The following document was compiled for use at the EU/ESO Workshop for European Astronomy Teachers held at the ESO Headquarters on November 25-30, 1994, in conjunction with the European Week for Scientific Culture (for more details, including the resulting Declaration on Teaching of Astronomy, see ESO Press Release 17/94 of December 2, 1994). A few items were added/corrected in early December. It gives an overview of some of the most important, recent developments during the interpretation of the data obtained in the course of the SL-9 campaign, as they were available to me by December 15, 1994. It is to a large extent based on information contained in the papers delivered at the AAS DPS conference on October 31, 1994, in Bethesda, near Washington, D.C., and also on comments on an earlier draft received from a number of individual scientists. Specific reference to the source is not given in all cases. Please note that the detailed information given below, especially that in Section 4, relates to on-going research projects and may no longer represent the current situation, as perceived by the involved groups. Most of it has not been checked by the mentioned groups and therefore represents my personal impression of the status of this research. The present summary information should therefore be used under this assumption and with some caution. Richard West European Southern Observatory December 15, 1994 UPDATE ON SL-9/JUPITER COLLISION 1. Main Interpretative Areas 1. The comet 1.1. Origin and earlier orbit 1.2. The break-up 1.3. Composition and structure 1.4. Size of nuclei 2. The approach 2.1. Final orbit 2.2. Effects near Jupiter 3. The bolide phase 3.1. Direct observations 3.2. Indirect observations (reflection, etc.) 4. The fireball 4.1. Direct observations of explosion 4.2. Early evolution (rise and fall) 4.3. Composition and early chemistry 5. The plumes 5.1. Long-term evolution of dark spots and their surroundings 5.2. Chemical evolution 6. Auroral effects 7. Seismic effects 8. The Io Torus and the Jovian dust ring 2. Current Main Questions 1. The comet 1.1. Sizes of parent object and individual nuclei after break-up 1.2. Internal constitution and composition 2. The approach 2.1. Behaviour of dust near Jupiter 3. The bolide phase 3.1. Depth of penetration 3.2. Physics of ablation and final explosion 4. The fireballs 4.1. Detailed processes after explosion (rise and fall) 4.2. Nature of preflashes 4.3. Origin of individual elements (comet and/or atmosphere) 4.4. Temperature evolution 5. The plumes 5.1. Nature of rings around impact sites 5.2. Composition and further chemical evolution 5.3. Vertical stratigraphy 5.4. Short- and long-term stratospheric development 6. Auroral effects 6.1. Source and mechanisms of effects in both hemispheres 7. Seismic effects 7.1. Were temperature variations observed? 7.2. Internal structure of Jupiter 8. The Io Torus and the Jovian dust ring 8.1. Why were so small or no effects observed? 9. General 9.1. Frequency of impact events 9.2. Terrestrial effects in case of similar event 3. Impact Times And Sites These data are taken from the latest list of Chodas and Yeomans (published at DPS meeting on October 31, 1994) and updated as of December 15, 1994. Dates in 1994; nominal uncertainties for the timings; Jovian coordinates in degrees. Timings with indicated seconds are based on Galileo light measurements. Fragment Date July UT Time 1 sigma Latitude Longitude A 16 20:11 4 min -43.15 185 B 17 02:53 4 min -43.17 069 C 17 07:12 4 min -43.38 224 D 17 11:54 3 min -43.46 034 E 17 15:11 3 min -43.48 153 F 18 00:33 5 min -43.55 134 G 18 07:33:32 10 sec -43.60 026 H 18 19:31:59 10 sec -43.74 100 K 19 10:24:14 10 sec -43.80 279 L 19 22:16:48 10 sec -43.92 349 N 20 10:29:17 10 sec -44.30 072 P2 20 15:23 7 min -44.64 250 Q2 20 19:44 6 min -44.26 046 Q1 20 20:13 3 min -44.05 063 R 21 05:34 3 min -44.07 045 S 21 15:15 5 min -44.16 033 T 21 18:10 7 min -44.99 141 U 21 21:55 7 min -44.43 276 V 22 04:23 5 min -44.43 150 W 22 08:06:12 10 sec -44.15 283 4. Update of Observations and Interpretations 1. The comet Comet Shoemaker-Levy 9 (SL-9) was discovered in March 1993. In May 1993, it was for the first time recognized that it will collide with Jupiter in mid-July 1994; full confirmation of this, based on longer series of accurate astrometry, was announced on November 22, 1994. 