__________________________________________________________________________ T H E O L E R O E M E R M E S S E N G E R _______________________________________________ JJJJ C G JJJJJJ I E JJJJ __________________________________________________________________________ Newsletter of the International Jupiter Watch Satellite Discipline E-mail issue 11 July 6, 1994 Editor and Discipline Leader: John Spencer Voice: (602) 774-3358 Lowell Observatory Fax: (602) 774-6296 1400 W. Mars Hill Rd. Internet: spencer@lowell.edu Flagstaff, AZ 86001 ----------------------------------------------------------------------- UV IMAGING OF IO WITH HST John Clarke reports on the successful series of Hubble Io observations taken in June: From clarke@sunshine.sprl.umich.edu Tue Jul 5 14:44:28 1994 John, We have beautiful UV Images of Io, at good exposure levels down to F218W plus weakly exposed F160W images due to faintness of emissions. We see Io at all wavelengths down to F160W. Following will be Gilda's DPS abstract, which describes the combined GTO/GO program we are pursuing: Ultraviolet Observations of Io with HST: WFPC2 Imaging and GHRS and FOS Spectroscopy G.E. Ballester, J. Clarke (U. of Michigan), J. Trauger, K. Stapelfeldt, D. Crisp (JPL), the WFPC2 Investigation Definition Team, J. Ajello (JPL), M. Combi (U. of Michigan), M. McGrath (STScI), N. Schneider (U. of Colorado) and D. Strobel (Johns Hopkins U.) A number of ultraviolet observations of Io with the Hubble Space Telescope were made in June 1994 to study the main constituents of Io's atmosphere, molecular SO2 and neutral oxygen and sulfur, and the surface reflectivity. Images were obtained with the Wide Field Planetary Camera 2 (WFPC2) as part of the WFPC2 Guaranteed Time Observer (GTO) program. A Guest Observer (GO) program was dedicated to disk-integrated spectroscopic observations with the Faint Object Spectrograph (FOS), and spectroscopic mapping with the Goddard High Resolution Spectrograph (GHRS). Images with a series of filters centered at ~ 2180, 2550, 3360 and 3800 A show the distribution of the surface reflectance and also of atmospheric SO2 gas absorption. As part of the GO program, disk-integrated spectra were obtained with the FOS that measure the SO2 atmospheric abundance from the ~2100 A absorption signature. Comparison of these two sets of data should add to their value in determining the spatial distribution of the main component of Io's atmosphere. The GO program also studied Io's neutral O and S components with spatial scans of their far-UV emissions with the GHRS. These scans should provide valuable information on the spatial distribution of the emissions. East/west and north/south scans of both optically thick and optically thin emissions were made, and depending on the details of the observing sequences, complimentary information may be obtained on the atmosphere and the dependence of the emissions on the Io plasma torus. WFPC2 images of Io were also obtained using the sodium Wood's filter covering ~1150-2100 A (with very low red-leak) for detection of neutral O and S far-UV emission features. The far-UV sunlight reflected by the surface is also measured in these images. Preliminary results will be presented. Re: future imaging, we have more images with WFPC 2 in our GTO program at both elongations in cycle 5, using filters F380 down to F160W, so I am afraid these are protected for one more cycle. The visible of course is wide open. We should talk about the comparison of our images toward developing a complete picture of Io, however, as we work on the respective data sets. p.s. the images we have from June 1994 are near west elongation. Regards, John --------------------------------------------------------------------------- IJW SATELLITE WORKSHOP AT DPS I'm planning a lunchtime meeting of the IJW Satellites Discipline during the DPS meeting in Annapolis in November. The meeting will be on a weekday, but the date is not set yet. Details later! --------------------------------------------------------------------------- OBSERVING IO'S VOLCANOS DURING THE COMET CRASH PERIOD For those with infrared cameras or photometers who have time during the