Download Lecture 22 Giant Planets Rings

Lecture 22
Giant Planets
Rings
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The planets – ring systems
None of the terrestrial planets (or Pluto) have ring
systems but all of the Jovian planets do
Although recently Pluto suggested to form dust
rings sporadically when KBOs collide with its
moons (Stern 2006)
And there’s a large amount of space debris in orbit
around the Earth…
They are interesting because:
•! circumplanetary disk processes similar to
circumstellar disk processes
•! test of planetary system dynamics
Planetary rings – Jovian rings overview
Jupiter: rings discovered in
1979 by Voyager 1; comprised
of dust <10 µm in diameter;
Main=122,800 km (30 km
thick); Halo=Extends from
main ring to Jupiter;
Gossamer= >129,000 km;
debris from smaller satellites;
affected by magnetic forces
Saturn: rings discovered in 1610
by Galileo; but Huygens in 1651
interpreted as rings; Cassini in
1675 discovered the first gap in
the rings; Voyager found
composed of ice few µm to 10s of
m; evidence for shepherding
moons; A-G rings; braided rings
Uranus: 11 rings
discovered in 1977; very
narrow, eccentric and
inclined; shepherded by
satellites; dark, carbon
particles up to m in size;
self-gravitational effects
Neptune: several
narrow rings;
including Adams ring
made up of
incomplete arcs and
clumps from satellite
interactions
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SATURN
- the ring
system!
12.4 Saturn’s Spectacular Ring
System
Saturn has an
extraordinarily large
and complex ring
system, which was
visible even to the first
telescopes
The first planet to have its rings detected was Saturn.
Saturn’s rings were seen by Galileo in 1614 but
disappeared soon afterwards. The rings appeared to
have a variety of forms when first detected:
Galileo did not fully understand what he was seeing
Huygens’ Explanation
Christiaan Huygens was the first person to explain
the rings (and their disappearance) when in 1659 he
worked out that Saturn must be surrounded by a thin
flat ring that does not touch the planet. The
appearance and disappearance of the rings was due
the different viewing geometries as seen from Earth.
These images were taken from the Hubble Space Telescope during a four-year period, from
1996 to 2000 (left to right), as Saturn moved along one seventh of its 29-year journey around
the Sun. As viewed from near the Earth, Saturn’s rings open up from just past edge-on to
nearly fully open as it moves through its seasons, from autumn towards winter in its northern
hemisphere.
Cassini
Division
between the
A and B
rings
the first
feature to
be
discovered
Edge-on view of Saturn’s rings
Titan and it’s shadow
These images were taken from the Hubble Space Telescope during a four-year period, from
1996 to 2000 (left to right), as Saturn moved along one seventh of its 29-year journey around
the Sun. As viewed from near the Earth, Saturn’s rings open up from just past edge-on to
nearly fully open as it moves through its seasons, from autumn towards winter in its northern
hemisphere.
Titan
4 satellites
When the Earth is in the plane of Saturn’s rings, an observer on the Earth views the rings edge
on. Because the rings are so thin, they are then barely visible. Saturn’s largest satellite, Titan, is
seen just above the rings (left); it is enveloped in a dark brown haze and casts a dark shadow on
Saturn’s clouds. Four other moons are clustered near the other edge of Saturn’s rings (right),
appearing bright white because their surfaces are covered with water ice.
The rings are thin and very flat: 10 to 100 meters thick
A and B are dense and bright
C and D are faint and sparse
F is a narrow ring at the edge of the system
" #
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Two major gaps: Cassini and Encke
Two major gaps:Spectacular
Cassini and Encke Ring
12.4 Saturn’s
System
12.4 Saturn’s
Spectacular Ring
System
Overview of the
ring system
Overview of the
ring system
B ring is the
brightest
B ring is the
brightest
Cassini Division has a
few faint rings
The narrow, mysterious F-Ring at the
outer edge of the A-ring
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Composition and Structure
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Ring particles range in size from fractions of a
millimeter to tens of meters
Composition: Water ice—similar to snowballs
Why rings?
