Uranus and Neptune Astronomy 311 Professor Lee Carkner Lecture 19

Compared to the clouds of Jupiter, the clouds of Saturn are, a)Much brighter b)More spread out in depth c)More likely to have water in them d)Flowing in only one direction e)At a higher temperature

Which best describes the structure of Saturn’s rings? a)A single disk of uniform density b)Denser away from Saturn and less dense near Saturn c)Large particles in the center and smaller particles at the edges d)A radial series of rings and sub-rings e)Chaotic with no regular structure

Why does Saturn have a lower density than Jupiter? a)It has a smaller core b)It has less iron c)It is made of lighter gases d)It has more helium e)It has less liquid hydrogen

Saturn’s rings are composed primarily of, a)House-sized particles b)Dust grain sized particles c)Fist-sized particles d)Molecule-sized particles e)Stadium-sized particles

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: 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 (CH 4 ): 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 Earth   Best images from Earth from the Hubble Space Telescope

Neptune Facts  Size: ~4 Earth diameters   Orbit: 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

The Interiors of Uranus and Neptune  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 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