impact observations, there is a chance to do some useful monitoring of Io's volcanos. Io will be eclipsed and occulted by Jupiter at the times in the following table. Time-resolved infrared photometry (2.3 microns or longer) of Jupiter occultation egress, which occurs in Jupiter's shadow, will give a 100-km resolution view of the spatial distribution of volcanic thermal emission across Io. Time resolution of a second or slower is useful. With the methane filters that everyone has handy for the comet crash, this is an easy observation at 2.3, 3.5, or 3.8 microns, and is good practice for the similar time-critical crash observations. Io emerges from behind Jupiter about 14 arcsec E, 11 arcsec S, from the center of the disk. It's also worth observing the Jupiter occultation ingress, which occurs in sunlight, especially at the longer wavelengths, as the occultation of bright volcanic hot spots, if any, can be seen even in sunlight. Absolute calibration of the photometry is useful to have. During the period of > 1 hour that Io is visible in Jupiter's shadow after occultation egress but before eclipse egress, multicolor photometry of the volcanic thermal emission is also valuable, at wavelengths of 1.7 microns or longer. If seeing is very much better than 1 arcsec, it is possible to directly resolve individual hot spots on the 1 arcsecond disk during this period. In May there was a 5-10 fold brightening of the volcanic thermal emission that lasted less than a few weeks: here is a chance to look for similar dramatic changes, or more subtle ones, during the most intensive monitoring of the Jupiter system that is ever likely to occur. Send questions or results to John Spencer, spencer@lowell.edu ---------------------------------------------------------- Occultation Ingress Occultation Egress UT ------------------- ------------------ Eclipse Date Start End Start End Egress ---------------------------------------------------------- 7/15 02:11:04 02:14:52 04:22:20 04:26:09 05:40 7/16 20:39:20 20:43:08 22:50:39 22:54:27 00:09* 7/18 15:07:47 15:11:35 17:19:09 17:22:57 18:38 7/20 09:36:13 09:40:01 11:47:38 11:51:25 13:07 7/22 04:04:47 04:08:35 06:16:14 06:20:02 07:36 7/23 22:33:19 22:37:07 00:44:49* 00:48:36* 02:04* ---------------------------------------------------------- *Following Day Occultation times are accurate to about 1 second. Eclipse egress times are taken from the Astronomical Almanac and are good to about 2 minutes. --------------------------------------------------------------------------- LATEST IMPACT PREDICTIONS This is from Paul Chodas: From PWC@GRAVI.JPL.NASA.GOV Tue Jul 5 23:04:44 1994 Below is the July 5 edition of our Predicted Impact Parameters table. This set of predictions is particularly important because the Galileo observation sequences will be keyed to the these impact times. The orbit solutions for these predictions were based on very recent astrometric positions, some of them obtained as late as yesterday, July 4! Helping matters further, some of the new measurements were reduced using the pre- release Hipparcos star catalog, making them highly accurate. We thank the many observers and measurers who worked long hours over the weekend to provide us with the very latest data. The predicted impact times are generally about 5 to 10 minutes earlier than those in our table of June 24, at least for the major fragments. The times for fragments A and D made the largest jumps (24 and 29 minutes later), and the time for W jumped 19 minutes earlier. Jumps like these are generally due to small measurement errors in the latest data used in the solution. Because of the new data, the impact time uncertainties have dropped substantially, down into the 6-8 minute range for many of the fragments. We expect future jumps in predicted impact times to be correspondingly smaller. Starting with this version of the table, we provide impact times to the nearest second, even though that is well below our current level of accuracy, and we provide impact time uncertainties to the nearest tenth of a minute. Paul Chodas 1994 July 5 ============================================================================== Predicted