Too close to planet for moon to form—tidal forces
would tear it apart
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Maxwell’s
Essay
James Clerk Maxwell won the Adams Prize Essay
in 1856 for his work on the stability of Saturn’s
rings. He showed that the rings could not be
solid but had to be composed of particles.
Rings not solid - stresses would tear them
apart
Saturn’s rings are a truly alien environment, consisting of
many small bodies in orbit around the planet.
Artist’s conception of the rings
Old ring model
Vrel <<< 15km/s
Ring
Vertical
Structure
15km/s
“Classical” ring model
New ring model
A more modern, densely packed ring model
30m thick
Gentle collisions & weak gravity between particles
give the rings the quality of a viscous fluid
The sizes of the particles have been determined by
observing the scattering of radio waves of various
wavelengths
There are also other dynamical features that give
information on the particle sizes - or the mass in the rings
- such as the wavelength of various waves seen in the ring
Particles in Saturn’s rings
Scattering of
Radio waves
0.94, 3.6, 13 cm
When
passedbehind
behind
Whenthe
theCassini
Cassini spacecraft
spacecraft passed
therings
ringsof
ofSaturn,
Saturn, it sent
the
sentthree
threesimultaneous
simultaneous
radiosignals,
signals, at
at 0.94,
0.94, 3.6
radio
3.6and
and13
13centimeter
centimeter
wavelength,through
through the
The
wavelength,
therings
ringstotoEarth.
Earth.
The
observedchanges
changes of
of each
observed
eachsignal
signalasasthe
thespacecraft
spacecraft
moved behind the rings provided a profile of the
moved behind the rings provided a profile of the
distribution of ring material as a function of distance
distribution
of ring material as a function of distance
from Saturn, or an optical depth profile.
from Saturn, or an optical depth profile.
The image shown here was constructed from these profiles, depicting the observed ring structure at about 10 kilometers in
resolution.
Color ishere
usedwas
to present
information
presence
or absences
of small ring
particles
in different
The
image shown
constructed
from about
thesethe
profiles,
depicting
the observed
ring
structure
at aboutregions
10 kilome
based on the
measured
effects
of the three
radio signals.
color indicates
regionsofwhere
a lack ofin
particles
of reg
resolution.
Color
is used
to present
information
aboutPurple
the presence
or absences
smallthere
ringisparticles
different
size less
thanmeasured
5 centimeters.
Green
and three
blue shades
indicate regions
there are particles
centimeters
based
on the
effects
of the
radio signals.
Purple where
color indicates
regionssmaller
where than
there5 is
a lack of partic
and
1
centimeter,
respectively.
The
saturated
broad
white
band
is
the
densest
region
of
the
B
ring,
which
blocked
of
size less than 5 centimeters. Green and blue shades indicate regions where there are particles smaller thantwo
5 centimet
the three radio signals. From other evidence in the radio observations, all ring regions appear to be populated by a broad
and
1 centimeter, respectively. The saturated broad white band is the densest region of the B ring, which blocked tw
range of particle sizes that extend to several meters across.
the three radio signals. From other evidence in the radio observations, all ring regions appear to be populated by a br
range of particle sizes that extend to several meters across.
KEY QUESTION
Why do these particles that
are gently colliding not stick
together and form a satellite?
The Roche limit
A large satellite (top) that moves well within
a planet’s Roche limit (dashed curve) will be
torn apart by the tidal force of the planet’s gravity.
The side of the satellite closer to the planet feels
a stronger gravitational pull than the side farther
away, and this difference works against the
self-gravitation that holds the body together.
A small solid satellite (bottom) can resist tidal
disruption because it has significant internal
cohesion in addition to self-gravitation.
Tidal forces
Radius of Roche limit scales with the radius of the planet
If the planet and the ring particles have
the same mean density then:
Roche Limit Radius = 2.4 x Planet Radius
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Roche limit for water ice near Saturn
Other asymmetric structures in dusty rings…
D ring
Roche
Divison
Cross-section of rings and satellites
All of Saturn’s main rings lie inside the Roche limit (dashed curve) within which the planet’s gravity
will tear a large satellite apart. The A and B rings have been observed for centuries. The more tenuous
C ring was discovered in the 19th century, and definite observations of the transparent D ring awaited
the arrival of the Voyager 1 spacecraft on 12 November 1980. The icy satellite Enceladus feeds the
tenuous E ring, also revealed from Voyager 1, as well as from the Cassini spacecraft.
