Transcript 19uranusneptune5s

Uranus and Neptune
Astronomy 311
Professor Lee Carkner
Lecture 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 offcentered 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 bluegreen 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 groundbased 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