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Periodic Comet Shoemaker-Levy 9 Collides with Jupiter

Periodic Comet Shoemaker-Levy 9 Collides with Jupiter

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TITLE Periodic Shoemaker-Levy 9 Collides with . Background Material for Science Teachers. INSTITUTION Jet Propulsion Lab., Pasadena, Calif. PUB DATE 94 NOTE 28p.; Several figures based on images taken from have marginal legibility. PUB TYPE Guides Classroom Use Teaching Guides (For Teacher) (052)

EDRS PRICE MF01/PCO2 Plus Postage. DESCRIPTORS *; Elementary Secondary Education; *Science Education; Space Sciences; Teaching Guides IDENTIFIERS *; *Jupiter ()

ABSTR,77 In July of 1994, fragments of Comet Shoemaker-Levy collided-with Jupiter. This document has been provided to better inform students of the work that will be done by scientists and others involved in the study of this event. This document offers some background material on Jupiter, comets, what has and possibly will happen, and how scientists propose to take advantage of the impact events. The following sections are included: (1) What is A Comet?; (2) The Motion of Comets; .(3) The Fragmentation of Comets; (4) The Discovery and Early Study of Shoemaker-Levy 9;(5) The Planet Jupiter;(6) The Final of Shoemaker-Levy 9;(7) The Collisions; (8) How Can These Impacts and Their Consequences Be Studied?; and (9) What Do Scientists Expect to Learn from All of This? (ZWH)

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-tu Periodic CometShoemaker-Levy 9

U.S. DEPARTMENT OF EDUCATION 01 lice et Educational Research andtaiprovemeni Collides with Jupiter EDUCATIONAL RESOURCES INFORMATION CENTER IERICI 1..Zsdocument has been reproduced as pined , the person or organization onginatrrig it 0 Minor changes have been madeto improve reproduction quality

Points of view or opinions stated in this doCu. rent do not necessarily represent offidial Background Material for Science OERI position or policy Teachers

2 BEST COPY AVAILABLE or a period of about six days centered on July 19, Stupendous as these collisions will be, they will occur on the F1994, fragments of Comet Shoemaker-Levy 9 are far side of a body half a billion miles from . There will expected to collide with Jupiter, the 's largest be no display yisible to the general public, not even a display_ planet. No such event has ever before been available for as obvious as a faint terrestrial meteor. Amateur astrono- study. The energy released by the larger fragments during mers may note a few seconds of brightening of the inner impact will be more than 10,000 times the energy released of Jupiter during the impacts, and they might by a 100-megaton bomb! Unfortunately for observe minor changes in the lovian cloud structure during observers, the collisions will occur on the. night side of the days following the impacts. The real value of this most Jupiter, which also will be the back side as seen from Earth. unusual event will come from scientific studies of the The collisions can still be studied. in many ways, nevertheless, comet's composition, of the imp& ihenomena-themselves, by spacecraft more advantageously located, by of the. and of the response of a planetary at..losphere and mag- collisions reflected from Jupiter's satellites, and by the effects netosphere to such a series of "insults." of the impacts upon the Jovian . (The impact This booklet offers some background material on Jupiter, sites will rotate into view from Earth about 20 minutes after comets, what has and possibly will happen, and how each collision.) scientists propose to take advantage of the impact events.

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Cover Comet Shoemaker-Levy 9 is expected to collide with Jupiter in July 1994. From this historic event, scientist hope to learn more about comet, Jupiter, and the physics of high ve- locity planetary impacts. s a Comet?

Comets are small, fragile, irregularly Comets, course,ust obey the same universal laws of shaped bodies composed of a mixture of non-volatile grains motion a do all o, er bodies. Where the ofplanets and frozen . They usually follow highly elongated paths around t e are nearly circular, however, theorbits of around the Sun. Most become visible,even in , comets re quite elongated. Nearly 100 known cometshave only when.they get near enough to the Sun for the Sun's ra- period (the time it takes them to makeone complete trip diation to start subliming the volatile gases, which in turn arou d the Sun) five to seven Earth years in length. Their far- blow away small bits of the solid material. These materials thest point from the Sun (their aphelion) isnear Jupiter'S or- expand into an enormous escaping atmosphere called the bit, with the closest point (perihelion) being muchnearer to , which becomes far bigger than a planet, and they are Earth. A few comets like Halley have their aphelionsbeyond forced back into long tails of andgas by radiation and (which is six times as far from the Sunas Jupiter). charged particles flowing from the Sun. Comets are cold Other comets come from much farther outyet, and it may bodies, and we see them only because the gases in their co- take them thousands or even hundreds of thousands of mae and tails fluoresce in (somewhat akin to a fluo- years to make one comp,ete orbit around the Sun. In all rescent light) and because of sunlight reflected from the cases, if a comet approaches near to Jupiter, it is strongly at- solids. Comets are regular members of the solar system fam- tracted by the gravitational pull of that giantamong , ily, gravitationally bound to the Sun.They are generally be- and its orbit is perturbed (changed), sometimesradically. lieved to be made of material, originally inthe outer part of This is part of what happened to Shoemaker-Levy9. (See the solar system, that didn't get. incorporated into the plan- Sections 2 and 4 for more details.) ets leftover debris, if you will. It is thevery fact that they are thought to be composed of such unchanged "primitive" The nucleus of a comet, which is its solid, persistingpart, has material that makes them extremely interesting to scientists been called an icy conglomerate, a dirty snowball,and other who wish to learn about conditions duringthe earliest pe- colorful but even less accurate descriptions.Certainly a riod of the solar system. contains silicates akin to some ordinary Earth rocks in composition, probably mostly invery small grains Comets are very small in size relative to planets. Their aver- and pieces. Perhaps the grains are "glued" togetherinto age diameters usually range from 750 m or less to about larger pieces by the frozen gases. A nucleusappears to in- 20 km. Recently, evidence has been foundfor much larger clude complex compounds and perhapssome free distant comets, perhaps having diameters of300 km or carbon, which make it very black in . Most notably,at more, but these sizes are still small compared to planets. least when young, it contains many frozengases, the most Planets are usually more or less spherical in shape, usually common being ordinary . In the low pressure condi- bulging slightly at the. equator. Comets are irregular in tions of space, water sublimes, that is, itgoes directly from shape, with their longest dimension often twice the shortest. solid to just like dry does on Earth. Water probably (See Appendix A, Table 3.) The best evidence suggests that makes up 75-80% of the volatile material inmost comets. comets are very fragile. Their tensile strength (the stress they Other common are (CO), carbondiox- can take without being pulledapart) appears to be only about ide (CO2), methane (CH4), ammonia (NH3), and formalde- 1,000 dynes/cm2 (about 2 Ibift.2). You could take a big hyde(H2C0). and solids appear to be fairly well piece of cometary material and simply pull it in two with your mixed throughout the nucleus of anew comet approaching bare hands, something likea poorly compacted snowball. the Sun for the first time. As a cometages from many trips

4 What Is a Comet? 1 clo5e to the Sun, there is evidence that it loses most of its the glare of the coma. A great deal was learned when the ices, or at least those ices anywhere near the nucleus sur- , the Soviet Union, and the Japanese face, and becomes just a very fragile old "rock" in appear- sent spacecraft to fly by Comet Halley in 1986. For the first ance, indistinguishable at a distance from an . time, actual images of an active nucleuswere obtained (see Figure 1) and the composition Of the dust andgases flowing A comet nucleus is small, so its gravitational pull is very from it was directly measured. Early in thenext century the weak. You could run and jump completely off of it (if you Europeans plan to send a spacecraft called toren- could get traction). The escape velocity is only about 1 m/s dezvous with a comet and watch it closely fora long period (compared to 11 km/s on Earth). As a result, the escaping of time. Even this sophisticated mission is not likelyto tell gases and the small solid particles (dust) that they drag with scientists a great deal about the interior structure of comets, them never fall back to the nucleus surface. Radiation pres- however. Therefore, the to reconstruct the sure, the pressure of sunlight, the dust particles back events that.occurred when Shoemaker-Levy 9 split and to into a dust tail in the direction opposite to the Sun. A study those that will occur when the fragment.are de- comet's tail can be tens of millions of kilometers in length stroyed in Jupiter's atmosphere is uniquely important (see when seen in the reflected sunlight. The gas are Sections 4, 7, and 8). torn apart by solar light, often losing and becoming electrically charged fragmentsor . The ions interact with the wind.of charged particles flowingout from the Sun and are forced back intoan tail, which again can extend for millions of kilometers in the direction opposite to the Sun. These ions can be seenas they fluo- resce in sunlight.

