07 Outer Planets

1. Summary of Jovian Planets

Basics of Jovians

  • Massive and dense
  • Large
  • Gaseous (Hydrogen, Helium and their compounds)
  • Far from the Sun
  • Fast rotation
  • High Magnetic Fields
  • Having Ring Systems

Interiors

  • Divided into two main configuration:
    • Jupiter and Saturn
    • Uranus and Neptune
  • Mainly:
    • Similar layers as Terrastials
    • However the content is purely Hydrogen
    • Hydrogen changes state and becomes liquid and metallic in lower layers.
    • Upper layers contain clouds of ammonia and their compounds

Atmosphere

  • Meaning of surface is very weak (see the figure)
  • Jupiter and Saturn atmospheres are studied by probes where they penetrated deep into the atmosphere
  • Therefore surface is defined where a temperature change (reflected from a pressure change) from cold to hot.
  • All four Jovians shows similar atmospheric conditions

Fast Rotation

  • When a ball of gas rotates it will deform its structure.
  • The deformation makes the object elongated.
    • Faster along equator
    • Slower on poles
  • Fast rotation of gas ball is also the reason of having a high magnetic field: Dynamo Effect

Magnetic Fields

  • They all have high magnetic fields
  • The largest is Jupiter and it extends until the Saturn's orbit
  • Magnetic Field axis not always alligned with the rotation axis.
  • Since Jovians are gaseous, centers of the magnetic fields (with respect to magnetic poles) don't always coincide with the planet's core; mostly for Uranus and Neptune

Moons of Jovians

  • Since Jovians are massive they will surely attract many minor objects beyond the planets.
  • Current Moon population
    • Jupiter: 4 major - 79 in total
    • Saturn: 7 major - 62 in total
    • Uranus: 5 major - 27 in total
    • Neptune: 1 major - 14 in total
  • They are mostly frozen rock.
  • Exceptions:
    • Io (Jupiter) - Lava flow
    • Titan (Saturn) - Atmosphere, Methane in Liquid form
    • Triton (Neptune) - Atmosphere

Ring System

  • All distances are expressed in planetary radii.
  • The red line represents the Roche limit, and all the rings lie within (or very close to) this limit of their parent planets. However, it is just an approximation

2. Jupiter (#5)

Jupiter’s Convection

The colored bands in Jupiter’s atmosphere are associated with vertical convective motion.

  • Upwelling warm gas results in zones of lighter color;
  • The darker bands overlie regions of lower pressure where cooler gas sinks back down into the atmosphere.

As on Earth, surface winds tend to blow from high- to low-pressure regions.

Jupiter’s rapid rotation channels those winds into an east–west flow pattern, as indicated by the three yellow-red arrows drawn atop the belts and zones.

Zonal Flow

The wind speed in Jupiter’s atmosphere, measured relative to the planet’s internal rotation rate. Alternations in wind direction are associated with the atmospheric band structure.

Jupiter's Rotation Period: ~ 10 hours

Atmosphere

  • Mostly molecular hydrogen and helium
  • Small amounts of methane, ammonia, and water vapor
  • Color is probably due to complex chemical interactions.


  • No solid surface; take top of troposphere to be at 0 km
  • Lowest cloud layer cannot be seen by optical telescopes
  • High wind speeds even at great depth (probably due to heating from planet, not from Sun)


  • Cloud color and cloud content reflect the chemistry of the layer

Great Red Spot

  • It has existed for at least 300 years, possibly much longer.
  • Color and energy source still not understood.
  • Complex turbulence to the left of both the Red Spot and the smaller white oval below it.

Bottom Left: The cyclic motion of the Great Red Spot, imaged by the Cassini spacecraft. Right Panel: Closeup detail of the Great Red Spot taken by NASA's Juno on 11 July 2017.

Red Spot Junior (a) Between 1997 and 2000, astronomers watched as three white ovals in Jupiter’s southern hemisphere merged to form a single large storm. Each oval is about half the size of Earth.

(b) In early 2006 the white oval turned red, producing a second red spot! The color change may indicate that the storm is intensifying.

Brown Oval A break in upper cloud layer (northern hemisphere). Deeper atmosphere looks brown. The oval’s length is approximately equal to Earth’s diameter.

Internal Structure

  • Jupiter radiates more energy than it receives from the Sun
    • Core is still cooling off from heating during gravitational compression.
  • Could Jupiter have been a star?
    • No; it is far too cool and too small for that.
    • It would need to be about 80 times more massive to be even a very faint star.
  • The density and temperature increase with depth, and the atmosphere gradually liquefies at a depth of a few thousand kilometers.
  • Below a depth of 20,000 km, the hydrogen behaves like a liquid metal.
  • At the center of the planet lies a large rocky core, somewhat terrestrial in composition.

