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 blue- green 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 

Observing Neptune  Neptune shows no detail from Earth  

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