Every comet then really has two tails, a dust tail andan ion tail. If the comet is faint, only one or neither tailmay be de- tectable, and the comet may appear just as a fuzzy blob of light, even in a big . The density of material in the coma and tails is very low, lower than the best vacuum that can be produced in most laboratories. In 1986 the spacecraft flew right through Comet Halley onlya few hun- dred kilometers from the nucleus. Though thecoma and tails of a comet may extend for tens of millions of kilometers and become easily visible to the in Earth's night sky, as 's were in 1976, the entire phenomenon Figure ;. A composite image of the nucleus of Comet Halley from is the product of a tiny nucleus only a few kilometersacross. images taken by the Giotto spacecraft of the European Space Agency (Courtesy of U. Keller, Max last for Aeronomy, Kadenburg, Lindau, Germany.) Because comet nuclei are so small, they are quite difficultto study from Earth. They always appear at mostas a point of light in even the largest telescope, if not lost completely in

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2 Wbat Is a Comet? 5 2 n of Comets

Comets necessarily obey the same In Figure B severa ellipses are drawn, all havingthe same physical laws as every other object. Theymove according semimaj axis bu different eccentricities. Eccentricity isa to the basic laws of motion and of universal gravitation dis- mathemical measure of departure from circularity. Acircle covered-by Newton in the 17th century (ignoring very small has zereccentricity, and most of the planets have orbits relativistic corrections). Ifone considers only two bodies which. re nearly circles. Only and haveeccen- either the Sun and a planet, or the Sun anda comet the tricities exceeding 0.1. Comets, however, havevery large ec- smaller body appears to follow an elliptical pathor orbit centricities, often approaching one, the value fora parabola. about the Sun, which is at one focus of the ellipse. The geo- Such highly eccentric orbits are justas possible as circular or- metrical constants which fully define the shape ofthe ellipse bits, as far as the laws of motion are concerned. are the semimajor axis a and the eccentricity e (see Figure 2). The semiminor axis b is related to thosetwo quantities by The solar system consists of the Sun; nine planets,numerous the equation b= a4(1-e2). The focus is located a distance ae satellites and , comets, and various small debris.At from the center of the ellipse..Three further constants are any given time the motion of any solar system body is af- required if one wishes to describe the orientation of the el- fected by the gravitational pulls of all of theothers. The lipse in space relative to some coordinate system, and a Sun's pull is the largest by far, unlessone body-approaches fourth quantity is required ifone wishes to define the loca- very closely to another, so orbit calculations usuallyare car- tion of a body in that elliptical orbit. ried out as twr.body calculations (the body inquestion and the Sun) with small perturbations (small added effectsdue to Semimaior Axis the pull of other bodies). In 1705 Halley noted in hisoriginal paper predicting the return of "his" comet that Jupiterun- doubtedly had serious effects on the comet's motion,and he presumed Jupiter to be the cause of changes in theperiod (the time required for one complete revolution aboutthe A) semolina Axis Sun) of the comet. (Comet Halley's period is usuallystated to be 76 years, but in fact it has varied between 74.4and 79.2 years during the past 2,000 years.) In thatsame paper Halley also became the first to note thevery real possibility of the collision of comets with planets, but statedthat he would leave the consequences of sucha "contact" or

"shock" to be discussed "by the Studious of PhysicalMatters." .

In the case of Shoemaker-Levy 9 we have the perfectex- ample both of large perturbations and their possible"conse- quences." The comet was fragmented and perturbed intoan orbit where the pieces will hit Jupiter one period later.In general one must note that Jupiter's gravity (or that of other Figure 2. A) An ellipse with eccentricityv 0.75. 8)A family of confocal planets) is perfectly capable of changing theenergy of a ellipses (ellipses sharing the same focus) and having thesame-length cornet's orbit sufficiently to throw it dear out of the solar semimajor axis. (Prepared by R. L. Newburn, Jr.) system (to give it escape velocity from the solar system) and

6 rbe Motion of Comets 3 has done so on numerous occasions. See Figure 3. This is ex- ergy) of Shoemaker-Levy 9 will be changed largely to ther- actly the same physical effect that permits using planets to mal energy when the comet is halted by Jupiter's atmo- change the orbital energy of a spacecraft in so-called "gray- sphere and destroyed in the process. When one body moves __ _Ay-assist maneuvers". such as were used_by the Voyager _ _ about another in the vacuum of space, the.totatenergy {ki- spacecraft to visit all the outer planets except Pluto. netic energy plus potential energy) is conserved.

One of Newton's laws of motion states that for every action Another quantity that is conserved is called angular momen- there is an equal and opposite reaction. Comets expel dust tum. In the first paragraph of this section, it was stated that and gas, usually from localized regions, on the sunward side the geometric constants of an el'ipse are its semimajor axis of the nucleus. This action causes a reaction by the cometary and eccentricity. The dynamical L. nstants of a body moving nucleus, slightly speeding it up or slowing it down. Such ef- about another are energy and an:,.Iar momentum. The total fects are called "non-gravitational forces" and are simply (binding) energy is inversely proportional to the semimajor rocket effects, as if someone had set up one or more rocket axis. If the energy goes to zero, the semimajor axis becomes motors on the nucleus. In general both the size and shape of infinite and the body escapes. The angular momentum is a comet's orbit are changed by the non-gravitational forces proportional both to the eccentricity and the energy in a not by much but by enough to totally confound all of the more complicated way, but, for a given energy, the larger celestial mechanics experts of the 19th and early 20th cen- the angular momentum the more elongated the orbit. turies. Comet Halley arrived at its point closest to the Sun (perihelion) in 1910 more than three days late, according to The laws of motion do not require that bodies move in the best predictions. Only after F. L. Whipple published his circles (or even ellipses for that matter), but if they have icy conglomerate model of a degassing nucleus in 1950 did some binding energy, they must move in ellipses (not count- it all begin to make sense. The predictions for the time of ing perturbations by other bodies), and it is then the angular perihelion passage of Comet Halley in 1986, which took into momentum which determines how elongated is the ellipse. account a crude model for the reaction forces, were off by Comets simply are bodies which in general have more angu- less than five hours. lar momentum per unit mass than do planets and therefore. move in more elongated orbits. Sometimes the orbits are so Much of modern physics is expressed in terms of conserva- elongated that, because we can observe only a small part of tion laws, laws about quantities.which do not change for a them, they cannot be distinguished from a parabola, which given system. Conservation of energy is one of these laws, is an orbit with an eccentricity of exactly one. In very general and it says that energy may change form, but it cannot be terms, one can say that the energy determines the size of created or destroyed. Thus the energy of motion (kinetic en- the orbit and the angular.momentum the shape.

Comet's New long -Period Orbit Comers Original longPenod Orbit Comers New ShortPencx1 Orbit

Comet's Orbit if it Hadn't Been , Deflected by Jupiter

Comet's Original ShortPeriod Orbit

Jupiter Comet Passes Closely 0 in Front of Jupiter Comet Passes Closely Behind Jupiter

Figure 3. A) By passing closely in front of Jupiter, a long-period comet B) By passing closely behind Jupiter, a short-period comet can gain can lose enough orbital energy (gain enough binding energy) to be enough orbital energy (lose enough binding energy) to become a captured into a much shorter period orbit about the Sun. (Prepared loraperiod comet. (Prepared by D. K. Yeomans.) by D. K. Yeomans.)

4 The Mottos of Comets 7 1

Every body is held together by two The Kreu forces, its self-gravitation and family ithe name given tomany comets which its internal strength dueto closely a molecular bonding. With roach t Sun from one direction in no external forces on it (andno ini- space. They always a' proach the Sunto within 3 million km tial rotation) a liquid body wouldform a perfect sphere or less, and just some h. e actually hit the Sun. The from self-gravitation (and fromvery weak molecular forces family was named for Heinri Kreutz who published extensive surface tension). Approachinganother body, the sphere monographs on would begin to elongate three'of these comets andsupported the idea that they toward that body. Finally,when the had a common origin, perhaps ina giant comet observed in difference in gravitational forceon the near side and far side 372 B.C. Today the Kreutzfamily has eight definite, of the former sphere exceededthe self-gravitation, the well- body studied members; 16 probable wouldbe torn apart. Thedistance from the larger members (that are listedas body at probable only because they which this disruptionoccurs is the so-called Roche *it, didn't survivepassage within 800,000 km of the Sunto permit further study); and named for the man who firststudied the problem. The dif- three more possible members. Extensive ferential gravitational effectsof the and the work by Brian Marsden Sun are suggests that all of these what raise the tides inEarth's oceans; and such may have resulted from the split- forces are ting of two comets around often referred toas tidal forces. 1100 A.D., which inturn may have been the parts of the of 372 B.C.Those Kreutz fragments which Solid bodies have intrinsicstrength due to their molecular survive their encounters withthe Sun are often found to bonds. Aluminum wiremay have a tensile strength of have split yet again! 2.4 109 dyneskm2 (5 million lb./ft.2) and goodsteel wire a tensile strength 10 times The classic Roche limit fora (fluid) body of density 1 g/cm3 larger still, which far exceedsthe tidal of anything short approaching Jupiter is about of a black hole. Asstated in 119,000 km above the cloud Section 1, comets have tops of the planet. It is about169,000 km for a body having very low tensile strength,near 1 x 103dynes/cm2 (2 Ibift.2). a density of 0.5 g/cm3. Morecomplete modern theories They can be pulledapart very easily by tidal force (or making different assumptionsresult in a somewhat smaller any other substantial force, for that matter). Some 25 limit. In 1886 CometBrooks 2 came within 72,000 comets have been observedto split over km of the past two centuries. Jupiter's clouds and splitinto two pieces. In July 1992 In other cases twoor more comets Comet have been discovered Shoemaker-Levy 9 came withinabout 25,000 km of Jupiter's in nearly the same orbit,and calcula- tions have indicated that clouds and fragmented into21 or more large pieces they were once a singlecomet. A and an few of these enormous amount of smaller debris cases have been obviously attributableto the down to micron or sub- tidal forces of Jupiter micron size. Details of thislast event follow. (Comet 2 and CometShoe- maker-Levy 9) or the Sun (the Kreutz comet family),while other splittings have to be attributed to less obviouscauses. For example, the loss of material from an activecomet, which tends to occur from a few localized areas, is boundto weaken it. It may be that a rapidly rotating cometcan be weakened to the point where the centrifugal forceis suffi- cient to cause large piecesto break off.

