The Jovian Planets

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The Jovian Planets (“Gas Giants”)Animation 1

Jupiter

Saturn

Uranus Neptune

Animation 2

No well-defined surface!

Interior Structure

The Chemical Compositionof Jupiter and Saturn

JupiterLargest and most massive planet in the solar system:

Contains almost ¾ of all planetary matter in the solar system.

Explored in detail by several space probes:

Pioneer 10, Pioneer 11, Voyager 1, Voyager 2,

Galileo

Most striking features visible from Earth: Multi-colored cloud belts

Visual image

Infrared false-color image

The Great Red Spot

8-year sequence of images of the Great Red Spot on Jupiter

Has been visible for over 300 years

Giant storm system similar to Hurricanes on Earth: Wind speeds of 430 km/h (= 270 miles/h)

Changes appearance gradually over time

Jupiter’s Rotation

Jupiter is the most rapidly rotating planet in the solar system:

Rotation period slightly less than 10 hr.

Considering the rapid rotation of Jupiter, would you expect it to be perfectly round?

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1. Yes.

2. No, it should be oblate.

3. No, it should be prolate.

Jupiter’s Rotation

Jupiter is the most rapidly rotating planet in the solar system:

Rotation period slightly less than 10 hr.

Centrifugal forces stretch Jupiter into a

markedly oblate shape.

Jupiter’s Atmosphere

Jupiter’s liquid hydrogen ocean has no surface:

Gradual transition from gaseous to liquid phases.

Only very thin atmosphere above cloud layers;

transition to liquid hydrogen zone ~ 1000 km below clouds.

Jupiter’s Atmosphere (II)

Three layers of clouds:

1. Ammonia (NH3)

crystals

2. Ammonia hydrosulfide

(NH4SH)

3. Water crystals

Would you think that the atmosphere of Jupiter gets warmer towards lower altitudes

because of the greenhouse effect?

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1. Yes: The atmosphere contains many greenhouse gases.

2. No, because of the dense cloud cover that reflects most of the sunlight.

3. No, because there’s no surface to absorb visible light and re-emit in infrared.

4. Both 2. and 3. are correct.

Jupiter’s Atmosphere (II)

Three layers of clouds:

1. Ammonia (NH3)

crystals

2. Ammonia hydrosulfide

(NH4SH)

3. Water crystals

Heating mostly from latent, internal heat

The Cloud Belts of JupiterDark belts and bright zones.

Zones higher and cooler than belts; high-pressure regions of rising gas.

The circulation patterns in the cloud belts are physically similar to which

phenomenon on the sun?

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1. Sun spots.

2. Prominences.

3. Granulation (sub-photospheric convection).

4. Solar wind.

5. Coronal Mass Ejections.

The Cloud Belts on Jupiter (II)Just like on Earth, high-and low-pressure zones

are bounded by high-pressure winds.

Jupiter’s Cloud belt structure has remained unchanged since humans began mapping them.

Jupiter’s Magnetic FieldMagnetic field at least 10 times stronger than

Earth’s magnetic field.

Magnetosphere over 100 times larger than Earth’s magnetosphere

Which factor might be the most important for Jupiter’s strong magnetic field?

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1. Its thick cloud belts.

2. A large liquid iron core.

3. A large solid iron core.

4. A large region of metallic hydrogen.

5. Its rapid rotation.

Aurorae on JupiterJust like on Earth,

Jupiter’s magnetosphere

produces aurorae concentrated in rings around the magnetic poles.

~ 1000 times more powerful

than aurorae on Earth.

At the location of Jupiter, is the flux of solar-wind particles (from the sun) stronger or weaker than at Earth?

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1. Stronger.

2. About the same as on Earth.

3. Weaker.

Aurorae on JupiterJust like on Earth,

Jupiter’s magnetosphere

produces aurorae concentrated in rings around the magnetic poles.

~ 1000 times more powerful

than aurorae on Earth.

Particles producing the aurorae originate

mostly from moon Io

The History of Jupiter• Formed from cold gas in the

outer solar nebula, where

ices were able to condense.