Jupiter's Magnetosphere

  • It is 30 million km across.
  • The plasma torus, a ring of charged particles associated with the moon Io.
  • Intrinsic field strength is 20,000 times that of Earth.
  • It can extend beyond the orbit of Saturn

Ring System

  • It has a faint ring. It is photographed (nearly edge-on) by Voyager 2.
  • Made of dark fragments of rock and dust possibly chipped off the innermost moons by meteorites.
  • It lies in Jupiter’s equatorial plane, only 50,000 km above the cloud tops.

Galileon Moons

  • Four major moons:
    • Io
    • Europa
    • Ganymede
    • Callisto
  • They have similarities to terrestrial planets:
    • Orbits have low eccentricity
    • Density decreases as distance from Jupiter increases

Io. Its surface is kept smooth and brightly colored by the moon’s constant volcanism. Orange color is probably from sulfur compounds in the ejecta.

Io is very close to Jupiter and also experiences gravitational forces from Europa. The tidal forces are huge and provide the energy for the volcanoes.

The plume measures about 150 km high and 300 km across.

Europa. (a) It has no craters; surface is water ice, possibly with liquid water below.

Tidal forces stress and crack ice; water flows, keeping surface relatively flat.

(b) Europa’s icy surface is only lightly cratered, indicating that some ongoing process must be obliterating impact craters soon after they form. The origin of the cracks crisscrossing the surface is uncertain.

(c) This picture shows a smooth yet tangled surface resembling the huge ice floes that cover Earth’s polar regions. This region is called Conamara Chaos.

(d) This picture shows "pulled apart" terrain that suggests liquid water upwelling from the interior and freezing, filling in the gaps between separating surface ice sheets.


Ganymede

It is the largest moon in the solar system; larger than Pluto and Mercury

Its history similar to Earth’s Moon, but it contains water ice instead of lunar rock.

(a) and (b) The dark regions are the oldest parts of the moon’s surface and probably represent its original icy crust.

The lighter, younger regions are the result of flooding and freezing that occurred within a billion years or so of Ganymede’s formation. The light-colored spots are recent impact craters.

(c) Grooved terrain on Ganymede may have been caused by a process similar to plate tectonics on Earth.

The image suggests erosion of some sort, possibly even caused by water

Callisto

(a) It is similar to Ganymede in overall composition, but is more heavily cratered.

The large series of concentric ridges visible at left is known as Valhalla. Extending nearly 1500 km from the basin center, the ridges formed when "ripples" from a large meteoritic impact froze before they could disperse completely.

(b) This picture displays more clearly its heavy cratering.

3. Saturn (#6)

  • Density: 700 kg/m3 (less than water!)
  • Rotation: Rapid and differential, enough to flatten Saturn considerably
  • Rings: Very prominent; wide but extremely thin

Atmosphere

  • It also shows zone and band structure, but coloration is much more subdued than Jupiter’s
  • Mostly molecular hydrogen, helium, methane, and ammonia
  • Helium fraction is much less than on Jupiter


  • Similar to Jupiter’s, except pressure is lower
  • Three cloud layers
  • Saturn’s weaker gravity results in thicker clouds and a more uniform appearance. We see only top layer.
  • The colors shown are intended to represent Saturn’s visible-light appearance.

Saturn’s Zonal Flow Winds on Saturn reach speeds even greater than those on Jupiter. As on Jupiter, the visible bands appear to be associated with variations in wind speed.

Saturn Storms (a) Circulating and evolving cloud systems on Saturn, imaged at approximately 2-hour intervals. (b) This infrared image, displayed in false color:

  • Blue coloration indicates regions where the atmosphere is relatively free of haze;
  • Green and yellow indicate increasing levels of haze;
  • Red and orange indicate high-level clouds.
  • Two small storm systems near the equator appear whitish.

Dragon Storm It generated bursts of radio waves resembling the static created by lightning on Earth.

Polar Vortex It is at south pole. It has a well-developed eye wall and calm winds at its center.

