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Geology 130F

Lecture Seventeen


(chapters 10 and 11 in Beatty and Chaikin)

Physical Characteristics

Jupiter contains most of the mass in the planets; it has 1/1047th the mass of the sun and 318 times the mass of the Earth. It dominates the dynamics of comets and asteroids. For example, it probably ejected most of the planetesimals originally formed in the outer solar system and it may have formed the Oort cloud of comets. It also dominates the secular dynamics (orbital precession) of the other planets. Historically it was important because it showed clearly that objects ---the Galilean satellites---orbit bodies other than the Earth and that Kepler's third law (stating that the orbital period is proportional to the 2/3s power of the size of the orbit) held as well. Eclipses of these satellites provided the first good measurements of the speed of light.

Orbital Period 11.86 Earth years
Rotational Period 9h52m (Shortest day of all planets)
Obliquity 3.12 degrees
Atmosphere (Jupiter has no solid surface)
Mostly H2 (90%)
and He (8%)
Temperature (at cloudtops)125K
Magnetic Field 20,000G (Earth's is about 1G)
Mass1.9x1030g (318 Earth masses; 1/1047 Solar masses)
Bulk Density 1.3g/cm3
Equatorial radius71,492km
Polar radius66,854km

Size and Composition

The size of a gravitationally bound body depends on both its mass and composition. For example, a Jupiter mass planet made of iron is much smaller than a Jupiter mass planet made of hydrogen (see the figure).

The figure is from Beatty and Chaikin "The New Solar System" chapter 10. It is clear that Jupiter is primarily hydrogen, while Uranus and Neptune must contain substantial amounts of denser (and more refractory) material. In fact the composition of Jupiter seems to be very similar to that of the sun.

Interior Structure

Below the visible cloud tops the structure has to be inferred. The upper layers of the planet probably consist of molecular hydrogen, H2. At greater depths the pressure reaches high enough levels that the molecules are dissociated. It is believed that the result is an ionized form of hydrogen that acts like metals, albeit in a fluid state. Jupiter may or may not have a silicate core---ideas on this are currently in flux. Jupiter is currently radiating away about 2 and a half times the energy it receives from the sun. This heat may be left over from the accretion of Jupiter, or it may result from ongoing planetary differentiation (helium diffusing to the core). It is thought that the energy emerging from the interior is responsible for the high velocity winds observed at the surface, which form the prominent bands one can see from Earth. One piece of evidence supporting this view is that Uranus, which is the only giant planet not emitting more energy than it receives from the sun, is also the only giant planet which has no band structure.


The bands consist of alternating wind flows, with velocities of around 100km/hr. The wind velocities on Saturn (about 500km/hr) and Neptune (up to 2,200km/hr) are much higher. In addition to the jets or bands, there are innumerable small eddies or storms, and a few large ones, the largest being the great red spot. The latter has been around for at least 400 years. The eddies appear to be driving the jets, and the small eddies feed the larger ones. Presumably the energy source of the eddies is the buoyancy and heat supplied by convection from the interior. The spots appear to be very shallow features---the great red spot is believed to be about 50km deep, while it is more than three Earth diameters across.

Regular Satellite Systems and Rings

All the giant planets have rings, and all have regular satellite systems. A regular satellite system consists of a few to a dozen or more moons on prograde circular non-inclined orbits. All four gas giants also have irregular moons, on inclined and eccentric orbits at larger distances from the planet. The irregular moons are believed to be captured, while the regular satellites were probably formed in a disk around the planet at the time the planet formed.


