Sections
The name gas giant is a bit of a misnomer since the gas giants' composition is dominated in inner layers by liquid phase, NOT gas phase, hydrogen and helium.
However, these elements are in the gas phase in the outer layers of the gas giants (which is what we directly observe) and are gas phase under ordinary terrestrial conditions and throughout most of the observable universe (in the form of stars, interstellar gas, intergalactic gas), and so the name gas giant has stuck.
We will NOT bother with the ice giant classification further in this lecture.
Additionally, improved ground-based and Earth-satellite observations (particularly from Hubble Space Telescope (HST)) have made great additions to our gas giant lore since the 1970s.
See videos Voyager 1 & 2 Trajectories to the Outer Planets | 1:18 and Juno Perijove 06 | 3:00 in Gas giant videos below (local link / general link: gas_giant_videos.html):
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Saturn's rings are previewed in the
figure above and below
(local link /
general link: saturn_rings_false_color.html).
EOF
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___________________________________________________________________________ Table: 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 farther out. A consequence is that Earth is approximately at the location of the Sun relative to the gas giants, and so they always look nearly full in planetary phase in Earth-based imagery such as from the HST. Note exoplanet gas giants are often NOT far out in their planetary system as we know now. Orbital period 11.857 29.424 83.747 163.723 (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 6.487(15) 9.796(18) 2.29(8) 1.71(13) [(R_e-R_p)/R_e] *100 (%) The gas giants 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 Solar System planetary mass The 4 gas giants altogether contain 99.5 % of the total Solar System planetary mass (HI-202). Mean Density 1.33 0.70 1.30 1.76 (g/cm**3) The mean density of the rocky planets are about 3--5.5 g/cm**3. Clearly, gas giants must have a different compostion from the rocky planets. Axis tilt 3.12 26.73 97.86 29.58 to the ecliptic axis (degrees) Jupiter won't have many seasonal effects and Uranus must have extreme seasonal effects. Known number 95 274 28 16 of moons These are the known moons, totalling 95+274+28+16=413, in 2025. New small moons are still found fairly often I think. Some day we will exhaust them. Maybe we have now, except very tiny ones. See Wikipedia: Jupiter's moons: 95 (2025), Wikipedia: Saturn's moons: 274 (2025), Wikipedia: Uranus's moons: 28 (2025), and Wikipedia: Neptune's moons: 16 (2025). 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 in part at least to methane (CH_4) which scatters blue light and absorbs red light (see Wikipedia: Neptune: Atmosphere). 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 domination is aided by the fact that the gas giants are much farther from the Sun than the Earth. The internal heat is, of course, primordial 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.
___________________________________________________________________________
As massive as the gas giants, they are almost negligible in mass compared to the Sun. The figure below (local link / general link: planet_sun.html) graphically illustrates this.
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The oblateness of the
gas giants is another interesting
factoid: it's
extreme for Jupiter and
Saturn.
A few images of the Jupiter and Saturn show clearly show their high oblateness---which is specified in the table above.
The high oblateness of Saturn is shown in the figure below (local link / general link: saturn_oblate.html).
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Some of the discussion is applies generally to gas giants in other planetary systems.
There are TWO THEORIES of the formation of the gas giants in planetary systems (HI-277).
THE TWO THEORIES:
These grains of ices then got entrained in the formation of planetesimals and protoplanets.
In the inner Solar System during Solar System formation, this did NOT happen nearly so much to the volatiles because it was hotter because of the great proximity to the Sun.
The upshot is that there was simply a lot more mass entrained into the formation of the protoplanets in the outer Solar System than in the inner Solar System.
So there were more massive rocky/icy protoplanets in the outer Solar System with much stronger gravity and these protoplanets were able to directly attract hydrogen and helium gas.
The added hydrogen and helium gas made them more massive still and a gravitational runaway occurred in which large amounts of hydrogen and helium were accumulated.
The icy/rocky protoplanets grew into being planets mostly made of hydrogen and helium---the gas giants: Jupiter, Saturn, Uranus, and Neptune.
Just by stochastic effects (i.e., by chance), there were fragments of the primordial solar nebula of high-density gas. These fragments started collapsing under self-gravity to form dense cores of gas and then gravitational runaway occurred leading to the gas giants.
In the inner Solar System, the extra heating by the Sun kept the pressure of the gas sufficiently high to prevent the collapse of fragments.
