ESO SL9 NEWS BULLETIN ===================== Issue : 11 Date : Wednesday, July 20, 1994, 08:00 UT (10:00 CEST; 04:00 Chilean time) Items : 11-A: La Silla clouded out ! 11-B: Q'ing up 11-C: Looking around the corner ? 11-D: The fireball from the K impact 11-E: L impact produces enormous plume 11-F: Images of the comet fragments just before impact 11-G. How deep did the fragments penetrate ? 11-A. LA SILLA CLOUDED OUT Throughout yesterday the clouds over La Silla were heavy and the astronomers were not optimistic about their chances of observing the SL9 impact. This was particularly distressing as the L-impact was very well timed for intensive study, especially with the TIMMI instrument. When the predicted time of impact approached some holes in the clouds appeared, but they were too few to make it possible to observe the initial phases of the fireball. The La Silla observers could do nothing but were content when they learned through the networks that that observations of this event were secured at several other observatories, although some these also had some problems with clouds. Only at the CAT and the 60-cm Bochum telescope was it possible to do a few observations. The Bochum pictures were obtained in the 893 nm (methane) filter and show the white plumes above the impact sites of fragments G, L and K. 11-B: Q'ING UP In the wake of the extremely violent impacts of fragments G, H, K and L, the expectations for impacts Q1 and Q2 on Wednesday evening (CEST) are mounting. Provided the weather improves, they will be observable from La Silla in the late afternoon, and will undoubtedly constitute the best remaining opportunity to complement the data obtained during the earlier impacts. If the brightness of the Q1 plume surpasses that of the other impacts, as expected, this may result in very detailed data about the components and dynamics of the impact plumes. Anticipating yet another dramatic event, the European media have shown great interest in being "present" during the upcoming observations of the Q-impacts. For this reason, another (informal) media session will take place at the ESO Headquarters this evening, starting at 20:00 CEST. During this time, we shall attempt to have telephonic contact with the observers at La Silla and to transfer and comment the new images as soon as possible. The new images will be ready for distribution during the usual Press Conference on the Thursday morning, 21 July. It is a strange coincidence that this spectacular event takes place, exactly (within a few hours) 25 years after Apollo 11 landed on the Moon. Preparing for this, two TV crews from Danmarks Radio and Bayerisches Fernsehen spent the first half of the night between July 19 - 20 at the ESO HQ in Garching. As it proved impossible to obtain a good image of the L-impact from La Silla, we were very grateful to receive an excellent one over the electronic network from the observers at the 2.5-metre Nordic Optical Telescope at La Palma (Canary Islands). It was made in the 10 micron waveband and shows Jupiter at the time when the fireball was rising over the L-impact site. The total intensity of the the plume is probably as high as the entire emission of the planet in this waveband. It is obvious that the L-event must have been about as energetic as the earlier G, H and K events. We wish to express our appreciation of this kind gesture by Pierre-Olivier Lagage and his collaborators at the NOT. This also served to illustrate the very positive and friendly collaboration among astronomers at the time of this once-in-a-lifetime event. 11-C. LOOKING AROUND THE CORNER It is well known that you cannot look around corners. Under normal circumstances, light cannot be bent, and what is behind a screen, cannot be seen. Therefore, the actual impacts of the fragments of comet SL9 on the rear side of Jupiter cannot be seen by telescope on and near the Earth, only by spacecraft like Galileo and Voyager 2 which have a direct line of sight. Or so it was thought until yesterday, when an impressive series of more than 1000 TIMMI images was obtained of the south-east part of Jupiter, one frame being taken approxmately every 3.5 seconds and starting about 15 minutes before the predicted time of impact, 19:26 UT (July 18). After the termination of the observations when Jupiter set at La Silla and all through the second half of the night, the observing team members, Tim Livengood (NASA), Ulli Kaeufl (ESO), Benoit Mosser (Institut d'Astrophysique de Paris, Universite Paris 6) and Marc Sauvage (Observatoire de Paris-Meudon, France). worked hard to perform a preliminary reduction of their data. They prepared a lightcurve of the H-plume, that is a graphic representation of how its intensity changed with time. The lightcurve that is reproduced here shows an enormous increase in intensity (brightness), as the plume rises above the limb, some minutes after the impact. It reaches a maximum within about 5 minutes and then begins to fade. The maximum is not symmetric, there is a pronounced "hump" on the fading branch. About 15 minutes after the