14-B: La Silla after the Storm
14-C: Summary of Recent Observations in Visible Light
14-D: Summary of Radio Observations
14-E: Overview of Current Status
14-F: Impact on the Net

14-A. INTRODUCTION

This is the final bulletin issued by the European Southern Observatory (ESO) on the impact of Comet Shoemaker-Levy 9 on the planet Jupiter. It was triggered by continued requests for information on the part of the media, and also by the discovery of an exciting new phenomenon: the appearance of infrared-bright features in the northern hemisphere, exactly opposite, with respect to the equator, of the large impact sites in the southern hemisphere. The cause and significance of these features are currently being investigated.

In addition to the usual condensed overview of recent observations this bulletin contains a synopsis which provides a high-level overview of the event. Please note that the observations are still in progress, and that a full scientific analysis has not even started. Thus all conclusions are, at this point, preliminary, and have to be taken "with a grain of salt".

For the first time an astronomical observing program has made extensive use of the Internet computer network. This provided the possibility for near real-time interactions among scientists on a global scale. It also provided the possibility for the interested public to access the data from any network-compatible computer. We provide some statistics of this usage of the ESO network links in this bulletin.

We have been trying to accommodate the requests of information from the media. We did not really expect media interest on such a large scale, therefore not everything went as smoothly we would have liked. We apologize. We also want to thank everybody who supported us in this effort.

14-B. LA SILLA AFTER THE STORM

Today, four days after the last impact on Jupiter, most observing programmes at La Silla have come to an end. Only the two largest telescopes, the 3.6-metre and the 3.5-m NTT, will continue to observe Jupiter for a few more nights in order to document in the greatest possible detail the effects of the recent bombardment.

Three nights were irrevocably lost due to bad weather, but six nights have been fine, some of them even excellent. Thousands of images and spectra were obtained and the observers will have a hard time to evaluate all of this. At the present moment many of them have already left La Silla, and the remaining "survivors" are now getting very tired. Even after 10 days, these observations are not yet a routine!

We present here a short summary of the latest developments at La Silla.

High resolution spectroscopic observations (resolution 100,000) of Jupiter were performed between July 18-25 by Anne-Marie Lagrange, Olivier Hainaut and Jean-Luc Bertaux with the 1.4-metre Coude Auxiliary Telescope (CAT). During this time, long slit (20 arcsecs) spectra of Jupiter were obtained with the Coude Echelle Sspectrograph (CES). The aim was to look for water vapour in the areas around the impact sites, originating either from the comet itself, or from the deeper layer of the Jovian atmosphere. At the same time it would be possible to look for signatures of other molecules (e.g., CH4, NH3).

Due to the adverse weather conditions, only a few high-resolution data of the spectral region near 9450 A (where some water lines are present) were recorded during the actual impacts. It appears that no strong water lines were detected, but only a detailed evaluation of the spectra, not possible so far, will make it possible to decide whether traces of fainter water absorption are present.

However, after the impacts, in the period July 22 - 25, good quality data were recorded in the 9450 A (water), 8940 and 5760 A (CH4) and 6450 A (NH3) regions. Even though a detailed reduction was not possible in real time, it is obvious that many Jupiter lines were observed in all regions. The effects of the impacts are clearly seen in the CH4 data, and probably also in the NH3 spectral regions.

The spectral observations at the New Technology Telescope with the IRSPEC spectrograph were done by Rita Schulz and Joachim A. Stuewe (Max-Planck Institut fuer Aeronomie, Katlenburg-Lindau, Germany), Guether Wiedemann (ESO) and Therese Encrenaz (Observatoire de Paris-Meudon, France). One of the most surprising results was that the monitoring of the l = -44 deg. and l = + 44 deg. latitude regions in the H2 and H3+ lines lead to the detection of (presumably) impact-induced emissions in the Northern hemisphere.

The impact region (l = -44 deg.) was monitored between July 22 UT 20:40 UT and July 23 4:00 UT, in the H2 (2.12 microns) and H3+ (3.53 microns) emission lines. The slit was aligned with the impacts' parallel and had a width of 4.4 arcsec. On July 22 (20:40 - 23:00 UT) the H3+ line was detected in sites C, (W, K, U) and L. A continuum, approximately equal to the H3+ line intensity, was observed on (W,K,U) and L, but was not present on C. The H3+ emission appeared more extended over the longitude range while the 3.5 microns continuum was more localized over the impact sites. Later, on July 23 03:30 UT, the (V, T, E, F) group and the H site were also detected in the H3+ line.