So the gas giants only formed in the outer Solar System.
The gas giants were too small ever to become hot and dense enough under contraction to start hydrogen burning and become stars.
The gas fragments contained all the elements of the primordial solar nebula composition, of course.
So all the elements got entrained in the formation of the gas giants.
The denser elements diffused to the center to create rocky/icy cores: i.e., chemical differentiation happened.
So just as in the 1st theory, the gas giants should have rocky/icy cores.
Some planetary systems have one operate and some the other. Or hybrids of the two theories may apply.
So they could both be right to a degree.
But which theory is favored?
It's hard to say since recent evidence has gone both ways:
The two results may be reconciliable. The disk dominated by hydrogen and helium may be blown out of planetary systems 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.
This suggests that refactories which are part of metallicity help to increase the gas giants abundance which suggests that refactories are key ingredients in their formation which is NOT the case in 2nd formation theory.
So this metallicity evidence favors the 1st theory.
Before the discovery of exoplanets
beginning in 1992
(see Wikipedia:
Exoplanets: Confirmed discoveries),
it was plausible that
the Solar System would be
a sort of average
planetary system
or a typical planetary system.
This idea is NOT true.
The distribution of
planetary system behaviors
is very broad, and NOT sharply peaked about
the Solar System.
There may be
4 planetary system architectures.
See the figure below
(local link /
general link: planetary_system_classification.html).
It still seems impossible that
gas giants could form in such
locations.
So it seems there must be
planetary migration:
gas giants form in the outer
planetary systems
and some migrate inward.
Exactly how this happens is NOT fully elucidated.
However, in a vague sense it is understood that
gas giants can interact
with a still existing protoplanetary disk
and other planets
and spiral inward in some cases and outward too in other cases.
Planetary migration
happens frequently, but maybe NOT always.
If it did happen in the Solar System,
it did NOT cause our
gas giants to migrate to the
inner Solar System.
Some set of initial conditions must decide on whether or NOT
significant planetary migration occurs
and how it evolves.
And that is all we will say here. But we can look at
the planetary migration
videos
Planet-Disk Interaction and Orbital Migration. Movie 1. Low Mass Planet | 0:33
(with narration)
and Planetary migration - FARGO3D | 2:28
(without narration)
in
Planetary system formation videos
below
(local link /
general link: planetary_system_formation_videos.html):
Since formation, the gas giants have probably undergone
some evolution, but
the instructor knows little of it, and so we will largely pass it by.
The gas giants do lose
heat energy, and are cooling off by
emitting infrared radiation---they
will cool off for quasi-forever losing
heat energy at ever
redder wavelengths as their
temperatures fall.
What of impactors?
Because the gas giants have powerful
gravity, they have always been
heavily impacted.
But, other than in formation itself, the impactors
have probably had little effect.
As we know from the Comet Shoemaker-Levy 9 impacts
on Jupiter in
1994
(see IAL 16: Small Astro-Bodies of the Inner Solar System and Target Earth:
Target Earth for a description),
large impactors create giant
chemical reaction
marks on gas giants.
But those marks disperse relatively quickly and leave no trace on the surface and
the refactory impactor material
sinks to the solid core with probably little lasting effect.
So impactors are probably negligible
for the evolution of gas giants though
the big ones are spectacular when they happen.
The MOON SYSTEMS of the gas giants were formed by two processes:
This disk is a
circumplanetary disk.
Formation from a
circumplanetary disk
is analogous to the formation planets around the
Sun.
Formation from
circumplanetary disk
is believed to be origin of at least the larger members
of the Jupiter system of moons
(certainly the Galilean moons:
Io,
Europa,
Ganymede, and
Callisto),
Saturn system of moons,
and Uranus system of moons
(see Wikipedia: Moons of Jupiter:
Origin and evolution;
Wikipedia: Titan: Formation;
Wikipedia: Protoplanetary disk: Planetary system).
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.
Remarkably, one large moon
of a gas giant planet
is a capture:
Neptune's
moon
Triton (2707 km, 7th largest moon
in the Solar System)
Among the rocky planets only
Mars may have
captured moons (i.e.,
the Martian moons
Phobos and Deimos).