8 The Fragmentationof Comets S Early Study ker-Levy 9

Comet Shoemaker-Levy 9 was discov- By 7 Marsd n had enough positions toattempt to ered photographically by the husband and wife scientific derive po ible orb One elliptical solution gave a close ap- team of Carolyn S. and Eugene M. Shoemaker and H. proach tJupiter ill July 1992. Also on March 27, Jane Luu Levy on March 24, 1993, using the 0.46-m (18-in.) Schmidt and Day' Jewitt took an image with the 2.2-mtelescope on telescope at in California. Its discovery Maun ea in Hawaii that showed as many as 17 separate was a serendipitous product' f their continuing search for sub - nuclei "strung out like pearlson a string" 50 arc sec- "near-Earth objects," and the "9" indicates that it was the onds long, and this was reported in Circular No. 5730two ninth short-period comet (period less than 200 years) discov- days later. Figure 4 shows an early image taken byScotti on ered by this team. Near-Earth objects are bodies whose or- March 30, 1993. This long exposure (440 secondson a CCD bits come nearer to the Sun than that of Earth and hence detector) brings out the faint detail of the debris field, have some potential for collisions with Earth. Theappear- though it overexposes the individual nucleus fragments. Fig- ance of the cornet was reported as. "most unusual"; the Ob- ure 5 is an image from the (HST), ject appeared as "a dense, linear bar about 1 arc minute taken by Harold A. Weaver and collaboratorson July 1, 1993 long" and had a "fainter, wispy 'tail.' " (A circle is divided (before the HST repair mission), that clearly showsat least into 360 degrees, each degree into 60 minutes, and each 15 individual fragments in one image frame of thetrain. minute into 60 seconds. The word "arc" is addedto denote an angular measure rather than time. The diameter of the Moon is near 30 arc minutes, for example, while theappar- ent diameter of Jupiter when closest to Earth is 50arc seconds.) The comet's brightness was reportedas about 14, more than a thousand times too faintto be seen with the naked eye.

The existence of this object wassoon confirmed by James V. Scotti of the program at the University of Ari- zona, and the Internatioial Astronomical Union's Central Bureau for Astronomical Telegrams immediatelyissued "Cir- cular No. 5725" reporting the discoveryas a new comet, :s giving it the provisional designation of 1993e(the fifth -441Pmell comet discovered or recovered in 1993). Scotti reportedat NNW& least five condensations in a "long,narrow train about 01110.'mum v., 47 arc seconds in length and about 11arc seconds in . We width," with dust trails extending 4.20arc minutes to the We east and 6.89 arc minutes to.the west and tails extending about 1 arc minute from elements of the nucleartrain. Bu- I I reau director Brian G. Marsden noted that the cometwas . 7 some 4° from Jupiter and that its motion suggested that it could be near Jupiter's distance from the Sun. Figure 4. Image of Shoerneker4.evy 9 on March 30,1093. 1,11442r40011 time 440 seconds with the 0.91-m Spacewetch telescope of the U. of Arizona Observatories on Kitt Peak Arizona. (Courtesy of J. V Scotti.)

The Dtscotry and Early Study ofShoemakerLevy-9 7 Figure S. Image of Shoemaker-Lew 9 taken by the Hubble Spec.Telescope on Ally 1, 1991 (Courtesy of H. A. Weaver and collaborators.)

In IAU Circular No. 5744, dated April 3, 1993, Marsden 1992, and approaching again, this time within 45,000 km of used positions covering a period'of 17 days (including two the center of Jupiter, on July 25, 1994. Marsden noted that prediscovery positions from March .15) and was able to re- this distance was less than the radius of Jupiter. In other port that no orbit of very long period (near parabolic)was words, the comet, or at least parts of it, couldvery well hit possible. The orbit had to be an ellipse of rather smalleccen- Jupiter. tricity relative to the Sun and relatively short period. Since it was not at all obvious where the center of mass of this new By October 18, 1993, Paul W. Chodas and Donald K. comet lay, most observers were just reporting the position of Yeomans were able to report at the annual American Astro- what appeared to be the center of the train. This made an nomical Society's Division of Planetary Sciences meeting that accurate orbit (or orbits) difficult to determine. Marsden sug- the probability of impact for the major fragments of Shoe- gested that a very close approach to Jupiter in 1992 contin- maker-Levy 9 was greater than 99%. The fragmentsappar- ued to be a distinct possibility, and the orbit he chose to ently would hit over a period of several days, centeredon publish was one with the comet at least temporarily" i:or- July 21.2, on the night side of Jupiter at latitude 44*S and bit around Jupiter. longitude 35° past the midnight meridian, accordingto available observations. This unfortunately is also the back By May 22 Marsden had almost 200 positions of thecenter .side of Jupiter as viewed from Earth. The 1992 approachto of the train. In Circular No. 5800 he reported on an orbit Jupiter that disrupted the comet was calculated to have computed May 18 by Syuichi Nakano that showed the been at a distance of 113,000 km from the planet's center comet approaching within 120,000 km of Jupiter on July 8, and only 42,000 km above its cloud tops. Furthermore, they

The Discovery and Early Study ofShoentrer-Levy 9 BEST COPY AVAILABLE found that the comet had.been in a rapidlychanging orbit around Jupiter for some time body, assuminga of 12 before this, probably forat hours, would depart least several decades. It with a velocity ofonly 0.72 m/s (1.6 did not fragment duringearlierap- mph) relative to the center of the comet._. _.__.proaches to- Jupiter, Some cometsappear to rotate-rifOre however,-because theSe were atmuch rapidly than once greater distances than that of1992. per half day, whilemany, such as Halley, rotate more slowly. In any case the centrifugalforce on un- attached pieces of After recovery of thecomet on December 9, following material lying on thesurface of a the comet is not normally sufficient rotating period during which to overcome the it was too near to the Sun inthe sky to gravity observe, Chodas and holding them there.Pieces do not fly off Yeomans found that theprobability of the nucleus was greater than 99.99% "spontaneously." Even whenthe tidal forces that-all the large fragmentswill hit overcome self- Jupiter. The er gravity the piecesseparate slowly, and they counter period is now centeredon July 19.5, continue to in- and orbits for individual teract gravitationally. Moreimportant, the pieces fragments are uncertainby about bang into 0.03 days (1 a). The one another, changing theirvelocities and perhaps impact site has moved closerto the limb frag- of Jupiter, menting further. now near 75° from the midnightmeridian and only a few degreesbeyond the dark limb as seen from Earth, In the case of Shoemaker-Levy but all pieces still impacton the back side. The 1992 9, Sekanina, Chodas,and ap- Yeomans estimate that proach that split thecomet is now calculated although fragmentationprobably be- to have oc- gan before closest approach curred on July 7.84 and only to Jupiter, dynamicindepen- 25,000 km (15,500 mi.)above dence of the pieces didn't the clouds. These datanow cover a much longer time occur until almost two hoursafter and are based base closest approach. Fora period of at least twothree upon calculations for individualfragments. hours, They are unlikely collisions dominated thedynamics of all but the to change significantly inthe future. The largest pieces, with each smallgrain suffering comet probably approachedJupiter no nearer than some 10 collisions per about second and the bigger 9 million km in theorbit prior to that of pieces being subjectedto many times 1992. this number of low velocity impacts by the smallparticles. All In a comprehensive of this convertedthe original rotational paper prepared for The velocities of the bits Astronomical and pieces of 0-2 mph Journai, ldenek Sekanina,Chodas, and Yeomans into a random "equilibrium"velocity report on distribution, with some smaller the details of the breakup pieces having velocitiessev- of Shoemaker -Levy 9as calculated eral times their original from the positions,motions, and brightness velocity. Once the piecesstopped of the frag- hitting one another, ments and debris. Theyused data from Jewitt, each continued tomove in its own in- Luu, and dependent orbit determined Chen taken in Hawaii,Scotti in Arizona, and mainly by the gravity ofJupiter Weaver's and the Sun. The Hubble Space Telescope pressure of light from the Sunalso had a (HST) observing team. Forexample, significant effect the 11 brightest fragments upon the smallest particles,creating a as measured with the HST,visual broad dust tail just (V) magnitude 23.7-24.8or about 15 million times as happens in a normalcomet. There has too faint been no evidence of to be seen by the nakedeye, had the brightness the presence ofgases from Shoemaker- one would Levy 9, either direct expect from spheres 4.3 downto 2.5 km in diameter, spectroscopic evidenceor motion of the as- dust particles that suming a normal cannot otherwise be explained.This is not. coinetary reflectivity for thefragments to say that there (about 4%). Ofcourse the fragments are are no gases, only that thereis no evidence not spheres, since for them. The only tidal disruption tendsto occur in planes perpendicular direct evidence we havethat Shoemaker- to the Levy 9 is really a comet direction of the objectcausing the disruption and not an asteroid isthe fact that it (Jupiter) and broke up so easily! since comets generally Asteroids are not thoughtto be so fragile. are not spherical to begin with.Nev- ertheless, addingup the sizes of these 11 fragments, the It is unlikely that the other fragments exact circumstances of the not precisely measured,and all of. the debris breakup of Shoemaker-Levy 9 will making up the trailsand tails, suggests that ever be known with certainty.How- the original ever, the physical model needed comet must have beenat least 9 km in average to reproduce the train -(of diameter, individual large fragments), and it could havebeen somewhat larger. the trails (of debrison either side This was a good- of the train), and the sized comet, aboutthe same size tails (of very small particlesin the anti- as Comet Halley. Sun direction) in many images like those shown inFigures 4 and 5 does set limits When comets split, thepieces do not go flying on the separation time, sizes,and ve- apart at a locities of the pieces high velocity, eachto immediately and particles makingup each element. go into its own indepen- The model of Sekanina, dent orbit. Theescape velocity from a non-rotating Chodas, and Yeomans,the most spherical complete at this writing, comet 5 km in radius witha density of 0.5 g/cm3 (half suggests that the originalcomet that cannot have been much smaller of water) is 2.65 mis(6.5 mph). If suddenly than 9 km inmean diam- freed of gravity eter, that it probably and molecular bonds,a particle at the equator of was rotating quite rapidly (perhaps that 10-km once in eight hours), and that the breakup, as definedby dy-

The Discoveryand Early Study of Shoemaker-Levy9 9 namical independence from collisions and limited mutual All of the large fragmentswere soon strung out in nearly a gravitational effects,.was not completed untilabout straight line that pointed at Jupiter, and they willremain so two hours after the closest approach to Jupiter. Thecomet until colliding with the planet (see Figure6). H. J._ Melosh nucleus was probably not very sphericalor the debris trails and P. Schenk have offered the intriguingsuggestion that on either side of the train of nucleus fragments would be linear chains of craters observedon Jupiter's satellites --nearly equal in length, which theyare-not: After the and Cal listo are the product ofimpacts by earlier sions ceased, the motion of the largest fragments was domi- comets fragmented by Jupiter. nated by Jupiter, with those fragments closestto Jupiter at breakup remaining closest and therefore movingwith a shorter period in accordance with basic mechanics. Thefrag- ment that started nearest to Jupiter will be the firstto return to Jupiter and hit the planet.

