Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 38: Uranus and Neptune

Key Ideas:

Outermost of the Jovian planets: Nearly identical structure and composition
Uranus:: Lacks internal heat and so nearly featureless; Axis tilted by 98 degrees, which causes extreme seasons
Neptune:: Has internal heat and an active atmosphere
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

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

Atmosphere of Uranus

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

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

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; 2007: Sun on equator, twilight at poles; 2027: South pole in full sunlight, north is dark

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

Rings of Uranus and Neptune

Uranus has thin rings: Dark, narrow rings only a few km wide; The wide "epsilon ring" is still only 100 km wide
Neptune has dark rings: Only a few km wide; Best seen back-lit by the Sun; Ring "arcs" are clumps in the outermost ring

Migration of Neptune

There was not enough mass in the early solar system to form Neptune at its current location
Saturn, Uranus, and Neptune all probably migrated outwards from their original location
They pushed the remaining icy planetesimals into the Kuiper Belt and Scattered Disk

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