Jupiter and SaturnChapter 23

As we begin this chapter, we leave behind the psychological security of planetary surfaces. We can imagine standing on the moon, on Venus, or on Mars, but Jupiter and Saturn have no surfaces. Thus, we face a new challenge—to use comparative planetology to study worlds so unearthly we cannot imagine being there. One reason we find the moon and Mars of interest is that we might go there someday. Humans may become the first Martians. But the outer solar system seems much less useful, and that gives us a chance to think about the cultural value of science. This chapter begins our journey into the outer solar system. In the next chapter, we will visit worlds out in the twilight at the edge of the sun’s family.

Guidepost

I. JupiterA. Surveying JupiterB. Jupiter's Magnetic FieldsC. Jupiter's AtmosphereD. Jupiter's RingE. Comet Impact on JupiterF. The History of Jupiter

II. Jupiter's Family of MoonsA. Callisto: The Ancient FaceB. Ganymede: A Hidden PastC. Europa: A Hidden OceanD. Io: Bursting EnergyE. The History of the Galilean Moons

Outline

III. SaturnA. Planet SaturnB. Saturn's RingsC. The History of Saturn

IV. Saturn's MoonsA. TitanB. The Smaller MoonsC. The Origin of Saturn's Satellites

Outline (continued)

JupiterLargest and most massive planet in the solar system:Contains almost 3/4 of all planetary matter in the solar system.

Explored in detail by several space probes:

Pioneer 10 (1973), Pioneer 11 (1974), Voyager 1 (1979), Voyager 2 (1979),

Galileo (1995-2003)

Most striking features visible from Earth are the multi-colored cloud belts

Visual image

Infrared false-color image

The Mass of JupiterMass can be determined from the orbit of any of

the innermost four Galilean moons

EarthJupiter

MoonIo

Relative sizes and distances are to scale

Using Kepler’s third law: MJupiter = 318 MEarth

27.3 day orbital period

1.8 day orbital period

click for orbital animation

Jupiter’s InteriorFrom volume and mass, average density of Jupiter is 1.34 g/cm3 [compare to Mercury (5.44), Venus (5.24), Earth (5.50), Mars (3.94)]

Therefore, Jupiter cannot be made mostly of rock, like earthlike planets, but consists mostly of hydrogen and helium.

Due to the high pressure, hydrogen is compressed into a liquid, and even metallic state.

T = 30,000 K(hotter than sun’s surface!)

The Chemical Composition of Jupiter and Saturn

Hydrogen gas Helium gas Water/ice

Methane

Ammonia

Jupiter’s Rotation

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

Rotation period slightly less than 10 hours.

Centrifugal forces stretch Jupiter into a oblate shape (like an M&M).

Jupiter’s Magnetic FieldDiscovered by observations of radio waves and microwaves

Magnetic field at least 10 times stronger than Earth’s magnetic field.

Magnetosphere over 100 times larger than Earth’s.

Intense radiation belts trap very high energy particles (electrons and protons).

Radiation doses are 100 times lethal amount for humans!

Auroras on JupiterJust like on Earth, Jupiter’s magnetosphere produces auroras concentrated in rings around the magnetic poles.

They are 1000 times more powerful than auroras on Earth!

Explorable Jupiter

(SLIDESHOW MODE ONLY)

The Io Plasma TorusSome of the heavier ions originate from Jupiter’s moon Io.

Jupiter’s magnetic field sweeps past Io, creating a donut-shaped plasma torus (donut-shaped ring of ions). Electrical current (over 1 million amps!) flows along the flux tube, creating bright spots in the aurora.

plasma torus flux tube

Jupiter’s AtmosphereJupiter’s liquid hydrogen ocean has no surface:

Gradual transition from gaseous to liquid phases as temperature and pressure combine to exceed the critical point.

Jupiter shows limb darkening, so hydrogen atmosphere exists above cloud layers.

Only very thin atmosphere above cloud layers.

Transition to liquid hydrogen zone about 1000 km below clouds.

