Gas Giants: Interiors

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1 Gas Giants: Interiors very different from the terrestrial planets. larger and more massive but much less dense all have populous satellite systems which include bodies at least as large as the Moon all have ring systems. primarily light elements: H, He, some C, N, O most models are consistent with near-solar compositions from measurements of moment of inertia, they are much more centrally concentrated than Earth Models predict a total mass of elements heavier than He (Z>2) in the range of 10-30M Earth, some of which is concentrated in a central core. strong magnetic fields indicate internal convection in electrically conducting regions The presence of significant cores in all four suggests cores formed and eventually began to accrete gas from the solar nebula, once the core mass reached ~10M Earth. Given the high mass fraction of H, He in Jupiter and Saturn, this must have occurred before the solar nebula dissipated i.e. <10 7 yr. The higher CNO fraction for Uranus and Neptune may be a clue that they formed closer to the Sun than did Jupiter and Saturn and then migrated to their current orbits.

2 Gas Giants: Atmospheres All except Uranus emit more heat (as IR blackbodies) than they receive from the Sun Jupiter: this is mostly due to gravitational contraction Saturn: must have an additional heat source (radioactivity?) Temperature structure of atmospheres are qualitatively similar to terrestrial planets Insulating cloud decks various compounds which give rise to the colours Steep temperature gradient means the interiors must be convective Atmospheres contain H under great pressure and temperature This forms a conducting metallic liquid. Jupiter and Saturn have strong magnetic fields Due to convecting, metallic H layers Neptune and Uranus do not have metallic H: their magnetic fields are stronger, and probably arise in their core oceans of hydrogen compounds, rock and metal.

3 Jupiter Atmosphere is distinguished by sharply defined bands in tinges of reddish brown and white with many storm-like features in a range of sizes. The combination of convection and rotation sets up strong zonal patterns 5 zones in each hemisphere (vs 3 on Earth). Clearly defined zones (whitish) and bands (brownish/red) remain stable for long periods of time, even though the details change. Within the bands and zones are eddies which appear to be circulation patterns generated by the atmospheric flow and high rotation. The coloured bands are warmer and deeper than the whiter zones which are places where the deeper view is blocked by clouds. The lowest cloud layer is water vapour, then ammonium hydrosulfide (reddish brown = NH SH) and still higher ammonia (NH = whitish). Rising air from the deeper layers cools and forms clouds as it rises Great Red Spot A large, cool eddy caused by rising hot gas and the Coriolis force colour may be due to the presence of red phosphorus. This spot has been there for at least 300 y: no land masses to disrupt storm.

4 Saturn Core mass may be as small as 1-2M Earth. Core is less massive than Jupiter s, but similar in physical size (less pressure). Saturn radiates more energy than can be explained by solar heating and gravitational collapse. Because it is cooler, He can separate from the H/He mixture and settle toward the interior These He droplets release energy by friction as they move through the H gas. He diffusion can account for the excess heat of Saturn and is consistent with the observed lower abundance of He in Saturn s atmosphere. Saturn also has multicoloured bands but both the bands, paler and yellowish, and their colours are less distinct than in Jupiter s atmosphere. Probably the result of sulphurous smog/haze above the cloud layers, which are deeper on Saturn because it is cooler. Also shows storms but they are harder to see and were not systematically viewed until the development of exceptionally high resolution telescopes, both ground and space. 3- zonal patterns like Jupiter, but they are much less dramatic.

5 Uranus and Neptune Atmospheres: The spectra of Uranus and Neptune are dominated by absorption due to methane (CH ), which produces the bluish-green colours Both very blue-green and show little in the way of bands or cyclonic storm features there is more atmospheric detail (zonal patterns and eddies) on Neptune than Uranus. Interiors: Mean densities are similar to Jupiter and Saturn but lower mass means less compression H and He are a much smaller mass fraction than for Jupiter and Saturn amounting to 1-2M Earth and making the heavy element component about the same in mass i.e. 15M Earth. The models which fit the data best give each of them a dense core of ~1M Earth and probably some iron; however reasonable models have been calculated which show no core at all. In both cases the mantles are ice-rich and make up ~80% of the mass with a H, He rich atmosphere above. The composition and physical conditions in the mantle are consistent with the mantles being hot, dense ionic oceans with conductivity high enough to produce a magnetic field.

6 Internal Heat Sources Uranus emits very little excess heat, just barely enough to be caused by radioactive decay Neptune has a very definite heat source other than this. One suggestion is that Uranus does have an internal heat source but it is not getting to the surface as efficiently because convection is somehow inhibited. Images of Uranus and Neptune are consistent with this in that Neptune does show large, long term storms while Uranus atmosphere does not. one idea is that major impacts (producing the unusual tilt of Uranus as well) disrupted the internal circulation by breaking up on impact and only partly mixing leaving lumps in the interior. Regardless, models show that convection in Uranus can t occur beneath ~ R planet and so the magnetic fields would originate in a thin shell region in the ionic ocean between ~ R. This is consistent with observational data which show the magnetic fields to be off centre from the planet s geometric centre and originating well away from the core.

7 Exercises 1. The average temperature of Jupiter is 160K. Is it in thermal uilibrium? Assume A v =0.3. The uilibrium temperature, assuming a perfect blackbody in the infrared, is given by 1/ 280 T ( 1 AV ) = 107K d / AU. At 160K, Jupiter is much hotter than this. Since Lout T = = 5 L T in We see that it radiates about 5 times as much energy as it receives. The difference is L out A L in = σ T T 2 ( T T ) = σt 1 = 29.7W / m Recall in Lecture 13 we saw that the rate of heat loss due to internal heating in the Earth is ~0.06 W/m 2. Multiplying by Jupiter s surface area, the luminosity difference is 1.9x10 18 W. Note that this is not the greenhouse effect, which means the surface can be considerably warmed by a blanket of cloud. In a planet with no internal heat source, the energy radiated must ual the energy received.