Monday April 19th 1999
Announcements:

Lecture notes:

Class 42
Saturn

Spacecraft studies.  Pioneer II Voyager 1 Voyager 2.

Mass and Density
Again use moons to get mass - 95 Earth masses.
now density is about 700 kg/m³  again mostly H He but less He than Jupiter.   92.4%H  7.4% He.

Rotation - Again differential rotation.
Equator - 10h14m.
High latitudes 10h 40m.
Bulges at equator by 10%
Again rocky core of about 15 Earth masses.

Climate and atmosphere.  Axial tilt is 27 degrees so expect seasons.  Again have internal heating.  3x as much radiation
emitted than arrives from sun.  "liquid helium rain" causes this heating.

Again has cloud layers.  But now thicker than Jupiter 200km due to less gravitational scrunching.  Due to this thickness we don't have as many holes so blander than Jupiter.

Rings (Galileo observed in 1610)  Can learn composition by stellar occultation and computer sims.  Made of many snowballs
and small rocks independently orbiting in equatorial plane.  400 000 km across but only a few meters thick.  Actually several
rings and each made of thousands of ringlets.  Rings can't be as  old as planet - darkened by micrometeorites in about 1000 million years.

Roche limit - destruction of moon?  Combined mass less than or equal to mass of any icy satellite 500 km across.  Or could be
from a comet/icy moon collision.  Some rings kept in place by small shepherd moons.  Too small for Roche destruction.

Uranus and Neptune.
Uranus period of orbit = 84.01 years.
Neptune = 164.8 yrs.
Uranus - 18.3-20.1 AU from sun.
Neptune - 29.8 - 30.4 AU from sun.

Spacecraft Studies.
Voyager 2
1986 Uranus
1989 Neptune.

Mass and Density.
Uranus 14 Earth masses 1200 kg/m³.
Neptune 17 Earth masses 1600 kg/m³
Rocky cores suggested by higher densities since gravitational compression should be less.

Rotation - both differential

Uranus faster at poles - weird.  16.5h at equator 14.2h at poles.
Neptune atmosphere rotates slower than core - weird!  17.3h atmosphere.  16.1h core.

Climate and Atmosphere
Uranus highly tilted 98 degrees.  Poles alternately plunged into darkness for 42 years at a time.  equatorial regions have 2
"summers" per orbit.

Neptune tilted by 30 degrees.
Colder than 70K in atmosphere so ammonia freezes and not important for color - methane gives bluish color - absorbs red
light more than blue.  Uranus - 2% methane in atmosphere.  Neptune has 3%.
When Voyager 2 passed by Uranus it was observed to have a bland featureless atmosphere.

Neptune - Great Dark Spot - about the size of Earth.
Scooters - white methane counterclockwise cirrus.
HST found that Great Dark Spot is gone.

Internal Heating.
None for Uranus.
Neptune emits 2.7 times as much as it gets - maybe again heat left over from formation.

Pluto - discovered in 1930.
Very different from other outer worlds.
Mass = 0.0023 Earth masses.  = 1.5E22kg
Radius = 1/5 of the radius of Earth.  = 1125 km. = .65 times the radius of Earth's moon.
Avg Density = 2300 kg/m³.

No spacecraft missions to Pluto yet so less well understood.

Most notable feature is the moon Charon.  Radius of Charon = 650 km 6.387 day orbit at 19640 km distance.

Pluto orbits the Sun in an inclined orbit of 248 years.
perihelion = 29.73 AU
aphelion = 49.33 AU
Tipped by 17.15 degrees relative to ecliptic.  From 1979-March 99 was closer than Neptune.

Some have suspected that Pluto is not a real planet because it is not obviously Jovian or terrestrial.  Also resembles one of the
ice Moons of Jovian planets - an escaped moon?

But Pluto has a moon of its own.