The size of this vortex is slightly larger than the entire Earth, and its winds are about twice that of a category 5 hurricane on Earth

Interiors

  • Similar to Jupiter
  • It also radiates more energy than it gets from the Sun, but not because of cooling:
    • Helium and hydrogen are not well mixed
    • Helium tends to condense into droplets and then fall
    • Gravitational field compresses helium and heats it up


  • It has a strong magnetic field, but only 5% as strong as Jupiter’s
  • It creates aurorae

Ring System

  • Ring particles range in size from fractions of a millimeter to tens of meters.
  • Composition: Water ice—similar to snowballs
  • Why rings: Too close to planet for moon to form—tidal forces would tear it apart.


  • Details of formation are unknown:
    • Too active to have lasted since birth of solar system
    • Either must be continually replenished, or are the result of a catastrophic event
  • The ringlets in the B ring, spread over several thousand kilometers.
  • Note the large number of tiny ringlets visible in the main rings.
  • The inset is an overhead view of a portion of the B ring.

Spokes in the Rings Saturn’s B ring showed a series of dark temporary “spokes”. The spokes are caused by small particles suspended just above the ring plane.

Shepherd Moon (a) Saturn’s narrow F ring appears to contain kinks and braids. Its thinness can be explained by the effects of two shepherd satellites that orbit near the ring—one a few hundred kilometers inside, the other a similar distance outside.

(b) One of the potato-shaped shepherd satellites (Prometheus here roughly 100 km across) can be seen at the right of this enlarged view.

F Ring Structure Kinks, waves, and other substructure in the F ring can be seen. A back-and-forth dance of the shepherd moons gravitationally sculpts clumps in the core of the rings.

Moons

  • Many of them are water ice
  • Siz medium sized: Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus
  • One large: Titan

Titan

  • Larger than Mercury and roughly half the size of Earth.
  • In the infrared some large-scale surface features can be seen.
  • The bright regions are thought to be highlands, possibly covered with frozen methane; nearly 4000 km across.

The structure of Titan’s atmosphere. The solid blue line represents temperature at different altitudes. The inset shows the haze layers in Titan’s upper atmosphere, depicted in false-color green

Titan Revealed

  • Cassini’s infrared telescopes revealed this infrared, false-color view of Titan’s surface in late 2004.
  • The circular area near the center may be an old impact basin.
  • The dark linear feature to its northwest perhaps mountain ranges caused by ancient tectonic activity.
  • Image on the left shows a surface feature thought to be an icy volcano, further suggesting some geological activity on this icy moon’s surface.
  • Huygens’s view (right panel) of its landing site, in approximately true color. The foreground rocklike objects are only a few centimeters across.

Titan’s Interior It appears to be largely a rocky-silicate mix.

The subsurface layer of liquid water, similar to that hypothesized on Jupiter’s Europa and Ganymede.

4. Uranus (#7)

Uranus

  • Discovered in 1781 by Herschel
  • The first planet to be discovered in more than 2000 years.
  • Little detail can be seen from Earth
  • Left arrow: Triton, Right arrow: Nereid.


  • (Right panel) almost planet’s true color.
  • But shows virtually no detail in the nearly featureless upper atmosphere.
  • Exceptions: a few wispy clouds in the northern hemisphere.

Seasons on Uranus

  • Uranus’s axial tilt is 98°
  • Therefore the planet experiences the most extreme seasons known in the solar system.
  • Each year the equatorial regions have
    • Two "summers": warm seasons, around the times of the two equinoxes.
    • Two “winters”: cool seasons, at the solstices.
  • The poles are alternately plunged into darkness for 42 Earth years at a time.

5. Neptune (#8)

In the closer view, resolved to about 10 km, shows cloud streaks ranging in width from 50 km to 200 km.

Neptune’s Dark Spot. Similar in structure to Jupiter’s Great Red Spot. The entire dark spot is roughly the size of planet Earth.

The cloud features (mostly methane ice crystals) are tinted pink because they were imaged in the infrared, but they are really white in visible light. Note that the Great Dark Spot has disappeared in recent years.

Moons of Uranus and Neptune

Miranda (Uranus)

It has a strange, fractured surface suggestive of a violent past, but the cause of the grooves and cracks is currently unknown

Triton (Neptune)

  • It is in retrograde motion.
  • The south polar region: deep ridges and gashes
  • Lakes of frozen water, all indicative of past surface activity.
  • The pinkish region at lower right is nitrogen frost, forming the moon’s polar cap.
  • The long black streaks at bottom left were probably formed by geysers of liquid nitrogen on the surface.
  • (Right panel) This lake like structure may have been caused by the eruption of an ice volcano. The water “lava” has since solidified, leaving a smooth surface. The absence of craters implies that this eruption was a relatively recent event.