Solar System Physics ISolar System Physics IDr Martin Hendry

5 lectures, beginning Autumn 2007

Department of Physics and Astronomy

Astronomy 1XSession 2007-08

Section 9: Ring Systems of the Jovian Planets

All four Jovian planets have RING SYSTEMS. e.g. Saturn’s rings are easily visible from Earth with a small telescope, and appear solid.

Section 9: Ring Systems of the Jovian Planets

All four Jovian planets have RING SYSTEMS. e.g. Saturn’s rings are easily visible from Earth with a small telescope, and appear solid.

The rings consist of countless lumps of ice and rock, ranging from ~1cm to 5m in diameter, all independently orbiting Saturn in an incredibly thin plane – less than 1 kilometre in thickness.

Section 9: Ring Systems of the Jovian Planets

All four Jovian planets have RING SYSTEMS. e.g. Saturn’s rings are easily visible from Earth with a small telescope, and appear solid.

The rings consist of countless lumps of ice and rock, ranging from ~1cm to 5m in diameter, all independently orbiting Saturn in an incredibly thin plane – less than 1 kilometre in thickness.

( Diameter of the outermost ring – 274000 km. If Saturn’s rings were the thickness of a CD, they would still be more than 200m in diameter! )

Section 9: Ring Systems of the Jovian Planets

All four Jovian planets have RING SYSTEMS. e.g. Saturn’s rings are easily visible from Earth with a small telescope, and appear solid.

The rings consist of countless lumps of ice and rock, ranging from ~1cm to 5m in diameter, all independently orbiting Saturn in an incredibly thin plane – less than 1 kilometre in thickness.

( Diameter of the outermost ring – 274000 km. If Saturn’s rings were the thickness of a CD, they would still be more than 200m in diameter! )

James Clerk Maxwell proved that Saturn’s rings couldn’t be solid; if they were then tidal forces would tear them apart. He concluded that the rings were made of ‘an indefinite number of unconnected particles’

Saturn’s rings are bright; they reflect ~80% of the sunlight that falls on them. Their ice/rock composition was confirmed in the 1970s when absorption lines of water were observed in the spectrum of light from the rings.

(See A1Y Stellar astrophysics for more on spectra and absorption lines)

Saturn’s rings are bright; they reflect ~80% of the sunlight that falls on them. Their ice/rock composition was confirmed in the 1970s when absorption lines of water were observed in the spectrum of light from the rings.

(See A1Y Stellar astrophysics for more on spectra and absorption lines)

Ground-based observations show only the A, B and C rings.

In the 1980s the Voyager spacecraft flew past Saturn, and observed thousands of‘ringlets’ – even in the Cassini Division(previously believed to be a gap).

A ringCassini division

B ring C ring

Saturn’s rings are bright; they reflect ~80% of the sunlight that falls on them. Their ice/rock composition was confirmed in the 1970s when absorption lines of water were observed in the spectrum of light from the rings.

(See A1Y Stellar astrophysics for more on spectra and absorption lines)

Ground-based observations show only the A, B and C rings.

In the 1980s the Voyager spacecraft flew past Saturn, and observed thousands of‘ringlets’ – even in the Cassini Division(previously believed to be a gap).

They also discovered a D ring, (inside the C ring), and very tenuous E, F and G rings outside the A ring, out to ~5 planetary radii.

A ringCassini division

B ring C ring

The structure of the F ring is controlled by the two ‘shepherd moons’ – Pandora and Prometheus – which orbit just inside and outside it.

The gravitational influence of these moons confine the F ring to a band about 100km wide

The F ring shows braided structure, is very narrow, and contains large numbers of micron-sized particles.

Solar System Physics ISolar System Physics IDr Martin Hendry

5 lectures, beginning Autumn 2007

Department of Physics and Astronomy

Astronomy 1XSession 2007-08

Jupiter’s ring system is much more tenuous than Saturn’s. It was only detected by the Voyager space probes. The ring material is primarily dust, and extends to about 3 Jupiter radii.

Ring Systems of the other Jovian Planets

Jupiter’s ring system is much more tenuous than Saturn’s. It was only detected by the Voyager space probes. The ring material is primarily dust, and extends to about 3 Jupiter radii.