Disruption at Close Approach to Jupiter on July 8. 1992

Impacts on Jupiter July 16-22, 1994

April 1993 Qat Nikilea se

Earth

Aclotwe 0 July 16. 1993 Q Figure 6. The fragments of Shoemaker -levy 9move around Jupiter. (This schematk could not be drawn to scale. For example, the distance to apojove is actuallyalmost 1,200 times the disruption distance, and the p true representation would be a long narrow ellipse,e > 0.99, that looks almost like a straight line out and back. The length of the line should be 350 times thediameter of Jupiter and the disruption a tiny dot less than a quarter of the diameter above Jupiter.)Mipared by t. Sekanina, 9. W. Chodas, and D. K. Yeomans.)

10 The Discovery and EarlyStudy of Sboenia2er-Levy 9 BEST COPY AVAILABLE lanet Jupiter

Jupiter is the largest of the nineplanets, more than 10 times the diameter of Earthand more than 300 times its mass. In fact themass of Jupiter is almost .2.5 times that of all the other planetscombined. Being com- posed largely of the light elementshydrogen and , its mean density is only 1.314 times that ofwater. The mean density of Earth is 5.245 times thatof water. The pull of gravity on Jupiter at the top of theclouds at the equatc,, is 2.4 times as great as gravity's pull at the surface of Earthat the equator. The bulk of Jupiterrotates once in 9^ 55.5m, al- though the period determinedby watching cloud features differs by up to five minutes dueto intrinsic cloud motions.

The visible "surface" of Jupiter isa deck of clouds ofammo- nia crystals, the tops of whichoccur at a level where the pressure is about half that at Earth's surface.The bulk of the atmosphere is made up of 89%molecular hydrogen (H2) and 11% helium (He). Thereare small amounts of gaseous ammonia (NH3), methane (CH4),water (H20), ethane (C2H6), acetylene (C2H2), carbon monoxide(CO), (HCN), and even more exoticcompounds such as phosphine (PH3) and germane (GeH4). Atlevels below the deck ofam- monia clouds thereare believed to be ammonium hydro- Figure 7. An image of Jupiter takenby the Voyager 1 spacecraft in 1979. sulfide (NH4SH) clouds andwater crystal (H20) clouds, fol- lowed by clouds of liquidwater. The visible clouds of Jupiter are very colorful. The cause of thesecolors is not yet known. ened and faded, the spot haspersisted for at least 162.5 years, the earliest definite "Contamination" by various polymersof sulfur (S3, S4, S5, drawing of it being and S8), which are yellow, red, Schwabe's of Sept. 5, 1831. (There and brown, has beensug- is less positive evidence gested as a possible cause of theriot of color, but in fact sul- that Hooke observed itas early as 1664.) It is thought that fur has not yet been the brighter zones are cloud-covered detected spectroscopically, andthere regions of upward are many other candidates as thesource of the coloring. moving atmosphere, while thebelts are the regions of de- scending gases, the circulationdriven by interior heat. The The meteorology of Jupiter spots are thought to be large-scale is very complex and not wellun- vortices, much larger and derstood. Even in small telescopes,a series c4 parallel light far more permanent thanany terrestrial weather system. bands called zones and darkerbands called belts is quite ob- vious. The polar regions of theplanet are dark. (See Fig- The interior of Jupiter is totallyunlike that of Earth. Earth has ure 7.) Also present are light and darkovals, the most a solid crust "floating" on a denser mantlethat is fluid on famous of these being "theGreat Red Spot." The Great Red top and solid beneath, underlain bya fluid outer core that Soot is larger than Earth,and although its color has bright- extends out to about half of Earth'sradius and a solid inner core of about 1,220-km radius. Thecore is probably 75%

BESI COPY AVAILABLE 13 The Planet Jupiter 11 iron, with the remainder nickel, perhaps silicon, andmany about 800-km width. Next inside comes a fainter ring about different metals in small amounts. Jupiter on the other hand 5,000 km wide, while very tenuous dust extends downto may well be fluid throughout, although it could have a the atmosphere. Again, the effects of the intrusion of the "small" solid core (say up to 15 times the mass of Earth!) of dust from Shoemaker-Levy 9 will be interesting to see, heavier elements such as iron and silicon extendingout to though not easy to study from the ground. perhaps 15% of its radius. Thebulk-oflupiter-is fluid-hydro- gen in two forms or phases, liquid molecular hydrogen on The innermost of the four large satellites of Jupiter,lo, has top and liquid metallic hydrogen below; the latter phaseex- numerous large volcanos that emit sulfur and sulfur dioxide. ists where the pressure is high enough,say 3-4 million atmor Most of the material emitted falls back onto the surface,but spheres. There could be a small layer of liquid helium below a small part of it escapes the . In space this material the hydrogen, separated out gravitationally, and thereis is rapidly dissociated (broken into its atomic constituents) clearly some helium mixed in with the hydrogen. The hydro- and ionized (stripped of one or more electrons). Onceit be- gen is convecting heat (transporting heat by mass motion) comes charged, the material is trapped by Jupiter's magnetic from the interior, and that heat is easily detected by infrared field and forms a torus (donut-shape) completelyaround Ju-- Measurements, since Jupiter radiates twice as much heat as piter in to's orbit. Accompanying the volcanic sulfurand oxy- it receives frorri the Sun. The heat is generated largelyby gen are many sodium ions (and perhaps some of the sulfur gravitational contraction and perhaps by gravitationalsepa- and oxygen as well) that have been sputtered (knockedoff ration of helium and other heavier elements from hydrogen, the surface) from lo by high energy electrons in Jupiter's in other words, by the conversion of gravitational potential magnetosphere. The torus also contains protons (ionizedhy- energy to thermal energy. The moving metallic hydrogen in drogen) and electrons. It will be fascinating tosee what the the interior is believed to be the source of Jupiter'sstrong effects are when large amounts of fine particulates collide magnetic field. with the torus.

Jupiter's magnetic field is much stronger than that ot Altogether, Jupiter has 16 known satellites. The two inner- It is tipped about 11.° to Jupiter's axis of rotation, similar to most, and Adrastea, are tiny bodies, having radiinear Earth's, but it is also offset from the center of Jupiter by 20 and 10 km respectively, that interact strongly with the about 10,000 km (6,200 mi.).The magnetosphere of rings and in fact may be the source of the rings. That is,the charged particles which it affects extends from 3.5 millionto rings may be debris from. impacts an the satellites. Amalthea 7 million km in the direction toward the Sun, depending and Thebe are still small, having mean radii of 86.2 and upon conditions, and at least 10 times that far in about 50 km, respectively, but they are close to Jupiter and the anti-Sun direction. The plasma trapped in this rotating, may serve as useful reflectors of light from some of the im- wobbling magnetosphere emits radio frequency radiation pacts. The Galilean satellites (the four discovered by measurable from Earth at wavelengths from 1 m or less to as in 1610), lo, , Ganymede, and ,range much as 30 km. The shorter wavesare more or less continu- in radius.from 1,565 km (Europa) to 2,634 km (Ganymede). ously emitted, while at longer wavelengths the radiation is (Earth's Moon has a radius of 1,738 km.) They lie at dis- quite sporadic. Scientists will carefully monitor the Jovian tances of 421,700 km (lo) to 1,883,000 km (Callisto) from magnetosphere to note the effect of the intrusion of large Jupiter. These objects will serve as the primary reflectors of amounts of cometary dust into the lovian magnetosphere. light from the impacts for those attempting to indirectly ob- serve the actual impacts. The outer eight satellites are all tiny The two Voyager spacecraft discovered that Jupiter has faint (less than 100-km radius) and at large distances (greater dust rings extending out to about 53,000 km above theat- than 11 million km) from Jupiter. They are expected to play mosphere. The brightest ring is the outermost, having only no role in impact studies.

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12 The Planet Jupiter 4 4 The Final ON aker-Levy

The motion of Comet Shoemaker- Levy 9 can technically be described as chaotic, which means Itipiter Jupiter's Orbit Plane that calculations based upon the input of comet positions having very tiny differences (small input errors)causes large differences in the results of calculations of the subsequent motion and the apparent prior motion. Large perturbations caused by successive close approaches to Jupiter havere- -10 sulted in each orbit beilig different in size, shape, andorien- tation. The orbits have not been the simple result ofa small . body in orbit around a large one but rather the productof a "tug of war" between Jupiter and the Sun,a classic "three- -20 body problem.", Near to Jupiter the planet's gravity has dominated the motion, but far from Jupiter the Sun ismore -25 ii.iportant. On July 16, 1993, at apojove (the pointfarthest from Jupiter) in the current orbit, Shoemaker-Levy 9 was -30 almost 1,200 times as far from Jupiteras at the time of breakup, a distance of 50 million km, equalto a third of the -35 1 ,. 1 distance of Earth from the Sun. The comet has been in a -5 0 5 10 15 20 25 30 35 closed orbit around Jupiter for many decades, but that orbit 106 km is far from stable. Figure 8 by Chodas showswhat this orbit will look like as viewed from the Sun at the time of Figure 8. Jovicentric orbit of Shoemaker-Levy 9as viewed from the impact. It Sun on July 21, 1994. (Prepared by P. W.- Chodas.) is tipped (inclined) 53° to Jupiter's equatoras measured at apojove and is bent about 20° more near to Jupiter.(A astrometric observations of each comet fragment made be- three-body orbit rarely lies in one plane like the simpletwo- body orbit.) tween December 1993 and the time of impact. A week be- fore the impacts the times should be knownat least to ±10 minutes (with 50% confidence), improving During this final orbit, after breakup in July 1992,Shoe- to perhaps ±5 minutes a half day before impact. maker-Levy 9 was followed carefully from discoveryin March 1993 until the time the comet's angular distance fromthe Sun became too small to permit observations. The last useful At a fail planetary astronomy meeting (DPS) Jewitt,Luu, and astrometric (positional) observations reported before' the Chen exhibited an image showing 21 distinct fragments in the Shoemaker-Levy 9 nucleus train, At discovery in March, fragments were lost in the glare of the Sunwere made on this train was about 50 arc seconds or 162,000 km in length July 11. The comet was recovered (found again afteralmost as projected on the sky. This angular distance had increased five months without observations) by Scotti andTom Gehrels by about 40% (and the true linear distance by about 50%, on December 9, with the comet rising above the horizona since Jupiter was then farther from Earth) by the time the bit more than three hours before the Sun in themorning sky. (The comet is so faint that it cannot be observed in twi- comet was lost in the glare of the Sun in July. The spreading is caused mainly by the fact that the piece closest light or too low on the horizon.) The quality ofthe predic- to Jupiter at breakup was some 9 km closer than the farthest piece tions for the time of impact of the individual fragmentson Jupiter will depend upon the number of high-quality (the diameter of the comet) and therefore entered a faster

