Chapter 13: Uranus and Neptune

William Herschel

Uranus was discovered by __________ in the year 1781.

barely visible

Uranus is just __________ to the naked eye, if you have very good eyes and know just where to look.

It was not traveling the path they predicted it would take. The gravity of a more distant planet must be affecting it’s observed position.

What did astronomers soon notice about Uranus’s orbit shortly after it was discovered?

John Adams and Urbain Leverrier; they were also credited for the discovery of this planet.

Who predicted the mass and location of the mystery planet that was thought to be just beyond Uranus using mathematical calculations in 1845 and 1846?

Johann Galle

Who used sky charts of the predicted positions of the mystery planet and viewed it through a telescope for the first time in 1846?

Neptune

What did astronomers name the mystery planet?

seen with the naked eye

Neptune cannot be_________, so a pair of binoculars or small telescope must be used.

differential rotation; faster

Uranus exhibits ________ but rotates ______ at the poles.

98°; seasons

The axial tilt of Uranus is ____, giving it extreme ________.

A result of 2 or more massive impacts gentle enough to allow the moons to retain their orbit.

What is believed to be the cause of Uranus’s extreme axial tilt?

retrograde rotation

Uranus is considered to have __________, as its north pole is below the ecliptic.

slowly

Neptune is unique because it is the only Jovian planet whose atmosphere rotates more ______ than its interior.

84% hydrogen and 14% helium (traces of methane)

The atmosphere of Uranus and Neptune are primarily made up of what?

Methane; blue

Neptune has more of what element than Uranus due to it being farther from the Sun? This causes Neptune to look like what color?

Increases

Methane increases in the atmosphere of all Jovian planets as distance from the Sun __________.

heat source

Uranus does not have an internal __________, so clouds are only found in low-lying atmospheric regions that are warmer. We must look deep into its atmosphere to see structure; we don’t see obvious ovals and bands like we do on Jupiter.

internal heating

Neptune’s _____________ makes its atmosphere warmer than the atmosphere of Uranus, which makes atmospheric structures easier to see.

Might be energy left over from its formation.

What is the source of Neptune’s internal heating?

2.7x

Neptune emits ___ the radiation it receives from the Sun.

Great Dark Spot

The __________ was discovered in 1989 but disappeared in the mid-90’s.

They’re over 100x stronger than Earth’s and a tenth of the strength of Saturn’s (weaker than Saturn/Jupiter, stronger than Earth’s). They are greatly tilted with respect to their rotation axis (60° for Uranus, 76° for Neptune); way off center.

What is unique about the magnetic fields of Uranus and Neptune?

A layer of slushy water that contains dissolved ammonia.

What is believed to create the magnetic fields on Uranus and Neptune?

Their pressures outside their cores are not great enough to create metallic hydrogen, so hydrogen stays in the molecular form.

Are the internal pressures outside the cores of Uranus and Neptune thought to be high enough to create metallic hydrogen?

28; tidally locked

Uranus is currently known to have ___ moons. All are ____________ and orbit on Uranus’ skewed equatorial planes.

No

Does Uranus have a large moon?

Ice mixed with rock

The densities of Uranus’ five major moons suggest they are made of what?

16

Neptune is currently known to have ___ moons.

Triton

Neptune’s largest moon is ________.

Not regular; many moons have inclined and/or eccentric orbits.

Describe Neptune’s moon system.

Triton has a retrograde orbit, which causes it to slowly spiral toward Neptune. It will eventually enter Neptune’s Roche limit and create a spectacular ring system.

What will eventually happen to Neptune’s largest moon, and why?

stellar occultation; a star and the rings dimmed its light

The rings of Uranus were discovered in 1977 when astronomers observed a _________, which essentially means that Uranus passed in front of a what?

There are 13 rings that are darker, more diffuse, narrower, and more widely spread out.

How are the rings around Uranus different from those around Saturn?

Two moons that work together by tugging on particles with equal force but in opposite directions. Material is kept in place between the orbits of the moons and is built up as the moons revolve around their parent planet.

Some of Uranus’ rings have shepherd moons that hold them together, which are what?

Five (three are very thin and two are broader and more diffuse, they are not uniform and show clumping).

Neptune has at least how many rings?

Astronomers are not sure and it is still being investigated.

Ring formation is probably common because rings are not expected to live very long. So, what causes rings to continue to form?