1.1. Origin and earlier orbit * passage near Jupiter at r = 1.6 R(Jupiter) on July 8, 1992 * earlier orbit very uncertain - no observations available * extrapolation back shows that: * must have orbited Jupiter for decades * 2 Jovian year oscillations in elements during this time * possible capture in 1920-1930 * before typical SP-comet of Jupiter family * most likely to have come from the inside, i.e. via the asteroidal belt 1.2. The break-ups * passage near Jupiter at r = 1.6 R(Jupiter) on July 8, 1992 * broke up into at least 20 pieces * original nucleus had very little internal strength * diameter of original nucleus uncertain, probably 4-5 km * further breaks-ups, e.g. of fragment P2 on March 30, 1994 * G observed to be double in June 1994 * HST observes complex behaviour of Q + P fragments (splitting, fading) * several impacts multiple, i.e., further break-ups near Jupiter likely 1.3. Composition * much dust soon after break-up, less later on * not obvious that more dust was produced long after primary break-up * no gas ever observed in comae: * HST and IUE see no OH in UV spectra * NTT sees no CN: production rate Q(CN) < 1-3 10E23/sec * this rate is low, but now exceptionally low at this distance * on April 15, optical spectra of G,H,K,L,Q,R identical (MPI/ESO 2.2 m) * on July 1, optical spectra of all nuclei similar (NTT) * photometry + polarimetry indicate all dust clouds of similar composition * possible colour trend along comet tails: sorting of particle sizes by SW ? * up to beginning of July, dust motion as predicted by gravitation from Sun and Jupiter + solar radiation pressure * fragment Q apparently bluer than S (Kitt Peak; July 1 - 16) 1.4. Sizes of nuclei * analysis of high-resolution HST images: * several fragments in each dust cloud possible (Sekanina) * no obvious point-sources visible, i.e. not possible to estimate sizes * energy measured at impact: * Galileo: measured luminous energy => min. 350 m diameter * energy released for fireball ascent => largest nuclei > 1-2 km diameter * sum of energy released (model) => progenitor diameter 4.1 +- 0.6 km * this issue still very uncertain! 1.4. Comet or asteroid ? * H2O, CO, HCN observed => comet * disappearence of several fragments => comet * loose adhesion during break-up => comet * P2 and G fragment broke up later => solid, not loose gravel * no gas observed => not much ice (?) * in summary: most probably comet 2. The approach The final approach of SL-9 was followed from the ground, mostly for astrometric purposes, and with the HST. Images of some of the individual fragments are available up to a few hours before impact. 2.1. Final orbit * astrometry => JPL orbits => prediction of impact times + sites * in the mean a delay of (observed * predicted time) = 5-7 minutes * probably mostly caused by lower accuracy of the mostly used astrometric reference catalogue (Guide Star Catalogue) * other effects not entirely excluded * gravitational effects of Jovian satellites introduce shifts of a few minutes only 2.2. Effects near Jupiter * clear elongation of dust clouds in the direction of Jupiter during the last days * effect not understood, but possibly related to magnetic field? * HST observes MgII (2800 A) emission on July 14 at distance 50 R(Jupiter) * charged grains => exploding ? * ionisation at crossing of bow shock ? * but unexpectedly no OH- observed => very little water ice? * UTR-2 (Ukraine) obs. radio pulses at 18-25 MHz, 25-17 min before A-impact * steep rise in X-ray emission (ROSAT), 3 min before K-impact * Pic du Midi observes pre-precursor at 22:16:36, i.e. 12 sec before L-impact * "sightings" in China of very elongated S and other fragments very near Jupiter * but most probably reflection in telescope 3.The bolide phase The impact sites were not directly observable from the Earth. However, the spacecraft Galileo (240 million km from Jupiter) and Voyager 2 (6,200 million km) had a direct view. Indirect observations from the ground were attempted by monitoring the light intensity from Jovian moons in order to detect possible reflections of the light emitted by the descending bolide, the subsequent explosion, and the beginning ascent of the resulting fireball. 