The Gas GiantsThe Gas Giants

The Gas GiantsThe Gas Giants

Jupiter Saturn Uranus Neptune

Mass (MEarth) 317.8 95.2 14.4 17.2

Distance from Sun (AU) 5.20 9.54 19.2 30.1

Equatorial Radius (REarth) 11.2 9.46 3.98 3.81

Average Density (kg/m3) 1330 710 1240 1670

Together they account for 99.5% of our planetary system

Size-Mass RelationshipSize-Mass Relationship•As with terrestrials, composition can be guessed from mean

density. But the high compressibility of volatiles must be accounted for

Initially, as they accrete mass they grow in radius But at a mass of ~300 earth masses, further accretion causes

the radius to decrease.

CompositionComposition

•Jupiter and Saturn are well matched by H+He models

Require some ice/rock as well

•Uranus and Neptune are much better represented by an icy composition.

T=0 K modelsT>0K

1

Recall: Moment of InertiaRecall: Moment of Inertia

•The moment of inertia is a measure of degree of concentration

Related to the “inertia” (resistance) of a spinning body to external torques

Shows giant planets are centrally concentrated: cores?

Body I/MR2

Sun 0.06

Mercury 0.33

Venus 0.33

Earth 0.33

Moon 0.393

Mars 0.366

Jupiter 0.254

Saturn 0.210

Uranus 0.23

Neptune 0.23

masspoint afor 0

shell hollow afor 670

sphere shomogeneou afor 4.0

2.

MR

I

ShapesShapes•Rotation induces

significant flattening of compressible material

Body a/(a-b) Rotation Period (d)

Sun 10000 25.4

Earth 298 1.0

Jupiter 16 0.41

Saturn 10 0.426

Heat BalanceHeat Balance

• The average temperature of Jupiter is 160 K. Is it in thermal equilibrium? (Assume a visible albedo of 0.43, but a perfect blackbody in the infrared).

• An infrared picture of Jupiter

AtmospheresAtmospheres

•Similar thermal structure to terrestrial planets

•Temperature of Jupiter and Saturn is never low enough to form a methane cloud deck – which dominates Neptune and Uranus

H at high temperatures and pressuresH at high temperatures and pressures

• Atmosphere of Jupiter and Saturn is mostly “liquid” H2.

• At very high pressures, H atoms dissociate from each other and their electrons

Forms a metallic liquid Good conductor

STP

Maximum of Earth-based laboratory experiments

InteriorsInteriors

•Jupiter and Saturn are dominated by an atmosphere of fluid, metallic hydrogen

•Neptune and Uranus are dominated by an icy mantle, probably as a fluid, conducting ocean, surrounded by a H and He atmosphere

Cloud patternsCloud patterns

• Surface features are due to forms and colourations of the highest cloud layers

MagnetospheresMagnetospheres

•The giant planets have strong magnetic fields Likely due to the convective, metallic hydrogen interior Interact with solar wind (and atmosphere of Io) to produce

spectacular aurorae

Summary: The Gas GiantsSummary: The Gas GiantsJupiter Saturn Uranus Neptun

e

Mass (MEarth) 317.8 95.2 14.4 17.2

Core (MEarth) ~15? ~15? ~13? ~13?

%Mass of Core ≤5% ≤15% ≤90% ≤76%

I/MR2 0.254 0.210 0.23 0.23

Heat Out/In 2.5 2.3 ~1.1 2.7

Wind Max (km/s) ~100 ≥400 ≤200 ≤200

Prot (hrs) 9.8 10.6 17.2 16.1

Magnetic Field (vs Earth)

20,000 6000 50 25

Convection? Y Y Y? Y

BreakBreak

JupiterJupiter

• This shows Voyager 1's approach during a period of over 60 Jupiter days.

• Notice the difference in speed and direction of the various zones of the atmosphere.

• The interaction of the atmospheric clouds and storms shows how dynamic the Jovian atmosphere is.

Cloud motions in Jupiter’s atmosphereCloud motions in Jupiter’s atmosphere

The Coriolis force diverts N-S motion into E-W motion, in distinct zonal bands (5 in each hemisphere).

Infared and Optical Infared and Optical

•Regions of white in visible light are dark in infraredinternal heat is blocked by the clouds

•Darkest visible bands are brightest in the infraredseeing deeper into the atmosphere where it is hotter

•Red spot also dark in IR: cool, high altitude storm

Jupiter’s atmosphereJupiter’s atmosphere

•Rising air from the deeper layers cools and forms clouds as it rises; we see deeper where the high ammonia clouds have been depleted by precipitation, much as on Earth rain will often mean clearer skies.

Great Red SpotGreat Red Spot

• Red colour probably from red phosphorous• A large eddy caused by rising hot gas and the Coriolis force

Oval BAOval BA

•A White oval storm, similar to the Red Spot but smaller Formed from colliding storms in 1998

•Recently turned Red May be bringing material to the upper atmosphere, where

reactions with UV solar rays change the colour.

Oval BAOval BA

•Near collision in July 2006 Is now the size of Earth with winds reaching 645 km/s, similar

to the Great Red Spot

Feb 2006

July 2006

SaturnSaturn

Saturn… views never seen beforeSaturn… views never seen before

•From the Cassini spacecraft http://saturn.nasa.jpl.gov

Saturn… views never seen beforeSaturn… views never seen before

•From the Cassini spacecraft http://saturn.nasa.jpl.gov•night side of Saturn is partly lit by light reflected from its own

ring system.

Earth

Faint E-ring, created by fountains from Enceladus

Earth from SaturnEarth from Saturn

Crossing the ring planeCrossing the ring plane

Internal heatingInternal heating

• Saturn radiates more energy than it receives, by the same amount as Jupiter does But Saturn is smaller, and this cannot all be gravitational energy It is thought that He forms droplets and sinks downward,

releasing gravitational energy

Thermal infrared picture of Saturn

Storms on SaturnStorms on Saturn

•Like Jupiter, Saturn shows large storms

•These are usually harder to see, however

Storm at the pole

Uranus and NeptuneUranus and Neptune

•Bluish green colour, with far fewer atmospheric features than Jupiter and Saturn

Uranus

Neptune

Colours of Uranus and NeptuneColours of Uranus and Neptune

Uranus and Neptune: InteriorsUranus and Neptune: Interiors

•Likely have a rocky core, but are dominated by a fluid, icy and ionic ocean

•Surrounded by an atmosphere rich in H and He

•Uranus is the only gas giant that does not emit much more heat than it receives from the Sun

Internal convection disrupted? Consistent with lack of storms

Uranus’ tiltUranus’ tilt

•Uranus has an axis of rotation pointing almost directly toward the Sun

Interestingly, prevailing winds are still E-W, so the Coriolis force dominates the weather patterns.