Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 9:30

Lecture 38:
Uranus & Neptune

Key Ideas:

• Nearly identical structure & composition.

Uranus:

• Lacks internal heat & so nearly featureless
• Axis is tilted by 90º, giving extreme seasons.

Neptune:

• Has internal heat & an active atmosphere

Comparison of the Jovian Planets

Orbits of Uranus & Neptune

Uranus

Semi-Major Axis: a = 19.2 AU

Orbital Period: P = 84 years

Eccentricity: e = 0.05

Inclination: i = 0.8°

Neptune

Semi-Major Axis: a = 30.1 AU

Orbital Period: P = 165 years

Eccentricity: e = 0.01

Inclination: i = 1.8°

Properties of Uranus & Neptune

Uranus

Equatorial Radius: R = 25,559 km (4.01 R_Earth)

Mass: M = 14.5 M_Earth

Rotation Period: 17^h 14^m

Axis Tilt: 97.9°!

27 moons, and thin, dark rings

Neptune

Equatorial Radius: R = 24,766 km (3.88 R_Earth)

Mass: M = 17.1 M_Earth

Rotation Period: 16^h 07^m

Axis Tilt: 29.6°

13 Moons and very dark, uneven rings and ring arcs.

Spacecraft studies

Voyager 2 flew by Uranus & Neptune:

• Uranus: January 1986
• Neptune: August 1989

Both have been extensively studied using the Hubble Space Telescope:

• Long-term monitoring of atmospheric weather patterns.
• Infrared imaging studies of their atmospheres, rings, and moons.

Uranus & Neptune the Twins

Uranus & Neptune are near twins:

• Nearly identical sizes and internal structures.

Both appear bluish/green in color:

• Sunlight passes through an outer haze layer and bounces off the cloud tops below.
• The haze is rich in methane, which strongly absorbs red & orange light, letting blue & green light pass through.
• Added factor is scattering in the haze layer.

Interiors of Uranus & Neptune

The interior structures of both are very similar:

• Atmosphere is of molecular hydrogen (H₂), Helium (He) & methane (CH₄).
• Thick, slushy mantle of water, ammonia & methane ices.
• Rocky cores about the size of the Earth.

Both lack the deep metallic hydrogen mantles found on Jupiter & Saturn because of their smaller size (pressure never gets high enough inside).

Uranus' Atmosphere

Uranus appears as a virtually featureless hazy blue ball.

• The reason is a lack of internal heat, unlike the other Jovian planets.
• Clouds on Uranus are cold and don't billow up above the top haze layer.
• Results in a generally uniform appearance.

Occasional clouds/storms seen in the infrared with the Hubble Space Telescope.

Neptune's Atmosphere

Neptune, like Jupiter & Saturn, radiates about 2.7x as much energy as it receives from the Sun.

• This internal energy gives Neptune an active, dynamic atmosphere:
• Dark belts and bright clouds of methane ice.
• Dark oval cyclonic storms.
• Great Dark Spot - a large storm that appeared in the mid-1980s then vanished by 1995.

Uranus' Extreme Seasons

The rotation axis of Uranus is tilted ~98°

• Uranus is lying on its side in its orbital plane.

Get extreme seasonal variations driven by uneven heating between hemispheres:

• 1985: North pole in full sunlight, south pole in total darkness.
• 2006: Sun on the equator, sunset at the north pole starts a 42 year night, sunrise at south pole
• 2027: South pole in full sunlight, north is dark.

Such extreme seasonal variations may help account for Uranus' lack of weather (the strong hemispheric difference may interfere with the processes in bands and zones that are seen on the other Jovian planets).

The Moons of Uranus & Neptune

Uranus has 27 moons:

• None are large enough to be "giant moons".
• 5 brightest are icy, spherical moons: Miranda, Ariel, Umbriel, Titania, & Oberon
• Remaining 22 are tiny, irregular bodies.
• Named for characters in Pope & Shakespeare.

Neptune has 13 moons:

• 1 giant moon: Triton, a very cold icy moon that orbits retrograde around Neptune.
• Remaining 12 are tiny, irregular bodies.

We will take a closer look at Triton along side Pluto and Charon which it closely resembles in its properties.

Comparison of the Jovian Planets

Jupiter & Saturn:

• Large planets, 318 M_Earth & 96 M_Earth.
• Dense, deep metallic hydrogen mantles
• Dense, rocky cores of 10-15 M_Earth

Uranus & Neptune:

• Similar size & mass (15 & 17 M_Earth)
• Slushy semi-liquid water & methane mantles
• Rocky cores of ~1 M_Earth.

Magnetic Fields

All Jovian planets have strong magnetic fields

• Jupiter's is strongest by far
• Saturn, Uranus, & Neptune have magnetic fields of similar strength & extent.
• The magnetic fields provide the best way to measure the rotation periods of the Jovian planets.

Uranus & Neptune have off-center fields:

• Magnetic & Rotation axes are very misaligned (59° for Uranus, 47° for Neptune).
• Fields are off-center of the planets (30% for Uranus, 55% for Neptune).

Nobody knows why, though a few ideas are being worked on.