The Gas Giants

JupiterSaturnUranusNeptune Mass (M Earth ) Distance from Sun (AU) Equatorial Radius (R Earth ) Average Density (kg/m 3 )

Size-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.

Composition 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 models T>0K

Recall: 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? BodyI/MR 2 Sun0.06 Mercury0.33 Venus0.33 Earth0.33 Moon0.393 Mars0.366 Jupiter0.254 Saturn0.210 Uranus0.23 Neptune0.23

Shapes Rotation induces significant flattening of compressible material Bodya/(a-b)Rotation Period (d) Sun Earth Jupiter Saturn

Heat 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

Atmospheres 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 Steep T gradient (especially in Jupiter) means interior is convective

H at high temperatures and pressures Atmosphere of Jupiter and Saturn is mostly “liquid” H 2. At very high pressures, H atoms dissociate from each other and their electrons  Forms a metallic liquid  Good conductor STP

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

Magnetospheres 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 Giants JupiterSaturnUranusNeptune Mass (M Earth ) Core (M Earth )~15? ~13? %Mass of Core≤5%≤15%≤90%≤76% I/MR Heat Out/In2.52.3~ Wind Max (km/s)~100≥400≤200 P rot (hrs) Magnetic Field (vs Earth)20, Convection?YYY?Y

Break

Jupiter 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 atmosphere The Coriolis force diverts N-S motion into E-W motion, in distinct zonal bands.

Infared and Optical Regions of white in visible light are dark in infrared  internal heat is blocked by the clouds Darkest visible bands are brightest in the infrared  seeing deeper into the atmosphere where it is hotter Red spot also dark in IR: cool, high altitude storm

Jupiter’s atmosphere The combination of convection and rotation sets up strong zonal patterns: 5 in each hemisphere 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 Spot Red colour probably from red phosphorous A large eddy caused by rising hot gas and the Coriolis force

Oval BA A White oval storm, similar to the Red Spot but smaller  Formed from colliding storms in 1988 Recently turned Red  May be bringing material to the upper atmosphere, where reactions with UV solar rays change the colour. Feb 27, 2006

Saturn

He Diffusion 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 Saturn Like Jupiter, Saturn shows large storms These are usually harder to see, however

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

Colours of Uranus and Neptune

Uranus 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’ 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.

Magnetic Fields Both Uranus and Neptune have magnetic field axes that are:  Not aligned with their rotation axis  Not centered at the geometric centre of the planet Probably due to complex fluid flow in the interiors Rotation axis Magnetic axis

Next lecture: Extrasolar planets