maximum, the intensity approaches a nearly constant level, after which it does not change much. But what is even more interesting is that a small intensity peak is seen on the curve, rising from the zero-level about four minutes before the major rise in intensity begins. The second curve shows this part of the lightcurve in greater detail. This obviously represents a "precursor" flash, seen at the very limb at about the time of the actual impact on the rear side. On this curve, the initial rise begins at 19:32:57 +- 3 sec (UT). Patricia Whitelock (SAAO) reports that Kaz Sekiguchi, imaging with the 0.75-m telescope at 2.2 microns (K-band), saw an initial brightening at 19:32 UT, then a partial fading, before the dramatic brightening at 19:38 UT. The same information appears in the trace from the 1.9-metre telescope, but the low level hump is hard to believe in since similar lumps appear in the trace earlier. Working with a Near Infrared Camera (1 - 2.5 microns) at Pic du Midi (France), J. Berthier (BDL Paris), F. Colas (BDL Paris), F. Deladerriere (OMP Pic du Midi P. Laques (OMP Pic du Midi), J. Lecacheux (DESPA Meudon), S. Pau (DESPA Meudon), D. Rouan (DESPA Meudon), A. Sanchez-Lavega (UPV Bilbao) and D. Tiphene (DESPA Meudon) also detected the same phenomenon: " We have fully observed the impact H at a wavelength of 2.2 microns. A strong "flash" at 19:33:04 UT was preceded by a short tiny flash and by a brightness increase of Io, and followed subsequently by the formation of a bright spot in the limb extending more than 5000 km above it. We have monitored subsequently the resulting low albedo cloud (in the continuum) which was similar in color and morphology to previous events." It is interesting to compare these timings with that contained in the following report about the most recent Galileo observations, prepared by Terry Z. Martin, Leslie K. Tamppari, and I. Claypool (JPL), L. Travis, A. Lacis, and J. Hansen (GISS, New York), and received in the early morning of July 20: "The Photopolarimeter Radiometer on the Galileo spacecraft observed Jupiter for 41 minutes during periods corresponding to earthbased observation times of UTC 198/02:22 to 198/03:03 for B and UTC 199/19:11 to 199/19:52 for H. The PPR has a single field of view that is about 4 times the size of Jupiter. A filter at 945 nm was used for these observations; the sample time was 0.4 sec. No evidence of the B impact was found. The H impact was seen at 19:31:59 UTC. The brightness corresponds to about 2 percent the brightness of Jupiter itself. The signal rises to peak value in about 2 sec and then decreases over 25 sec to background levels. Galileo is positioned to see the impact sites directly, at a distance of 240 million km; the phase angle of Jupiter is 51 degrees." Thus, the observers at Pic du Midi, Sutherland and La Silla most likely were direct witnesses to the very early development of the plume while it was still hidden behind the limb of Jupiter. They were most probably able to see its light, because of refraction in the atmosphere (as on the Earth, before sunrise and after sunset), or because it was reflected in the cloud layers near the limb (as it is possible to see distant lightning reflected in thunder clouds on Earth). A detailed analysis of the very early images, obtained at the time of the impact and within the first few minutes will now be most interesting and may provide unique information about these layers. It therefore appears that Earth-based observers may after all observe the impacts (almost) ! 11-D. THE FIREBALL FROM THE K IMPACT Another demonstration of this effect came during the K impact. J. Watanabe, T. Yamashita, H. Hasegawa, S. Takeuchi, M. Abe, Y. Hirota, E. Nishihara, A. Mori and S. Okumura report from the Okayama Astrophysical Observatory observations of the plume of nucleus K with Near-IR camera attached to 188-cm telescope. The initial flash was detected at 10:25 UT on July 19. It was fainter than Io, and its brightness decreased rapidly. Then the brightness increased again from around 10:31 UT, and reached a maximum around 10:38 UT. The maximum brightness was roughly 20 times or more of the brightness of Io. This event was visible from the Pacific region and other reports came from several observatories in this area. Here are some extracts: Anglo-Australian Observatory and Australian National Observatory at Siding Spring (the K-impact flash was 400 times brighter than Jupiter's southern polar cap at 10:33 UT; no reflected emission from Europa observed); Perth observatory in Western Australia (starting at 10:35 UT, images with a narrow methane absorption filter at 8930A showed a definite prominence above the apparent limb of Jupiter, having approximately the same surface brightness as other portions of that latitude belt. Note that this is probably already on the front side of Jupiter as the limb in the continuum is well above the limb in the strong methane filter. This suggests that condensation occurs very rapidly after the fireball is ejected; Mike A'Hearn); W.M. Keck telescope on Hawaii (weather was nice and clear from about 4:00-9:30 UT. We imaged Jupiter many times in the filters: 1.23, 1.65, 2.1, 2.3, 3.1, 3.4, and 4 micron. An image with pictures of the planet at each wavelength is submitted to the NASA select account. We also imaged Jupiter's ring,and fragments K and M. We do not yet know if we detected the fragments; Imke de Pater, James Graham, Garrett Jernigan, and collaborators); National Central University, Taiwan (despite the clouds which hindered the observations of the K plume, we successfully monitored the crossing of the impact site K with the 0.61-m reflector at the National Central University. CCD images were obtained in the I, V, or R band. The dark spot is most prominent in the V band, and has a size about 15 percent of the Jovian diameter; W. P. Chen, W. H. Tsay, W. H. Sun, J. Li, E. S. Liang, H. J. Kuo, G. L. Chang, Y. L. Su, J. Lee and C. C. Ho). 11-E. L-IMPACT PRODUCES ENORMOUS PLUME The impact of fragment L was duly observed near the predicted time, 22:07 UT on July 19. The first report this time came from Mike Skrutskie and Sven Aas at the Whately Observatory (Mass., USA) where the flash was detected at 22:17 UT the NICMASS camera at the 40-cm telescope through a 2.23 - 2.29 micron filter. The plume steadily brightened until the observers had to switch from the this filter to a 1 percent (transmission) filter centered at 2.29 micron due to saturation. But the plume continued to brighten until the image saturated in this filter as well. The plume began to dim and was still at least twice as bright as Io at 22:40 UT. Next, Pierre-Olivier Lagage reported the observation of impact L at 10 microns with the CAMIRAS camera mounted on the Nordic Optical Telescope at 22:24 UT. It was about as bright as the H impact, may be even brighter. This impact was also observed with the 3.5 meter telescope at Calar Alto. The initial brightening was first seen at 22:14:15 UT with a 2.3 micron filter. At 22:23 UT, the new spot was as bright as the K impact site (visible just to the west) and still increasing. At 22:23 UT, the observers switched to a K grism to obtain a spectrum in the 2.0-2.4 micron range. they saw strong emission lines in the 2.3-2.4 micron range. At 22:36 UT imaging observations showed the spot to be far brighter than spot K, saturating the detector in 0.15 seconds. Spots K, C, and A were visible to the west. At the Pico-dos-Dias Observatory (Brazil), D. Lazzaro, J.L. Kohl-Moreira, D. Foryta, P.D. Singh, R.D.D. Costa, A.A. de Almeida, A.M. Magalhaes, V.Ellinger-Margoniner did not detect the flash from fragment L as reflected from Europa. Europa was monitored with a rapid photometer binned on 0.1s. in the V-filter from 21:32:54 UT until 22:27:51 UT. At 22:23:45 UT they noticed an extremely bright plume on the limb of Jupiter. It increased in intensity from 22:23:45 UT until 22:27:51 UT when it began to fade away. In the raw data appeared a very strange, unexplained and symmetrical decrease in intensity around 22:14:06 UT by nearly 20% of mean intensity. Not far from here, the members of the observing team at the 61-cm telescope of the University of Sao Paulo, Antonio Mario Magalhaes, Vera Ellinger Margoniner and Claudia Rodrigues, were able to image (CCD, 9900 A filter) what turned out to be an impressive plume from fragment L. The fireball was first detected at around 22:20 UT. The feature in that filter was still visible well after the visual sight had faded. The resulting spot seen against Jupiter's disk was impressive, spanning more than 2 arcsec in angular size. For the SPM/Camila Team, S. Levine, J. Klavetter, L .Cruz-Gonzalez, L. Carrasco, L. Salas, F. Cruz, C. Desiderio and E. Ruiz reported about the L observations at San Pedro Martir (Baja California, Mexico): "A bright spot from Fragment L was first seen at 22:24 UT (19 July) and subsequently brightened to roughly the magnitude of Io (K about 3) with an extended diameter of roughly 10 arcseconds. It still seems to brighten and expand." Dan Gezari, Drake Deming, Bob Loewenstein, Frank Varosi and Larry Woods observed an extremely bright 10.3 micron source on Jupiter, following the impact of comet fragment L, using the Goddard Infrared Array Camera at the ARC Apache Point 3.5-meter telescope. Because of difficult weather conditions the source was acquired at 22:42 UT. The flux density was about 3000 Janskys (about 5 times the flux density of Alpha Boo), and tit appeared to fade over the following hour by about 50 percent. The event was also seen as a dramatic dark spot in visible light at the focal plane of the 3.5-meter telescope. SPIREX at the South Pole observed the L plume appearing over the limb at 22:21 UT, imaged at 2.36 and 2.22 microns. The peak brightness appeared to be comparable to that created by the H impact. Further observations were reported during the night from Palomar where site L was observed briefly through thick clouds at 22:40 - 23:00 UT, and again at 23:30 - 23:35 UT. Low-resolution 8-14 micron spectra show it to be 10 times the surface brightness of Jupiter at 10 micron, where the spectrum seemed to peak (uncorrected for telluric absorption!). The site was extended 2-3 arcsec along the limb at 3.6 and 10 microns. Images were also obtained at 5 and 8.8 microns, where the site was also very bright (Phil Nicholson). The dimensions of this event is illustrated by the visual observation of the L impact site by Mark Sykes in daylight at 00:30 UT (July 20) from the Steward Observatory 53 cm telescope with no filter, through light clouds. It appeared to be similarly dark and of the same size as the G impact site observed 19 hours earlier. 