5 The Final Orbit of Shoemaker-Levy 9 13 (-)

orbit. The orbits are all so elongated that from Earth they ap- apart after one day. Moving jointly at a velocity which pear to be nearly a straight line with the fragments strung reaches 60 km/s at impact, thepieces would hit within a few out along it. The fragment nearest to Jupiter at breakup re- seconds of each other. The effect of further splittingupon mains nearest to it and will be the first toimpact. At this the impact phenomena would be fargreater and is dis- writing, Chodas and Yeomans predict that thetrain will cussed in the, next ______----reach-an-a-pparent length-esberid 1,286 arc Secondsat the time the first of the fragments enters Jupiter's atmosphere. Figure 9 by David Seal shows the finalsegment of the comet The true length of the train will be 4,900,000km at that fragment's . They will impactnear a latitude of time, and it will rcoire 5.5 days for all of the major frag- S and a longitude 70' past the midnightmeridian, still ments to impact. 10' beyond the limb of Jupiteras seen from Earth, impacting the atmosphere at an angle of about 42' fromvertical. The new data taken following solarconjunction (the closest apparent approach to the Sun as projectedagainst the sky) North more than doubled the length of time since discovery for Jupiter Pole Limb As Seen which cometarY positions we(r available. Withfils new from Earth data, it appears that the impacts will be centeredon about July 19.5, a day and a.half earlier than the firstpredictions. The approach to Jupiter that shatteredthe comet appears to have been even closer than first thought,about 96,000 km from the planet's center and only 25,000km above the clouds. The revised orbit has also moved theimpact points closer to the visible hemisphere, but unfortunatelystill on Earth the backside as seen from Earth. Thebrightest fragment, of --"'" Sun which there is some indication that it itself isfragmented, will impact on about July 20.78 and contains.about10% of the total mass of the comet. The other20 observed frag- ments contain more than 80% of themass. The remaining Mass is contained in all of the dustand small pieces in the train, the trails, and the tails. Most of-thismass also will hit Shoemaker-Levy 9 Time Ticks of Shoemaker-Levy 9 impact of Train Center Drawn Every 10 minutes Jupiter over a period of several months beginningabout July 10, but it probably will cause fewor no-detectable effects.

Meanwhile, the dust trails of small debris willcontinue to spread, as will the major fragments. The east-northeast trail Figure 9. The impact site of the fragments of CometShoemaker- is expected to reach a maximumapparent length of some Levy 9 on Jupiter. (Prepared by D. Seal.) 70 arc minutes in late June of 1994 and thendecrease again in apparent length with the tip turning around into a "V" Observing conditions from Earth will not be idealat the time shape. The west-southwest trail may reach a length of al- of the impacts, since there will be only abouttwo hours of most 100 arc minutes before impacts begin. Only the larger good observing time for large telescopesat any given site trail material will actually impact Jupiter. The earliest dust after the sky gets good and dark and beforeJupiter comes will begin to hit about July 10, and impacts will continue too close to the horizon to observe. At least it will besum- into October. The smaller dust will be moved into the tail by mer-in the northern hemisphere, and there will bea better solar and will miss the planet completely. chance for good weather where many observatoriesare lo- cated. With 21 pieces hitting over a 5.5-day period, there If upon further study it is found that the pieces of Shoe- will be an impact on average aboutevery 6 hours, so any maker-Levy 9 have continued to fragment, then predicting given site should have about one chance in three ofobserv- impact times will be much more difficult and the predictions ing at the actual time of an impact each night.Since the im-. less reliable. Such continued fragmentation ofpieces already pacts will be on the back side of Jupiter, light from the badly fractured is very possible, but fragmentation in the last impacts can only be observed by reflection fromJupiter's day or two, when it is most likely to occur, will have no sig- moons or perhaps from the rings or the dust comae of the nificant effect on the predicted times of impact. The pieces comet fragments. Those attempting observations of the ef- typically separate with a velocity of less than a meter per sec- fects of the impacts on Jupitercan begin about 20 n, nutes ond. There are 86,400 seconds in a day, so even pieces after the impacts, when the impactarea rotates into view separating at a full meter per second would be only 86.4km from Earth.

14 The Final Orbit of Shoemaker-Levy 9A6 Ile Collisions

I Exactly what will happen as the frag- tribute toe plasma. At higher temperatures, energy trans- ments of Shoemaker-Levy 9 enter the fer by radi t ion becdmes more important than conduction. is very uncertain, though there are many predictions. If the Ultimately he temperatures of the plasma and the surface process were better understood, it would be less interesting. of the intruding body are determined largely by the radiation Certainly scientists have never observed anything like this balance /The temperature may rise to 50,000 K (90,000° F) event. There seems to be complete agreement only that the or more for very large bodies such as the fragments of Shoe- major fragments will hit Jupiter and that these collisions will maker-Levy 9 entering Jupiter's atmosphere at 60 km/s. The occur on the back side of Jupiter as seen from Earth. loss of material as gas from the impacting body is called thermal ablation. The early manned spacecraft (Mercury, Any body moving through an atmosphere is Stowed byat- Gemini, and Apollo) had "ablative heat shields" made ofa mospheric drag, by having to push the molecules of thatat- material having low heat conductivity (through to thespace- mosphere out of the way. The kinetic energy lost by the craft) and a high vaporization temperature (strong molecular body is given to the air molecules. They move a bit faster bonds). As this material was lost, as designed, it carried (become hotter) and in turn heat the moving body bycon- away much of the orbital energy of the spacecraft reentering duction. This frictional process turns energy of mass motion Earth's atmosphere. (kinetic energy) into thermal energy (molecular motion). The drag increases roughly as the square of the velocity. Inany There are other forms of ablation besides thermal ablation, medium a velocity is finally reached at which the atmo- the most important being loss If solid material in pieces. Ina spheric molecules can no longer move out of theway fast comet, fragile to begin with and further weakened and/or enough and they begin to pile up in front of the moving fractured by thermal shock and by melting, such spallation body. This is the speed of sound (Mach 1 331.7 m/s or of chips or chunks of material has to be expected. Turbu- 741 mph in air on Earth at sea level). A discontinuity inve- lence in the flow of material streaming from the front of the locity and pressure is created which is calleda shock wave. shock wave can be expected to strip anything that is loose Comet Shoemaker-Levy 9 will enter Jupiter's atmosphereat away from the comet and send it streaming back into the about 60 km/s, which would be about 180 times the speed wake. The effect of increasing temperature, pressure, and of sound on Earth (Mach 180!) and is about 50 times the vibration on an intrinsically weak body is to crush it and speed of sound even in Jupiter's very light, largely hydrogen cause it to flatten and spread. Meanwhile the atmosphere is atmosphere. also increasing in density as the comet penetrates to lower altitudes. All of these processes occur at an ever increasing At high supersonic velocities (much greater than Mach 1) rate (mostly exponentially). enough energy is transferred to an intruding body that it be- comes incandescent and molecular bonds begin to break. On Earth a sizable iron or even some relatively The surface of the solid body becomes a liquid and thena low velocity stony can survive all of this and im- gas. The gas atoms begin to lose electrons and become ions. pact the surface, where we collect them for study and exhi- This mixture of ions and electrons is calleda plasma. The bition. (Small bodies traveling in space are called meteoroids. plasma absorbs radio waves and is responsible for the,com- The visible phenomena which occur as a meteoroid enters munication blackouts that occur when a spacecraft suchas the atmosphere is called a meteor. Surviving solid fragments the Space Shuttle reenters Earth's atmosphere. Theatmo- are called . There is no sharp size distinction be- spheric molecules are also dissociated and ionized andcon- tween meteoroids and asteroids. Normally, if the body has