Impact Parameters for Fragments of P/Shoemaker-Levy 9 --------------------------------------------------------------- P.W. Chodas, D.K. Yeomans and Z. Sekanina (JPL/Caltech) P.D. Nicholson (Cornell) Predictions as of 1994 July 5 Date of last astrometric data in these solutions: 1994 July 4 The predictions for all fragments except Q2 are based on independent orbit solutions; our orbit reference identifier is given. The orbit solution for fragment Q2 was obtained by applying a disruption model to the orbit for Q1, and using astrometric measurements of Q2 relative to Q1. Except for fragment Q2, uncertainties in the impact parameters are given immediately below the predicted values. These uncertainties are 1-sigma values obtained from Monte Carlo analyses; we have made an effort to make them realistic: they are not formal uncertainty values. NOTE: To obtain a 95% confidence level, one should use a +/- 2 sigma window around the predicted values. The uncertainties for Q2 have not been quantified, but are probably comparable to those for fragment T. The dynamical model used for these predictions includes perturbations due to the Sun, planets, Galilean satellites and the oblateness of Jupiter. The planetary ephemeris used was DE245. ------------------------------------------------------------------------------- Frag- Impact Jovicentric Merid. Angle Satellite Longitudes ment Date/Time Lat. Long. Angle E-J-F Orbit at Impact (deg) July (UTC) (deg) (deg) (deg) (deg) Ref. Amal Io Eur Gany ------------h--m--s------------------------------------------------------------ A = 21 16 19:53:40 -43.08 175 64.05 99.04 A18 203t 344 106+ 76+ 8.4 .19 5 .77 .57 4 1 1 0 B = 20 17 02:49:03 -43.04 67 63.54 99.40 B17 52+ 42+ 136+ 91+ 8.3 .20 5 .75 .56 4 1 1 0 C = 19 17 06:55:36 -43.22 215 64.85 98.42 C14 175t 77+ 153+ 99+ 8.4 .17 5 .74 .55 4 1 1 0 D = 18 17 11:41:50 -43.45 27 65.27 98.08 D16 319 117+ 173 109+ 8.7 .18 5 .77 .57 4 1 1 0 E = 17 17 15:03:51 -43.42 149 65.78 97.72 E31 61+ 146+ 187 117+ 8.0 .08 5 .48 .34 4 1 1 0 F = 16 18 00:28:15 -43.52 131 64.49 98.61 F22 344o 226 226 136+ 6.8 .12 4 .57 .41 3 1 0 0 G = 15 18 07:28:00 -43.58 23 66.60 97.09 G30 194t 286 255 151+ 5.7 .07 3 .38 .27 3 1 0 0 H = 14 18 19:25:48 -43.70 96 66.87 96.86 H29 194t 27+ 305 176 5.6 .07 3 .38 .27 3 1 0 0 K = 12 19 10:17:58 -43.77 275 67.76 96.21 K30 282 152+ 8+e 207 6.5 .07 4 .40 .28 3 1 0 0 L = 11 19 22:06:58 -43.88 343 68.11 95.93 L31 278 253 59+ 232 6.1 .07 4 .39 .28 3 1 0 0 N = 9 20 10:18:37 -44.19 65 67.80 96.09 N19 285 356 o 111+ 257 8.6 .12 5 .68 .48 4 1 1 0 P2= 8b 20 15:05:10 -44.53 240 66.40 97.01 P17 69+ 37+ 132+ 267 7.4 .09 4 .59 .41 4 1 1 0 Q2= 7b 20 19:31:36 -44.31 39 68.86 95.32 202t 74+ 150+ 277 Q1= 7a 20 19:59:04 -44.02 55 69.28 95.07 Q34 216 78+ 152+ 278 7.3 .07 4 .41 .29 4 1 1 0 R = 6 21 05:22:04 -44.05 35 69.28 95.06 R28 138 157 191 297 7.2 .08 4 .47 .33 4 1 1 0 S = 5 21 15:07:13 -44.13 28 69.72 94.73 S38 72+ 240 232 318 6.9 .08 4 .42 .29 3 1 0 0 T = 4 21 18:04:14 -44.99 138 67.37 96.23 T12 161t 266 244 324 15.2 .16 9 1.00 .70 8 2 1 1 U = 3 21 21:47:00 -44.47 271 68.68 95.41 U13 273 297 259 332 16.1 .19 10 1.13 .79 8 2 1 1 V = 2 22 03:57:25 -44.31 135 68.43 95.60 V13 99+ 349 285 345 12.4 .17 7 1.00 .71 6 2 1 0 W = 1 22 07:53:17 -44.17 276 70.23 94.36 W30 217 23+ 302 353 9.1 .10 6 .53 .37 5 1 1 0 Satellite Codes: + impact is visible from satellite o satellite is occulted by Jupiter at impact e satellite is eclipsed but not occulted at impact t satellite is in transit across Jupiter ------------------------------------------------------------------------------- Notes: 1. Fragments J=13, M=10, and P1=8a are omitted because they have faded from view. The March'94 HST images show that P2=8b and G=15 have split; we do not have sufficient data to obtain independent predictions for the sub-components. 2. The impact date/time is the time the impact would be seen at the Earth (if the limb of Jupiter were not in the way); the date is the day in July 1994; the time is given as hours and minutes of Universal Time. The impact time uncertainty is a 1-sigma value in minutes. 