For clarity, the thickness of the rings has been exaggerated.
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particles closest to the planet move fastest
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How old are the rings?
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Details of formation are unknown:
Probably too active to have lasted since birth of
solar system
Not all rings may be the same age
Either must be continually replenished, or are
the result of a catastrophic event
The age of the rings is not known
Saturn’s Rings
A laboratory for planetary dynamics
Saturn’s rings of ice
hundreds of narrow rings!
The narrow angle camera aboard the
Cassini spacecraft took this image from
beneath Saturn’s ring plane on 21 June
2004. The brightest part of the ring
The narrow angle camera aboard the
system, extending from the upper right
Cassini spacecraft took this image from
to the lower left, is the central B ring.
beneath Saturn’s ring plane on 21 June
It is separated from the outermost A ring
2004.
The brightest
partDivision,
of the ring
by
the wide,
dark Cassini
system, extending
from
upper right
discovered
in 1675 by
the the
Italian-born
to the lower
left, isGiovanni
the central
B ring.
French
astronomer
(Gian)
It is separated
the outermost
Domenico
(Jeanfrom
Dominique)
CassiniA ring
by the wide,Below
dark Cassini
Division,
(1625-1712).
the B ring,
closer
the Italian-born
todiscovered
the planet,in
is 1675
the C by
ring.
French astronomer Giovanni (Gian)
Domenico
(Jean
Dominique)
All
three rings
are composed
of Cassini
innumerable particles of water ice. The different shades of the
(1625-1712).
Below
the B ring,
closerof contamination by other materials such as rock or
rings
are attributed
to different
amounts
to the planet,
is theWhen
C ring.
carbon
compounds.
viewed close up, the broad icy rings break up into thousands of
individual wave-like ringlets.
All three rings are composed of innumerable particles of water ice. The different shades of the
rings are attributed to different amounts of contamination by other materials such as rock or
carbon compounds. When viewed close up, the broad icy rings break up into thousands of
individual wave-like ringlets.
The gravitational tugs from Saturn’s moons
are responsible for a wide variety of
structures in Saturn’s rings:
Sharp edges
Density variations
Vertical Warps
Arcs
The moons’ effects are most important
when the moons are near the rings or at
various resonances…
Satellites perturb the ring particles particularly near resonances
Resonance occurs when
ratio of period of particle to period of
perturbing satellite
= ratio of two small integers
Formation of waves near resonances
10.4 Density waves and bending waves
(a)
493
(b)
Planet
Planet
Fig. 10.11. Schematic diagrams of the coplanar particle paths that give rise to trailing
all particles
their
own
keplerian
but spiral
spiral density
waves nearfollow
a resonance
with an
exterior
satellite. (a)orbits
The two-armed
density wave
associated
with the 2:1forms
(m = 2) ainner
Lindblad
resonance.
(b) The seventheir
distribution
spiral
wave
pattern
armed density wave associated with the 7:6 (m = 7) inner Lindblad resonances. The
These localized disturbances can even lead to
changes in the apparent brightness of the ring
visible from a distance
Ring particle’s orbital Period=
3/4 Janus’ orbital Period
4/5 Janus’ orbital Period
3/5 Mimas’ orbital Period
5/6 Janus’ orbital Period
Two major gaps: Cassini and Encke
wo major gaps: Cassini and Encke
The orbital periods of particles at the inner edge of the
12.4
Saturn’s
Spectacular
Ring
ital periods
of
particles
at
the
inner
edge
of
the
Cassini Division
are half the orbitalRing
period of Mimas
4i Division
Saturn’s
Spectacular
are half the orbital period of Mimas
System
System
this is a 2:1 resonance
Overview of the
w of ring
the system
em
B ring is the
is the brightest
test
Two waves in Saturn’s rings
These are not separate
rings but spirals
Gaps and wave-like concentrations in ring
particles are due to the gravitational influence
of Saturn’s moons. A small, nearby moon
orbiting at varying distance from Saturn’s
rings is thought to produce waves of density,
causing the ring particles to bunch together
Gaps
and wave-like
concentrations
and
disperse
like the crests
and troughs in ring
of ocean
waves
particles
are due to the gravitational influence
of Saturn’s moons. A small, nearby moon
orbiting at varying distance from Saturn’s
This
image,
which spans
about 220
kilometers,
was taken from the Cassini spacecraft on 29
rings
is thought
to produce
waves
of density,
October
2004.