• Rapid growth

• Soon able to trap gas directly through gravity

• Heavy materials sink to the center

• In the interior, hydrogen becomes metallic (very good

electrical conductor)

• Rapid rotation → strong

magnetic field

• Rapid rotation and large size → belt-zone cloud pattern

• Dust from meteorite impacts onto inner moons trapped to form ring

Jupiter’s RingNot only Saturn, but all four gas giants have rings.

Jupiter’s ring: dark and reddish; only

discovered by Voyager 1

spacecraft.

Galileo spacecraft image of Jupiter’s ring, illuminated from behind

Composed of microscopic particles of rocky material

Location: Inside Roche limit, where larger bodies (moons) would be destroyed by tidal forces.

Who discovered the largest moons of Jupiter?

1) 2) 3) 4) 5)

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1) Ptolemy

2) Galilei

3) Copernicus

4) Kepler

5) Newton

Jupiter’s MoonsAbout 2 dozen moons known at this time, but

more are being discovered …

Largest and most prominent: 4 Galilean Moons (already discovered by Galilei in 1610):

Io EuropaGanymede Callisto

What happens to a soft rubber ball if you keep squishing and releasing it?

1) 2) 3)

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1) It will get gradually colder.

2) It will get gradually warmer.

3) None of the above.

Jupiter’s Influence on its MoonsPresence of Jupiter has at least two

effects on geology of its moons:

1. Tidal effects: possible source of heat for interior of Ganymede

2. Focusing of meteoroids, exposing nearby satellites to more impacts than those further out.

Io: Bursting EnergyMost active of all Galilean moons; no impact craters visible at all.

Over 100 active volcanoes!

Interior is mostly rock.

Activity powered by tidal interactions

with Jupiter.

Interactions with Jupiter’s Magnetosphere

Io’s volcanoes blow out sulfur-rich gases

→ tenuous atmosphere, but gases can not be retained by Io’s gravity

→ gases escape from Io and form an ion torus in Jupiter’s magnetosphere.

→ Aurorae on Jupiter are fueled by particles from Io

Europa: A Hidden Ocean

composition: mostly rock and metal; icy surface.

Close to Jupiter → should be hit by many meteoroid impacts; but

few craters visible.

→ Active surface; impact craters rapidly erased.

The Surface of Europa

Cracked surface and high albedo (reflectivity) provide further evidence for geological activity.

The Interior of EuropaEuropa is too small to retain its internal heat →

Heating mostly from tidal interaction with Jupiter.

Europa has a liquid water ocean ~ 15 km below the icy surface.

Would there, in principle, be life possible in the sub-surface ocean of Europa?

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1. Yes.

2. No, because there’s no light which is necessary for plant life.

3. No, because it’s too cold.

4. No, because it’s too warm.

But it would have to be able to make use of heat provided by internal heat from the planet.

Ganymede: A Hidden PastLargest of the 4 Galilean moons.

• Rocky core • Ice-rich mantle • Crust of ice

1/3 of surface old, dark, cratered; rest: bright, young, grooved terrain

Bright terrain probably formed through flooding

when surface broke

Callisto: The Ancient FaceTidally locked to Jupiter, like all of Jupiter’s moons.

composition: mixture of ice and rocks

Dark surface, heavily pocked with craters.

No metallic core: Callisto never differentiated to form core and mantle.

→ No magnetic field.

Layer of liquid water, ~ 10 km thick, ~ 100 km below surface, probably heated by radioactive decay.

Saturn2nd largest planet; about 16 % smaller than Jupiter.

1/3 of Jupiter’s mass; density

less than water: Saturn would

float in water!!

Atmospheric cloud structures similar to Jupiter’s, but less obvious.

Rare, isolated storm systems.

Rotates about as fast as Jupiter, but is twice as oblate → No large core of heavy elements.

Saturn2nd largest planet; about 16 % smaller than Jupiter.

1/3 of Jupiter’s mass; density

less than water: Saturn would

float in water!!

Atmospheric cloud structures similar to Jupiter’s, but less obvious.

Rare, isolated storm systems.

