A false color image of Saturn's rings.
Credit:
NASA.
A false color image of Saturn's rings.
Credit:
NASA.
Sections
In brief: They are:
___________________________________________________________________________
Gas Giant Facts
___________________________________________________________________________
Quantity\Planet Jupiter Saturn Uranus Nepture
___________________________________________________________________________
Mean distance 5.203 9.537 19.191 30.069
from Sun (AU)
Mars is at 1.524 AU: the gas giants are much
further out. But this is not true of all
extrasolar gas giants in their solar systems it seems.
Revolutionary 11.857 29.424 83.747 163.723
Period
(years)
Rotation Period 0.413 0.444 0.718 0.671
(days)
The rotation periods are for the deep interior
not the surface layers???. The surface
bands have steady east or west winds that give them
different rotational periods
(FMW-220).
Equatorial radius 11.209 9.449 4.007 3.883
(Earth radii)
Oblateness 6.49 9.79 2.29 1.71
[(R_e-R_p)/R_p]
*100 (%)
These planets are fast rotators and this gives them
large centrifugal forces, and thus makes them highly
oblate.
Mass 317.82 95.161 14.371 17.147
(Earth masses)
Jupiter contains 71 % of the total planetary mass
The 4 gas giants altogether contain 99.5 % of the
total planetary mass
(HI-202).
Mean Density 1.33 0.70 1.30 1.76
(g/cm**3)
The mean density of the rocky bodies are about 3--5.5 g/cm**3.
Clearly, gas giants must have a different compostion from
the rocky bodies.
Axis tilt 3.12 26.73 97.86 29.58
to orbital pole
(degrees)
Jupiter won't have many seasonal effects and Uranus must
have extreme seasonal effects.
Known number 16 18 17 8
of moons
New small moons are still found fairly often I think.
Some day we will exhaust them.
Composition
Probably close to solar H and He in all cases with denser
substances ices, rocks, and metals forming a stratified core.
We know this from studying the surface, knowing the mean
density, and our understanding of formation and chemical
differentiation.
There are many trace chemicals. These are important for
the colors. H and He are colorless gases.
The colors of Jupiter and Saturn are not well understood,
but are probably due to compounds containing sulfur and/or
phosphorus.
The white, can be due to ammonia,
ammonia hydrosulfide, and water crystals.
The blue colors of Uranus and Neptune are due
to methane (CH_4) which scatters blue light and absorbs
red light
(Se-499;
SRJ-212--213).
Factor of 2 2 slight 2.7
energy excess
over solar For the rocky planets the internal heat emitted at the
surface is minute compared to impinging solar heat.
For the gas giants internal heating of the surface
dominates over solar by the above factors.
The internal heat is, of course, residual heat of formation
and past and present radioactive heat and, in the case
of Saturn at least, the release gravitational potential
energy of helium settling toward the interior
(SRJ-210--211).
___________________________________________________________________________
References: Cox-294--296,303--304;
Se-499;
SRJ-212--213.
THE TWO THEORIES:
These the icy protoplanets could directly attract and hold H_2 and He gas and thus grow even more massive until they become mostly H_2 and He: the GAS GIANTS: Jupiter, Saturn, Uranus, and Neptune.
There were regions of compressed gas where dense cores of gas formed and then gravitational runaway occurred leading to the gas giants.
But the GAS GIANTS were too small ever to become hot and dense enough under contraction to start hydrogen burning and become stars.
The main reason is that 2nd theory allows for FAST FORMATION and increasingly that seems to be necessary since protoplanetary disks seem to have very brief lives: recent evidence suggests less than 5 Myr (Irion, R. 2003jun06, Science, 300, 1498).
The two results may be reconciliable. The disk dominated by hydrogen and helium may be blown out of the solar system in a few million years, but collisions among rock and icy may recreate dusty disk continuously or repeatedly.
This may be a much grander version of the process that produces planetary rings.
The gas giants had to form out the hydrogen and helium disk.]
The controversy still rages.
Since formation, the GAS GIANTS have probably undergone some evolution, but the instructor knows little of it, and so we will pass it by.
The MOON SYSTEMS of the GAS GIANTS were formed by two processes:
This formation is analogous to the formation planets around the Sun.
The Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto) probably formed this way????.
This process of capture probably started early on and continued intermittently.
The great mass and therefore large gravity field of the gas giants compared to the rocky planets makes capture a more important process for the gas giants.
Among the rocky planets only Mars has captured moons (i.e., Phobos and Deimos).
Let's look at the Galilean moons of Jupiter that were probably form in situ and Jupiter's 4 innermost moons (all small) that were probably captures.
The Moons are in their right relative positions (inner to outer Io, Europa, Ganymede, Callisto), but not to scale. The images were taken by Voyager 1 in 1979mar.
Credit: NASA.
Collage Jupiter's inner moons Thebe, Amalthea, and Metis.
Galileo probe, 1997nov.
Jupiter has 4 small moons within the orbital radius of the innermost Galilean moon Io: Metis, Adrastea, Amalthea, and Thebe. ADRASTEA is missing from the collage.
In the image north is approximately up in the images and the moons are in their correct relative sizes.
In the image counting downward are Thebe (1, 2), Amalthea (3, 4), and Metis (5).
The two Thebe images are rotated by 50 degrees. The large Crater Zelthus is on the far side of Thebe: i.e., on the hemisphere permanently turned away from Jupiter.
The 1st Amalthea image shows the ``hemisphere'' that faces along the orbital path of Amalthea. The 2nd image is the far side of Amalthea.
These small inner moons are probably captured asteroids or maybe comets. I suppose it is possible that some formed in situ. Certainly, the outer small moons of Jupiter are believed to be captures (HI-221).
The small inner moons play an essential role in the rings of Jupiter as we will see below.
___________________________________________________________________________
Jupiter's Inner Moon Facts
___________________________________________________________________________
Quantity Metis Adrastea Amalthea Thebe
(Jupiter V)
___________________________________________________________________________
Mean distance 1.79 1.80 2.53 3.11
from Jupiter
(Jupiter eqatorial
radii)
Revolutionary 0.294780 0.29826 0.49817905 0.6745
Period
(days)
Amalthea and Thebe are synchronously tidally
locked to Jupiter: i.e., they always turn one face
to Jupiter. The rotation periods of Metis
and Adrastea were unknown as of 2000, but they
are probably synchronously tidally locked too.
Radius 20 x 20 13 x 10 x 8 131.0 x 73.0 x 67.0
(km) 55 x 45
These moons are too small to be pulled into spherical
shapes by there own gravity.
____________________________________________________________________________
References: Cox-303--307.
____________________________________________________________________________Credit: NASA: image #PIA02530.
But knowing size, density, and probable composition, plus MODELING we think we have some plausible models of the interiors (FMW-216, HI-207).
A cartoon of possible models of the gas giant interiors.
Note these models are NOT considered definitive.
In particular, whether liquid metallic hydrogen exists in Uranus and Neptune is uncertain: the cartoon shows none.
Notes on structure:
For example, the pressure and density at Jupiter's center is estimated to be 100 million atmospheres (i.e., Earth atmosphere units) and 31 g/cm**3.
It takes a lot of pressure to squeeze iron to 31 g/cm**3. Uncompressed iron has density of about 8 g/cm**3 recall.
The hydrogen electrons become unbound from the protons and become free like electrons in a metal: hence, the substance is a metal.
Liquid metallic hydrogen has been produced in the laboratory in the late 1990's (HI-207). But it was theoretically predicted to exist in Jupiter long before.
As you go down in the density rises and the molecular hydrogen becomes a liquid. But there is NO PHASE TRANSITION.
The distinction between liquid and gas vanishes at high temperatures and pressures such as one finds inside the gas giants (Se-497).
The upper region of a gas giant which is clearly gas (i.e., the atmosphere) is a very thin film.
Jupiter's, for example, is only about 1000 km deep and the radius of Jupiter is about 71,000 km (Se-498).
But their behavior is probably not too well understood???.
In the magnetic fields of Jupiter and Saturn probably arise from electrical currents in the liquid metallic hydrogen.
In Uranus and Neptune the situation is not so clear.
Jupiter has the strongest planetary magnetic field in the solar system (SRJ-216).
Jupiter's magnetic field gives rise to some very interesting behavior like strong auroras on Jupiter, but we can't cover everything.