3. The impact latitude is Jovicentric (latitude measured at the center of Jupiter); the Jovigraphic latitudes are about 3.84 deg more negative. 4. The impact longitude is System III, measured westwards on the planet. The large uncertainty in impact longitudes is due to Jupiter's fast rotation. 5. The meridian angle is the Jovicentric longitude of impact measured from the midnight meridian towards the morning terminator. This relative longitude is known much more accurately than the absolute longitude. At the latitude of the impacts, the Earth limb is at meridian angle 76 deg and the terminator is at meridian angle 87 deg. 6. Angle E-J-F is the Earth-Jupiter-Fragment angle at impact; values greater than 90 deg indicate a farside impact. All impacts will be just on the farside as viewed from Earth; later impacts will be closer to the limb. 7. Satellite longitudes are given for Amalthea, Io, Europa, and Ganymede. The longitudes are measured east from superior conjunction (the anti-Earth direction). Longitude uncertainties listed as "0" are simply < 0.5 deg. 8. According to these predictions, the only impact certain to occur during a satellite eclipse is K=12 with Europa eclipsed. --------------------------------------------------------------------------- NATURE ARTICLE ON THE COMET CRASH BY PAUL WEISSMAN From pweissman%issac.span@noao.edu Wed Jul 6 16:00:33 1994 to be published in Nature, News and Views, July 14. --------------------------------------------------- The Big Fizzle is Coming Paul Weissman How will the fragments of comet Shoemaker-Levy 9 meet their end, with a bang or a whimper? That is the question on everyone's mind as the icy fragments rush toward their cosmic rendezvous with Jupiter, beginning July 16. Will Jupiter's atmosphere be torn with massive explosions, each greater than the sum of all the nuclear weapons on Earth, or will it be a giant fizzle? We are about to find out. Whatever the outcome, the breakup of comet Shoemaker-Levy 9 has provided fresh clues as to the structure of cometary nuclei and their bulk density. One fascinating example is the paper by Eric Asphaug (NASA Ames Research Center) and Willy Benz (University of Arizona) on page XXX of this issue. Asphaug and Benz used a high-speed computer workstation to model the breakup of Shoemaker-Levy 9 when it passed within Jupiter's Roche limit two years ago. They assumed that the comet was a "primordial rubble pile," a collection of hundreds to thousands of dirty snowballs, held together only by their own self-gravity. This model for comets was independently proposed a decade ago by myself,1 and by Bertram Donn (NASA Goddard Space Flight Center) and David Hughes (University of Sheffield),2 who referred to their idea as the "fractal model." An improved description of how such 50-meter diameter dirty snowballs (or more aptly, frozen mudballs) might form in the primordial solar nebula and then come together to form kilometer-sized nuclei was recently provided by Stuart Weidenschilling (Planetary Science Institute).3 Asphaug and Benz's dynamical simulations show the nucleus of tightly packed snowballs being torn apart by Jupiter's gravity during the close approach, the hundreds or thousands of snowballs stretching into a long column in space. But as the column lengthens and moves away from Jupiter, the individual snowballs begin to clump together due to their own self-gravity. The truly amazing result is that the number of clumps formed appears to be a function of the density of the individual snowballs. At a density less than 0.4 g/cm3, no clumping occurs; at a density of 2.4 g/cm3 all the snowballs come back together to form a single body. But at intermediate values, in particular between 0.4 and 0.9 g/cm3, the snowballs form 15 to 20 clumps. Comet Shoemaker-Levy 9 consisted of 21 individual nuclei when it was discovered last year. (Note, the densities quoted here refer to the density of the individual snowballs; Asphaug and Benz use the bulk density of comet Shoemaker-Levy 9 before it broke up, which is about 27% less because of the voids between the packed snowballs). Results are modified if the original comet nucleus was rotating. Asphaug and Benz's simulations rule out a retrograde