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

Herschel had built a very high quality telescope
and was systematically observing the brighter
stars when he found Uranus
Observing Uranus
Even with the best Earth based
telescopes Uranus appears as a small
featureless blue-green disk

Voyager 2 is the only spacecraft to visit
Uranus and Neptune

Uranus Facts
Size:
Orbit:
Description:
The Rotation of Uranus


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
The atmosphere of Uranus is similar to
those of Jupiter and Saturn, with one
key difference




Structure of Atmosphere
Uranus is too cool to have the same high altitude
clouds that Jupiter and Saturn have


Careful observations have determined that Uranus
does not have alternating zones and bands

Uranus -- True and False
Color
Uranus’s Rings
The ring system of Uranus was first
seen through stellar occultation


Rings are composed of dark material

The Rings of Uranus
The Moons of Uranus
Uranus has 5 major moons (discovered from Earth)
and 10 minor moons (discovered by Voyager 2)

Moons are composed of mixture of ice and rock

Two of the moons shepherd the Epsilon ring

Uranus’s Moons and Rings
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

Magnetic Fields Fields on
Uranus and Neptune
Voyager data indicate that Uranus and
Neptune have magnetic fields


The centers of the magnetic fields are
offset from the center of the planet

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

Neptune -- God of the Sea

The name is
appropriate due to
Neptune’s bluegreen color
The Discovery of Neptune
In the early 1800’s it was noticed that
Uranus’s orbit differed from predictions

Two scientists J. C. Adams and
U. J. Le Verrier tried to calculate where
such a planet should be in 1845-1846

Observing Neptune
Neptune shows no detail from Earth


Neptune Facts
Size: ~4 Earth diameters

Orbit: 30.11 AU

Description:
Neptune’s Atmosphere
Neptune shows much more distinct cloud bands and
storms than Uranus
Neptune has visible storms like Jupiter, but they
appear to be short lived


Clouds are formed into bands which rise and fall
due to convection

Composition and Heating
Atmosphere’s composition similar to
Uranus’s



Neptune has more internal heat to
power cloud motions

Neptune from Voyager
Spots on Neptune
Clouds on Neptune
The Rings of Neptune
Neptune’s rings were discovered via
occultation


Caused by the gravity of a near-by moon
Inner narrow ring has shepherd moons
Neptune’s Rings and Arcs
The Moons of Neptune
Neptune has 7 small satellites and
Triton (diameter= 2700 km)


Triton may be a captured Kuiper Belt
object
Neptune and Triton
The Interiors of Uranus and
Neptune
We can model each planet with a similar interior



The two planets have relatively more heavy elements
and less hydrogen than Jupiter and Saturn

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

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 accident (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