Lecture 21: Saturn

"..one ring to rule them all..."
Tolken- Lord of the Rings

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Date: November 22, 1994
Reading Assignment: pp. 260-281

Description : introduction to Saturn- the ring, the planet, and the moons

Objectives

Be able to describe composition of Saturn's atmosphere and why it is different than Jupiter.

Be able to describe the characteristics and composition of the ring of Saturn.

Be able to describe the Roche limit.

Be able to describe why Saturn gives off three times more energy than it receives from the Sun.

Be able to describe the rotation and basic orbital properties of Saturn.

Be able to describe why the ring of Saturn disappears during some parts of its orbit.

Be able to define gaps, ringlets, and spokes and discuss why they occur.

Be able to describe the basic characteristics of Saturn's moons and how they differ from Jupiter's moons.

Be able to define the term albedo.

Be able to describe the moon Titan and its atmosphere.

Lecture Outline

Slide # 1: Lecture 21- Saturn

normal or just a ringer?

Slide # 2: Saturn

orbit and rotation

composition and internal structure

surface features and history

atmosphere

magnetosphere

moons

rings

Slide # 3: Planetary Structure

core

mantle

crust

hydrosphere

atmosphere

magnetosphere

Slide # 4: Jovian Planet Sizes Slide # 5: Planetary Orbits

outer solar system

Slide # 6: Saturn- Revolution

semimajor axis = 9.54 AU

elliptical orbit

minimum radius = 9.01AU

maximum radius = 10.07 AU

orbital period = 29.5 Earth Years

Slide # 7: Moons

Mercury, Venus - none

Earth - 1

Mars - 2

Jupiter - 16

Saturn - 18

Uranus - 15

Neptune - 8

Pluto - 1

Slide # 8: Planetary Masses

Mercury = 1/20

Venus = 4/5

Earth = 1

Mars = 1/9

Jupiter = 318

Saturn = 95

Uranus = 15

Neptune = 17

Pluto = 0.002

Slide # 9: Saturn's Rotation

rotation is usually measured by observing surface features

Saturn, like Jupiter, has no surface

Slide # 10: Saturn's Rotation

10h 14m at the Equator

10h 40m near the poles

differential rotation

clouds are rotating at different sppeds

Slide # 11: Differential Rotation

Saturn is NOT a solid planet

Slide # 12: Saturn's Shape

oblateness of 0.11

Slide # 13: Atmospheric Composition of Saturn

hydrogen (92.4%)

helium (7.4%)

traces of other gases (<1%)

methane

ammonia

Slide # 14: Cloud Layers of Saturn

structure of the atmosphere

Slide # 15: Why the Boring Colors?

Saturn has an atmosphere similar to Jupiter

the outer layers of Saturn are thin haze which blocks some of the colors

Slide # 16: Atmospheres

molecular speed and escape velocity

Slide # 17: Origin of the Atmospheres

primary atmosphere- Hydrogen and Helium

gases common in the solar system

nearly identical to Sun's composition

too cold and too much gravity for any gases to escape

Slide # 18: Atmospheres of Jupiter and Saturn

why are they different?

Slide # 19: Atmospheric Composition

Jupiter and Saturn are probably composed of exactly the same gases in the same

because of the low temperatures on Saturn, much of the helium sank into the co

the sinking of the helium probably is releasing energy and increasing the temp

Slide # 20: Weather

zonal flow similar to Jupiter

higher speed winds which are more uniform

some turbulent storms

white spots observed that last weeks

similar to the Great Red Spot

average temperature is 97K

radiates 3 times more energy than it absorbs

caused by helium rain

Slide # 21: Internal Structure

no seismic information is possible

we must rely on computer models

some tests possible through observations

Slide # 22: Exploration of Saturn

Pioneer 11 (December 1974)

Voyager 1 (1979)

Voyager 2 (1979)

Slide # 23: Magnetic Fields

two conditions needed to form magnetic fields

liquid metal core

rapid rotation

Saturn has rapid rotation and a large metallic hydrogen core

Slide # 24: Magnetosphere

1,000 times stronger than Earth's magnetic field

magnetosphere extends past the rings

magnetosphere aligned with the rotation axis

Slide # 25: Saturn's Ring

first seen by Galileo (1610)

identified as a ring by Huygens 1659

Cassini found a dark band in the ring in 1675

Slide # 26: The Ring of Saturn

what is it?

