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Update on SL9/Jupiter Collision



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projects and may no longer represent the current situation,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

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 
  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
  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
  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

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
* 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
* 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
* fragment Q apparently bluer than S (Kitt Peak; July 1 -

1.4.  Sizes of nuclei

* analysis of high-resolution HST images:
  * several fragments in each dust cloud possible
  * no obvious point-sources visible, i.e. not possible to
    estimate sizes
* energy measured at impact:
  * Galileo: measured luminous energy => min. 350 m
  * 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
* 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 +
  * 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 
  * 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
* 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 
* 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
  * Photopolarimeter/radiometer (PPR)
  * Solid State Imaging Camera (SSI)
  * UltraViolet Spectrometer (UVS)
  * Near Infrared Mapping Spectrometer (NIMS)
* Galileo can only communicate via omdirectional antenna (10
* 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
  * at 60 km/sec, this corresponds to total path of about
    120 km
  * entry angle 45 deg and vertical path correspondingly
  * 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
  * 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
  * 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,
* Keck: possible preflash at R-impact => reflection in dust?
* Chinese observatories: flash reflections at E, K, N, P2, S-
* 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
  * 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
  * 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
  * 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
* 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
  * 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,
  * 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 
* 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
  * 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
* 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
  * also C2H2 and C2H6 seen
  * unidentified lines at 10.5 mum
  * 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
  * CO at 230 GHz, in emission
  * HCN detected in emission; most probable origin: formed
    in the region of the explosion by shock induced
  * 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
  * NH3 at 1 mbar persisted at least 8 days after impacts
  * C2H6 present
  * NH3: 3 10E12 g at p > 15 mbar
  * C2H4 mixing ratio 5 10E-9
  * CH4 at 7.85 mum at 10 mbar
  * 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 

* 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
* 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
* 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
* 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
  * 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 
  * 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-
* Jovian radio emission enhanced after impacts
  * no detection (yet announced) of obvious change from
  * 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
* 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
* 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 
  * 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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