.1.7 The ColilstonsIS r)

been detected telescopically before entering the atmo- before the terminal explosion and the velocity is still almost sphere, it has been called an asteroid.) Many meteoroids suf- 60 km/s. During that last second the energy of perhaps fer what is called a "terminal explosion" when crushed while 10,000 100-megaton bombs is released. Much of the still many kilometers above the ground. This is what hap- cometary material will be heated to many tens of thousands --pened in-Tunguska,- Siberia;-in-1908: There-a bodyrwith--a --of-degrees, vaporized, and-ionized-along with-a substantial mass of some 109 kg (2.2 billion lb.) and probably 90 to amount of Jupiter's surrounding atmosphere. The resulting 190 m in diameter entered Earth's atmosphere at a low fireball should balloon upward, even fountaining clear out of angle with a velocity of less than 15 km/s. It exploded at an the atmosphere, before falling back and spreading out into altitude of perhaps 5-10 km. This explosion, equivalent to Jupiter's atmosphere, imitating in a non-nuclear fashion 1'0-20 megatons of TNT, combined with the shock wave some of the atmospheric hydrogen bomb tests of the 1950s. generated by the body's passage through the atmosphere Once again, the total energy release here will be many thou- immediately before disruption, leveled some 2,200 km2 of sands of times that of ,..ny hydrogen bomb ever tested, but Siberian forest. The Tunguska body had a tensile strength of the energy will be deposited initially into a much greater vol- some 2 x 108 dynes/cm2, more than 100,000 times the ume of Jupiter's atmosphere, so the energy density will not strength of Shoemaker-Levy 9, but no surviving solid frag- be so high as in a bomb, and, of course, there will be no ments of it (meteorites) have ever been found. The fragile gamma rays or neutrons (nuclear radiation or particles) flying Shoemaker-Levy 9 fragments entering an atmosphere of vir- about. The energy of these impacts will be beyond any prior tually infinite depth at a much higher velocity will suffer al- experience. The details of what actually occurs will be deter- most immediate destruction. The only real question is mined by the observations in July 1994, if the observations whether each fragment may break into several pieces imme- are successful. diately after entry, and therefore exhibit multiple smaller ex- plosions, or whether it will survive long enough to be If differential gravitation (tidal forces) should further frag- crushed, flattened, and obliterated in one grand explosion ment a piece of the comet, say an hour or two before im-

and terminal fireball. pact, the pieces can be expected to hit within a second of . each other. In one second a point at 44° latitude on Jupiter Scientists have differed in their computations of the depths will rotate 9 km (5.6 mi.), however, so the pieces would en- to which fragments of given mass will penetrate Jupiter's at- ter the atmosphere some distance apart. Smaller pieces will mosphere before being completely. destroyed. If a "terminal explode at higher altitudes but not so spectacularly. If explosion" occurs above the clouds, which are thought to lie smaller pieces do explode above the clouds, they may be at a pressure level of about 0.5 bar or roughly 0.5 Earth at- more "visible" than larger pieces exploding below the mosphere (see Section 5), then the explosion will be very clouds. It is also possible that implanting somewhat less en- bright and easily observable by means of light reflected from ergy density over a wider volume of atmosphere might cre- Jupiter's satellites. Using ablation coefficients derived from ate a more visible change in Jupiter's atmosphere. Sekanina _observation of many terrestrial fireballs, Sekanina predicts notes that pieces smaller than about 1.3-km mean radius that the explosions indeed will occur above the clouds. should not be further fragmented by tidal_ forces unless they Mordecai-Mark Mac Low and Kevin Zahnle have made cal- were already weakened by earlier events. culations using an astrophysical hydrodynamic code (ZEUS) on a supercomputer. They assume a fluid body as a reason- One of the more difficult questions to answer is just how able approximation to a comet, since comets have so little. bright these events will be. Terrestrial fireballs have typically strength, and they predict that the terminal explosions will exhibited perhaps 1% luminous efficiency. In other words occur near the 10-bar level, well below the clouds. Others about 1% of the total kinetic energy has been converted to have suggested still deeper penetration, but most calcula- visible light. The greater magnitude of the Jupiter impacts tions indicate that survival to extreme depths is most un- may result in more energy appearing as light, but let's as- likely. The central questions then appear to be whether sume the 1% efficiency. Then Sekanina calculates that a terrestrial experience with lesser events can be extrapolated 10'3-kg fragment, a reasonable value for the largest piece, to events of such magnitude and whether all the essential will reach an apparent visual magnitude of 10 during the physics has been included in the supercomputer calculations. terminal explosion. This is 1,000 times Jupiter's normal We can only wait and observe what really happens, letting brilliance and only 10 times fainter than the full moon! teach us which predictions were correct. Sekanina, of course, calculates that the explosions will occur abovc the clouds. And, remember that, unfortunately, these O.K. So an explosion occurs at some depth. What does that impacts will occur on Jupiter's back side as seen from Earth. do? What happens next? Sekanina calculates that about There will be no immediate visible effect on the appearance 93% of the mass of a 10'3-kg fr4gment remains one second of Jupiter. The light of the explosion may be seen reflected

1$ The Collisions s from the Galilean satellites of Jupiter, if they are properly mospheric gas to very high altitudes. The fireballmay carry placed at the times of impacts. Ganymede, for example, -with it atmospheric gases that are normally to be found only might brighten as much as six times, while lo could brighten far below Jupiter's visible clouds. Hence theimpacts may to 35 times its. normal brilliance for a.second before fading. give astronomers an opportunity-to-detect-gases which have slowly, if the explosions occur above the clouds. This would been hitherto hidden from view. As the gaseous fireball rises certainly be visible in an amateur telescope and could con- and expands it will cool, with some of the gases it contains ceivably be visible to the naked eye at a dark mountain site condensing into liquid droplets or smal' solid particles. Ifa. as a tiny flash next to Jupiter at the location of the normally sufficiently large number of particles fc.rm, then the clouds invisible satellite. Emphasis on "tiny"! The brightness ofex- they produce may be visible from E:Irth-based telescopes af- plosions occurring below the clouds will be attenuated bya ter the impact regions rotate Onto the visible side of the factor of at least 10,000, making them most difficult to ob- planet. These clouds may provide the clearest indication of serve. In the best of cases, these. events will be spectacles for the impact locations after each event. the mind to imagine and big telescopes to observe, nota free fireworks display. After the particles condense, they will grow in size by collid- ing and sticking together to form larger particles, eventually The most recent predictions are that at least some of the im- becoming sufficiently large to "rain" out of the visible part pacts will occur very close to the planetary limb, the edge of of the atmosphere. The length of time spent by the cloud the planet's disk as seen from Earth. That edge still has 11° particles at altitudes where they can be seen will depend to rotate before it comes into sunlight. This means that the principally on their average size; relatively large particles tops of some of the plumes associated with the rising fire- Would be visible only for a few hours after an impact, while balls may be just visible, although with a maximum predicted small particles could remain visible for several months. Un- height of 3,000 km (0.8 arc second as projected on the sky) fortunately, it is very difficult to predict what the number they will be just "peeking" over the- limb. The newly repaired and average size of the particles will be. A cloud of particles Hubble Space Telescope (HST), with its high resolution and suspended in the atmosphere for many days may signifi- low scattered light, may offer the best chance tosee such cantly affect the temperature in its vicinity by changing the plumes. By the time they reach their maximum altiti 'de the amount of sunlight. that is absorbed in the area. Such a tern- plumes will be transparent (optically thin) and not nearlyso perature change could be observed from Earth by searching bright as they were near the clouds. Some means of block- for changes in the level of Jupiter's emitted infrared light. ing out the bright lightfrom Jupiter itself may be required in order to observe anything. A number of observers plan to Glenn Orton notes that large regular fluctuations of atmo- look for evidence of plumes and to attempt tomeasure their spheric temperature and pressure will be created by the size and brightness. shock front of each entering fragment, somewhat analogous to the ripples created when a pebble is tossed into a pond, It also is difficult to predict the effects the impacts on and will travel outward from the impact sites. These may be Jupiter's atmosphere. Robert West points out thata substan- observable near layers of condensed clouds in thesame way tial amount of.material will be deposited even in the strato- that regular cloud patterns are seen on the leeward side of sphere of Jupiter, the part of the atmosphere above the mountains. Jupiter's atmosphere will be sequentially raised visible clouds where solar heating stabilizes the atmosphere and lowered, creating a pattern of alternating cloudyareas against convection (vertical motion). Part of this material will where ammonia gas freezes into particles (the same way come directly from small cometary grains, which vaporize that water condenses into cloud droplets in our own atmo- .during entry and recondense just as do meteoritic grains in sphere) and clear areas where the ice particles warm up and the terrestrial atmosphere. Part will come from volatiles (am- evaporate back into the gas phase. If such waves are de- monia, water, hydrogen sulfide, etc.) wellingup from the tected, measurement of their wavelength and speed will deeper atmosphere as a part of the hot buoyant fireballscre- allow scientists to determine certain important physical prop- ated at the time of the large impact events. Many millimeter- erties of Jupiter's deep atmospheric structure that are very sized or larger pieces from the original breakup will also difficult to measure in any other way. impact at various times-for months and over the entire globe of Jupiter. There is relatively little mass in these smaller Whether or not "wave" clouds appear, the ripples spreading pieces, but it might be sufficient to create a haze in the from the impact sites will produce a wave structure in the stratosphere. temperature at a given level that may be observable in infra- red images. In addition there should be compressionwaves, James Friedson notes that the fireball created by the terminal alternate compression and rarefaction in the atmospheric explosion will expand and balloon upward and perhaps pressure, which could reflect from and refract within the spew vaporized comet material and Jupiter's entrained at- 9 The Collisions 17 BEST COPY AVAILABLE higher particles will reflect deeper atmosphere, much as seismic wavesreflect and re- in the atmosphere, because the sunlight better. Much of the light is absorbedbefore reach- fract due to density changes inside Earth. Orton suggests clouds in gaseous that these waves might be detected "breaking up"in the ing the lower particles. Observing these they lie in the shallow atmosphere on-the opposite side of theplanet from-- ---absorption bands.will then teil_us how high atmosphere, and observations over a periodof time will indi- the impacts. Others suggest the possibilityof measuring the pushing them. The small temperature fluctuations wherever the wavessurface, cate how fast high-altitude winds are will be a measure but this requires the ability to mapfluctuations in Jupiter's speed with which these clouds disappear settle out visible atmosphere of a few millikelvin (a fewthousandths of of particle sizes in the clouds, since large particles much faster than small ones, hours as compared todays or a degree) Detectionof any of these waves will require a very fine infrared array detector (a thermal infraredcamera). months.