Uranus’ rings were discovered in 1977, during the occultation of a star, and first studied in detail by Voyager 2 in 1986There are 11 rings, ranging in width from 10km to 100km. The ring particles are very dark and ~1m across. Some rings are ‘braided’, and the thickest ring has shepherd moons. There is a thin layer of dust between the rings, due to collisions.

Ring Systems of the other Jovian Planets

Jupiter’s ring system is much more tenuous than Saturn’s. It was only detected by the Voyager space probes. The ring material is primarily dust, and extends to about 3 Jupiter radii.

Uranus’ rings were discovered in 1977, during the occultation of a star, and first studied in detail by Voyager 2 in 1986There are 11 rings, ranging in width from 10km to 100km. The ring particles are very dark and ~1m across. Some rings are ‘braided’, and the thickest ring has shepherd moons. There is a thin layer of dust between the rings, due to collisions.

Neptune’s rings were first photographed by Voyager 2 in 1989.There are 4 rings: two narrow and two diffuse sheets of dust. One of the rings has 4 ‘arcs’ of concentrated material.

Ring Systems of the other Jovian Planets

Why are the ring systems so thin?

Collisions of ring particles are partially inelastic.

Consider two particles orbiting e.g. Saturn in orbits which are slightly tilted with respect to each other.

Collision reduces difference of y components, but has little effect on x components

this thins out the disk of ring particles

x

y

The ring systems of the Jovian planets result from tidal forces. During planetary formation, these prevented any material that was too close to the planet clumping together to form moons. Also, any moons which later strayed too close to the planet would be disrupted.

Consider a moon of mass and radius , orbiting at a distance (centre to centre) from a planet of mass and radius .

Section 10: Formation of ring systems

r A

PM

SM SRr PM PR

( Assume that the planet and moon are spherical )

SM

The ring systems of the Jovian planets result from tidal forces. During planetary formation, these prevented any material that was too close to the planet clumping together to form moons. Also, any moons which later strayed too close to the planet would be disrupted.

Consider a moon of mass and radius , orbiting at a distance (centre to centre) from a planet of mass and radius .

Section 10: Formation of ring systems

r

PM

SM SRr PM PR

( Assume that the planet and moon are spherical )

Force on a unit mass at A due to gravity of moon alone is

2S

SG R

MGF (10.1)

A

GFSM

The ring systems of the Jovian planets result from tidal forces. During planetary formation, these prevented any material that was too close to the planet clumping together to form moons. Also, any moons which later strayed too close to the planet would be disrupted.

Consider a moon of mass and radius , orbiting at a distance (centre to centre) from a planet of mass and radius .

Section 10: Formation of ring systems

r

PM

SM SRr PM PR

This follows from eq. (4.3) putting

Tidal force on a unit mass at A due to gravity of planet is

(10.2)

SR

3

2rRMGF SP

T A

TFSM

We assume, as an order-of-magnitude estimate that the moon is tidally disrupted if

In other words, if

This rearranges further to

23

2

S

SSP

RMG

rRMG

GT FF (10.3)

(10.4)

SS

P RMMr

3/13/12

(10.5)

We can re-cast eq. (10.5) in terms of the planet’s radius, by writing mass = density x volume.

Substituting

So the moon is tidally disrupted if

33

4PPP RM

33

4SSS RM

PS

P Rr3/1

3/12

(10.6)

We can re-cast eq. (10.5) in terms of the planet’s radius, by writing mass = density x volume.

Substituting

So the moon is tidally disrupted if

More careful analysis givesthe Roche Stability Limit

33

4PPP RM

33

4SSS RM

PS

P Rr3/1

3/12

(10.6)

Pm

P Rr3/1

456.2

(10.7)

e.g. for Saturn, from the Table of planetary data

Take a mean density typical of the other moons

This implies

Most of Saturn’s ring system does lie within this Roche stability limit. Conversely all of its moons

lie further out!

-3mkg700P

-3mkg1200m

PPRL RRr 05.21200700456.2

3/1

(10.8)

Roche stability limit

Tidal forces also have an effect (albeit less destructive) outside the Roche stability limit.