3.1. Direct observations (Galileo, Voyager 2) * Galileo observations obtained with four on-board instruments * Photopolarimeter/radiometer (PPR) * Solid State Imaging Camera (SSI) * UltraViolet Spectrometer (UVS) * Near Infrared Mapping Spectrometer (NIMS) * Galileo can only communicate via omdirectional antenna (10 bits/sec) * it is not possible to transfer all observations of impacts; from Jan. 1995, Galileo must begin preparations for Jupiter encounter * PPR time resolution 0.23 sec (sample time): * H,L impacts 2 sec rise time, assumed to be light from bolide * at 60 km/sec, this corresponds to total path of about 120 km * entry angle 45 deg and vertical path correspondingly shorter * then roughly constant light intensity during about 29 sec * light disappears after a few seconds more * brightness of L-impact > that of G-impact by 20 percent * G impact about 15 percent of total light from Jupiter at maximum * observations in two filters (678 and 945 nm): * correspond to colour temperature at maximum at about 18,000 K * SSI time resolution 2.5 sec * data returned for K, N, V and W impacts * W-impact clearly seen as light point with 1 percent intensity of Jupiter: * total duration as observed with PPR * UVS observes at 292 nm * G-impact seen at 07:33:32; 20 percent increase in total Jovian brightness * secondary impact at 07:34:36, 1/4 as intensive as G itself * NIMS * IR spectra of full disk determines begin of G-impact at 07:33:37 * they also show warming in the normally dark 3.0-4.4 mum band at 07:39:41 * this is interpreted as heating by fall-back ejecta onto the upper atmosphere * Voyager 2 observes July 8 - August 15 * 126 channels in 500 * 1700 A spectral interval * no unambiguous detection (yet) of any changes associated with impacts * HST directly images G-funnel at Jupiter terminator 3.2. Indirect observations (reflection, etc.) * high-speed, multi-channel photometry at La Silla plagued by bad sky * no certain detection in intensity-ratio of the brightness of two moons * high-speed photometry at Perth: no detections at D, E, K, N-impacts * Keck: possible preflash at R-impact => reflection in dust? * Chinese observatories: flash reflections at E, K, N, P2, S- impacts? * Kiev: double flash on Io at Q1, Q2 impacts? * Brazil: no flash observed * Mauna Kea: no flashes observed at C and R-impacts * Catania: Na-emission at Io during one impact ? * Kitt Peak: Spacewatch spectral changes at Io during impact probably sky effect * IUE: * no flash observed in reflection at A, H-impacts * identification of Lyman-alpha and FUV H2-bands just above Q, W-impact sites, possibly excited in comae train just before impact, or in fireball 4. The fireball The fireballs were first observed by ground-based IR cameras. HST visual imaging also shows the rising material above the edge of Jupiter. The exact sequence is not yet clear, in particular because it is difficult to deduce the size, shape and temperature of the fireballs from the observed total IR intensity, due to the complicated geometric effects, including the changing altitude of the fireball as it rotates into view at the edge. 4.1. Direct observations of explosion * Galileo SSI observes K, N, V, W-impacts (resolution 2.5 sec) * K: 5 sec rise and then roughly constant during 49 secs at 15 percent of total Jupiter intensity * N: 5 sec rise and then constant during 15 sec at half intensity of K * W: by chance simultaneous observation by HST in same filter, when the fireball is still 140 km below limb as seen from HST! * can this effect be due to reflection in high-level incoming dust? * this demonstrates the difficulty of interpreting the images * luminuous energy 0.8-2.0 10 E24 erg in CH4 890 nm filter * this corresponds to total lum. energy from explosion of 2.1-4.4 10 E26 erg * this fixes a minimum impactor radius of 250-350 metres * but impactor most certainly significantly larger, since much of the kinetic energy did not transform into luminous energy, but was deposited inside atmosphere * PPR measurement of fireball temperature up to 18,000 K (see above) * initial fireball 2-4 km in diameter 4.2. Early evolution (rise and fall) * HST documents development of G-impact in great detail * A, E, G, W fireballs all reach altitude above cloud deck of about 3200 km * fireball image first round, later much wider and flattened * effects from fragments in main "comet line" apparently in general larger than from those which were displaced sideways (e.g. B, F, P, T) * some fireballs faded more quickly at 10 mum than others * differences in composition? * does it mean that they did not get down to NH3 cloud layer? * TIMMI at 3.6 m telescope: more than 120,000 images * H-presursor observed 63 secs after PPR impact time at h = 438 km * temperature of R-impact 500-700 K after about 10 minutes * "bumps" in L, H, Q IR lightcurves at approx. same time, after main peak * do they represent dust that is heated as it re-enters the upper atmosphere? * theoretical interpretation of fireballs still very uncertain * in general, two scenarios: * "large" impactors (Ahrens et al., Crawford et al.): ~10E28-E29 erg * "small" impactors (Zahnle, McLow): energy ~ 10E27 erg * both appear plausible, but "large" scenario predicts observed altitudes quite well, while in "small" model it critically depends on temperatures reached * Ahrens et al. (Caltech) scenario * large objects, total energy deposited 3-4 10E30 erg = 6-9 10E7 MT * this corresponds to 4-kilometer parent body * biggest fragments break up at -385 km altitude * 2 km objects reach H2O cloud layer * observed max height + total brightness of G-impact => 2 km diameter body * K, W less bright and therefore smaller objects * observation of S + H2O in lower plume => T initially above 10,000 K at center * Crawford et al. (Sandia) scenario: * model includes melting, vaporization, dissociation, ionization * test spheres of 1, 2, 3 km; density = 1.0 (silicate), 1.0-0.3 (H2O ice), or 0.2-0.01 (loose mix of these) * after 120 sec, total mass in fireball above 1 bar is 5 times mass of impactor (1.4 10E16 g), i.e. much more material from atmosphere than from comet present in fireball * Zahnle and McLow (Chicago) scenario: * five points in favour of smaller impactors and less deep penetration: * lack of atmospheric H2O in fireballs * high abundance of sulphur in fireballs * fireballs rather dim at visual wavelengths * tidal break-up models favour 1.5 km diameter parent body to match obs. length of fragment train, i.e. individual fragments ~ 0.5 km * plume sizes of several 10,000 km fit this model * main disagrement in fireball maximum height, which depends strongly on the temperature in the fireball, i.e. the speed of rise * currently most plausible scenario: * impact along 45 deg funnel; visible on HST images * lower part of fragment still moves downward, while upper part of fireball already begins to rise * most of kinetic energy deposited deep down in atmosphere * biggest fragments reach H2O cloud layer * most material in fireball from Jovian atmosphere * fireball temperature initially above 10,000 K * cools very rapidly while rising and expanding * after 3-10 min, cools to 400-700 K * after 12 min, cools to 500 K (other determination) * material moves in ballistic orbits and reaches altitude of about 3200 km * maximum height reached after about 500 sec (HST) * then rains down on upper atmosphere causing heating * > 10,000 km diameter "pancakes" form at about 1 mbar level in stratosphere * at 2-3 mbar, T > 200 K for some minutes * at 2-3 mbar level, DeltaT > 10 K (above ambient temperature) for some hours * at 10 mbar level, DeltaT ~ 3-4 K for several weeks * at 150 mbar level, DeltaT ~ 4 K for some weeks * at 400 mbar level, DeltaT ~ 1.5 K for some days * optical depth of aerosols ~0.01 above 1 mbar * aerosol particles 0.15-0.3 mum (UV reflectance) * troposphere later largely unaffected, possibly only localized effects (little change in 6 cm radio emission) 4.3. Composition and early chemistry * many gaseous components of fireballs observed at the very beginning * quick condensation into aerosols, as temperature drops * HST UV spectra: * H2S, NH3, CS2, S2 (19 bands) in absorption * FeI, FeII, SiI, CS, MgI, MgII in emission * CS (2 10E14 g); S2 (10E14 g); CS2 (3 10E10 g); NH3(3 10E10 g); H2S (5 10E10 g) * Si absorption at 2305 A? * IUE UV spectra (1150-3300 A): * NH3, CS in absorption * SiII, NaI, MgII tentatively identified in emission in fresh impact sites, disappearing in a few tens of minutes * NTT: CH4 (nu-3 bands) at T = 400-700 K * disappears after ~ 10 min (cooling effect) * TIMMI: C2H2 detected * KAO: detection of water at high temperature (not immediately obvious): * H2O: five lines at 22.6-23.9 mum * H2O/CH4-ratio ~ 2000 (at T = 500 K) * H and O most likely from comet? * HDO