The H2 S(1) line (2.12 micron) has been monitored on July 22 UT 23:00 - 00:00. Two emissions were recorded, in the L site and at the position of the (R,S,G,Q) group. At the L site, the continuum emission was stronger, but the H2 line was weaker than in the other groups.

Following a suggestion by Benoit Mosser (observing at the 3.6-metre telescope) that near-IR emission might appear in the northern hemisphere at the same longitude as the impact sites, as a result of particle transfer along the magnetic field lines, the NTT team also observed the l = +44 deg. region on July 23 between 00:25 and 01:45 UT, i.e., 18 hours after the last impact. They detected emissions, both in H2 and H3+ lines, which seem to be associated to impacts located at the same longitude. The point located at the same longitude as the (R,S,G,Q) group showed an emission at 2.12 micron, with a continuum about 3 times weaker than at l=-44 deg., and a H2 line of comparable intensity. It also corresponded to a maximum of H3+ emission, although this emission was much more extended over the longitude range than the H2 one. At 00:45 UT, they observed the adjacent northern region at lower latitude (l = +30 deg.) in the H3+ line. A very weak H3+ emission was detected near the longitude of the (R,S,G,Q) group (thus indicating some extension of its northern counterpart), but no continuum was detectable.

On July 23 04:00 UT, two other H3+ emission maxima were observed at l = +44 deg., at the same longitude as the A site and the (V,T,E,F) group.

The NTT Team continues to observe Jupiter and took images of the planet and the region below the south pole in the visible with the SUperb Seeing Imager (SUSI) in several different filters (including CH4 at 8900A) between July 23 23:46 UT and July 24 03:31 UT.

On July 24 23:15 UT they started mapping the planet in several different infrared regions (including H3+ at 3.53 micron and H2 at 2.12 micron). The impact sites and their counterparts in the northern hemisphere are still visible. They will continue this mapping until 31 July.

The observations with TIMMI at the 3.6-metre telescope also continued and long series of infrared images were obtained in many different wavebands; they were performed by Tim Livengood (NASA), Ulli Kaeufl (ESO), Benoit Mosser and Marc Sauvage (Observatoire de Paris-Meudon, France). These observations support the search for global oscillations and inertia-gravity waves, they measure the winds, chemical and thermal alterations, thermal-IR auroral effects, etc. The W region at an age of 10 hours was clearly identified at the 3.6-metre telescope at the beginning of the night July 23 - 24. A number of other impact regions were identified. The strength of thermal emission from these impact residuals did not correlate in an obvious way with either the `maturity' or reported plume brightness of the impacts. Perhaps lower altitudes and further reflection will produce results.

The ESO/MPI 2.2m programme (Klaus Jockers, Max-Planck Institut fuer Aeronomie, Katlenburg-Lindau, Germany) ended with a final night on 23 July, obtained many near-infrared images of Jupiter.

Due to the unfavourable weather conditions, the search for light echoes of SL9-impacts had to be confined to only two (impact B and F) out of four events observable from La Silla in the visible wavelength range. Around the predicted time for impact B, Europa and Ganymede were monitored through cloudy skies. Using movable optical fibers, each feeding a prism-spectrograph, Heinz Barwig and Otto Baernbartner (Munich Observatory) obtained simultaneous UBVRI-photometry at the ESO 1m telescope with a time resolution of 1 second in the time interval July 17 02:30 - 03:50 UT. The multi-channel photometry allows compensation of atmospheric transparency variations, but no significant flash features that might have been reflected from Jovian satellites were detected during impact B. However, it should be possible to reduce the photometric errors to the 1 percent level by means of more detailed analysis.

During impact F observations were made simultaneously of Ganymede, Callisto and the sky background through variable cloud cover. The observations cover the time interval July 18 0:00 - 01:36 UT. The photometric accuracy of our measurements due to photon statistics is about 1 percent for 1 second integrations when the clouds were thin. In these lightcurves no significant deviation from constant levels is seen, neither in the colours of a given moon, nor in the brightness ratio of two moons.

14-C. SUMMARY OF RECENT OBSERVATIONS IN VISIBLE LIGHT

One of the many surprise effects of the collision of SL-9 with Jupiter was that very conspicuous dark features were seen in visible light. These are persisting although their structure is slowly changing. Large numbers of amateur observers all around the world are following these changes with quite small telescopes and this will allow almost continuous monitoring of Jupiter over the next weeks and months. The HST is also observing the planet through many filters at very high resolution but cannot provide such continuous coverage.