Jupiter’s Atmosphere (2): CloudsThree layers

of clouds:

1. Ammonia (NH3) crystals

3. Water crystals

2. Ammonia hydrosulfide

(NH4SH)

Planetary Atmospheres

(SLIDESHOW MODE ONLY)

The Cloud Belts on JupiterDark belts and bright zones.

Zones are higher and cooler than belts since they are high-pressure regions of rising gas.

The Cloud Belts on Jupiter (2)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.

The Great Red SpotSeveral bright and dark spots mixed in with cloud structure.Largest and most prominent is the The Great Red Spot.

~ 2 DEarth

It has been visible for over 330 years.

Formed by rising gas carrying heat from below the clouds, creating a vast, rotating storm.

The Great Red Spot (2)

Structure of Great Red Spot may be determined by circulation patterns in the liquid interior

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.

Ring material can’t be old because radiation pressure and Jupiter’s

magnetic field force dust particles to spiral down

into the planet. Rings must be constantly re-supplied with new dust.

Roche Limit

(SLIDESHOW MODE ONLY)

Comet Impact on Jupiter

Impacts released energies equivalent to a few megatons of TNT (Hiroshima

bomb was 0.15 megaton)!

Visual: Impacts seen for many days as dark spots

Impact of 21 fragments of comet SL-9 in 1994

Impacts occurred

just behind the horizon

as seen from Earth, but came into view about 15 min. later.

Impact sites appeared

very bright in the infrared.

video clip

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

causes strong magnetic field

• Rapid rotation and

large size cause belt-zone cloud

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

video clip

Jupiter’s Family of MoonsOver five dozen moons known now and

new ones are still being discovered!

Four largest moons discovered by Galileo in 1610 are called the Galilean moons

Io Europa Ganymede Callisto

Each moon has interesting and diverse individual geologies.

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

Density is 1.8 g/cm3

Composition is mixture of ice and rocks

Dark surface, heavily pocked with craters.

No metallic core because it never differentiated to form core and mantle.

No magnetic field.

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

Ganymede: A Hidden PastLargest of the all moons in the solar system.

• Density is 1.9 g/cm3

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

1/3 of surface old, dark, cratered. 2/3 is bright, young, grooved terrain.

Bright terrain probably formed through flooding

when surface broke

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 Gany-mede

2. Pull of gravity on meteoroids, exposing nearby satellites to more impacts than those further out.

Europa: A Hidden OceanDensity is 3,0 g/cm3

Composition is mostly rock and metal with icy surface.

Close to Jupiter so should be hit by many meteoroid impacts, but few craters visible. Why?

It has an active surface, so impact craters rapidly erased.

The Surface of Europa

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

The Interior of EuropaEuropa is too small to retain its internal heat. Heating mostly from tidal interaction with Jupiter.

Core not molten so No magnetic field.

Liquid water ocean 15 km below the icy surface.

Io: Bursting EnergyMost active of all Galilean moons, with no impact craters visible.

Over 100 active volcanoes!

Geologic activity powered by tidal

interactions (heating) with

Jupiter.

Density is 3.6 g/cm3, so interior is mostly rock.

Interaction with Jupiter’s Magnetosphere

Io’s volcanoes blow out sulfur-rich gasses

Io has a weak atmosphere, but gasses can not be retained by

Io’s gravity

Gasses escape from Io and form

an ion torus in Jupiter’s

magnetosphere

The History of the Galilean Moons• Minor moons are probably captured asteroids• Galilean moons probably formed together with Jupiter.

• Moon densities decreasing outward – moons probably formed in a “mini solar nebular disk around Jupiter, similar to how the planets formed around the sun.

Galilean moons are probably a second generation of moons (earlier moons spiraled into Jupiter.

Io, Europa, and Ganymede are in orbital resonance with 1:2:4 ratio of periods

orbit animation

SaturnMass is 1/3 of mass of Jupiter

Radius is 16 % smaller than Jupiter

Density: 0.69 g/cm3 So low it would float in water!