causing
the ring particles to bunch together
and disperse like the crests and troughs
of ocean waves
This image, which spans about 220 kilometers, was taken from the Cassini spacecraft on 29
October 2004.
#a < 1.75
mp
(10.62)
a.
Satellites near rings cause
waves
satellite
Fig. 10.14. A schematic diagram showing how the variation of the amplitude and
wavelength with separation from a satellite leads to the formation of a wake in adjacent
ring material upstream from an exterior satellite. The lower arrow indicates the direction
of motion of the particles with respect to the satellite.
Satellite appears to repel the ring particles
- while the ring particles repel the satellite!
Small satellites in some gaps
10.7 The F ring of Saturn
515
Pan
Fig. 10.23. A schematic diagram showing how Pan is responsible for edge waves as
it maintains the Encke gap by a shepherding mechanism, while simultaneously keeping
coorbital material in a horseshoe orbits.
implying the presence of more than one satellite. Cooke (1991) points out
that there are a number of strong inner Lindblad resonances in the vicinity:
the 18:17 resonance with Pandora at 136,457 km, and the 32:31 resonance with
Prometheus at 136,481 km (see Fig. 10.5). However, the mechanism responsible
Small satellites in some gaps
Nearby moons are
responsible for two
narrow gaps in the
outer parts of
Saturn’s rings…
Pan
Daphnis
narrow gaps in the
outer parts of
Saturn’s rings…
Daphnis
Enceladus vents water jets
Gaps and wave-like concentrations in ring
particles are due to the gravitational influence
of Saturn’s moons. A small, nearby moon
orbiting at varying distance from Saturn’s
rings is thought to produce waves of density,
causing the ring particles to bunch together
and disperse like the crests and troughs
of ocean waves
This image, which spans about 220 kilometers, was taken from the Cassini spacecraft on 29
October 2004.
Geysers near the south pole of Saturn’s moon Enceladus (middle of ring) send water ice and
water vapor tens of thousands of kilometers into space, where they are trapped by Saturn’s
gravity into orbit around the planet, forming the E ring.
Dramatic plumes, both large and small, spray water ice particles, water vapor and organic
compounds out from many locations along tiger stripe fractures near the south pole of Saturn’s
moon Enceladus.
This backlit view shows the fainter F, G, and E rings
G and E rings are very sparse - hardly exist!
Active Enceladus feeds Saturn’s E ring
Gaps and wave-like concentrations in ring
particles are due to the gravitational influence
of Saturn’s moons. A small, nearby moon
orbiting at varying distance from Saturn’s
rings is thought to produce waves of density,
causing the ring particles to bunch together
and disperse like the crests and troughs
of ocean waves
This image, which spans about 220 kilometers, was taken from the Cassini spacecraft on 29
October 2004.
Geysers near the south pole of Saturn’s moon Enceladus (middle of ring) send water ice and
water vapor tens of thousands of kilometers into space, where they are trapped by Saturn’s
gravity into orbit around the planet, forming the E ring.
Sparse ring of ice particles
of ocean waves
Dramaticenormous
plumes, both
largering
and small, spray water ice particles, water vapor and organic compounds out
Saturn’s
infrared
Thismany
image,locations
which spans
kilometers,
from the
Cassini
spacecraft
onEnceladus.