11-F. IMAGES OF THE COMET FRAGMENTS JUST BEFORE IMPACT David Jewitt and Paul Kalas (University of Hawaii) have succeeded in obtaining R-band coronagraphic images of fragments of comet SL9 inside the Jovian magnetosphere. They used the University of Hawaii 2.2 meter telescope on Mauna Kea under very good seeing conditions. The observations were taken UT 94/07/19.2 - 19.3 using coronagraphic optics with a mask that "covers" the image of Jupiter and therefore allows to see even quite faint objects in the immediate neighbourhood. The fragments P2, Q1, Q2, S, R and W were detected at high signal/noise ratio. Fragments N, P1, T, U and V were beneath the detection limit, while L fell outside the fields of view employed. Images of fragment K were obtained just 2 hours prior to impact. The measured positions were communicated to Paul Chodas and Don Yeomans (JPL) for all observed fragments except K. The non-appearance of V was surprising, based on its pre-entry brightness (comparable to W). The fragments resembled aligned tadpoles, with the tails pointing towards Jupiter along the axis of the comet string (i.e. the tadpoles are falling into Jupiter tail-first). Jewitt and Kalas saw no evidence for the radiation pressure-swept dust tails that have dominated the morphology since early 1994. The tadpole morphology was most pronounced for Q1 and S, but is probably present in all nuclei. The images presumably trace the distribution of large "dust" or boulder-sized solids that are immune to scattering by the Lorenz force. Preliminary photometry provides no evidence that the central (1 arcsec) cores of the fragments have faded since immersion in the magnetosphere, although further work will be needed to clarify this point. 11-G. HOW DEEP DID THE FRAGMENTS PENETRATE ? The upper cloud layer on Jupiter contains ammonia, NH3. A detection of this molecule in a plume over an impact site would therefore be a strong indication that the corresponding cometary fragment detonated at or below this layer. R. Boreiko, A. Betz, M. Bester and D. Hale (ISI Observatory, Mt. Wilson, California) now appear to have made such an observation. They observed the G impact site between 04:30 - 06:15 UT on 19 July with the 10 micron heterodyne spectrometer on one of the 1.65 m telescopes of the Infrared Spatial Interferometer (ISI) and two lines of 14NH3, at 10.737 and 10.784 microns respectively, were detected in emission. Antenna temperatures were about 2 K, corresponding to a brightness temperature of 200 K if the emission fills the 1.5 arcsec beam. The linewidths were approximately 1 km/s, but rotational broadening accounts for a large part of this. This information may been considered together with the following report by Gordon Bjoraker (NASA GSFC) Terry Herter, Susan Stolovy, Bruce Pirger, George Gull (Cornell): "The Kuiper Airborne Observatory (KAO) has just landed in Melbourne, Australia, after observing methane emission from the K impact at 7.7 microns at a resolution of 9000. It was even more spectacular than the G fragment. At approximately 10:39 UT (July 19), the signal soared to roughly 25 times the brightness observed 10 minutes earlier. The fireball appeared in two of our spatial pixels, implying that its spatial extent was at least 5 arcsec, and possibly as large as 10 arcsec. Interestingly, we also noticed some bright spectral features which are probably not due to methane, and are at this point unidentified. We again observed at 22.6 and 23.9 microns to look for water vapour, but decided to wait until the fireball dimmed considerably. Data analysis is in progress on the water vapour observations, but at first glance, there was no evidence for Jovian water." Does this mean that the explosions took place between the ammonia and water cloud layers ? If this is indeed so, then we may have for the first time a rather precise determination of the altitude at which this happened. ----------------- This daily news bulletin is prepared for the media by the ESO Information Service on the occasion of the July 1994 collision between comet Shoemaker-Levy 9 and Jupiter. It is available in computer readable form over the ESO WWW Portal (URL: http://http.hq.eso.org/eso-homepage.html) and by fax to the media (on request only). News items contained therein may be copied and published freely, provided ESO is mentioned as the source. ESO Information Service European Southern Observatory Karl-Schwarzschild-Strasse 2 D-85748 Garching bei Muenchen Germany Tel.: +49-89-32006276 Fax.: +49-89-3202362