Uranus and Neptune

Astronomy 311Professor Lee

CarknerLecture 19

Uranus -- God of the Sky

He gives his name to Urania, the Greek muse of astronomy

Discovery of Uranus

The other planets can only be seen with a telescope

Uranus was discovered in 1781 by William Herschel while surveying the sky Herschel had built a very high quality telescope

and was systematically observing the brighter stars when he found Uranus

Observing Uranus

Most of our information about Uranus comes from Voyager 2 and HST

No future missions planned

Uranus Facts

Size: Orbit: 19.22 AU Description: blue-green,

featureless, tilted on its side

The Rotation of Uranus The tilt of Uranus’s axis is 98 degrees

Extreme tilt may be due to a large

impact when Uranus was forming

The large tilt produces seasons where half of the planet is in sunlight and half in darkness for long periods of time

Seasons on Uranus

Composition of Atmosphere

Hydrogen: 84 % Helium: 14 % Methane (CH4): 2%

The large amount of methane gives Uranus its bluish color

Structure of Atmosphere

Ammonia, Ammonium hydrosulfide and water have frozen out in the lower atmosphere where we can’t see them

Careful observations have determined that Uranus does not have alternating zones and bands Winds mostly blow east

Uranus’s Rings

As Uranus moved past a star, the star dimmed several times before being occulted by the planet

Rings are composed of dark material They reflect very little light and are difficult to

observe at optical wavelengths

The Moons of Uranus Uranus has 5 major moons and 22 minor

moons

Moons are composed of mixture of ice and rock

Two of the moons shepherd the Epsilon ring

The other rings may also have shepherd moons that are too small to see

Radiation Darkening Why are the moons and rings of Uranus

(and Neptune) so dark?

Impacts by high energy electrons from the magnetosphere break off the carbon atoms Carbon soot builds up on the ice

making it dark

Magnetic Fields Fields on Uranus and Neptune

May be formed by motions of a liquid water mantle containing ions

The centers of the magnetic fields are offset from the center of the planet How can the dynamo effect produce an off-centered

field?

Rotation and Magnetic Axis

Determining Mass

How are the masses of planets determined?

We can measure the period and the orbital radius of a moon or spacecraft The relationship between them depends

on mass

Neptune -- God of the Sea

The name is appropriate due to Neptune’s blue-green color

The Discovery of Neptune

Was an undiscovered planet altering the orbit?

In 1846 J. G. Galle used Le Verrier’s calculations to find Neptune after a 30 minute search

Observing Neptune

Neptune shows no detail from ground-based telescopes

Best images from Earth from the Hubble Space Telescope

Neptune Facts

Size: ~4 Earth diameters

Orbit: 30.11 AU

Description: more distant, cloudier Uranus

Neptune’s Atmosphere

Neptune has visible storms like Jupiter, but they appear to be short lived

The white features in Neptune’s atmosphere are high altitude methane clouds

All seem to be moving east rather than in opposite directions

Composition and Heating

84 % Hydrogen 14 % Helium 2 % Methane

Neptune may be still contracting

The Rings of Neptune

But stars were dimming by different amounts

Caused by the gravity of a near-by moon Inner narrow ring has shepherd moons

The Moons of Neptune

Triton’s orbit is unstable, in 100 million years it will be inside of the Roche limit giving Neptune a spectacular ring system

Triton may be a captured Kuiper Belt object

Interiors We can model each planet with a similar interior

Mantle of water and ammonia (Windex)

The two planets have relatively more heavy elements and less hydrogen than Jupiter and Saturn They also do not have enough gravity to produce

liquid metallic hydrogen

The Interiors of Uranus and

Neptune

The Formation of Uranus and Neptune

At 20-30 AU the planetesimals were fewer and more widely dispersed than at 5-10 AU

By the time they formed much of the hydrogen and helium was dispersed

Next Time

Read Chapter 11.5 and 12.5

Summary Information comes from Voyager and HST Blue-green in color with white clouds

Caused by methane Radiation darkening produces dark, soot colored rings

and moons Interior composed of rocky core, water and ammonia

mantle and hydrogen atmosphere Offset magnetic field Formed more slowly than Jupiter and Saturn and so

captured less gas

Summary: Uranus

Discovered by survey (1781) Faint cloud patterns

Due to low internal heat Tilted on its axis

Causing non-uniform solar illumination

Summary: Neptune

Discovered through use of Newton’s laws (1846)

Most distant gas giant Has more internal heat and

stronger cloud features than Uranus