Consider the Moon’s tide on the Earth (and vice versa).

The tidal force produces an oval bulge in the shape of the Earth (and the Moon)

Section 11: More on Tidal Forces

Tidal forces also have an effect (albeit less destructive) outside the Roche stability limit.

Consider the Moon’s tide on the Earth (and vice versa).

The tidal force produces an oval bulge in the shape of the Earth (and the Moon)

There are, therefore, two high and low tides every ~25 hours. (Note: not every 24 hours, as the Moon has moved a little way along its orbit by the time the Earth has completed one rotation)

Section 11: More on Tidal Forces

The Sun also exerts a tide on the Earth.

Now, so

and

so that

3rMF P

T 3

Sun

Moon

Moon

Sun

Moon,

Sun,

rr

MM

FF

T

T (11.1)

kg10989.1 30Sun M kg1035.7 22

Moon M

m10496.1 11Sun r m10844.3 8

Moon r

5.03

Sun

Moon

Moon

Sun

Moon,

Sun,

rr

MM

FF

T

T (11.2)

Spring tides occur when the Sun, Moon and Earth are aligned (at Full Moon and New Moon). High tides are much higher at these times.

Neap tides occur when the Sun, Moon and Earth are at right angles (at First Quarter and Third Quarter). Low tides are much lower at these times.

The Sun and Moon exert a tidal force similar in magnitude. The size of their combined tide on the Earth depends on their alignment.

Earth

Moon

Even if there were no tidal force on the Earth from the Sun, the Earth’s tidal bulge would not lie along the Earth-Moon axis. This is because of the Earth’s rotation.

Earth’s rotation

Earth

Moon

The Earth’s rotation carries the tidal bulge ahead of the Earth-Moon axis. (The Earth’s crust and oceans cannot instantaneously redstribute themselves along the axis due to friction)

Moon’s orbital motion

Earth’s rotation

Earth

Moon

A ‘Drag’ force

The Moon exerts a drag force on the tidal bulge at A, which slows down the Earth’s rotation.

The length of the Earth’s day is increasing by 0.0016 sec per century.

Moon’s orbital motion

At the same time, bulge A is pulling the Moon forward, speeding it up and causing the Moon to spiral outwards. This follows from the conservation of angular momentum.

The Moon’s semi-major axis is increasing by about 3cm per year.Earth’s rotation

Earth

Moon

A ‘Drag’ force

Moon’s orbital motion

Earth’s rotation

Earth

Moon

A ‘Drag’ force

Given sufficient time, the Earth’s rotation period would slow down until it equals the Moon’s orbital period – so that the same face of the Earth would face the Moon at all times.

(This will happen when the Earth’s “day” is 47 days long)

In the case of the Moon, this has already happened !!!

Moon’s orbital motion

Given sufficient time, the Earth’s rotation period would slow down until it equals the Moon’s orbital period – so that the same face of the Earth would face the Moon at all times.

(This will happen when the Earth’s “day” is 47 days long)

In the case of the Moon, this has already happened !!!

Tidal locking has occurred much more rapidly for the Moon than for the Earth because the Moon is much smaller, and the Earth produces larger tidal deformations on the Moon than vice versa.

Given sufficient time, the Earth’s rotation period would slow down until it equals the Moon’s orbital period – so that the same face of the Earth would face the Moon at all times.

(This will happen when the Earth’s “day” is 47 days long)

In the case of the Moon, this has already happened !!!

Tidal locking has occurred much more rapidly for the Moon than for the Earth because the Moon is much smaller, and the Earth produces larger tidal deformations on the Moon than vice versa.

The Moon isn’t exactly tidally locked. It ‘wobbles’ due to the perturbing effect of the Sun and other planets, and because its orbit is elliptical. Over about 30 years, we see 59% of the Moon’s surface.

Many of the satellites in Solar System are in synchronous rotation, e.g.

Mars: Phobos and DeimosJupiter: Galilean moons + AmaltheaSaturn: All major moons, except Phoebe + HyperionNeptune: TritonPluto: Charon

The moons The moons of Jupiterof Jupiter

Many of the satellites in Solar System are in synchronous rotation, e.g.

