An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30
Lecture 38: The Ice Giants
Uranus & Neptune
Uranus & Neptune are the outermost of the Jovian planets.
- Composed of mostly ices with shallow hydrogen atmospheres.
- Smaller and less Hydrogen/Helium than Jupiter & Saturn
- Lacks internal heat & so nearly featureless
- Axis is tilted by 90º, giving extreme seasons.
- Has internal heat & an active atmosphere
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.
Voyager 2 flew by Uranus & Neptune:
- Uranus: January 1986
- Neptune: August 1989
Both have been extensively studied using the Hubble Space
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.
- Long-term monitoring of atmospheric weather patterns.
- Infrared imaging studies of their atmospheres, rings, and
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
- Added factor is scattering in the methane haze layer.
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).
- Thick, slushy mantle of water, ammonia & methane ices.
- Rocky cores about the size of the Earth.
- Shallow atmospheres of molecular hydrogen (H2),
Helium (He) & methane (CH4).
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 appears as a virtually featureless hazy blue ball at visible
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.
- 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.
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
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.
- 1986: North pole in full sunlight, south pole in total darkness.
- 2007: Sun on the equator, sunset at the north pole
starts a 42 year night, sunrise at south pole.
The Uranus equinox is 2007 December 7.
- 2028: South pole in full sunlight, north is dark.
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:
We will take a closer look at Triton along side Pluto and Charon which
it most closely resembles in its properties.
- 1 giant moon: Triton, a very cold icy moon in a retrograde orbit
- Remaining 12 are tiny, irregular bodies.
Comparison of the Jovian Planets
Jupiter & Saturn are Gas Giants
- Large planets, 318 MEarth & 96 MEarth.
- Deep hydrogen/helium atmospheres
- Deep, dense, metallic hydrogen mantles
- Dense, rocky cores of 10-15 MEarth
Uranus & Neptune are Ice Giants
- Similar size & mass (15 & 17 MEarth)
- Shallow Hydrogen/helium atmospheres
- Mostly slushy semi-liquid water & methane ice mantles
- Rocky/icy cores of ~1 MEarth.
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:
Nobody knows why, though a few ideas are being worked on.
- Magnetic & Rotation axes are very misaligned (59°
for Uranus, 47° for Neptune).
- Fields are off-center of the planets (30% for Uranus, 55%
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Updated: 2007 November 10
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