A very detailed description of the recent appearance of Jupiter in a modest telescope at a very good site has been provided by Daniel Fischer, an experienced Jupiter observer, who is currently visiting the Cerro Tololo Interamerican Observatory in Chile. This is reproduced here in slightly edited form.

Following extensive visual studies of all major dark spots on Jupiter with a 16-inch Cassegrain here is a summary of what the individual regions looked like to the eye (no filters available) under ideal conditions on the nights 22/23 and 23/24 and under deteriorating seeing on the evening of July 24th. The "Visibility Index" indicates how easy a region is to see. Index I features are very conspicuous and should be visible in small (10cm) telescopes easily, Index III features will require quite large (40cm) high-quality telescopes under excellent conditions. The number of days from impact at the time of observation are given in backets.

A (T+7): A pretty faint elongated mass of dark material, oriented in a more or less North-South direction, connected (?) to the STrB. No dark nucleus visible: Index III. C (T+6): A grey mass, extended both in NS and EW direction. No dark nucleus evident: Index II-III. E(T+6): Consists of a dark Core (Index I) and an extended winglike halo, with two spikes pointing towards the West, the more northern one is connected to the STrB. Whole Structure: Index I-II. G(T+5&7): By far the most impressive feature, no aging whatsoever visible over three days! The dark Core is extremely long (and connected to the much smaller but very dark R site), to the south is an extended Halo, almost as dark. The structure looks like a squirrel. Its extent in longitude was measured to be roughly 30 000 km: Index I. H(T+4): Probably the most bizarre impact site. There is a dark Core, but from it emanate long wisps of dark material to both the SW and N: Index I. (T+6): Structure resembles a smoking volcano, sitting on the STrB, with 'smoke' extending to the WSW: Index I-II. K (T+4&5): The second most impressive impact region after G, looks more like a sunspot group than any other. There's a very long (25-30000 km) 'penumbra' in EW direction, containing by two big dark 'umbrae', another 'umbra' accompanies these spots to the south: Index I. L (T+3&4&5): Another quite bizarre region, with several dark Cores and halo structures, extending along a SE - NW axis: Index I. Q (T+2): Mainly a dark Core (Index I-II) with a Halo (Index II-III). (T+4): Another, even smaller spot (Index II-III) precedes Q.

"General Impression: All Regions of Visibility Indices I and II are much darker than anything else on the planet except the occasional satellite shadow in transit."

As a complement to these observations the following notes on the latest HST imaging were provided by Heidi Hammel on behalf of the HST Jupiter Imaging Team:

Based on the timing provided by Spencer et al., the dark "antipodal feature" they report appears to correspond to a dark wave-like structure along the south edge of the North Tropical Zone. It was prominent in pre-impact HST images. It is probably not related to the impacts. The impact features are clearly being rapidly sheared and spread by Jovian winds. Looks like "one heck of a mess." No evidence has been found for large rings in Hubble imaging at any wavelength (aside from the ejecta veils, which are more visible when near the limbs of the planet). However, HST does not have continuous temporal sampling. More careful timings from those who have reported them would help in verifying their existence. It is noted that the Q2 and N sites are so small that they are difficult to detect in Wide Field Camera images (0.1 arcsec/pixel).

14-D. SUMMARY OF RADIO OBSERVATIONS

Francisco Reyes (University of Florida) who is coordinator of the IJW Decametric Wavelength Network provides a summary of the radio observations so far at decametric wavelengths during the period of the impacts. Only partial information is available from some of the observatories and nothing yet from some stations.

No emission clearly associated with the impacts was detected. Several episodes of what appears to be normal Jovian decametric emission were detected during the days of the impacts, a few of them near the time of the impacts. All the episodes of emission are listed under each station. Further data analysis is being conducted by each group to check on the possibility that these events could in some way be related to the entry of the fragments into the Jovian magnetosphere. In particular, the detection of emission on July 18 around 07:00 UT which occured at about 40 minutes before impact of fragment G is being investigated.

The observations were made at the following sites: Univ. of Florida Radio Observatory (T.D. Carr, F. Reyes - emission seen July 17 00:15-00:20 UT), Owens Valley (Associated with U.F.) (J.A. Phillips, T.D. Carr, F. Reyes - emission seen July 17 06:37-07:00 UT), Maipu (Chile) (U.F. and U. of Chile.) (J. May, T.D. Carr, F. Reyes - emission seen July 20 19:32-19:35 UT), Nishi-Harima (Japan) (K. Maeda - emission seen July 17 07:50-08:40 UT & July 18 06:57-07:07 UT), Kochi (Japan) (K.Imai - no emission detected), Bruny Island (Tasmania) (W.C. Erickson - no emission detected), Culgoora (Australia) (N. Prestage, R. Luckhurst - emission seen July 17 07:45-08:30 UT, July 18 07:07-07:10 UT).