The capture theory has rival theories
(see Wikipedia: Moons of Mars: Origin).
php require("/home/jeffery/public_html/astro/planetary_systems/planetary_system_classification.html");?>
A particular unexpected feature of
other planetary systems
is that many have gas giants
very close to their host star.
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EOF
The figure below (local link / general link: jupiter_galilean_moons_collage.html: with detailed caption in section The Galilean Moons of Jupiter) shows the Galilean moons of Jupiter (which were probably formed in situ) and figure below (local link / general link: jupiter_inner_moons.html) that shows Jupiter's 4 innermost moons (all small which were probably captures).
php require("/home/jeffery/public_html/astro/jupiter/moons/jupiter_galilean_moons_collage_2b.html");?>
php require("/home/jeffery/public_html/astro/jupiter/moons/jupiter_inner_moons.html");?>
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We can only have a brief look the atmospheres.
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 gas giant bands.
Jupiter and its gas giant bands are shown in the figure below (local link / general link: jupiter_full_aurora.html).
php require("/home/jeffery/public_html/astro/jupiter/jupiter_full_aurora.html");?>
As you can see from the figure above,
(local link /
general link: jupiter_full_aurora.html)
the Jupiter bands
are immensely complex in detail.
This is because they are highly turbulent.
The Jupiter 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 in detail (Se-499)---probably only since the advent of astrophotography in the mid 19th century.
To see some of the fine detail of the Jupiter bands, see video Juno Perijove 06 | 3:00 in Jupiter videos below (local link / general link: jupiter_videos.html):
What are the Jupiter bands?
They are the tops of rising and sinking convection cells
organized as bands parallel to the Jovian equator.
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 Jupiter 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.
The Earth atmospheric circulation
is shown in the figure below
(local link /
general link: hadley_cell.html).
Venus has two large bands,
on in the northern hemisphere and one in the southern hemisphere, as we saw in
IAL 13: Venus: Venusian Weather.
The Jupiter bands
must be rather different though than those of
Earth and Venus.
For one thing, on
Earth
and Venus
it is solar energy that drives the
convection.
On Jupiter
and the other gas giant planet,
their own
primordial-radiogenic heat geology
(see also Wikipedia:
Earth's internal heat budget: Radiogenic heat: Primordial heat)
is probably the main driver of
convection.
Modeling and Galileo orbiter probe sent into the
Jupiter atmosphere
suggested that the Jupiter bands
extend to great depth into the interior
(Se-499).
But here we will just consider the surface as it is currently understood by looking at the cartoon in
the figure below.
Caption: A cartoon of the
Jupiter bands.
Credit/Permission: ©
David Jeffery,
2003 / Own work.
Image link: Itself.
The CLOUD LAYERS in the Jupiter 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 (H_2) gas and
helium gas are colorless.
Thus, the COLORS the
Jupiter 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 Jupiter bands, but actual
measurements don't show a simple pattern
(FMW-220).
The high speeds are probably partly due to the
Coriolis force
which
causes the main band winds on Earth.
But probably are also due to deep circular motions
(Se-499).
The Coriolis force on
Earth and
other planets
is explicated in the figure below
(local link /
general link: coriolis_force.html).
But it's considered a powerful rotating storm---it is
a Jovian vortex.
The rotation period of the is about 6 days
(FMW-221).
Caption: Jupiter's
Great Red Spot
in false color from
Voyager 1, 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/Permission: NASA,
Voyager 1 (1977--c.2025),
1979 /
Public domain.
php require("/home/jeffery/public_html/astro/jupiter/jupiter_videos.html");?>
EOF
Question: What physical condition must be essential in
imposing the band structure
of bands parallel to the Jovian equator?
Answer 1 is right. Rotation is what breaks the spherical symmetry.
Earth has convection bands too. The ground-level
wind patterns associated with are:
php require("/home/jeffery/public_html/astro/earth/atmosphere/hadley_cell.html");?>
php require("/home/jeffery/public_html/astro/mechanics/coriolis_force.html");?>
Now the Great Red Spot
(see the figure below)
on Jupiter is a high pressure region
and therefore formally an anticyclone and with
counterclockwise swirl since it is in the southern hemisphere.
Download site: NASA:
NSSDCA Photo Gallery.
Image link: Itself.
Form groups of 2 or 3---NOT more---and tackle Homework 15 problems 2--7 on the gas giant planets.
Discuss each problem and come to a group answer.