rotation, because the snowballs then form a large central clump and smaller outlying clumps; this was not observed for Shoemaker-Levy 9. But if the comet had a prograde rotation, one obtains 15-20 clumps if the density of the snowballs is higher, perhaps 1.3 g/cm3. Asphaug and Benz's results also suggest that the original comet nucleus was fairly small, at most 1.5 km in diameter, in agreement with work by Scotti and Melosh.4 Past estimates of the bulk density of cometary nuclei have ranged from 0.1 to 1.3 g/cm3, based on comparisons of the predicted effects of gases jetting from the sunlit surface of comet Halley, with detailed observations of Halley's orbital motion.5,6,7 But the many free parameters in such comparisons make the estimates highly uncertain. More recently, meteoriticists have measured the density of microscopic cometary dust grains recovered by U-2 aircraft high in the Earth's atmosphere;8 those values are typically between 1 and 2 g/cm3. Asphaug and Benz's results clearly rule out the lower range of values from the estimates of jetting forces, but may be in conflict with some of the higher values from the cometary dust grains. A question not answered by Asphaug and Benz is whether the individual dirty snowballs in each clump of Shoemaker-Levy 9 have reaccreted into a single body, or whether they are only gravitationally bound dynamical swarms, like bees buzzing around a hive. Several of the clumps in Shoemaker-Levy 9 have been observed to split, well away from Jupiter's tidal pull, suggesting that within each clump, several sub-nuclei may reaccrete, but that a single solid body did not form. Other clumps have dissipated completely with time, suggesting that the snowballs don't reaccrete and/or do sublimate away. What does this say about the coming impacts on Jupiter? As the clumps approach Jupiter for their final plunge into the atmosphere at 60 km sec-1, Jupiter's gravity will again pull them apart. Rather than hitting as a single solid body, they will likely come in as an elongated shotgun blast of smaller pellets. Because of Jupiter's rapid rotation, the impact sites will be spread in longitude, like machine gun bullets lacing into a moving target. Each snowball will individually ablate and burn up like a meteor in Jupiter's upper atmosphere. Lacking the momentum and the structural integrity of a single solid body, they will likely not penetrate deeper into the atmosphere where they might explode with multi-thousands of megatons of energy. Thus the giant impacts will produce a spectacular meteor shower of bright bolides, but not the massive fireball explosions that have been predicted by some researchers. The impacts will be a cosmic fizzle. The cometary meteors may resemble the bolide which exploded harmlessly at 25-34 km altitude over the south Pacific on February 1 of this year, with an estimated yield of 15-20 kilotons. The Shoemaker-Levy 9 snowball explosions may be closer to about 30 megatons each, but still far less than the 100,000 megaton explosions that some have predicted. Nevertheless, Shoemaker-Levy 9's legacy will likely be an improved understanding of the nature of cometary nuclei. It will provide a dramatic confirmation of the primordial rubble pile and fractal models, and will provide the first definitive bounds on the bulk density of cometary nuclei. Or maybe, it won't. Paul Weissman is a Research Scientist in the Earth and Space Sciences Division of the Jet Propulsion Laboratory in Pasadena, CA. References: 1. Weissman, P. R. Nature 320, 242-244 (1986). 2. Donn, B. & Hughes, D. Proc. 20th ESLAB Symposium on the Exploration of Halley's Comet (eds. Battrick, B., Rolfe, E. J. & Reinhard, R.) pp. 523- 524 (1986). 3. Weidenschilling, S. J. Nature 368, 721-723 (1994). 4. Scotti, J. V. & Melosh, H. J. Nature 365, 733-735 (1993). 5. Rickman, H. In The Comet Nucleus Sample Return Mission (ed. Melita, O.) ESA SP-249, pp. 195-205 (1986). 6. Sagdeev, R. Z., Elyasberg, P. E. & Moroz, V. I. Nature 308, 240-242 (1988). 7. Peale, S. J. Icarus 82, 36-49 (1989). 8. Love, S. & Brownlee, D. Icarus, in press (1994). ---------------------------------------------------------------------------