  1. Closest to the planet is a system of tenuous rings. Click here to see a schematic of the rings and inner moons.
  2. Small inner moons
    1. Metis
    2. Adrastea
    3. Amalthea
    4. Thebe
  3. Galilean Moons
    Name Diameter (km) Orbital Period (days) Density (g/cm3) I/MR2
    Io 3630 1.769 3.57 0.378
    Europa 3138 3.551 2.970.347
    Ganymede 5262 7.155 1.94 0.311
    Callisto 4800 16.689 1.860.406

  4. Small outer moons
    1. Leda
    2. Hemalia
    3. Lysithea
    4. Elara
  5. Retrograde moons (``irregular moons'')
    1. Anake
    2. Carme
    3. Pasiphae
    4. Sinope


Io's relatively high density (3.57 g/cm3) suggests that it is composed mostly of silicates. The surface has a orange or yellow color, marked by large discolorations. These consist of sulfer (S) or sulfer dioxide (SO2) frost, volcanic craters, and extensive ``lava'' flows. In fact, Io is the most volcanically active body in the solar system. The surface of Io is the youngest in the solar system; no impact craters are known. The primary heat source driving the activity on Io is tidal heating. Io is on a slightly eccentric orbit; the eccentricity is forced by a 2-1 orbit-orbit resonance with Europa. As Io moves around its orbit, it also moves radially toward and away from Jupiter. When Io moves inward, the tidal deformation produced by Jupiter increases; when it moves out, the deformation decreases. During these repeated deformations friction in the interior of the moon generates tremendous amounts of heat. Other geologic features (besides volcanoes) include

  1. A thin lithosphere
  2. No evidence for tectonic plates
  3. Scarps, graben, other tensional faults
  4. Long lava flows
  5. Mountain ranges near the poles


Europa is the smallest of the Galilean moons. It shows very little vertical relief, and no sign of volcanoes. It has a high albedo, which coupled with its density, suggests that the surface consists of water ice. Dramatic photos taken by the Galileo spacecraft show features reminiscent of ice floes on Earth (have a look at the source, located at JPL ). While its surface is clearly very young, there are a number of impact craters. There are also a number of palimpsests, ringed crater-like structures with no central depression (no ``crater''). These may be old craters in which the crater floor has risen or been filled in by ice flows. Similar structures are seen on Ganymede and Callisto.

Europa is criss-crossed by numerous fractures and ridges. There are also features reminiscent of flows.

It is believed that the surface of Europa consists of 10-30km of ice, which lies over a very deep (100-200km) water ocean, possibly maintained in a liquid state by tidal heating.


Ganymede is the largest satellite in the solar system. Its low density indicates that it contains large amounts of ice relative to its silicate content. Like Io and Europa, the fact that the moment of inertia (divided by MR2 is less than 0.4 indicates that it is differentiated. It is also similar to Io and Europa in that it is in a 2:1 resonance, with Europa. However, it has a negligible eccentricity, and is relatively far from Jupiter, so the tidal heating is not significant. On the other hand, Ganymede does possess a magnetic field, suggesting that its interior is fluid. Its surface is mostly water ice, divided roughly equally into bright lightly cratered terrain and dark heavily cratered terrain. There are also complex tectonic features, including grooves; note the rather low crater density in this bright, relatively young region.


Callisto shows no signs of internally driven geologic processes. In fact, it is the most heavily cratered object in the solar system, suggesting that it has the oldest surface in the solar system. Its moment of inertia suggests that it is not differentiated, while the density suggests that water ice is the dominant constituent. It differs from Ganymede in that it does not have a magnetic field. Taken together, these two facts suggest that the interior has never been in a fluid state. This is surprising, given that the accretion energy, if released over a reasonable (short) time should have been more than enough to melt Callisto.

The Roche Limit

Any body in orbit around a planet is subject to tidal forces that act to pull the body apart. Material on the the side of the satellite most distant from the planet experiences a weaker gravitational force than material on the side of the satellite facing the planet. If the satellite is held together by its own gravity, then it cannot approach the planet too closely, or the tidal forces will overcome the self gravity, and the moon will be torn apart. The distance at which this occurs is known as the Roche limit; it is approximately 2.5 times the radius of the planet. Note that this does not mean that no moons can exist inside the Roche limit---many such moons are known, including Metis and Adrastea around Jupiter, and most of the moons around Neptune. Moons larger than about 100km in radius are held together by self-gravity, but smaller moons may be partially bound by electrical (chemical) forces.

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