• Rotates about as fast as Jupiter, in 10 hr 40 min, but is twice as oblate since it has no large core of heavy elements.

• Mostly hydrogen and helium with liquid hydrogen core.

• Saturn radiates 1.8 times the energy received from the sun. • Probably heated by liquid helium droplets falling towards center,

similar to how sun heats while it contracts.

Saturn’s MagnetosphereSaturn’s magnetic field:

• has weaker radiation belts

• not inclined (tilted) to rotation axis

• Auroras are centered around poles of rotation

• is 20 times weaker than Jupiter’s

• driven by dynamo effect

Saturn’s AtmosphereHas zone-belt structure, formed through

the same processes as on Jupiter,

but not as distinct and colder than on Jupiter since Saturn is

twice as far from the Sun.

Saturn’s Atmosphere (2)

Three-layered cloud structure, just like on Jupiter

Main difference to Jupiter is fewer wind zones, but much stronger winds than on Jupiter. Winds up to 1100 mph near the equator!

Saturn’s RingsRing consists of 3 main segments: A, B, and C ring separated by empty regions called divisions.

A Ring

B Ring

C Ring

Cassini Division

Rings must be replenished by fragments of

passing comets & meteoroids.

Rings did not form with Saturn because ice material would have been heated at the time of formation.

Composition of Saturn’s Rings

Rings are composed of ice particles moving at large but equal speeds around Saturn, so the astronaut shown here could “swim” through the ring.

Shepherd Moons

Some moons on orbits close to the rings focus the ring material, keeping the rings confined.

Divisions and Resonances

• Some moons act as “shepherds” that “herd” material into rings with gravitational pull.

• Moons can also create divisions (gaps) when the orbital period of a moon is a small ratio of the orbital period of material in the disk (for example “2:3 resonance”).

Titan• About the size of Jupiter’s

moon Ganymede. • Rocky core, but also large

amount of ice.• Thick atmosphere, hiding

the surface from direct view.

video clip video clip

Titan’s AtmosphereBecause of the thick, hazy atmosphere, surface features are only visible in infrared images. Many of the organic compounds in Titan’s atmosphere may have been precursors of life on Earth.

Surface pressure is 50% greater than air pressure on Earth

Surface temperature is -290 oF

Methane and ethane are liquid!

Methane is gradually converted to ethane in the atmosphere

Methane must be constantly replenished, probably through breakdown of ammonia (NH3).

Saturn’s Smaller MoonsSaturn’s smaller moons, formed of rock and ice, are heavily cratered and appear geologically dead.

Tethys

Heavily cratered and marked by 3 km deep, 1500 km long crack.

Iapetus

Leading (upper right) side darker

than rest of surface because of dark deposits.

Enceladus

Possibly active with regions of fewer craters,

containing parallel grooves, possibly filled

with frozen water.

Saturn’s Smaller Moons (2)Hyperion is too small to pull itself into spherical shape.

All other known moons are large enough to attain a spherical shape.

video clip

The Origin of Saturn’s Satellites• No evidence of common

origin, as for Jupiter’s moons.

• Probably captured icy planetesimals.

• Moons interact gravitationally, mutually affecting each other’s orbits.

• Co-orbital moons (orbits separated by only 100 km) periodically exchange orbits!

• Small moons are also trapped in Lagrange points of larger moons Dione and Tethys.

Coorbital Moons

(SLIDESHOW MODE ONLY)

oblatenessliquid metallic hydrogendecameter radiationdecimeter radiationcurrent sheetIo plasma torusIo flux tubecritical pointbeltzoneforward scatteringRoche limitgossamer ringsgrooved terraintidal heatingshepherd satellitespoke 

New Terms

1. Some astronomers argue that Jupiter and Saturn are unusual, while other astronomers argue that all solar systems should contain one or two such giant planets. What do you think? Support your argument with evidence.

2. Why don’t the terrestrial planets have rings?

Discussion Questions