14-E. OVERVIEW OF CURRENT STATUS

Note: this summary was produced for this bulletin. The intention is to provide a high level overview of the SL-9 impact event for use by the interested public and by the media. This is NOT an account of the scientific results available at this time.

The Impacts

All impacts took place at very nearly the same position relative to the centre of the planet The fragments approached almost exactly from (Jovian) south to north, striking the atmosphere at an angle of about 45 degrees. The fragments hit at about 45 degrees south latitude, 4-9 degrees in longitude behind the limb as seen from Earth. As Jupiter rotates the impact sites form a patchy belt around the planet.

Timeline of an impact
   
T + 0 min : impact flash. Visible from the Galileo spacecraft

T + 1 min : impact flash becomes visible from Earth through reflection in the
            Jovian atmosphere

T + 5 min : plume rises above the limb and becomes visible from Earth.

T + 10 min (approx) : plume rotates into view and becomes fully visible
                      at the limb.
T + 20 min : plume reaches maximum infrared brightness

T + 60 min : plume reaches maximum extension

The collapsed plumes ("pancakes") persist for days, and they are still visible in near infrared light. They are progressively smeared out due to motions in the Jovian atmosphere.

Impact timing

The trajectory of the comet was calculated on the basis of data from observers world wide by Don K. Yeomans at the Jet Propulsion Laboratory (JPL). It was noted that the actual impact times were generally several minutes later than the predicted impact times. The reason for this was found to be the following: the positions of the comet fragments were determined relative to the position of background stars. The coordinates of these background stars were taken from the Guide Star Catalog, a catalogue of about 20 million stars used for pointing the Hubble Space Telescope. While the GSC positions are, on the average, accurate to 0.3 arcseconds, individual stars can be off by up to one arcsecond. This probably was the case, and it produced impact times which were too early by several minutes. Post-impact analysis has secured the timing of the impacts to about +/- 1.5 minutes.

The Impact Flash

It was expected that it would be possible to observe the impact flash by reflection off the Jovian moons. This has not apparently been possible. For one impact a change of the colour of Io was observed, which lasted for several minutes. This was not caused by the impact, but by the developing plume.

Some of the impact flashes were observed from space by Galileo, and from the ground through refraction in the Jovian atmosphere. Differences in timing are probably caused by the fact that the observations were done at different wavelengths, so different interactions were registered.

The Plume

Plumes have been observed at IR wavelengths for all impacts with the exception of impacts T and U. Within about one hour the plume collapses into a flat structure ("pancacke"), which is typically the size of the Earth or bigger. The plume fades quicker at longer IR wavelengths than at shorter IR wavelengths. It is transparent at optical and UV wavelengths.

The "Impact Crater"

At optical and UV wavelengths we see a craterlike structure in the cloud deck of Jupiter. This has been observed for all fragments which impacted. There are indications of rings being formed and extending out from the impact site. No mesurements of expansion rate exist yet. There is considerable asymmetry, probably because of the impact angle. The "ejected material" contains matter from the comet.

The central "hole" of the crater consists of material with low reflectance in the UV. Spectral analysis with HST is continuing.

Northern Latitude Counterpart Features

After the last impacts we observed the appearance of features, at infrared wavelengths, on the northern Jovian hemisphere exactly opposite with respect to the equator to the large impact areas on the southern hemisphere. While there is the possibility that these are seismic waves which have travelled through the planet, it is current thinking that the features are caused by the transport of material, which was lifted up by the impacts, to the northern hemisphere by the very strong Jovian magnetic field, producing an aurora-like activity at these low latitudes. (A process of this nature was observed 45 minutes after the impact of fragment K, but in the ultraviolet light, and near the northern pole). This has not been totally resolved.

Negative Results

No changes have been observed in the dust ring of Jupiter, or in the Io-torus (a hose-like structure, in which the Jovian moon Io orbits, which is filled with material from volcanic activity on Io). There is also no change in the radio emission from Jupiter, indicating that there is probably less comet dust than was expected, or that the interaction of the dust are weak.

What did the impacts NOT do to Jupiter

Date            MBytes          Server (http)  Requests on same day
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Sun 17th July:   463               20500          124    
Mon 18th July:   625               37330          816   
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Sat 23th July:    55                6411          234