Orton also notes-that in the presence of anatural wind shear Between the water and other condensable gases(volatiles) speeds and/or direc- brought with the comet fragments and thoseexhumed by (a region with winds having different the tace of Jupiter, a the rising fireballs, it is fairly certain that a cloudof con- tions) such as exists commonly across long-lived cyclonic feature can be created whichis actually densed material will form at the location of the impacts quite stable. It may gain stability bybeing fed energy from themselves, at high altitudes where such gasesseldom, if the wind shear, in much the same waythat the Great Red ever, exist in the usual courseof things. It may be difficult to be stabilized. differentiate between the color or brightness of these con- Spot and other Jovian vortices are thought to is some- densates and any bright material below. them in spectra at Such creation of new, large, fixed "storm" systems what controversial, but this is a most intriguingpossibility! most visible wavelengths. However, atwavelengths where gaseous methane and hydrogenabsorb sunlight, a distinc- tion can easily be made between particleshigher and lower

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1$ The Collisions 20 - -

o Impacts and Their Cons Be Studied?

Space-Based There are at least four spacecraft the fact d befor the impact times are knownto better Galileo, , Voyager 2, and with some than a.' t 20 minutes with 95% certainty. A latercom- potential to observe the Jovian impacts from different van- mand t at simply triggers the entire commandsequence tage points than that of Earth. There is also the Hubble may b possible. A lot of data frames can be stored inthe Space Telescope (HST), in orbit around Earth, which will view Galil tape recorder, but only about 5% of themcan be the event with essentially the same geometry as any Earth- transmitted back to Earth, so the trick will beto decide based telescope. HST, however, has the advantages of which 5% of the data are likely to include theimpacts and perfect "seeing" (no atmospheric turbulence), very low scat- to have the greatest scientific value, without being ableto tered light, ultraviolet sensitivity, and the ability to observe look at any of them first! After the fact, theimpact times much more than two hours each day. HST is scheduled to should be known quite accurately. This knowledgecan help devote considerable time to the observation of Shoemaker- to make the decisions about which data to returnto Earth. Levy 9 before as well as during the impacts.

The Ulysses spacecraft was designed for solarstudy and used The Galileo spacecraft has the best vantage point from a gravity assist from flying close to Jupiter to change its incli- which to observe the impacts. It ison its way to Jupiter and nation (the tilt of its path relative to the plane ai theplanets) will be only 246 million km away from the planet, less than a so it can fly over the poles of the Sun. In July 1394 it will be third the distance of Earth from Jupiter at that time. All of about 378 million km south of the plane of theplanets (the the impacts will occur directly in the field of view of its high ) and able to "look" over the south pole ofJupiter di- resolution camera and 20 -.25° of Jovian longitude from the rectly at the impact sites. Unfortunately, Ulysseshas no cam- limb. Images of Jupiter will be 60 picture elements (pixels) era as a part of its instrument complement. It does havean across, although the impact site will still be smaller than the extremely sensitive receiver of radio frequency signals from resolution of the camera. Several instruments besides the 1 to 1000 kHZ (kilohertz, or kilocycles in olderterminology) camera have potential use, including an ultraviolet spectrom- called URAP (Unified Radio and Plasmawave experiment). eter, a near infrared mapping spectrometer, and a photopo- URAP may be able to detect thermal radiation fromthe im- larimeter radiometer. This last suite of instrumentscould pact fireballs once they rise sufficiently high above interfer- acquire light curves (plots of intensity versus time) of the en- ence from the Jovian ionosphere (upper atmosphere) and to try and fireball at many wavelengths from ultraviolet to ther- measure a precise time history of their rapid cooling. mal infrared (from wavelengths much shorterthan visible light to much longer). The Voyager 2 spacecraft is now far beyond Neptune(its last object of study back in 1989 after visitingJupiter in Using Galileo to make these observations will bechalleng- 1979, in 1981, and in 1986) and is about ing. The amount of data the spacecraft can transmit back to 6.4 billion km from the Sun. it can look directly backat the Earth is limited by the capability of its low-gain antenna and dark side of Jupiter, but the whole of Jupiter isnow only two the time available on the receiving antennas of the National picture elements in diameter as seen by its high-resolution Aeronautics and Space Administration's (NASA's) Deep camera, if that instrument were to be used. In fact the cam- Space Network here on Earth. The "commands" that tell the era has been shut down for several years, and the engineers spacecraft what to do must be sent up several weeks before who knew how to control it havenew jobs or are retired. It would be very expensive to take thecamera "out of moth- 21 How Can These ImpaCts and TheirConsequences Be Studied? 19 balls" and probably of limited scientific value. Voyagerdoes 10,000 km or more around each fragment. Moving at 60 km/s, it will be almost three minutes before allof the have an ultraviolet spectrometer which is still takingdata, . and it will probably be used to acquire ultravioletlight curves dust also impacts Jupiter. Proper interpretation of such ob- the (brightness versus time) of the impact phenomena.The pos- servations will be difficult, however, because the area of sibility of using one or two other instruments isbeing con- "reflector," the coma dust particles, will be changing as the -sidered, though useful-results- from them -seemless likely - --observations -are -made. Another. complication is the bright-_ ness of Jupiter itself, which will have to bemasked to the greatest extent possible. Observations in visible light re- A new small spacecraft called Clementine waslaunched on January 25 of this year, intended to orbit theMoon and then flected from the satellites will be relatively straightforward and can be done with small telescopes and simple photom- proceed on to study the asteroid Geographos.Clementine small enough has good imaging capabilities, but its viewpointwill not be eters or imaging devices. This equipment is much different from. Earth's. The impact siteswill still be just that it can be transported to appropriate sites. over the limb, and Clementine'sresolution will be only a few Spectroscopy of the entry phenomena via reflected light picture elements on Jupiter. Since thespacecraft will be in from one of the Galilean satellites could be used to deter- cruise mode at the time, on its way to Geographosand not mine the composition of the comet and the physicalcondi- terribly busy, it seems probable that attemptswill be made above to observe "blips" of light on the limbof Jupiter, from the tions in the fireball, if the terminal explosions occur clouds, entering fragments or the fireballs or perhapslight scattered Jupiter's clouds. If the explosion occurs below the with from cometary material (coma) that has not yet enteredthe there willbetoo little light to do useful spectroscopy atmosphere. Useful light curves could result. even the largest telescopes.

The impact zone on Jupiter will rotate into sightfrom Earth about 20 minutes after each impact, though quiteforeshort- ened as initially viewed. Extensive studies of the zoneand Ground-Based studies Manylarge telescopes will be available the area around it can be made at that time. Such surely will include imaging, infrared temperature measure- on Earth with which to observethe phenomena associated largest tele- with the Shoemaker-Levy 9 impacts on Jupiterin visible, in- ments, and spectroscopy using many of the will continue for some weeks; frared, and radio wavelengths. Small portable telescopes can scopes on Earth. These studies fill in gaps in existing observatory locations for some pur- if there is any evidence of changes in Jupiter's atmosphere and cloud structure as a result of the impacts. poses. Imaging, photometry, spectroscopy,and radiometry will certainly be carried out using a multitudeof detectors. For example, astronomers will use spectrometers tolook for Many of these attempts will fail, but someshould succeed. evidence of chemical changes in Jupiter's atmosphere.Some in the Apart from the obvious difficulty that theimpacts will occur of the species observed might be those only present deep atmosphere and carried up by the fireball(if the ex- on the back side of Jupiter as seenfrom Earth, the biggest result of problem is that Jupiter in July can only be observedusefully plosion occurs deep enough). Others will be the taking for about two hours per night from any.givensite. Earlier the changes to the chemistry of the upper atmosphere, impacts sky is still too bright and later the planet is tooclose to the place because of the energy deposited there by the particulates. horizon. Therefore, to keep Jupiter under continuoussurveil- or because of the additional lance would require a dozen observatories equallyspaced in be longitude clear around the globe. A dozen observatoriesis The faint , mentioned in Section 5, can wavelengths. feasible, but equal spacing is not. There will be gapsin the usefully observed from the ground at infrared Shoemaker-Levy 9 debris might bring in new ringmaterial by coverage, notably in the PacificOcean, where Mauna Kea, hitting the two small.satellites embedded in therings (Metis Hawaii, is the only astronomical bastion. and Adrastea). The rings surely will bemonitored for some which are sensi- Measuring the of the enteringfragments and the time using infrared imaging array detectors, long as red vis- post-explosion fireball can be done only by measuringthe tive to wavelengths more than eight times as light reflected from something else, one of Jupiter'ssatellites ible light. , or perhaps the dust comaaccompanying the fragment. That dust coma could still be fairly dense out to distancesof

20 How CanThen Impacts and Their ConsequencesBe Studied? In Section 5, note was made of the Jovian magnetosphere, In summary, the phenomena directly associated with each which makes its presence known at radio wavelengths,, impact from entry trail to rising fireball will last perhaps and the lo torus of various ions and atoms, which can t. three minutes. The fallback of ejecta over a radius of a few mapped spectroscopically. Either or both of these could be thousand kilometers will last for about three hours. Seismic affected sufflcientlybythe intruding dust from Shoemaker- waves from each impact might be detectable for 0,day,and___ Levy 9 to be detectably changed. Radio telescopes will surely atmospheric waves for several days. Vortices and atmo- monitor the former and optical telescopes the latter for spheric hazes could conceivably persist for weeks. New weeks or months looking for changes. Jupiter's intense elec- material injected into the Jovian might be tromagnetic environment is responsible for massive auroral detectable for years. Changes in the magnetosphere and/or emission near the planet's poles and less intense phenomena the lo torus might also persist for some weeks or months. across the face of the planet. These may also be disrupted by There. is the potential to keep planetary observers busy for a the collisions and/or the dust "invasion," making auroral long time! monitoring a useful observing technique.

