PHY 113 Lecture Notes - Lecture 13: Orbital Period, Occultation, Retrograde And Prograde Motion

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9 Jun 2018

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Chapter 13 Uranus and Neptune
13.1 The Discoveries of Uranus and Neptune
Uranus was discovered in 1781 by Herschel; first planet to be discovered in more than 2000
He was charting faint stars and came across a curious object he thought it was a nebula or
comet at first (He almost named the planet after King George III, at first!)
Arrows point to three of Uranus’s moons
Just barely visible in best conditions with naked eye
More detail can be seen in this image taken by Voyager 2 at a distance of 1 million km
(More on the color in a few slides)
Neptune was discovered in 1846, after analysis of Uranus’s orbit indicated a few peculiarities
(due to gravitational tugs from an unknown outer planet)
Details of Neptune cannot be made out from Earth either; arrows again point to moons
More detail is visible in these Voyager 2 images, also taken from a distance of 1 million km
13.2 Orbital and Physical Properties
Uranus and Neptune are very similar in many ways
Mass Radius Density
Uranus 14.5 x Earth 4.0 x Earth
Peculiarity of Uranus: Axis of rotation lies almost in the
plane of its orbit. Seasonal variations are extreme.
Best theory is some kind of catastrophic collision, but we have no evidence either way.
Neptune’s is so far out, the orbital period is 163.7 years
Since its discovery, it’s only now orbiting all the way around for the first time!
13.3 The Atmospheres of Uranus and Neptune
Outer atmospheres of Uranus and Neptune are similar to those of Jupiter and Saturn, mostly
H, He
As go from Jupiter Neptune Ammonia decreases and Methane increases, since Ammonia
freezes in outermost planets
Methane gives the dominant blue colors for Uranus and Neptune (Neptune = colder higher
concentrations darker blue)
Uranus is very cold; clouds only in lower, warmer layers.
These images show Uranus rotating (ac), and its ring (d)
Band structure of Neptune is more visible; it had a “Dark Spot” similar to Jupiter’s storms
(now vanished)
Uranus seems to lack any kind of internal heat source. It has a very cold surface temperature.
Oddly, Neptune does have an internal energy source it radiates ~3 times more energy than it
receives from the Sun.
The cause is still unknown!
Possibly, the thick methane has insulated the planet, so it still has some heat left over from
13.4 Magnetospheres and Internal Structure
Uranus and Neptune both have substantial magnetic fields, but at a large angle to their
rotation axes.
Magnetic center in both Uranus and Neptune are significantly off center.
Magnetic fields of Uranus and Neptune must not be produced by dynamos, as the other
planets’ fields are (no liquid H in the interior too cold)
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