29 October
from
alongabout
tiger220
stripe
fractureswas
neartaken
the south
pole
of Saturn’s
moon
2004.
This artist’s conception illustrates the
Geysersglow
near of
thecold
south
pole
of Saturn’s
infrared
dust
particles
in moon Enceladus (middle of ring) send water ice and water vapor
tens of thousands
of kilometers
space, where they are trapped by Saturn’s gravity into orbit around
Saturn’s
largest ring,
discoveredinto
using
theSpitzer
planet,Space
forming
the E ring.
the
Telescope
in 2009.
Dramatic plumes, both large and small, spray water ice particles, water vapor and organic compounds out
from many locations along tiger stripe fractures near the south pole of Saturn’s moon Enceladus.
The very tenuous collection of ice and dust particles is spread out in an enormous belt at the far reaches o
Saturn’s
system,
with anillustrates
orbit tilted
This artist’s
conception
the27 degrees from the main ring plane. The bulk of its material starts
about
six million
away from
infrared
glow of kilometers
cold dust particles
in the planet, and extends outward another 12 million kilometers.
Saturn’s largest ring, discovered using
The
is equivalent
to roughly 300 times the diameter of Saturn. The planet appears as just a
thering’s
Spitzerdiameter
Space Telescope
in 2009.
small dot in the middle of this portrayal. The inset shows an enlarged image of Saturn, as seen by the W.
M. Keck Observatory at Mauna Kea, Hawaii, in infrared light. Saturn’s retrograde moon Phoebe circles
within
the newfound
ring, and
the particles
source ofisits
material.
The very
tenuous collection
of is
icelikely
and dust
spread
out in an enormous belt at the far reaches of
Saturn’s system, with an orbit tilted 27 degrees from the main ring plane. The bulk of its material starts
about six million kilometers away from the planet, and extends outward another 12 million kilometers.
The ring’s diameter is equivalent to roughly 300 times the diameter of Saturn. The planet appears as just a
small dot in the middle of this portrayal. The inset shows an enlarged image of Saturn, as seen by the W.
M. Keck Observatory at Mauna Kea, Hawaii, in infrared light. Saturn’s retrograde moon Phoebe circles
within the newfound ring, and is likely the source of its material.
Phoebe’s ring has an angular width on
the sky of about one degree
- this is two Moon diameters!!!
This ring is very sparse - better than what
we would call perfect vacuum in the lab
Other edges and divisions in rings are also the result of
resonance
“Shepherd” moon defines outer edge of A ring through
gravitational interactions
“Shepherd satellites can also produce narrow rings
10.5 Narrow rings and sharp edges
505
outer
satellite
inner
satellite
Fig. 10.18. A schematic diagram (adapted from Dermott 1984) illustrating the basics
of the shepherding mechanism whereby a narrow ring (shaded area) is confined by two
small satellites on either side of it.
In Fig. 10.18 we have assumed that the excited eccentricity produced by the
encounter is eventually reduced to zero by the effect of collisions with other
ring particles. This results in the wave being damped such that the particle is
again in a circular orbit when it next encounters the satellite at a time 2π/U later.
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0%./112%.3+451($%&#)"%$.(5&65%
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impacts onto moon Mab; past disruption of
moon
origin for
R2;doubted
chaos ->- moons
Shepherding
of F-ring
now
current
unstable and will collide in few Myr ->
research
youthful dynamic system
Prometheus
Discovery of single one-armed spiral
structure in Saturn’s F ring (Charnoz et al.
2005): not shepherded by Pandora and
Prometheus as thought, but disrupted
(Prometheus steals material) and S/2004 S6
moonlet discovered by Cassini also involved
Discovery of the thin rings of Uranus
An unexpected discovery the first rings to be discovered
after the rings of Saturn
Astronomers
recording
theoflight
starexpected
that was expected
Astronomers recording
the light
a starof
thata was
to disappear
disappear behind
Uranus,
on 10 March
unexpectedly
to
behind
Uranus,
on 101977,
March
1977, unexpectedly
recorded short
dipsdips
in theinstarlight
before the
star passed
behind
recorded
short
the starlight
before
the star
passed behind
the planet (top). The same pattern was repeated when the star
the
planet(bottom),
(top). The
samethat
pattern
when the star
reappeared
indicating
narrowwas
ringsrepeated
briefly block
reappeared
(bottom),
narrow
rings
out the starlight
at the sameindicating
distance on that
opposite
sides of
the briefly block
planet.
out
the starlight at the same distance on opposite sides of the
planet.