The most obvious common feature of the gas giant atmospheres is their BAND STRUCTURE.
The atmospheres and band structure vary considerably among the gas giants.
We will just consider the case of JUPITER which is the best studied and has the most prominent bands.
Jupiter from HST. 1995feb13.
This image is probably close to true color, but perhaps the colors are enhanced to emphasize features.
One can see the dramatic band structure of Jupiter.
The Great Red Spot is giant anticyclonic storm. It's longest dimension is about twice the Earth's diameter (Se-499).
The Spot may have existed in the 17th century, but has certainly been identified since the 19th century.
There are also three white ovals which are smaller anticyclonic storms???. The outer two formed in the 1930s.
Essentially, it is the huge nature of Jupiter that causes its weather features to be so enduring by comparison to Earth.
See the official caption for more information.
Credit: NASA/HST.
As you can see the bands are immensely complex in detail. This is because they are highly turbulent.
The bands change in a bit in color and brightness, but at least for Jupiter, they have been unchanging in position for as long we've been mapping them (Se-499).
What are the bands?
They are the tops of rising and sinking convection cells organized as bands parallel to the equator.
Answer 1 is right. Rotation is what breaks the spherical symmetry.
It is an essential ingredient to band structure.
But just saying ``rotation'' is not a full explanation We won't give a full explanation of the bands.
Thus, six bands in all counting the northern and southern hemisphere mirror image versions separately.
But on Earth the bands are not nearly so stable and often they are invisible since they are often just moving clear air.
Venus had two large bands as we saw in IAWL 13: Venus.
Modeling and the probe the Galileo orbiter sent into the Jupiter atmosphere suggested that the bands extend to great depth into the interior (Se-499).
But here we will just consider the surface as it is currently understood.
A cartoon of the bands on Jupiter.
The CLOUD LAYERS in the bands are made of ice crystals of various kinds.
The upper layer is ammonia (NH_3), the next layer in is ammonia hydrosulfide, and the 3rd layer is water.
Spectroscopy tells us about the cloud material. But these crystals are all white.
Molecular hydrogen and helium are colorless.
Thus, the COLORS OF JUPITER'S BANDS are thought to be due to trace amounts of sulfur and/or phosphorus in compounds (Se-499).
At the top of the clouds the temperature is about 150 K. At the bottom about 300 K.
As one goes inward it gets hotter. The internal heat of Jupiter accounts for that.
There are high speed east and west winds of hundreds of kilometers per hour in the bands, but actual measurements don't show a simple pattern (FMW-220).
The high speeds are probably partly due to the CORIOLIS EFFECT which causes the main band winds on Earth. But probably are also due to deep circular motions (Se-499).
The Coriolis effect.
The coriolis effect explains CYCLONES and ANTICYCLONES.
Cyclones and anticyclones in a northern hemisphere.
Actually cyclones are a bit tricky in that as the winds spiral in close to the center they start bending opposite to the Coriolis effect direction (SWT-520).
I assume this is because winds from different directions start pushing each other in or something like that.
Now on Earth ANTICYCLONES are usually good weather and CYCLONES are usually bad weather: i.e., stormy weather.
The most severe CYCLONES are the things we call hurricanes, typhoons, and cyclones in another usage of the term.
Now the GREAT RED SPOT on Jupiter is a high pressure region and therefore formally an ANTICYCLONE and with COUNTERCLOCKWISE SWIRL since it is in the southern hemisphere.
But it's considered a storm.
The rotation period of the spot is about 6 days (FMW-221).
Jupiter's Great Red Spot in false color from Voyager 1979jun06.
North is at the top and bottom to top is 24,000 km.
The colors enhance reds and blues and suppress greens.
As one can see, a complicated turbulent flow around the big vortex.
See the official caption for more information.
Credit: NASA/HST.
All the moons are unique and have interesting features---but maybe not that interesting.
To save us from exhaustion let us just consider the 4 GALILEAN MOONS discovered by Galileo in 1610 (HI-214).
These moons show a wide range of interesting behavior.
The Moons are in their right relative positions (inner to outer Io, Europa, Ganymede, Callisto), but not to scale. The images were taken by Voyager 1 in 1979mar.
Credit: NASA.