Rotates about as fast as Jupiter, but is twice as oblate → No large core of heavy elements.

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Saturn’s Atmosphere

Three-layered cloud structure,

just like on Jupiter

Main difference to Jupiter:

Fewer wind zones, but much stronger winds than on Jupiter:

Winds up to ~ 500 m/s near the equator!

Saturn’s Magnetosphere

Magnetic field ~ 20 times weaker than

Jupiter’s

Aurorae centered around poles

Saturn’s Rings

Consisting of ice particles, from microscopic to

several meters in size.

Several sub-divisions:

A-ring

B-ring

C-ring

Cassini Division

Why did the material in Saturn’s rings not clump together to form another moon of Saturn?

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1. It was too cold to melt and clump together to form a moon.

2. It is mostly ice, which doesn’t stick together.

3. Tidal forces of the nearby planet kept pulling rocks apart.

4. It is too far away from Saturn, so it is not gravitationally bound to the planet.

Shepherd Moons

Some moons on orbits close to the

rings focus the ring material, keeping

the rings confined.

What will happen to a small piece of rock orbiting Saturn, if its orbital period is exactly twice the orbital

period of a moon that is orbiting further out?

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1. Nothing special.

2. It will be destroyed by tidal forces.

3. It will gradually be pushed inward and fall onto Saturn.

4. It will gradually be pulled outward, away from Saturn.

Divisions and Resonances

Moons do not only serve as “Shepherds”.

Where the orbital period of a moon is a small-number fractional multiple (e.g., 2:3) of the orbital period of material

in the disk (“resonance”), the material is cleared out

→ Divisions

The Moons of Saturn

Largest moon:

Titan: The only moon in the solar system with an atmosphere:

Mostly Nitrogen, but also traces of organic compounds like

Methane and Ethane

Uranus

Hubble-Space-Telescope image (rings artificially enhanced)

4 times larger than Earth (in radius)

14.5 times the mass of Earth

Faint rings, indirectly detected through

occultations

Discovered in 1781

Rotation axis inclined by 970

against the orbital axis: Uranus is “rolling” on its orbit

The Rings of UranusRings of Uranus and Neptune are similar to Jupiter’s rings.

Confined by shepherd moons; consist of dark material.

Rings of Uranus were discovered through

occultations of a background star

Apparent motion of star behind Uranus

and rings

The Atmosphere of UranusLike other gas giants: No surface.

Gradual transition from gas phase to fluid interior.

Mostly H; 15 % He, a few % Methane, ammonia and water vapor.

Optical view from Earth: Blue color of methane

Cloud structures only visible after artificial computer enhancement of optical images taken

from Voyager spacecraft.

The Structure of Uranus’ Atmosphere

Only one layer of Methane clouds (in

contrast to 3 cloud layers on Jupiter and Saturn).

Cloud layer is very deep in Uranus’

atmosphere.

Uranus’ cloud layer difficult to see

because of thick atmosphere above it.

The Interior of UranusAverage density ≈ 1.29 g/cm3 → larger portion

of rock and ice than Jupiter and Saturn.

Ices of water, methane, and

ammonia, mixed with

hydrogen and silicates

Is the Earth’s magnetic north pole exactly in line with the north pole of its rotation?

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1. Yes.

2. No, the magnetic north pole is slightly offset (in the direction of Canada).

3. No, it is substantially offset (near central Europe).

4. No, it is completely independent of the rotation: The magnetic north pole is actually on the Equator.

The Magnetic Field of UranusNo metallic core → no magnetic field was expected.

But actually, magnetic field of ~ 75 % of Earth’s magnetic field strength was discovered:

Offset from center: ~ 30 % of planet’s radius!

Inclined by ~ 60o against axis of rotation.

Possibly due to dynamo in liquid-water/ammonia/methane solution in Uranus’

interior.

The Moons of UranusOberon

Umbriel Ariel

Miranda

Titania

D = 1550 km

480 km

1610 km

1190 km1160 km

Would you expect the moons of Uranus to be volcanically active?

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1. Yes, because their interiors should be very hot due to tidal interactions with Uranus.

