IAL 15: Gas Giants

Don't Panic


  1. Introduction
  2. Basic Facts
  3. Formation
  4. Interiors
  5. Atmospheres of the Gas Giants
  6. The Galilean Moons of Jupiter
  7. The Rings of the Gas Giants
  8. What and Why Rings?
  9. Why are the Ring Systems so Complex?
  10. The Moons of Saturn
  11. The Fate of the Gas Giants

  1. Introduction

  2. The gas giant planets---Jupiter, Saturn, Uranus, and Neptune---are very different from the rocky or terrestrial planets.

    In brief, in comparison/constrast to the rocky or terrestrial planets, the gas giants:

    1. are massive.
    2. have powerful gravity sources.
    3. are low in mean density.
    4. are in outer Solar System beyond 5 AU where it is generally pretty cold.
    5. have compositions dominated by hydrogen and helium. Only dominated in the envelopes in the case of Uranus, and Neptune (see the discussion of ice giants below).
    6. have extensive moon systems.
    7. have complex ring systems.

    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 know far more about the gas giants than before the 1970s thanks mainly to the probes that have been sent to the outer Solar System: Voyager 1 (1977--), Voyager 2 (1977--), Galileo spacecraft (1989--1995-arrival--2003), Cassini spacecraft (1997--2017), Juno spacecraft (2011--), etc.---see the videos below.

    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):


    Saturn's rings are previewed in the figure above and below (local link / general link: saturn_rings_false_color.html).

  3. Basic Facts

  4. We can peruse some gas giant facts in the two tables below.

    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 
    Rotation Period      0.413        0.444        0.718        0.671
                         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
    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)
    *100 (%)
                    The gas giants are fast rotators and this gives them
                    large centrifugal forces, and thus makes them highly
    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 
    Mean Density         1.33         0.70         1.30         1.76
                     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 
                     Jupiter won't have many seasonal effects and 
                     Uranus must
                     have extreme seasonal effects.
    Known number         95          146           27           14
    of moons
                     These are the known moons, totalling 282,
                     circa 2023.
                     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,
                     Wikipedia:  Saturn's moons,
                     Wikipedia:  Uranus's moons,
                     and Wikipedia:  Neptune's moons.
                    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
                    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
                    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
                    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
    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.

    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).

  5. Formation

  6. In this section we deal with the formation of the gas giants and their moons.

    Some of the discussion is applies generally to gas giants in other planetary systems.

    1. Theories of Formation:

      There are TWO THEORIES of the formation of the gas giants in planetary systems (HI-277).


      1. In the outer Solar System during Solar System formation, volatiles could readily condense to form grains of ices.

        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.

      2. The gas giants formed directly from hydrogen and helium gas like stars.

        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.

      The TWO THEORIES are NOT exclusive.

      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:

      1. The 2nd theory allows for FAST FORMATION of gas giants and increasingly that seems to be necessary since protoplanetary disks seem to have very brief lives: recent evidence suggests less than 5 Myr (R. Irion, 2003jun06, Science, 300, 1498).

          Annoyingly, even more recent evidence points disks of dust around stars hundreds of millions of years old have been recently reported based on infrared data from the Spitzer Space Telescope (SST) (D. Savage & W. Clavin, 2004oct19, JPL Latest News).

          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.

      2. Evidence from Kepler probe suggests that gas giants abundance in planetary systems increases with the metallicity of those systems. See J. Matson, 2012jun13, Scientific American.

        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.

      The controversy still rages.

    2. Planetary Migration:

      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. See the figure below (local link / general link: planetary_system_classification.html).

      This hypothesis may still be true, but it is also true that the distribution of
      planetary system behaviors is very broad, and NOT sharply peaked about the Solar System.

      A particular unexpected feature of other planetary systems is that many have gas giants very close to their host star.

      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):


    3. Evolution Since Formation:

      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 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.

    4. Formation of the Moons of the Gas Giants:

      The MOON SYSTEMS of the gas giants were formed by two processes:

      1. The larger moons in some cases probably formed from a miniature protoplanetary disk that formed about the gas giant itself.

        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).

      2. Other moons were probably captured small bodies such planetesimals, protoplanets, asteroids, and maybe comets.

        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). Triton has retrograde orbit (i.e., it revolves clockwise as seen from the NCP), and so CANNOT have formed in situ.

        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).

      It probably isn't easy to decide in all cases which mechanism (in situ formation or capture) gave rise to a given moon?????.

      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).

  7. Interiors

  8. Obviously, we don't have as much information about the interiors of the gas giants as we do about the rocky planets.

    But knowing size, density, and probable composition, plus modeling, we think we have some plausible models of the interiors (FMW-216, HI-207).

    The figure below gives the basic idea of the interiors.

    Notes on structure:

    1. Note that we only show the dominant element/elements in each layer. The stratification of helium is NOT shown and may be a bit uncertain.

    2. Chemical differentiation has occurred. The densest materials have sunk to the lowest positions.

    3. The inner ice, rock, and metal is in a far different condition than anything we know on the surface of the Earth because of the high pressure in the interior.

      For example, the pressure and density at Jupiter's center is estimated to be 100 million atmospheres (i.e., 100 million times the Earth atmosphere surface pressure) 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.

    4. Liquid metallic hydrogen is what hydrogen turns into under pressures of over about a million atmospheres.

      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 is a actually theoretical entity since experimentally it has NOT been created definitively it seems---it is believed in theoretical entity.

      Metallic hydrogen (but perhaps NOT liquid metallic hydrogen) has possibly been produced in the laboratory since the 1996 in various ways, but results are still in some dispute in all cases it seems (see Wikipedia: Metallic Hydrogen: Experimental pursuit before 2011).