___________________________________________________________________________
Galilean Moon Facts
___________________________________________________________________________
Quantity Io Europa Ganymede Callisto
___________________________________________________________________________
Mean distance 5.903 9.386 14.967 26.339
from Jupiter
(Jupiter equatorial
radii)
Revolutionary 1.769 3.551 7.155 16.689
Period
(days)
The Galilean moons are all synchronously tidally
locked to Jupiter: i.e., they always turn one face
to Jupiter.
Equatorial radius 1820. 1565. 2634. 2403.
(km)
Io is rather asymmetric: 1820 km is a rough average.
The Galilean moons are all among the 10 largest
rocky/ice bodies in the solar system: in order one
has Earth, Venus, Mars, Ganymede, Titan (a moon of
Saturn), Mercury, Callisto, Io, the Moon, Europa.
Largest rocky/icy bodies in the solar system.
Mean Density 3.55 2.970 1.900 1.790
(g/cm**3)
Composition silicates water ice water ice water ice
sulfur carbonaceous carbonaceous carbonaceous
metals silicates silicates silicates
metals metals metals
Note that in the outer system water ice and other
ices (e.g., of CO_2 and CH_4) could condence out
of the solar system and become primary planet
building materials.
So could carbon in the form of graphite and other
black sooty compounds which we just label collectively
as carbonaceous.
Ices and carbonaceous compounds tend to cover the
surfaces. Old surfaces are black because because
a small amount of carbonaceous material give ices that
color or because the ice has been vaporized by
micrometeoritic impacts.
Silicates and metals probably mostly chemically
differentiated and sank to form cores: an inner
one of iron and an outer one of silicates.
On Io all the volatiles have evaporated and escape due
to Io's intense tidal heat geology. Thus sulfur
is left as the abundant surface material.
The mean densities tell us that Ganymede and Callisto
have a large fraction of water ice. Recall
water ice density is almost exactly 1 g/cm**3.
Tidal heat extreme significant low low
geology
The Galilean moons have slightly eccentric orbits.
This means that Jupiter's tidal force will continuously
flex them.
Internal friction leads to heating that
can drive geology. The closer to Jupiter, the greater
the tidal force and greater the tidal heating.
The heat energy must ultimately come the rotational
kinetic energy of the system, but exactly where is
probably a tricky question.
___________________________________________________________________________
References: HI-213ff;
Cox-303--307.
___________________________________________________________________________Now let's focus on IO, EUROPA, and CALLISTO since they present interesting contrasting cases.
GANYMEDE is sort of intermediate in character between CALLISTO and EUROPA, and so doesn't make such an interesting contrasting case.
Io in approximately true color. Galileo, 1999jul03.
On the image 3 km features can be resolved.
The blacks and reds mark volcanic features. They are sulfur compounds. The white is solid sulfur dioxide. The green is ???: sulfur compounds too I suppose.
Molten sulfur lava is black and solidified sulfur lava is red or orangy or yellowy (HI-216).
Io is the most geologically active body in the solar system with the geology being driven by tidal flexing due to Jupiter. Many large-scale features have altered even during the lifetime of the Galileo mission.
Io has no (or almost no) significant impact craters since its surface is continually being renewed.
The flexing occurs even though Io's orbital and rotational periods are synchronized so that Io always turns the same face to Jupiter.
Gravitational perturbations from the other Jovian moons keep Io's orbit from being perfectly circular. The tidal flexing occurs as Io moves closer and farther from Jupiter.
Io is stretched by several kilometers along the Io-Jupiter line. The tidal flexing causes the surface of Io to rise and fall in places by about 100 m.
The interior of Io must be molten sulfur, rock, and metals.
The surface is cold away from the volcanoes: maybe 80 to 150 K. The lava temperatures are typically 400 to 600 K, but some be has hot as 2000 K (HI-215)
Io long ago has lost its volatiles such as hydrogen, helium, water, and CO_2. They were evaporated by volcanic heat and then escape because of Io's low gravity.
On Io the lavas are sulfur, sulfur compounds, and, for the hottest volcanoes, silicates.
The gas which explosively evaporates from liquid to gas when the pressure of the lava falls as it rises to the surface is SO_2. In terrestrial volcanoes it is water fulfills this role.