Slide # 27: What are Rings made of?

cannot be solid

force of gravity would cause it to be unstable

made of many separate orbiting particles

observed different redshifts (Keeler 1895)

high albedo (0.8)

very reflective

probably made mostly of ices

Slide # 28: The Rings are Thin

the axial tilt of Saturn is 27 degrees

Slide # 29: The Rings are Thin

Saturn changes orientations during its orbit from this axial tilt

rings are aligned with the equatorial plane

at times the rings are edge-on

we cannot see the rings when they are edge-on

Slide # 30: Voyager Measurements of the Rings

the Rings are composed mostly of water ice

particle sizes vary

average size = 10 cm

largest 10 m

smallest 0.01 cm

ring thickness few hundred meters

ring size 200,000 km

Slide # 31: Why are the Rings thin?

collisions cause most particles to stay in a single plane

Slide # 32: Gaps, Spokes and Ringlets

features seen in the ring

Slide # 33: Gaps

first gap seen in 1675

Cassini Division

easy to see from Earth

orbital period of the Cassini division is exactly 1/2 of the orbital period of

Slide # 34: Gaps

every one orbit of Mimas equals two orbits in the gap

Slide # 35: Ringlets

thousands of high and low density regions are seen in the rings

caused by two things

interaction between particles inside the ring

moons inside the rings

Slide # 36: The F-Ring

kinks and braids in a ring

Slide # 37: A Single Moon in the Ring

a single moon in the ring will create a gap

Slide # 38: Shepherd Satellites

moons inside and outside of the ring "herd" particles

Slide # 39: The Roche Limit

moons are held together by gravity

moons are are also pulled by tidal forces

at some point, the tidal forces from the planet will be larger than the gravit

Slide # 40: Roche Limit

moons cannot form very close to a planet

Slide # 41: Roche Limit

tidal forces are larger near the planet

Slide # 42: Roche Limit

finally, the tidal forces pull the moon apart

Slide # 43: The Roche Limit

the radius where tidal forces are greater than self-gravity is called the Roch

the exact position depends on the density of the planet and the moon

all planetary rings are found within their Roche limit

Slide # 44: Moons of Saturn

at least 18 moons

three major groups of moons

tiny rocks (<300 km in diameter)

medium sized moons (400 km to 1500 km)

Titan

Slide # 45: Medium Sized Moons

Rhea

Dione

Tethys

Mimas

Encleadus

Iapetus

Slide # 46: Rhea

crater density similar to moon

mostly water ice

albedo 0.6

density 1.3 gm/cm3

wispy terrain

Slide # 47: Rhea

valleys, craters and wispy terrain

Slide # 48: Dione

very similar to Rhea

wispy terrain

mostly water ice

albedo 0.6

density 1.4 gm/cm3

Slide # 49: Dione

wispy terrain

Slide # 50: Dione

some maria

Slide # 51: Tethys

ices again

albedo 0.8

density 1.3 gm/cm3

big impact crater 2/5 diameter of the moon

giant canyon probably formed during Odysseus impact

Ithaca Chasma

Slide # 52: Tetheys

giant canyon

Slide # 53: Mimas

low mass- 1/100 of Rhea

responsible for the Cassini Division

very large crater- Herschel

1/3 the diameter of the moon

high albedo 0.8

low density 1.3 gm/cm3

Slide # 54: Mimas

big crater, small moon

Slide # 55: Encleadus

young surface- recent water volcanos?

Slide # 56: Iapetus

albedo varies from 0.03 to 0.5

dark material is probably organic molecules

origin is unknown

Slide # 57: Iapetus

dark and light sides

Slide # 58: Titan

thick atmosphere

1.5 times the surface pressure of Earth

possible methane/ethane rain

no surface features are yet visible

Slide # 59: Titan

atmosphere hides the surface

Slide # 60: Saturn's Moons

all moons seem to be made mostly of ice

high albedo

low density

some evidence of recent geology

not directly related to distance from Saturn

some water volcanos

many unexplained surface features

wispy terrain

dark marks on Iapetus

thick atmosphere around on Titan