Oh, 5--10 minutes.
See Solutions 15.
The winners get chocolates.
php require("/home/jeffery/public_html/astro/videos/ial_0000_standards.html");?>
php require("/home/jeffery/public_html/astro/videos/ial_015_gas_giants.html");?>
php require("/home/jeffery/public_html/astro/art/art_c/chocolate_hot_2.html");?>
The new ones have evaded previous detection because they are all very small and inconspicuous. Someday, we will exhaust the population of objects large enough to be called moons, but that day may be far off and someone will have define how large astro-body has to be to be a moon. One can keep finding planet-orbiting astro-bodies until we are at the level of interplanetary dust.
Maybe there are a huge number of meteoroid-size-scale (10-meter-size-scale) moons, but are they moons?
We can't do everything re moons of the gas giants, so we will just consider the Galilean moons of Jupiter to save us from exhaustion.
Jupiter has 95 known moons circa 2022 (See Wikipedia: Jupiter's moons: 95 (2024), Wikipedia: Saturn's moons: 146 (2024), Wikipedia: Uranus's moons: 28 (2024), and Wikipedia: Neptune's moons: 16 (2024).)
Most of Jupiter's moons are very small and all the large ones were found long ago.
All Jupiter's moons are unique and have interesting features---but maybe NOT that interesting.
The Galilean moons are by far the largest of Jupiter's moons.
The 4 Galilean moons discovered by
Galileo (1564--1642) in
1610 (HI-214).
The Galilean moons are shown in
two collage figures below
(local link /
general link: jupiter_galilean_moons_collage.html;
local link /
general link: jupiter_galilean_moons_collage_far.html).
We can first look that this range of behavior below
in Table: Galilean Moon Orbital Facts.
References:
Cox-303--307, HI-213ff,
Wikipedia: Galilean moons.
The 3 innermost Galilean moons exhibit a 1:2:4
Laplace resonance as illustrated
in the animation
in the figure below
(local link /
general link: jupiter_galilean_moons_resonance.html).
See below
Table: Galilean Moon Size Facts.
References:
Cox-303--307, HI-213ff,
Wikipedia: Galilean moons.
The figure below
(local link /
general link: rocky_icy_body.html)
shows how the
Galilean moons
rank in size among the Solar System
rocky bodies
and rocky-icy bodies.
From the figure below
(local link /
general link: moons_interesting.html),
we can see that the
Galilean moons
among the moons of the
Solar System
have 1st, 3rd, 4th, and 6th largest sizes.
In fact, only the Moon (5th largest)
and Titan (2nd largest) break up
the dominance in size of the Galilean moons.
See below
Table: Other Galilean Moon Facts.
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.
But even though we neglect poor old
Ganymede as
less interesting, it is still
the largest moon
and the 4th largest
rocky-icy body
in the Solar System.
In the figure below
(local link /
general link: jupiter_io_full.html),
behold Io.
Whenever an image is taken of
Io, there is always
one or more
volcanic eruptions
in progress---as the images and
videos illustrate
in the figure below
(local link /
general link: io_003_eruption.html).
And then there is Europa as seen
in the image and videos in the figure below.
Caption: Europa:
the 2nd innermost Galilean moon of
Jupiter.
Image from Voyager 1,
1979 Mar04.
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 of
Jupiter.
But the
Galileo spacecraft (1989--1995-arrival--2003)
saw no fracturing during its time
in orbit around Jupiter,
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
to cause liquid water.
But the heating must 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 as we know it.
This is a very exciting prospect.
On the other hand, the liquid water may be far too acidic for
life as we know it as
we know.
Circa 2004 someone???
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.
For more information on Europa,
see HI-216,
SRJ-218,
Se-506, and
Cox-303--307.
See also the
video
Europa: Ocean World | 4:12
in Jupiter videos
below
(local link /
general link: jupiter_videos.html):
EOF
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Credit/Permission: NASA,
1979 /
Public domain.
We skip merrily over
Ganymede---the largest
moon
in the Solar System, but
just so boring---and arrive
at the last and least
Galilean moon,
Callisto.
See Callisto
in the figure below.
So much for Callisto
and the Galilean moons.
Caption:
Callisto
the outermost
Galilean moon of
Jupiter.
A true color image
from the
Galileo spacecraft (1989--1995-arrival--2003),
2001 May.
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???.