The plumes of sulfur ash can rise up 100 km and some material escapes Io altogether.
See FMW-234, HI-215, Se-508, and Cox-303--307.
Credit: NASA.
Io with two volcanic eruptions.
This is a Galileo probe image from 1997nov17.
Two eruptions are seen: one on the limb and one near the
center close to the terminator with a shadow extending to
the right.
North is at the top. The resolution is about 2 km.
The colors are false I think. But Io is very colorful because of all the sulfur and sulfur compounds.
The Io surface is largely sulfur and sulfur dioxides and silicates.Credit: NASA.
Europa: the 2nd innermost moon. Voyager 1, 1979mar04.
This is a 3-color filter image, that may be close to true color: i.e., an orangy, off-white (SRJ-218, HI-217).
From a distance the surface of EUROPA looks bland except for the crinkly fractures that extend up to of order 3000 km and are about 100 km wide. The fractures are only of order 100 m deep.
The variation in height is small on EUROPA which gives it a smooth appearance.
But looked at closely there are all kinds of fine features which we won't consider in detail here.
EUROPA probably chemically differentiated. There may be an iron core and rocky mantle. On the surface is an icy layer that may be of order 100 km thick (HI-217).
The comparatively bland surface has few impact craters and is probably less than a few hundred million years old.
This implies resurfacing is ongoing today.
Perhaps the fractures are due due to flexing by the tidal force. But the Galileo orbiter saw no fracturing during its lifetime, and so the time scale must be longer than a few years.
When the fracturing occurs, the surface stretches and cracks and liquid water from below wells up and then freezes on the surface.
On the other hand or in addition there may be ICE VOLCANOES that break up the surface with ``lava'' flows of water.
In any case over time, there is resurfacing.
Note that liquid water from below is needed in this scenario.
Tidal flexing of the interior must be heating Europa inside, but significantly less than in the case of Io. This is understandable Io is closer in to Jupiter where the tidal force is stronger.
There may be an ocean of liquid water 10 km or more in thickness below the icy surface.
If liquid water exists, then there is a possibility of life. This is a very exciting prospect.
On the other hand the water may be far too acidic for life as we know. Circa 2004 someone??? has proposed that Europa water is full of stuff like sulfuric acid (H_2SO_4) from sub-surface volcanism.
There is still a great deal of uncertainty about Europa's structure and evolution.
See the official caption, but note it is out-of-date in many respects. See also HI-216, SRJ-218, Se-506, and Cox-303--307.
Credit: NASA/Voyager 1.
Callisto in true color????. Galileo image from 2001may.
Callisto is the outermost of the Galilean moons to Jupiter.
Mean distance 26.34 Jupiter equatorial radii.
Callisto is probably about half water ice and half silicates. There is probably a rocky/iron core, a rock/ice mantle, and an ice crust???.
But micrometeoritic impacts have probably vaporized the surface ice leaving superficial layer of carbonaceous regolith.
Bright spots are relatively recent impact craters that have exposed underlying ice.
Crater counts (including old dark ones) suggest that Callisto's surface has not been renewed since about 4 Gyr. Thus, there is probably little internal-heat geology.
But Callisto does have a weak magnetic field. It has been suggested that there is a dynamo effect due a subsurface of ocean of water in which electrical currents flow due to ions in the water.
See HI-220 and Cox-303--307.
Credit: NASA.
GALILEO in 1610 observed that Saturn seemed to have protuberances, but he couldn't make out their real shape (No-335).
A few years later they seemed to vanish and Galileo wondered ``Does Saturn devour his children?''
CHRISTIAN HUYGENS by 1656 had concluded that the Saturn's features constituted a thin ring and that its disappearance twice every Saturnian year was a consequence of seeing the ring edge-on (No-344).
Saturn over 1996--2000.
Saturn's rings orbit Saturn's equator. Both rings and equator have a tilt of 26.73 degrees to the Saturn's orbital plane: the tilt is constant---over short times anyway.
As viewed from the inner solar system, the rings will be seen edge-on twice per Saturnian year at the Saturnian equinoxes when Saturn's axis is perpendicular to the Sun-Saturn line.
Because the rings are very narrow---only about 100 m or less thick (HI-209)---the rings practically vanish at the equinoxes.
