The Outer Solar System Note the different scale of the inner and outer solar system. Note that Mercury and Pluto have the largest orbital inclinations. Note the tilts of the different planets (Venus upside down, Uranus sideways). We used to think these were due to giant impacts (and maybe they are), but recent work has also shown that frictions between core, mantle, and atmosphere can lead to large tilt changes. The Moon may act to stabilize the Earths tilt.

The “Gas Giants” The outer planets have no solid surface, and are much bigger. The primary gases are hydrogen and helium (like the Sun), although Uranus and Neptune have substantial (15 Earth mass) rock/ice cores. This is mostly to discuss relative sizes. Jupiter and Saturn are true gas giants; Uranus and Neptune are more like “ice giants” (though with deep gaseous exteriors). Can mention that it takes a core about the size of their cores to attract gas directly from the solar nebula (which needs to be defined).

Jupiter – King of the Planets Mass = 0.001 solar (300 earths), Radius = 11.2 Earths, density = 1.3 x water Distance: 5.2AU; Orbital Period: 11.8 years; Rotation period: 9.9 hours. Point out the clouds, whose colors come from organic chemicals we don’t fully know. Note that “organic” means compounds with H,C,N,O, not “live”. Shadow of Io. Red Spot. Bands. Other smaller storms (white ovals).

Hydrostatic Equilibrium – Pressure balance “Hydrostatic equilibrium” governs the structure of all gaseous bodies (planets or stars). The inside has higher temperature and density because of the weight of the overlying material. Temperature at center is ~20000K. Note relation between temperature and pressure – pressure needed to prevent collapse (and more upper weight requires more pressure). Note odd state of metallic hydrogen (conducting – semi-free electrons). Core is uncertain (current limit about 5 earth masses – that’s odd).

Giant Interiors Jupiter and Saturn have similar structures. Both are still collapsing slowly, and the gravitational energy released makes them “shine” more heat out than they get from the Sun. It is carried out by convection. Helium is also slowly settling faster than hydrogen. Note the putative core in Jupiter is about the size of the Earth. The temperature increases inward because the pressure is increasing. Jupiter is almost as big as a non-star can be. More than twice its mass causes the density in the body to go up faster and it actually gets more compact.

Atmospheres of Jupiter and Saturn The clouds we see are mostly ammonia. Talk about the most common elements: H,He,C,N,O. He is inert. H forms molecules with the others: 0H_2 (water), NH_3 (ammonia), CH_4 (methane). These are among the most common molecules around (along with forms involving C + O or N). Note that there are layers with room temperature water clouds (and only a few atmospheres pressure). We don’t understand the trace constituents that give the clouds color. The temperature keeps increasing toward the interior.

Colorful Clouds The Great Red Spot is a cyclone the size of the Earth that has lasted at least 300 years.

Red Spot Movie

Banded Structure of Clouds Repeat that convection is bringing heat outward. As the cells spread at the surface, the rapid rotation of the planet brings Coriolis forces to bear (same as generate hurricanes on Earth, but we aren’t going to try to explain this in detail). These end up generating “jet streams” with different directions. How do we decide what the “real” rotation rate is (some winds are faster, some slower, so they look like they go in opposite directions). We use the magnetic field from the interior.

Jupiter’s Magnetosphere – Bigger than the Sun The metallic hydroden is conducting. Jupiter’s field is quite strong.

Making Magnetic Fields Magnetic field arises when there is a conducting, convecting medium in a rapidly rotating body. This makes a “dynamo”; the same mechanism in the Sun, the Earth and other contexts. Jupiter has by far the strongest field. Venus may be too slow a rotator – no field. The magnetic fields trap high energy particles, making “radiation belts”. Astronauts have to stay out of them, and spacecraft near Jupiter have to be specially designed. Particles from Io are trapped in a doughnut or “torus” around Jupiter. Note the off-center fields for Uranus and Neptune.

Auroral Zones The high energy particles come down the field lines and hit the atmosphere near the poles, causing the gases to glow. Just like on the Earth, this makes an “aurora” in a ring-like zone. These auroral pictures are taken in UV light.

Saturn Mass = 95 Earths, Radius = 9.4 Earths, density = 0.7 x water (floats) Distance: 9.5 AU; Orbital Period: 29.4 years; Rotation period: 10.6 hours. Although it is impossible to think of Saturn without its rings, they are of no planetary consequence, and are temporary. All the other outer planets also have rings systems (but not as nice). We’ve already discussed most of the interesting planetary aspects of Saturn, since it is much like Jupiter. We’ll come back to the rings later.

The Cassini Mission to Saturn Already passed Jupiter, will reach Saturn in July 2004. We will learn much more about the planet, rings, and moons. The Huygens probe will drop into Titan’s atmosphere, hopefully reaching and analysing the surface.

Uranus Mass = 14.5 Earths, Radius = 4.0 Earths, density = 1.3 x water Distance: 19.2 AU; Orbital Period: 84 years; Rotation period: 17.2 hours. Featureless in visible light, because clouds are below haze layer of methane (colder than Saturn). Can see a little in UV light (right, false color).

The Interior of Uranus (and Neptune) Uranus and Neptune have relatively thin hydrogen layers on top; they may have just begun to accrete large amounts of gas when the solar nebula dissipated, interrupting their formation.

A Very Tilted Pole The seasons on Uranus are extreme. Half the planet shares the fate of the pole in not seeing the Sun for half the Uranus year (40 years). Nonetheless, the temperature is fairly uniform around the planet; the gases redistribute the heat.

Neptune Mass = 17 Earths, Radius = 3.9 Earths, density = 1.76 x water Distance: 30 AU; Orbital Period: 163 years; Rotation period: 16.1 hours. Voyager showed it is more like Jupiter than Uranus in appearance. Recently, we have developed the ability to see its storms from Earth, using “adaptive optics” in infrared.

Clouds and Storms on Neptune High clouds are made of methane ice crystals. The heat flow is greater than expected, giving more storms. The Great Dark spot was an upwelling, but has already disappeared.

Pluto Discovered in 1930 by Clyde Tombaugh. Charon discovered at USNO in 1978. Pluto: radius = 1145km, mass = 0.002 Earths, density = 2.1x water Charon: radius = 600 km, mass = 0.1-0.2 Plutos Pluto was closer than Neptune in the late 1990s, but never crosses Neptune’s actual path (2:3 orbital resonance, tilted). Adaptive Optics image Image on left from HST, on right from AO on ground. Pluto goes around twice while Neptune goes around 3 times, but inclination means they won’t collide.

Pluto and Charon Last decade Charon’s orientation carried it through a number of eclipses, giving us good sizes and even maps for the 2 bodies. Inclination of Pluto's equator to its orbital plane About 112 degrees (between 98° and 122°) Pluto's rotation period 6.387 days retrograde Charon's rotation period 6.387 days Charon's orbital period Charon's average distance from Pluto 19,130 km (11,889 mi)

What Pluto probably looks like NASA is considering a Pluto mission now. Neptune’s moon Triton is likely a good model for Pluto now (Triton is twice its size); eventually Pluto’s atmosphere will freeze out onto its surface as it recedes from the Sun. Triton has ice volcanoes erupting methane. Triton is a retrograde moon of Neptune (one wonders whether impacts or encounters played a role in that). Triton’s volcanoes are driven by changing solar flux. Note the dark plumes on the surface which mark eruptions (maybe more like geysers).