not looked for * T-increase from ambient 160 K to 500 K; 3300 km altitude, implies total energy of ~10E28 erg, i.e. impactor of 1 km size * H2O maximum line intensity after 12 minutes after impact * also C2H2 and C2H6 seen * unidentified lines at 10.5 mum * IRAM: * CO, CS, OCS detected; CS line maximum after 1.2 days * CS increases faster than CO * CO 5 days in emission, then in absorption; 2 10E14 g at p < 0.3 mbar * CS ~ 6 10E11 g at p < 0.7 mbar * OCS ~ 6 10E12 g at p < 0.5 mbar * CO:OCS:CS ratio = 320:10:1; O/S = 70 in volume * SEST: * CO at 230 GHz, in emission * JCMT: * HCN detected in emission; most probable origin: formed in the region of the explosion by shock induced chemistry * HCN decreases in a few days because of cooling * HCN observed in absorption during August-October * stratospheric thermal profile now substantially cooler than pre-impact * upper limits for H2CO, H2S, OCS, SO * IRTF: * NH3 at 1 mbar persisted at least 8 days after impacts * C2H6 present * IRSHELL: * NH3: 3 10E12 g at p > 15 mbar * C2H4 mixing ratio 5 10E-9 * MIRAC2: * CH4 at 7.85 mum at 10 mbar * UKIRT: * CO, NH3, H2O, H2S "long" after impacts * no organic features observed at 3.3 mum * small particles in plumes * Palomar: * Si-emission at 11 m? * Pic du Midi (5500-7000 A) and La Palma (INT; 4000-6000 A): * Na, Li, Fe, Ca, K, Mg, H-alpha in emission; disappear after 30 min * most of these are most probably from comet * origin of Lithium (never observed before in comets)? 5. The plumes After about 30 minutes, the fireballs have been transformed into black clouds (also referred to as dark spots, plumes), easily visible at all wavelengths and observable with even very small telescopes. 5.1. Short- and long-term evolution of dark spots and their surroundings * fireball debris settles at flat clouds at ~1 mbar level * this is confirmed by: * at most visual wavelengths, they are dark on bright background of the clouds below, but in methane band images (visual and IR), they are bright, because they are above most of the strongly absorbing methane and reflect the sunlight * short term evolution (within first 5 hours) monitored with IUE for A, B, E, G, K, Q, R, S, W: * ejected material darkens rapidly * the evolution timescale is wavelength dependent * HST high-resolution images show complex structures: * detailed observations of A, E, R, Q impact areas * horseshoe-shaped diffuse feature in direction of impactor entry * two sets of rings * outer reaches 5000 km size in 9000 seconds, velocity about 450 m/sec * better measurement: 499 m/sec (outer); 310 m/sec (inner) * not sure what this represents, because these velocities apparently do not fit: * predicted pressure wave velocity: 775 m/sec * predicted gravity wave velocity: 130 m/sec, but depends on H2O density in stratosphere and other factors * CO, CS (IRAM) and HCN (JCMT) absorption persist for several months * for the first time tracers of stratospheric motion available * further development depend on mixing and shearing * at levels between 0.34-200 mbar, there is total material mass of ~32 g/cm2 * HST finds that spots wider and deeper in Far-UV * 20 m/sec measured in N-S expansion * IUE: FUV/NUV spectra of most impact sites monitored until mid-August * Pic du Midi measures combined GDSR cloud: * spreads between -37 deg to -61 deg latitude until August 1; i.e. lateral (north-south) velocity of 6 m/sec * otherwise expansion along impact latitude up to 31 m/sec * development of G-area documented by HST up to end of August * major structures still clearly visible in late September 5.2. Chemical evolution * complex compounds undoubtedly present in dark clouds * now referred to as "gunk" (greasy, light-flowing, unpleasant mixture) * further chemical development: * shock-induced chemistry * mixture of comet and Jovian components * NH3 still in cloud after 2 months at 4 ppb * years to relax in stratosphere * polymerisation of HCN - as on Halley surface * total aerosol content corresponds to about 1 km diameter sphere * S- and N-rich organics are main candidate compounds in these * optical depth increases in UV, decreases in Vis. + IR * silicates, graphites not possible (flat reflection curve 300-1000 nm) * theoretical chemical evolution difficult to predict * much S and N * model with 92 different species + 801 different reactions: * S2 and CS2 