Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 38: Ice Giants: Uranus and Neptune

Key Ideas:

The outermost planets: Nearly identical structure and composition; Called Ice Giants because of chemical content, not because they are solid ice
Uranus:: Lacks internal heat and so nearly featureless; Axis tilted by 98 degrees, results in extreme seasons
Neptune:: Has internal heat and an active atmosphere; One Giant Moon: Triton
Migration of the Outer Solar System

Spacecraft studies

Voyager 2 flew past Uranus and Neptune: Uranus: January 1986; Neptune: August 1989
Both planets have been extensively studied using the Hubble Space Telescope: Long-term monitoring of weather patterns; Infrared imaging studies of their atmospheres, rings, and moons

Interiors of Uranus and Neptune

Rocky cores
Slushy, "ice" mantles
Molecular hydrogen in their outer layers
Presence of Methane in the outer atmosphere gives them their characteristic blue appearance

Magnetic Fields

All Jovian planets have strong magnetic fields: Jupiter's is the strongest by far; The other are similar in extent and strength; Provide the best way to measure rotation periods
Uranus and Neptune have off-center fields: Magnetic and rotation axes are very misaligned (59 degrees for Uranus, 47 degrees for Neptune); Fields are off-center as well (30 percent for Uranus, 55 percent for Neptune)

Uranus' Atmosphere

Uranus is a virtually featureless, hazy blue ball: Lacks source of internal heat; Clouds are cold and do not billow above the top layers; Results in a generally uniform appearance
Occasional clouds and storms seen in the infrared

Uranus' Extreme Seasons

Uranus' axis is tilted ~98 degrees: Lying on its side in its orbit
Extreme seasonal variations:: 1985: North pole in full Sun, South pole in darkness; 2008: Sun on equator, twilight at poles; 2027: South pole in full sunlight, north is dark

Neptune's Atmosphere

Neptune radiates 2.7x as much energy as it gets from the Sun: Active atmosphere; Dark belts; Bright clouds of methane ice; Dark oval cyclonic storms; "Great Dark Spot" and methane cirrus clouds

Moons of Uranus and Neptune

Uranus has 27 moons: None are large enough to be "giant moons"; 5 icy, spherical moons; Miranda, Ariel, Umbriel, Titania, Oberon; 22 tiny, irregular icy moons
Neptune has 13 moons: 1 Giant Moon: Triton, a very cold, icy moon; Triton orbits retrograde; 12 tiny, irregular icy moons; Triton was likely captured by Neptune after the planet formed; This capture probably destroyed the original moon system

Migration of Neptune

There was probably not enough mass in the early solar system to form Neptune at its current location
Neptune, and probably Saturn and Uranus, all migrated outwards from their original location
They pushed out the remaining icy planetesimals
Neptune captured many into orbital resonances

Planetary Migration

Occurs when a planet interacts with a disk of gas or planetesimals
Angular momentum is exchanged via 'gravitational scattering'
Semimajor axes change leading to inward or outward migration
Outward Migration of Neptune led to the resonant capture of Pluto (and other Plutinos) in the 3:2 orbital resonance

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.