Jupiter and SaturnChapter 8

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 and 11, Voyager 1 and 2,

Galileo,Juno arrives July ’16

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

Visual image

Infrared false-color image

The Mass of JupiterMass can be inferred from 2 things: 1)The orbit of Io,

which is the innermost of the 4 Galilean Moons:

EarthJupiter

Moon

Io

Relative sizes, and distances, to scale

2)Using Kepler’s third law MJupiter = 318 MEarth

Jupiter’s InteriorFrom radius and mass Average density of Jupiter ≈ 1.34 g/cm3

=> Jupiter can not be made mostly of rock, like earthlike planets.

Jupiter consists mostly of hydrogen and helium.

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

T ~ 20,000 K and 1x108 the pressure of Earth

The Chemical Composition of Jupiter and Saturn

Jupiter’s RotationJupiter 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. And the rapid spin with a large metallic hydrogen interior creates a HUGE magnetic field

Jupiter’s Magnetic FieldDiscovered through observations of decimeter (radio) radiation

Magnetic field is nearly 20,000 times stronger than Earth’s magnetic field.

Magnetosphere over 100 times larger than Earth’s.

Extremely intense radiation belts: Very high energy particles can be trapped and surround the rings and inner moons. That means radiation enough to kill unprotected humans instantly

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. The particles causing them are thought to come from the inner moons, not the Sun as on Earth

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

Inclination of Jupiter’s magnetic field against rotation axis leads to wobbling field structure passing over Io

Acceleration of particles to high energies.

Visible UV

Io flux tube Io 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 hydrogen atmosphere above cloud layers.

Only very thin atmosphere above cloud layers;

transition to liquid hydrogen zone ~ 1000 km below clouds.

Galileo probe mission

Jupiter’s Atmosphere (2): CloudsThree layers

of clouds:

1. Ammonia (NH3) crystals

3. Water crystals

2. Ammonia hydrosulfide

The Cloud Belts on Jupiter

Dark belts and bright zones.

Zones higher and cooler than belts; 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 basically unchanged since humans began mapping them in

1600’s.

The Great Red Spot

Several bright and dark spots mixed in with cloud structure.

Largest and most prominent: The Great Red Spot.

~ 2 DEarth

Has been visible for over 350 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. 3 parts: Main, Halo(closer to J), and Gossamer

Main ring ~6400 km wide ~123,000 km from J Halo is ~23,000 km wide ~ 100,000 km from J

Gossamer is ~85,000 km wide outside of the main ring

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

The Roche limit is a distance, the minimum distance that a smaller object (e.g. a moon) can exist, as a body held together by its self-gravity, as it orbits a more massive body (e.g. its parent planet); closer in, and the smaller body is ripped to pieces by the tidal forces on it.Read more: http://www.universetoday.com/56538/roche-limit/#ixzz2DXWFSN5Q

Image of Uranus and its rings with moons.

Image credit: Hubble

Comet Impact on Jupiter

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

bomb: ~ 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.

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 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 Io and Europa

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

Jupiter’s Family of MoonsOver five dozen moons known now; 50 are named,

with another 17 still to be named the latest 2 just discovered in 2011.

Four largest moons discovered by Galileo in 1610: They are now called Galilean moons

Io Europa Ganymede Callisto

Interesting and diverse individual geologies.

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

Av. density: 1.86 g/cm3

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.

Ganymede: A Hidden PastLargest of the 4 Galilean moons. Largest in SS and

even larger than Mercury• Av. density = 1.9 g/cm3

• Iron core = Mag. field • Rocky 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

Europa: A Hidden OceanAv. density: 3 g/cm3

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.

Core not molten No magnetic field.

Europa has a liquid water

ocean ~ 15 km below the icy

surface.

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

Over 100 active volcanoes!

Activity powered by tidal interactions

with Jupiter.

Av. density = 3.55 g/cm3 Interior is mostly rock.

Interaction with Jupiter’s Magnetosphere

Io’s volcanoes blow out sulfur-rich gasses

tenuous 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, ~32 of them and they mostly have retrograde motions.• Galilean and other ‘inner’ moons probably formed together with Jupiter.• Densities decreasing outward Probably formed in a disk around Jupiter, similar to planets around the sun.

Simulations suggest earlier generations of moons around Jupiter may have been lost and spiraled into Jupiter;

Today’s Galilean moons are possibly a fifth generation of moons.

Saturn

Av. density: 0.69 g/cm3 Would float in water!

Most distant of the planets you can see unaided. In 1610 Galileo saw the rings but drew them as handle-like. In 1659 Christian Huygens said it was a thin flat ring that surrounded Saturn. And in 1675 Dominique Cassini saw that there was a gap in the rings (we now call it the Cassini Division) between the A and B set of rings

Cassini Division

SaturnMass: ~ 1/3 of mass of Jupiter

Radius: ~ 16 % smaller than Jupiter

Av. density: 0.69 g/cm3 Would float in water!

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

Mostly hydrogen and helium; liquid hydrogen core.

Saturn radiates ~ 1.8 times the energy received from the sun.

Probably heated by liquid helium droplets falling towards center.

Saturn’s MagnetosphereMagnetic field ~ 20 times weaker than Jupiter’s

weaker radiation belts

Magnetic field not inclined against

rotation axis

Aurorae centered around poles of

rotation

Saturn’s Atmosphere

Cloud-belt structure, formed through the same processes as on Jupiter,

but not as distinct as on Jupiter; colder than on Jupiter.

Saturn’s Atmosphere (2)

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 RingsRing consists of 3 main segments: A, B, and C Ring and fainter D,E,F, and Gseparated by empty regions: divisions/gaps

A Ring

B Ring

C Ring

Cassini Division

Rings must be replenished by fragments of

passing comets & meteoroids.

Rings can’t have been formed together with Saturn because material would have been blown away by particle stream from hot Saturn at time of formation.

Composition of Saturn’s Rings

Rings are composed of mainly ice

particles, believed to be from shattered

comets/asteroids, or moons.

Moving at large velocities around Saturn, but small

relative velocities (all moving in the same direction). They vary in size from tiny dust

sized ice grains to Mt. sized

Shepherd Moons

Some moons on orbits close to the rings focus the ring material, keeping the rings confined. And two moons even travel in gaps

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

Electromagnetic Phenomena in Saturn’s Rings

Radial spokes in the rings rotate with

the rotation period of Saturn:

Magnetized ring particles lifted out of the ring plane

and rotating along with the magnetic-field

structure

Titan• About the size of Jupiter’s moon Ganymede (but slightly smaller).

• Rocky core, but also large amount of ice.

• Thick hazy atmosphere, hiding the surface from direct view.

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: ~60% greater than air pressure on Earth

Surface temperature: 94 K (-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; heavily cratered and appear geologically dead. Over 60 total moons

Tethys:

Heavily cratered; 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; regions with fewer craters, containing parallel

grooves, possibly filled with frozen water.

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

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

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.

oblatenessliquid metallic hydrogenIo plasma torusIo flux tubebeltzoneforward 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