photolyzed in 1 day * S2 => S8 and sulfanes (H2Sx), CS2 * formation of nitriles * if S8 not formed so rapidly, then H2CS, CH3SH, etc. * NH3 photolyzed in 1 week * N2 stable 6. Aurora and other magnetic field effects The Jovian aurora, best observed on UV images, was temporarily influenced by the impacts. Surprisingly, some effects were apparently larger in the northern hemisphere than in the southern. * both northern and southern aurora well observable in H2 and Ly-alpha (1216 A), mapped with HST/WFPC2 (1150- 1650 A), HST/FOC (1460-1670 A) and IUE (1150-1650 A) * complete oval around poles observed in days before impacts * enhanced emission (arcs) at the northern conjugate point of the K-impact site, 45-54 minutes after the impact * duration of enhancement probably 10-15 minutes (beginning not certain) * IUE observes a bright, blinking spot at south polar cap, 1.5-1 hour prior to P2 impact * when mapped along magnetic field lines, seems to match Q-complex * could be signature of particle precipitation driven by field-aligned current closing in large, active comae and Jovian ionosphere * EUVE detects He I 584 A emission * reaches 25 Rayleigh after H and K-impacts * this means that He is lifted into high atmosphere, no longer any absorption by H2 above these atoms * NTT: full disk survey of H2 and H3+ after impacts over * symmetric H2 and H3+ emission at +44 deg latitude at high level * H3+ detected at all latitudes, first in south, then in south+north, then more in north and less in south, then only in north * ROSAT detects X-ray emission in north after K, P2 and W- impacts * Jovian radio emission enhanced after impacts * no detection (yet announced) of obvious change from Ulysses * Nancay detects 30-40 percent increase in synchrotron emission; maximum around July 20-22 * gradual increase of about 20 percent at 13 cm, similar at other, longer wavelengths, confirmed by several telescopes 7. Seismic effects The energies deposited in the Jovian deep atmosphere during the impacts appear to have been large enough to excite internal oscillations. There is therefore some hope that effects will be found in the data which will allow to study the interior structure of Jupiter for the first time. * the Jovian seismologists are looking for: * primary waves * antipodal heating * eigenmodes * T detectable, if deposited E > 10E27 erg; this is quite likely * international collaboration (3.6m-La Silla + NOT-La Palma + CFHT-Mauna Kea) * more than 40 hours IR high-speed imaging during impacts * about 40 hours in the days after the impacts * broad-band filter (9-10.4 mum) corresponds to 0.5 bar level * quick reduction of CFHT data: possible antipodal heating maybe seen 8. The Io torus and the Jovian dust ring The amount of dust predicted to be injected into the Jovian environment was believed to be barely sufficient to cause observable effects in the Io torus or to result in a significant change in the Jovian dust ring. This seems to have been confirmed. * HST FOS does not detect obvious dust trapping in Io torus * ESO 1.5-m observations of Io torus failed * Io extreme-UV luminosity decreased (EUVE) * possible due to temporarily less electrons in Io torus, reason unknown * no effects observed in Jovian dust ring 9. General Jupiter has been telescopically observed since Galileo first pointed his telescope towards this planet on January 7, 1610. On some occasions, new features have been seen. * dark bands first seen around 1630 * motions on surface first described by Giambattista Riccioli (1598-1671) * "spots" first described by Giovanni Cassini (1625-1712) and Robert Hooke (1635-1702) * detailed description of Great Red Spot from about 1870, but it has most probably been seen long before * it cannot be excluded that some "dark spots" seen before may have been caused by (smaller?) impacts * but it seems unlikely that any event like the present one (number of impacts, energies, effects) occurred during the past two centuries * thus, the frequency of such events is probably not very high * possibly one per 100-1000 years? * comet impact frequency at the Earth not well known * Tunguska event (1908) has some similar characteristics: * "funnel" (dust trail) observed * bright nights (sunlight reflected in high aerosols?) * comet or stone meteoroid?
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