Micrometeoroid
impacts have probably largely vaporized the surface
water ice leaving a superficial layer of
carbonaceous
regolith.
Bright spots are relatively recent impact crater
that have
exposed underlying water 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 liquid water in
which electrical currents
flow due to ions in the
water.
For more information, see
HI-220 and
Cox-303--307.
Credit/Permission: NASA,
2001 /
Public domain.
Recall that we briefly looked at
Jupiter's has 4 small
moons that are within the orbital radius of the
innermost Galilean moon
Io
(see subsection
Formation of the Moons of the Gas Giants).
They are Metis,
Adrastea,
Amalthea,
and Thebe.
See file
jupiter_inner_moons.html
for these 4 small
moons.
The 4 Galilean moons are fascinating because
they show a remarkable range of behavior determined by
tidal force geology.
The
tidal force is due, of course, to
Jupiter and decreases from
very strong for the innermost
Galilean moon
Io
to rather weak for the outermost
Galilean moon
Callisto.
php require("/home/jeffery/public_html/astro/jupiter/moons/jupiter_galilean_moons_collage.html");?>
php require("/home/jeffery/public_html/astro/jupiter/moons/jupiter_galilean_moons_collage_far.html");?>
The Galilean moons show a wide range of interesting behavior.
___________________________________________________________________________
Table: Galilean Moon Orbital Facts
___________________________________________________________________________
Quantity Io Europa Ganymede Callisto
___________________________________________________________________________
Mean distance 5.903 9.386 14.967 26.339
from Jupiter
(Jupiter equatorial
radii)
Orbital period 1.769 3.551 7.155 16.689
(days)
The Galilean moons are all
tidally locked
to Jupiter: i.e., they always turn one side
to Jupiter and their
orbital periods and
rotational periods
are continually driven toward exact equality by the
tidal force.
The orbits of
of Io,
Europa, and
Ganymede are in a
1:2:4 Laplace resonance.
___________________________________________________________________________
___________________________________________________________________________
php require("/home/jeffery/public_html/astro/jupiter/moons/jupiter_galilean_moons_resonance.html");?>
___________________________________________________________________________
Table: Galilean Moon Size Facts
___________________________________________________________________________
Quantity Io Europa Ganymede Callisto
___________________________________________________________________________
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 (the larget moon of
Saturn), Mercury,
Callisto, Io,
the Moon,
Europa.
___________________________________________________________________________
___________________________________________________________________________
php require("/home/jeffery/public_html/astro/solar_system/rocky_icy_body.html");?>
The Galilean moons
must, of course, also rank high in size among
the moons of the
Solar System.
php require("/home/jeffery/public_html/astro/solar_system/moons_interesting.html");?>
___________________________________________________________________________
Table: Other Galilean Moon Facts
___________________________________________________________________________
Quantity Io Europa Ganymede Callisto
___________________________________________________________________________
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 condense 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:
Cox-303--307, HI-213ff,
Wikipedia: Galilean moons.
___________________________________________________________________________
php require("/home/jeffery/public_html/astro/jupiter/moons/jupiter_io_full.html");?>
Io is the most geologically
active body in the Solar System.
Earth is a comparatively distant number 2.
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Download site: NASA:
NSSDC Photo Gallery Europa.
Image link: Itself.
Download site: NASA:
Color global image of Jupiter's satellite Callisto.
Image link: Itself.
Form groups of 2 or 3---NOT more---and tackle Homework 15 problems 20--26 on the Galilean moons.
Discuss each problem and come to a group answer.
Oh, 5--10 minutes.
See Solutions 15.
The winners get chocolates.
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php require("/home/jeffery/public_html/astro/videos/ial_015_gas_giants.html");?>
php require("/home/jeffery/public_html/astro/art/art_c/chocolate_swiss_2.html");?>
Galileo (1564--1642) 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?"---an allusion to the Greek god Cronus---who was identified with the much more benign Roman god Saturn---who was a prototype for Santa Claus.
Christiaan Huygens (1629--1695) 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).
The figure below (local link / general link: saturn_rings_orientation_perspective.html) illustrates the variation in the orientation of Saturn's rings as seen from the perspective of the Earth.
As the centuries rolled by,
more than one ring was identified for Saturn.
