Update on SL9/Jupiter Collision
UPDATE ON SL-9/JUPITER COLLISION (December 15, 1994)
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projects and may no longer represent the current situation,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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