2. Yes, because their interiors should be very hot due to radioactive decay of heavy elements.

3. Yes, because they might be heavily bombarded by meteorites.

4. No, because they might be very cold and have a very deep crust and solid mantle region.

The Moons of UranusOberon

Umbriel Ariel

Miranda

Titania

D = 1550 km

480 km

1610 km

1190 km1160 km

Mostly inactive; heavily cratered surfaces.

Neptune

Discovered in 1846 at position predicted from gravitational

disturbances on Uranus’ orbit.

Why does Neptune appear blue?

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1. It’s the natural color of hydrogen in the Neptune’s atmosphere.

2. It’s the natural color of methane in the Neptune’s atmosphere.

3. It’s the natural color of ammonia in the Neptune’s atmosphere.

4. Because of large-angle scattering of sunlight towards Earth (the same effect that makes our sky appear blue).

Neptune

Discovered in 1846 at position predicted from gravitational

disturbances on Uranus’ orbit.

Blue-green color from methane in the atmosphere

4 times Earth’s diameter; 4 % smaller than Uranus

The Atmosphere of Neptune

Cloud-belt structure with high-velocity winds; origin not well understood.

Darker cyclonic disturbances, similar to Great Red Spot on Jupiter, but not long-lived.

The “Great Dark Spot”

White cloud features of methane ice crystals

The Rings of Neptune

Made of dark material, visible in forward-

scattered light.

Interrupted between denser segments (arcs)

Focused by small shepherd moons embedded in the

ring structure.

Ring material must be regularly re-supplied

by dust from meteorite impacts on the moons.

When viewed from the North, all planets and moons are orbiting …

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1. Clockwise.2. Counterclockwise.3. All counterclockwise, except for Venus

which orbits clockwise.4. Some clockwise, some

counterclockwise. There’s no preferred sense of orbital motion.

5. Planets are orbiting clockwise around the sun; moons are orbiting counterclockwise around their planets.

The Moons of NeptuneTwo moons (Triton and Nereid) visible from Earth;

6 more discovered by Voyager 2

Unusual orbits:

Triton: Only satellite in the solar system

orbiting clockwise, i.e. “backward”.

Nereid: Highly eccentric orbit; very long orbital period (359.4 d).

The Surface of Triton

→ Triton can hold a tenuous atmosphere of

nitrogen and some methane; 105 times less

dense than Earth’s atmosphere.

Very low temperature (34.5 K)

Surface composed of ices: nitrogen, methane, carbon monoxide, carbon dioxide.

Possibly cyclic nitrogen ice deposition and re-vaporizing on

Triton’s south pole, similar to CO2 ice polar cap cycles on Mars.

The Surface of Triton (II)

Ongoing surface activity: Surface

features probably not more than 100 million years old.

Large basins might have been flooded multiple times by liquids from the interior.

Pluto and Charon

Hubble-Space-Telescope image

Size of Earth to scale

Pluto: discovered 1930

Average distance from the sun: ≈ 40 AU

Orbit highly inclined against plane of the solar system

Most eccentric planetary orbit in the solar system; part of

orbit is inside Neptune’s orbit!

Moon Charon about half the size of Pluto; occasional

eclipses of Pluto and

Charon

Would you think that Pluto and Charon have a similar history as the four gas giants?

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1. Yes, because they are in the same region of the solar system.

2. Yes, because they are very similar in size and composition to the gas giants.

3. Both 1. and 2. are true.

4. No, because they are very different in size and composition from the gas giants.

5. No, because they have very unusual orbits.

6. Both 4. and 5. are true.

The Origin of Pluto and CharonProbably very different history than

neighboring Jovian planets.

Modern theory: Pluto and Charon members of Kuiper belt of small, icy objects.

Collision between Pluto and Charon may have caused the peculiar orbital patterns and large

inclination of Pluto’s rotation axis.

The Tenth “Planet”Discovered on Jan. 8, 2005

• Bigger than Pluto

• Currently 97 AU from the sun (3 times further than Pluto)

• Composition and surface structure probably similar to Pluto’s