The Outer Solar System

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.

Jupiter – King of the PlanetsMass = 0.001 solar (300 earths), Radius = 11.2 Earths, density = 1.3 x waterDistance: 5.2AU; Orbital Period: 11.8 years; Rotation period: 9.9 hours.

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.

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.

Atmospheres of Jupiter and Saturn

Colorful CloudsThe 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

Jupiter’s Magnetosphere – Bigger than the Sun

Making Magnetic FieldsMagnetic 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.

Auroral ZonesThe 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.

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).

The Cassini Mission to SaturnAlready 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.

UranusMass = 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.

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 PoleThe 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.

PlutoDiscovered in 1930 by Clyde Tombaugh. Charon discovered at USNO in 1978.

Pluto: radius = 1145km, mass = 0.002 Earths, density = 2.1x waterCharon: 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

Pluto and Charon

Inclination of Pluto's equator to its orbital plane

About 112 degrees (between 98° and 122°)

Pluto's rotation period 6.387 daysretrograde

Charon's rotation period 6.387 days

Charon's orbital period 6.387 days

Charon's average distance from Pluto19,130 km (11,889 mi)

Last decade Charon’s orientation carried it through a number of eclipses, giving us good sizes and even maps for the 2 bodies.

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.