      But liquid metallic hydrogen was theoretically predicted to exist in Jupiter long before there was anything experimental evidence for it.

    5. Only the icy/rocky/metallic core is solid. So the "surface" of the gas giants is a long way down.

    6. The atmospheres are dominated by molecular hydrogen (H_2) gas.

      As you go down in the density rises and the molecular hydrogen (H_2) gradually becomes dense enough to be generally considered a liquid---there is NO PHASE TRANSITION---no interface between gas and liquid.

      The distinction between gas and liquid 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).

    7. The fluids (i.e., liquid and gas) are convecting heat. Thus there are convection cells inside the gas giants.

      But their behavior is probably NOT too well understood???.

    8. The gas giants have dipole magnetic fields.

      The dipole magnetic fields of Jupiter and Saturn probably arise from electrical currents in the liquid metallic hydrogen layer.

      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.

  9. Atmospheres of the Gas Giants

  10. The atmospheres of the gas giants are usually considered those upper levels where the material can be considered gas and where one finds clouds.

    We can only have a brief look the atmospheres.

    1. Band Structure:

      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).

      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):


    2. What Are the Jupiter Bands?

      What are the Jupiter bands?

      They are the tops of rising and sinking convection cells organized as bands parallel to the Jovian equator.

        Question: What physical condition must be essential in imposing the band structure of bands parallel to the Jovian equator?

        1. Jupiter's fast rotation.
        2. Jupiter's magnetic field.
        3. The solar wind.

        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 Jupiter bands.

    3. The Driver of Convection on Jupiter and the Other Gas Giants:

      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.

    4. The Cloud Layers:

      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).

      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.

      But it's considered a powerful rotating storm---it is a Jovian vortex.

      The rotation period of the is about 6 days (FMW-221).

  11. The Galilean Moons of Jupiter

  12. The gas giants have altogether 182 known moons (circa 2018). It may be a few new very small ones will be discovered, but we may have exhausted all that are larger than kilometer size scale.

    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.

    1. The Moons of Jupiter:

      Jupiter has 80 known moons circa 2022 (see Wikipedia: Jupiter's moons).

      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.

    2. Galilean 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).

      Galilean moons show a wide range of interesting behavior.

      We can first look that this range of behavior below in Table: Galilean Moon Orbital Facts.

      Table:  Galilean Moon Orbital Facts
      Quantity             Io          Europa       Ganymede     Callisto      
      Mean distance        5.903       9.386        14.967        26.339
      from Jupiter
      (Jupiter equatorial
      Orbital period       1.769       3.551        7.155         16.689
                           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.

      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).

    3. The Galilean Moon Size Facts:

      See below Table: Galilean Moon Size Facts.

      Table:  Galilean Moon Size Facts
      Quantity             Io          Europa       Ganymede     Callisto
      Equatorial radius    1820.       1565.        2634.          2403. 
                           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, 

      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.

      Galilean moons must, of course, also rank high in size among the moons of the Solar System.

      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.

    4. Other Galilean Moon Facts:

      See below Table: Other Galilean Moon Facts.

      Table:  Other Galilean Moon Facts
      Quantity             Io          Europa       Ganymede     Callisto
      Mean Density         3.55        2.970        1.900          1.790 
      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
                           have a large fraction of water ice.  Recall
                           water ice density is almost exactly 1 g/cm**3. 
      Tidal heat           extreme    significant    low         low
                           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.

    5. Focus on Io, Europa, and Callisto:

      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.

    6. Io:

      In the figure below (local link / general link: jupiter_io_full.html), behold Io.

      Io is the most geologically active body in the Solar System. Earth is a comparatively distant number 2.

      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).

    7. Europa:

      And then there is Europa as seen in the image and videos in the figure below.

    8. Callisto:

      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.

  13. The Rings of the Gas Giants

  14. All the gas giants have planetary rings, but only Saturn has conspicuous planetary rings.

    1. The Early History of Rings:

      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.

    2. More Rings:

      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.
        Download site: NASA/HST: Alas, a dead link.
        Image link: Itself.

      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).

    3. Dark Rings:

      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)????.

  15. What and Why Rings?

  16. What and why?

    1. Planetary Rings in General:

      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.

      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.

        Question: What would happen to the bits of an orbiting object broken up by the tidal force?

        1. They would fall into the planet.
        2. They would escape from the gravitational field of the planet.
        3. They would just carry on orbiting the planet for the most part.

        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.

      Now the gas giant rings ARE NOT believed to be the remnants of bodies torn apart by the tidal force---except maybe for the exceptional rings of Saturn.

      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.

      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.

        Question: Why are rings usually flat?

        1. The tidal force overcomes self-gravity to prevent the particles from coalescing into a moon, but self-gravity can still pull the particles into a thin disk since the tidal force doesn't oppose that.

        2. Recall that the particles must orbit the center of mass of Jupiter. Thus if they arn't orbiting in single plane they will tend to collide at high speeds.

          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.

        Both right?

        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.

      What decides on final orbital plane that the rings particles end up on?

      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.

        Also the dust particles that make the rings came off small moons that may be orbiting equatorially or relatively close to equatorially???.

        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.

      Why don't the rings get perfectly flat?

      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????.

    2. Saturn's Rings:

      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).

      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.

  17. Why are the Ring Systems so Complex?

  18. The short answer is the subtle gravitational interactions of the many moons, particularly small ones, that mingle with the rings.

    Also the tidal forces of the planets probably cause complications too.

    To conclude the story, we'll just look at a few figures below.

  19. The Moons of Saturn


    Under construction, but see Cassini videos below (local link / general link: saturn_videos.html):


  21. The Fate of the Gas Giants

  22. The gas giants will just cool off forever---nearly fovever.

    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)) ).