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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 38: The Ice Giants
Uranus & Neptune

Uranus & Neptune

Key Ideas:

Uranus & Neptune are the outermost of the Jovian planets.

Ice Giants



Comparison with the Gas Giants

Orbits of Uranus & Neptune

Semi-Major Axis: a = 19.2 AU
Orbital Period: P = 84 years
Eccentricity: e = 0.05
Inclination: i = 0.8°

Semi-Major Axis: a = 30.1 AU
Orbital Period: P = 165 years
Eccentricity: e = 0.01
Inclination: i = 1.8°

Properties of Uranus & Neptune

Equatorial Radius: R = 25,559 km (4.01 REarth)
Mass: M = 14.5 MEarth
Rotation Period: 17h 14m
Axis Tilt: 97.9°!
27 moons, and thin, dark rings

Equatorial Radius: R = 24,766 km (3.88 REarth)
Mass: M = 17.1 MEarth
Rotation Period: 16h 07m
Axis Tilt: 29.6°
13 Moons and very dark, uneven rings and ring arcs.

Spacecraft studies

Voyager 2 flew by Uranus & Neptune:

Both have been extensively studied using the Hubble Space Telescope:

New studies are being enabled by the use of adaptive optics to "detwinkle" the atmosphere of the Earth and provide views of Hubble-type clarity from the ground.

Uranus & Neptune the Twins

Uranus & Neptune are near twins:

Both appear bluish/green in color:

Ice Giants

The interior structures of Uranus and Neptune are very similar: Both lack the deep metallic hydrogen mantles found on Jupiter & Saturn because of their smaller size (pressure never gets high enough inside).

Because they are mostly ices without deep hydrogen mantles, Uranus and Neptune are often called Ice Giants, to distinguish them from the Gas Giants, Jupiter and Saturn.

Uranus' Atmosphere

Uranus appears as a virtually featureless hazy blue ball at visible light. More detail can be seen at mear infrared wavelengths, especially those infrared wavelengths that can peer through the methane haze layer. These infrared images reveal occasional clouds and cyclonic storms, as well as weak bands and zones.

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:

Uranus' Extreme Seasons

The rotation axis of Uranus is tilted ~98°

Get extreme seasonal variations driven by uneven heating between hemispheres:

Such extreme seasonal variations may help account for Uranus' relative lack of weather when Voyager 2 passed the planet in 1986. The strong hemispheric difference are thought to interfere with the processes that allow formation of the bands and zones that are seen on the other Jovian planets. However, as the Uranus Equinox has approached, more weather and banding have started to be visible in Uranus' atmosphere.

The Moons of Uranus & Neptune

Uranus has 27 moons:

Neptune has 13 moons:

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

Comparison of the Jovian Planets

Jupiter & Saturn are Gas Giants

Uranus & Neptune are Ice Giants

Magnetic Fields

All Jovian planets have strong magnetic fields

Uranus & Neptune have off-center fields:

Nobody knows why, though a few ideas are being worked on.
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Updated: 2007 November 10
Copyright Richard W. Pogge, All Rights Reserved.