude winds are pushing them. The speed with which these clouds disappear will be a measure of particle sizes in the clouds, since large particles settle out much faster than small ones, hours as compared to days or months.
Orton also notes that in the presence of a natural wind shear (a region with winds having different speeds and/or directions) such as exists commonly across the face of Jupiter, a long-lived cyclonic feature can be created which is actually quite stable. It may gain stability by being fed energy from the wind shear, in much the same way that the Great Red Spot and other Jovian vortices are thought to be stabilized. Such creation of new, large, fixed "storm" systems is somewhat controversial, but this is a most intriguing possibility!