The rings are most full at the two Saturnian solstices.
Recall that the Saturnian year is 29.424 Julian years: a Julian year is exactly 365.25 standard days.
Credit: NASA/HST.
As the centuries rolled by more than one ring was identified: going outward one has the well known C ring, B ring, Cassini division (a gap region), and the A ring (Se-513).
Saturn from the HST, 1998jan04.
This is a false color image showing infrared light reflection of Saturn.
The A and B rings are board and greyish and separated by the Cassini division (or gap): the A ring is on the outside.
The Cassini division is empty of large particles, but probably contains small dust grains similar to those that make up Jupiter's rings (HI-209).
The brown ring inside the B ring is the C ring.
Credit: NASA/HST.
The Voyager 1 and 2 spacecraft showed showed that there are 10 rings at least and the rings themselves have intricate structure (Cox-311).
A false color image of Saturn's rings.
This image is based on visible and ultraviolet light.
The A, B, and C rings and the Cassini division can be identified.
The Cassini division and the C ring are shown as blue. The Cassini division is not a pure gap: there are particles of scale size perhaps 10 microns in the division. They are probably rocky dust (HI-209, SRJ-227).
Chemical and physical variations in the rings can be brought out by image enhancement techniques with pretty colors used to distingish the regions.
The rings are not simple when looked at closely---but this shouldn't have been a surprise.
For more information see NASA image caption.
Credit: NASA.
Rings around Uranus had been suspected before the Voyager probes, but the Voyagers found DARK RINGS around Jupiter, Uranus, and Neptune.
Because those rings are dark and reflect very little light, they arn't easy to see from the Earth.
The VOYAGERS observed them best (and maybe only) when they (i.e., the VOYAGERS) were eclipsed by the planet. Then the faint reflection was not overwhelmed by contrast with brighter sources: i.e., the Sun and the planet itself reflecting sunlight.
Perhaps, the rings could also be seen as diminished sunlight when the VOYAGERS looked back at the Sun through the rings (HI-209)????.
They are so close to Saturn that Saturn's TIDAL FORCE would tear a solid disk apart, unless the solid disks were made of some super-strong material.
The TIDAL FORCE recall is the actually the difference in gravitational force between two parts of an object.
In general it will try to pull the object apart.
The TIDAL FORCE difference gets bigger for larger objects and also the closer you get to a strong source of gravitation.
A cartoon of the tidal force pulling a body apart.
(Correct ``too'' to ``to'' when you are able to.)
For example, the Earth's Moon would be torn apart eventually by the Earth's TIDAL FORCE if the Moon were only about 3 Earth radii away (HI-139).
The Moon is safe at 60 Earth radii recall. It is stretched along the Earth-Moon line a little by the tidal force but doesn't pull apart.
Answer 3 is right.
The bits altogether still have much the same ``sideways'' kinetic energy (i.e., angular momentum) for orbiting that they had when they were stuck together.
So the bits would go into individual orbits around the gravitational source.
The individual orbits would have slightly different periods and speeds depending on orbital distance: longer periods and lower speeds the greater the distance.
The bits would not recoalesce under their mutual gravitational force because the TIDAL FORCE cancels that effect.
But the rings are so close to the gas giants that the TIDAL FORCE prevents them from coalescing into new moons.
Actually sufficiently small moons can co-exist with rings and are necessary for the existence of the non-Saturnian rings: recall the size of the TIDAL FORCE depends on the size of the object: small enough objects arn't torn apart.
A diagram of Jupiter's ring system.
The main ring is the most visible of ring of the system.
The gossamer rings and thick halo are very transparent.
The rings are probably made of dark rocky dust that is knocked off Jupiter's inner minor moons by meteoritic impacts.
The small moons Adrastea, Metis, Thebe, and Amalthea are all closer to Jupiter than the closest large moon Io.
The dust grains may have size scale of order 10 microns (SRJ-227).
Jupiter's tidal force keeps the dust from coalescing into clumps under the dust's own gravity or falling back onto the moons.
The ring material is actually believed to be spiralling into Jupiter and must be continuously replenished (HI-210). Some perturbations must account for the in-spiral.
How the rings form.