How Can Tbese Impacts and Their Consequences Be Studied?21 s Expect to oAll of This?

Tgive a simple and succinct answer to If suffici t materi. impacts Jupiter's rings or especially the the title of this section, scientists hope to learn more about ring sate tes, the there should be local brightening caused comets, more about Jupiter, and more about the physics of by the irease in reflecting area due to the introduction of high velocity impacts into a planetary atmosphere. Some- new m. erial. These new ring particles will each take up thing has already been learned about comets from the be- their n orbits around Jupiter, gradually spreading out and havior of Shoemaker-Levy 9 during its breakup, as discussed causing local brightening followed by slow fading into the in Section 4. Bits and pieces of what everyone hopes will be general ring background. Careful mapping of that brighten- learned have been noted in Sections 5 through 8. A more ing and fading will reveal a great deal. about the structure complete summary follows. and dynamics of the rings. Many believe that impacts on those small inner satellites are the source of the rings, the If the fragments explode above the clouds, there should be reason for their existence. Enhancement of the rings from enough light reflected from various Jovian satellites to take Shoemaker-Levy 9 impacts would be strong confirmation of spectra of explosions. Since the atmosphere of Jupiter this idea. Similarly, the interaction of cometary dust with the contains very few heavier elements to contaminate the spec- magnetosphere and with the to torus will be quite informa- tra, they could give a great deal of information about the tive, if the dust density proves sufficient to cause observable composition of cometary solids. If the fragments explode be-. effects. Radio telescopes will be active in the magneto- low the clouds, then spectroscopy must wait until the impact spheric studies, along with optical spectroscopy of the ions sites rotate into view from Earth: By that time everything will and atoms in the torus. have cooled a great deal, and the cometary component will have been diluted by mixing with the lovian atmosphere, Last, but far from least, the physics of the impact phenom- making such study difficult. In that case the Jovian material ena themselves, determined from the reflected light curves itself may prove of interest, with spectroscopic study giving and from spectra, will be most instructive. Note the inability new knowledge of Jupiter's deeper atmospheric composition. of scientists to agree on the level of Jupiter's atmosphere at which the terminal explosion will occur. (A few even believe It seems somewhat more certain that new knowledge of that there will be no terminal explosion or that it will occur Jupiter's atmosphere Will be obtained, even if predictions dif- so deep in that atmosphere as to be completely unobsetv- fer as to exactly what that new knowledge will be. There is able.)`Entry phenomena on this scale cannot be reproduced, nary unanimous agreement that the impacts will cause ob- even by nuclear fusion explosions, and have never before servable changes in Jupiter, at least locally at the impact been observed. Better knowledge of the phenomena may sites. These may include changes in the visible appearance of allow scientists to predict more accurately just how serious the douds, locallyor more widely, measurable temperature could be the results of future impacts of various-sized bodies fluctuations, again locallyor more widely, composition on Earth, as well as to determine their effects in the past as changes caused by material broughtup from below the registered by the fossil record. clouds (if the fragments penetrate-that deeply), and/or chemical reactions brought about by the thermal pulse and the introduction of cometarymaterial. Any dynamicpro- cesses such as these will give a new and better understand- ing of the structure of Jupiter's atmosphere, perhaps of its motion as well as its static structure. 4 Wbat Do Scientists Expect to Learn from All of TbtsP22 Appendixz1

Table 1. Energy comparisons.'

Two 3,500-lb. cats colliding head-on at 55 mph

Explosion of 1 U.S. ton of TNT 4.2 x 109 4,271

Explosion of a 20-megaton fusion bomb 8.4 x 10'6 87,500.000,000

Total U.S. annual electric 1 x 10'9 10,400,000.000.000 power production. 1990

Energy released in last second of 10'3-kg - 9 x 1021 9,375,000,000,000.000 fragment of Shoemaker-Levy 9

Total energy released by 10'3-kg 1.8x 1022 18,750,000,000,000,000 fragment of Shoemaker-Levy 9

Total sunlight on Jupiter for one day 6.6 x 1022 68.750,000.000,000,000

'187U 252 (small) calories1.01512.93 X 10-4 itWb.

Table 2. Power comparisons.'

Power Producer Power, MW Power, Relative

Hoover Dam 1,345 1

Grand Coulee Dam, final plant 9,700 7.2

Annual average, sum of all U.S. power plants 320,000 238

Average, impact of 1013-kg fragment of - 9 x 100 6,700,000,000,000 Shoemaker-Levy 9, final second

Sun 3.8 x 1073 280.000,000,000,000,000

'1 bonspouvr745.7 W 7.457 x 10-4,WW.

25 Appendix A Comparative Tables25 DES COPY AVAILABLE Table 3. Size comparisons.

Jupiter 71.350 (Equatorial) 1.4 x 10'5 _ _ 67,310 (Polar).,

Earth 6.378 (Equatorial) 1.1 x 10'1 6,357 (Polar)

Comet Shoemaker-Levy 9 4.5 (Equivalent sphere) 382

Comet Halley 7.65 x 3.60 x 3.61 365

.1mi. i.go9 km

Table 4. Brightness comparisons.

Object Magnitude ti.. Relative Brightness

Largest fragment of Shoemaker-Levy 9 - -10 . 1.000 during last second

Jupiter -2.5 1

Ganymede 4.6, 1/692

lo 5.0 1/1,000

largest fragment of Shoemaker-Levy 9 23.7 1/30,000,000.000 as viewed today

1 2$ Appendix A Comparative Tables Appendix1)

e_KL_,-T Event

Sixty-five million years ago about 70% taken 10 ears earl. r by geophysicists looking for oil in the of all species then living on Earth disappeared withina very Yucatangion ofexico. There a 180-km (112-mi.) diam- short period. The disappearances included the last of the eter ring tructure called "Chicxulub" seemed to fit what great dinosaurs. Paleonzoiogists speculated and theorized for would expected from a 65-million-year-old impact, and .many years about what could have caused this "mass extinc- furthe studies have largely served to confirm its impactori- tion," known as the K-T event (Cretaceous-Tertiary Mass Ex- gin. The has been age dated (by the 443Ar/ tinction event). Then in 1980 Alvarez, Alvarez, Asaro, and 39Ar method) at 65 million years! Suc:ian impact would Michel reported their discovery that the peculiar sedimentary cause enormous tidal waves, and evidence of just such clay layer that was laid down at the time of the extinction waves at about that time has been found all around the Gulf showed an enormous amount of the rare element . of Mexico. Similarly, glassy debris of appropriateage called First seen in the layer near Gubbio, Italy, the same enhance- tektites (and their decomposition products), which arepro- ment was soon discovered tobe worldwide in that one par- duced by large impacts, have been found all around the ticular 1-cm (0.4-in.) layer, both on and at sea. The Gulf. Alvarez team suggested tnat the enhancementwas the product of a huge asteroid impact. One can never prove that an asteroid impact "killed the di- nosaurs." Many species of dinosaurs (and smaller flora and On Earth most of the iridium and a number of other rare ele- fauna) had in fact died out over the millions of years preced- ments such as platinum, osmium, ruthenium, rhodium, and ing the K-T event. The impact of a 10-km asteroid would palladium are believed to have been carried down into most certainly have been an enormous insult to on Earth. Earth's core, along with much of the iron, when Earthwas Locally there would have been enormous shock wave heat- largely molten. Primitive "chondritic" meteorites (andpre- ing and fires, a tremendous earthquake, hurricane winds, sumably their asteroidal parents) still have the primordialso- and trillions of tons of debris thrown everywhere. It would lar system abundances of these elements. A chondritic have created months of darkness and cooler temperatures asteroid 10 km (6 mi.) in diameter would contain enough globally. There would have been concentrated nitric acid iridium to account for the worldwide clay layer enhance- rains worldwide. Sulfuric acid aerosols may have cooled ment. This enhancement appears to hold for the other ele- Earth for years. Life certainly could not have been easy for ments mentioned as well. those species which did survive. Fortunately such impacts oc- cur-only about once every hundred million years. Since the original discovery many other pieces of evidence have come to light that strongly support the impact theory. The high temperatures generated by the impact would have caused enormous fires, and indeed soot is found in the boundary clays. A physically altered form of the mineral quartz that can only be formed by the very high pressures associatedwith impacts has been found in the K-T layer.

Geologists who preferred other explanations for the K-T event said, "show us the crater." In 1990 a cosmochemist named Alan Hildebrand becameaware of geophysical data

Appendix B The K-T Event27 elhis booklet is the product of many scientists, all of The writing and production of this materialwas made whom have cooperated enthusiastically to bring possible through the support of Jurgen Rahe and Joe Boyce, their best Information about this exciting event to a wider Code SL, NASA, and of Dan McCleese, Jet Propulsion audience. They have contributed paragraphs, words, Laboratory (JPL). diagrams, and prepiliits as well as their critiques to For help in the layout and production of this booklet,on a this document, .which attempts to present an event that no very tight schedule, additional thanks go to the Design one is quite sure how to describe. Sincere thanks go to Mike Services Group of the /PL Documentation Section. A'Hearn, Paul Chodas, Gil Clark, Janet Edberg, Steve Edberg, Jim Friedson, Mo Geller, Martha Hanner, Cliff Heindl, David All comments should be addressed to the author: Levy, Mordecai-Mark Mac Low, Al Metzger, Marcia Neugebauer, Glenn Orton, Elizabeth Roettger, Jim Scotti, Ray L. Newburn, Jr. David Seal, Zdenek Sekanina, Anita Sohus, Harold Wearer, Jet Propulsion Laboratory, MS 169-237 4800 Oak Grove Dr. Paul Weissman, Bob West, and Don Yeomans and to Pasadena, CA 911048099 those who might have been omitted. The choice of material and the faults and flaws in the document obviously remain the responsibility of the author alone.

NASA National Aeronautics and Space Administration Jet Propulsion Laboratory California Institute of TechnolOgy Pasadena. California