Going outward,
one has the well known C ring, B ring, Cassini division (a gap region), and the
A ring (Se-513).
See the figure below.
Caption: 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/Permission:
NASA,
HST,
1998 /
Public domain.
The Voyager 1 and
Voyager 2
showed showed that
there are 10 rings at least and the rings themselves have intricate structures
(Cox-311).
For the intricacy of
Saturn's rings,
see the figure below
(local link /
general link: saturn_rings_false_color.html).
Rings around Uranus had been suspected
before
Voyagers 1 and 2---which
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)????.
php require("/home/jeffery/public_html/astro/saturn/saturn_rings_orientation_perspective.html");?>
Download site: NASA/HST:
Alas, a dead link.
Image link: Itself.
php require("/home/jeffery/public_html/astro/saturn/saturn_rings_false_color.html");?>
In 1859, James Clerk Maxwell (1831--1879) showed that the Saturn's rings could NOT be solid disks.
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. This is illustrated in the figure below.
Caption: A cartoon of the tidal force pulling a body apart. (Correct "too" to "to" when you are able to.)
Credit/Permission: ©
David Jeffery,
2003 / Own work.
Image link: Itself.
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.
Planetary rings that are dark rings are explicated in the figure below.
Image 1 Caption: A diagram of Jupiter's ring system.
Features:
The dust grains may have size scale of order 10 microns (SRJ-227).
More images of Jupiter's rings are shown in the figure below.
Caption: 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/Permission: NASA,
before or circa 2005 /
Public domain.
Download site: NASA:
PIA01622.html.
Alas, a dead link.
Image link: Itself.
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 protoplanetary 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 that relatively clos to each other ompared to their size.
(HI-209,
Se-512).
See the artist's conception
in the figure below
(local link /
general link: saturn_rings_artist_conception.html).
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).
Thus, Saturn's rings are likely unusual and temporary and
the non-Saturnian rings are likely to be normal.
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Perturbations should make the ring material spiral inward, and so
the rings will probably last only of order of
hundreds of millions of years.
Also the tidal forces of the planets probably cause complications too.
To conclude the story, we'll just look at a few figures below.
Caption: 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/Permission: NASA,
before or circa 2005 /
Public domain.
Download site: NASA:
saturn92.html.
Alas, a dead link.
Image link: Itself.
Capiton: 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/Permission: NASA,
before or circa 2005 /
Public domain.
Download site: NASA:
c3493048.html.
Alas, a dead link.
Image link: Itself.
Capiton: 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/Permission: NASA,
before or circa 2005 /
Public domain.
Download site: NASA:
c1141251.html.
Alas, a dead link.
Image link: Itself.
UNDER CONSTRUCTION BELOW
Under construction, but see Cassini videos below (local link / general link: saturn_videos.html):
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EOF
The residual heat of formation will become exhausted and the heat from long-lived radioactive isotopes will decline as those radioactive isotopes decay away (see, e.g., radiogenic_heat.html). The gas giants will escape being vaporized by the post-main-sequence Sun and will continue to orbit white dwarf Sun (i.e., the white dwarf remnant of the Sun) for of order 10**6 gigayears, but in larger orbits since the white dwarf Sun will only be about half the mass of the current Sun (see Wikipedia: Sun: After core hydrogen exhaustion; Timeline of Solar System evolution).
On the time scale of 10**6 gigayears, gas giants and any other remaining Solar System objects will be ejected from the Solar System by passing stars. Such ejections are super rare events, but they will happen if there is enough time.
The gas giants will then be rogue planets.
Note evolution of the observable universe to 10**6 gigayears is highly speculative since we are extrapolating well beyond the age of the observable universe = 13.797(23) Gyr (Planck 2018) since the Big Bang. For that past time frame we have a good theory the Λ-CDM model which even if it needs replacement CANNOT be wrong in rough outline---there is lots of evidence for it. But extrapolating the Λ-CDM model for even 10s of gigayears into the future is highly speculative. Many cosmic factors of which we know little or nothing can come into play. But for highly speculative extrapolation, see Wikipedia: Timeline of the far future, Wikipedia: Future of an expanding universe, Wikipedia: Graphical timeline from Big Bang to Heat Death (but note that the left-hand vertical scale is tricky: for > 10 years, it is x=100*log(log(t_year)), and so t_year=10**(10**(x/100)) ).