The strong and abrupt absorption of starlight indicates that the narrow rings are quite opaque and have well-defi
above the Indian Ocean aboard the Kuiper Airborne Observatory. (Courtesy of James L. Elliot.)
The strong and abrupt absorption of starlight indicates that the narrow rings
are quite opaque and have well-defined edges.
These observations were taken from high above the Indian Ocean aboard
the Kuiper Airborne Observatory. (Courtesy of James L. Elliot.)
The rings of Uranus
dense, narrow rings
+
shepherd satellites
This Voyager 2 image, taken on
23 January
shows
fiveonof
This
Voyager1986,
2 image,
taken
the
rings1986,
of Uranus
23 nine
January
shows that
five had
of
been
previously
from
the nine
rings of inferred
Uranus that
had
Earth-based
observations
of their
been previously
inferred from
brief
occultation
of a star’s
Earth-based
observations
oflight.
their
In
thisoccultation
view, sunlight
striking
the
brief
of a star’s
light.
In thisparticles
view, sunlight
strikingback
the
rings
was reflected
rings particles
was reflected
toward
the camera,
showingback
toward
camera,
that
the the
dense
parts showing
of the rings
that the consist
dense parts
of the rings
system
of narrow
rings
system
consist
with
wide
gaps.of narrow rings
with wide gaps.
In contrast, Saturn’s main rings are broad with narrow gaps.
In contrast, Saturn’s main rings are broad with narrow gaps.
Eight of Uranus’s small satellites circle the planet
Neptune’s rings
just outside its bright epsilon ring. This image,
taken with the Hubble Space Telescope on 28 July
1997, is a false-color composite of three images
takenClumps
at differentare
infrared wavelengths in which
associated
with dim but the rings and
Uranus
appears relatively
moons
do not. Thewith
satellites range in size from
resonances
40 kilometers across, for Bianca, to 150 kilometers
nearby
satellites
for
Puck. The
arrows denote their direction of
revolution about Uranus. White clouds are seen
just above the planet’s blue-green methane atmosphere.
As Voyager 2 left Neptune in August 1989,
the planet’s narrow rings were backlit by the
Sun, enhancing the visibility of the rings’
dusty particles. The outer ring consists of
at least three dense clumps of orbiting debris,
named Liberté, Egalité and Fraternité,
which stand out from the thinner remainder
of the ring
taken at different infrared wavelengths in which
Uranus appears relatively dim but the rings and
moons do not. The satellites range in size from
40 kilometers across, for Bianca, to 150 kilometers
Jupiter’s
ringsarrows denote their direction of
for Puck. The
revolution about Uranus. White clouds are seen
just above the planet’s blue-green methane atmosphere.
Wisps
of
As Voyager 2 left Neptune in August 1989,
dustnarrow rings were backlit by the
the planet’s
Sun, enhancing the visibility of the rings’
dustyfrom
particles. The outer ring consists of
at least
three dense clumps of orbiting debris,
nearby
named Liberté, Egalité and Fraternité,
which
stand out from the thinner remainder
satellites
of the ring
The upper atmosphere of Jupiter and the planet’s main ring can be seen when the Sun is behind the planet,
and an imaging spacecraft is in Jupiter’s shadow peering back toward the Sun. In such a configuration,
very small dust-sized particles are accentuated so both the ring particles and the smallest particles in the
upper atmosphere of Jupiter are highlighted. It is somewhat like looking back at a movie projector in a
dusty theater or at a bright light in a smoky room, which permits you to see the dust or smoke in the air.
The small particles in Jupiter’s rings are believed to have human-scale lifetimes, and must be continuously
replenished if the ring persists.