The rings of Uranus and Neptune and Cassini's division about Saturn are believed to also be rings of the same nature as those of Jupiter (HI-209,232-234).
Credit: NASA for both images.
A Galileo mosaic of Jupiter's main ring and halo in two panels.
The two panels use different lighting scales so that the top one brings out the halo.
The image was taken under dark conditions when Galileo was eclipsed by Jupiter. This is necessary since the rings are dark and reflect very little light.
Rings are usually very flat.
The halo is vertically extended because electromagnetic forces acting on the charges on the ring particles keep the particles pushed out of the plane.
Because of its strong magnetic field and complex flows of ions from the solar wind and outgassing from Io, Jupiter has a pretty complicated electromagnetic environment.
Credit: NASA.
The collisions will tend to cancel the opposing momenta between the particles, and thus on average cause them to leave the collision going more in the same direction than when the entered. (The particles don't just reverse their opposing momenta because some kinetic energy is lost to heat energy on collision. The lost kinetic energy also causes a spiraling in toward the planet.)
This complex process will continue until they the particles are orbiting in thin disk in which collisions are minimized.
You could call this a flattening process collisional relaxation to a disk.
Well one authoritative reference gives the 2nd answer, and so it must be the main reason.
But I think the 1st answer must play a role too. The individual particles have very small gravity, but there is a collective gravitational effect of all the particles on each one.
Actually both effects??? are used to explain the formation of protoplanetary disks. But those disks are a bit different. They are much more massive and initially consist of gas that doesn't undergo simple collisions like particles.
Well rings end up orbiting on the EQUATORIAL PLANE of the planet. This is because of the planet oblateness.
The gravity field of an non-spherical body is NOT spherically symmetric.
The gravity of oblate bodies ends up favoring relaxation of the particles to a disk in the EQUATORIAL PLANE of the planet. See another authoritative reference.
So a lot of perturbation to get the ring particles into equatorial orbits may not be necessary.
The small moons themselves probably were probably gravitationally perturbed into equatorial orbits.]
Probably weak gravitational perturbations from the moons and planet and particle-particle collisions keep churning the particles up to a small degree and keep their orbits from perfectly aligning????.
Now small dust particles account for the non-Saturnian rings, but SATURN'S RINGS are clearly different.
SATURN'S RINGS are CHUNKS OF WATER ICE from billiard ball to house size (HI-209, Se-512).
The fact that they are ICE (plus some other stuff one guesses) makes them highly reflective and thus very visible from Earth.
SATURN'S RINGS at their densest would allow an astronaut to swim through them by pushing him/herself from particle to particle (Se-515).
The origin of SATURN'S RINGS is NOT known definitively.
A plausible scenario is that within the last hundreds of millions of years, an impactor shattered an ICY MOON of Saturn and the rings are the debris (HI-212).
Perturbations should make the ring material spiral inward, and so the rings will probably last only of order of hundreds of millions of years.
Thus, SATURN'S RINGS are likely unusual and temporary and the non-Saturnian rings are likely to be normal.
Also the tidal forces of the planets probably cause complications too.
We'll just look at a few images to conclude the story.
A Voyager 2 image of Saturn's F ring and moons Pandora and Prometheus.
The F ring is a very narrow ring with bends, kinks and bright clumps that give the illusion of a braided strand.
These unusual features are at least in part due to complex gravitational interactions with the small moons Pandora and Prometheus.
Such close, interacting moons have been called shepherd moons (SRJ-228).
Credit: NASA.
A Voyager 1 image of Saturn's F ring.
The F ring is a very narrow ring with bends, kinks and bright clumps that give the illusion of a braided strand.
These unusual features are at least in part due to complex gravitational interactions with the small moons Pandora and Prometheus.
Such close, interacting moons have been called shepherd moons (SRJ-228).
Credit: NASA.
Neptune and two of its narrow rings.
LeVerrier is the inner ring and Adams is the outer one.
LeVerrier is actually uniform in structure (although it doesn't quite look that way), but Adams shows concentrations of particles that turn up as bright arcs.
Adams has four arcs of which three are visible in this image: Liberte, Egalite, Fraternite.
The arcs are probably due to gravitational perturbations of the 150 km diameter moon Galatea (SRJ-229 which is not seen in this image.
Credit: NASA.