Mars: Phobos and DeimosJupiter: Galilean moons + AmaltheaSaturn: All major moons, except Phoebe + HyperionNeptune: TritonPluto: Charon

Pluto and Charon are in mutual synchronous rotation: i.e. the same face of Charon is always turned towards the same face of Pluto, and vice versa.

Many of the satellites in Solar System are in synchronous rotation, e.g.

Mars: Phobos and DeimosJupiter: Galilean moons + AmaltheaSaturn: All major moons, except Phoebe + HyperionNeptune: TritonPluto: Charon

Pluto and Charon are in mutual synchronous rotation: i.e. the same face of Charon is always turned towards the same face of Pluto, and vice versa.

Triton orbits Neptune in a retrograde orbit (i.e. opposite direction to Neptune’s rotation).

Neptune’s rotation

Neptune

Triton

ATriton’s orbital

motion

In this case Neptune’s tidal bulge acts to slow down Triton. The moon is spiralling toward Neptune (although it will take billions of years before it reaches the Roche stability limit)

Tidal forces have a major influence on the Galilean Moons of Jupiter

Name Diameter Semi-major Orbital Period Mass (m) axis (m) (days) (kg)

Io 610642.3

610120.3

610268.5

610800.4

Europa

Ganymede

Callisto

The Moon

Mercury

610476.3

610880.4

810216.4

810709.6

910070.1

910883.1

810844.3

1.769

3.551

7.155

16.689

27.322

2210932.8

2210791.4

2310482.1

2310077.1

2210349.7

2310302.3

Section 12: The Galilean Moons of Jupiter

Io Europa

Ganymede Callisto

The orbital periods of Io, Europa and Ganymede are almost exactly in the ratio 1:2:4. This leads to resonant effects :

The orbit of Io is perturbed by Europa and Callisto, because the moons regularly line up on one side of Jupiter. The gravitational pull of the outer moons is enough to produce a small eccentricity in the orbit of Io. This causes the tidal bulges of Io to ‘wobble’ (same as the Moon) which produces large amount of frictional heating.

The surface of Io is almost totally molten, yellowish-orange in colour due to sulphur from its continually erupting volcanoes.

Tidal friction effects on Europa are weaker than on Io, but still produce striking results. The icy crust of the moon is covered in ‘cracks’ due to tidal stresses, and beneath the crust it is thought frictional heating results in a thin ocean layer

Inside Europa

Interior structure of the Galilean Moons

IoEuropa

Ganymede

Callisto

Structure of the Galilean Moons

Their mean density decreases with distance from Jupiter

The fraction of ice which the moons contain increases with distance from Jupiter

This is because the heat from ‘proto-Jupiter’ prevented ice grains from surviving too close to the planet. Thus, Io and Europa are mainly rock; Ganymede and Callisto are a mixture of rock and ice.

The surface of the Moons reflects their formation history:

Io: surface continually renewed by volcanic activity. No impact craters

Europa: surface young ( < 100 million years), regularly ‘refreshed’ – hardly any impact craters

Structure of the Galilean Moons

Their mean density decreases with distance from Jupiter

The fraction of ice which the moons contain increases with distance from Jupiter

This is because the heat from ‘proto-Jupiter’ prevented ice grains from surviving too close to the planet. Thus, Io and Europa are mainly rock; Ganymede and Callisto are a mixture of rock and ice.

The surface of the Moons reflects their formation history:

Ganymede: Cooled much earlier than Io and Europa. Considerable impact cratering; also

‘grooves’ and ridges suggest history of tectonic activity

Structure of the Galilean Moons

Their mean density decreases with distance from Jupiter

The fraction of ice which the moons contain increases with distance from Jupiter

This is because the heat from ‘proto-Jupiter’ prevented ice grains from surviving too close to the planet. Thus, Io and Europa are mainly rock; Ganymede and Callisto are a mixture of rock and ice.

The surface of the Moons reflects their formation history:

Ganymede: Cooled much earlier than Io and Europa. Considerable impact cratering; also

‘grooves’ and ridges suggest history of tectonic activity

Callisto: Cooled even earlier; extensive impact cratering