Download Satellites and Rings II

Lecture #31: Satellites and Rings II
The Main Point
• Ring Systems
– Overview of rings:
All of the giant planets have rings, which are
just billions of small dusty, rocky, or icy
moons traveling in similar orbits around a
planet according to Kepler’s laws.
• Jupiter,
Jupiter Saturn
Saturn, Uranus,
Uranus Neptune.
Neptune
– What are rings made of?
– How do they form?
• Reading: Chapter 11.3.
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Planetary Ring Systems
Overview
• All four of the giant planets have rings.
• Each ring system consists of billions of small
"moonlets" orbiting close to their planet.
• Ring systems exhibit complex structures and
patterns caused by interactions with larger moons.
Properties of Ring Systems
________________________________________________________________________________________________________________________________________
Planet
Jupiter
Saturn
Uranus
Neptune
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Outer Radius
(km)
Rplanet
128,000
1.8
140,000
2.3
51,000
2.2
63,000
2.5
Mass
(kg)
~1010
1019
1014
1012
Reflectivity
(%)
~5
60
~5
~5
3
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What are Rings?
Rings and Tides
• Rings exist inside
the "tidal stability
limit" of a planet.
• Within this limit,
known as the
Roche limit,
limit a
planet's gravity
should break up
satellites.
• Only very small
moons and rings
exist within this
zone.
• Billions of moons!
• marble-sized to house-sized (Saturn)
• dust-sized (Jupiter, Uranus, Neptune)
• Rings
g are collections of huge
g numbers of small bodies,, each
moving on its own, independent, Keplerian orbit.
• Because of Kepler's Third Law, inner ring particles must
move faster than outer ring particles (differential rotation):
- Rings don't rotate as a solid body.
• Particles collide & interact gravitationally with each other:
- Waves and other phenomena are generated in the process.
Cloud Tops
Each planet's ring system drawn to a different scale.
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Saturn's Rings
Where Do Rings Come From?
• Small impacts on small moons create dust-sized
ring particles that disperse into an orbiting disk.
• Occasional larger impacts on small moons can
shatter the moon and release many
man boulder-sized
bo lder si ed
ring particles.
One of the most
beautiful sights in
the solar system!
Saturn's Ring System
_________________________________________________________________________________________________________________________________
Ring
F
A
Cassini Division
B
C
Outer Edge
(km)
Rplanet
140,180
2.324
136,780
2.227
122,170
2.025
117,580
1.949
92,000
1.525
Width
(km)
90
14,600
4,590
25,580
17,490
CB A F
Cassini Division
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Cassini Images of Saturn’s Rings
• There are actually
thousands of rings and
gaps.
• Gaps can be cleared by
so-called gap moons
orbiting within the rings.
• Other gaps are due to
orbital resonances with
small inner moons.
• Two nearby gap moons
can shepherd a narrow
ring.
• Some rings are dense &
bright, others thin &
transparent.
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Rings in their natural color
Cassini s first image
Cassini’s
of the F ring
"Spokes" seen in the rings
are thought to result from
interactions between ring
particles and Saturn’s
magnetosphere.
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False color image of two
densityAstro
waves
in Saturn's
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A ring
Jupiter's Rings
Rings are Thin!
•
• Saturn's rings are about 6 times wider than Earth.
• But they are less than 100 meters thick!
• Sometimes the rings appear edge-on from Earth.
•
Discovered from infrared telescopes
and Voyager 1.
Very different from Saturn's!
•
Three main components:
– Very dark, very small particles.
– Halo
alo
• Broad, faint inner torus.
Saturn
"ring plane
crossing"
observed
with HST
in 1996.
– Main Ring
• 7000 km wide.
• Edge radius is 129,000 km.
– Gossamer Rings
• Broad, faint outer disks.
•
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Dust particles from the small inner
satellites of Jupiter appear to be the
source of ring material.
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How were
Jupiter’s rings
observed?
Light Scattering
Observing rings at
different viewing
geometries provides a
way to determine the
sizes of ring particles.
• Jupiter's rings are very dark and
very faint.
• Hard to see against the glare of
Jupiter itself.
• The best images were obtained
when the Galileo spacecraft flew
into Jupiter's shadow and looked
back towards the Sun
• Amazing navigation feat!
Astronomers have
also used radar to
estimate the sizes of
ring particles.
Confirmed by Voyager
radio occultation data.
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Uranian Rings
Distance (km)
------------38000
41840
42230
42580
44720
45670
47190
47630
48290
50020
51140
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Neptune's Rings
Ring
------Diffuse
Inner
Plateau
Main
10 main narrow rings,
1 diffuse ring.
Ring
------1986U2R
6
5
4
Alpha
Beta
Eta
Gamma
Delta
1986U1R
Epsilon
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Width (km)
---------2,500
1-3
2-3
2-3
7-12
7-12
0-2
1-4
3-9
1-2
20-100
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Distance
(km)
-------41900
53200
53200
62930
Width
(km)
----15
15
5800
< 50
Name
------Galle
LeVerrier
Lassell, Arago
Adams
(distance is from Neptune's center to the ring's
inner edge)
• 3 main narrow rings, 1 diffuse.
• Main rings appear “clumpy”:
• Once thought to be arcs, not rings.
• Voyager showed that they are rings.
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(distance is from Uranus' center to the ring's
inner edge)
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Keeping it all together: Shepherds
Ring Composition
Small inner moons of the giant
planets can gravitationally
"shepherd" the ring particles,
maintaining their stability.
• Saturn's rings are bright and slightly red:
– Spectra show evidence for water ice.
– Red component
p
unknown... "dirt"? "organics"?
g
• Jupiter's, Uranus', and Neptune's rings are
all very dark and gray:
Pandora and Prometheus,
Shepherding Saturn's tenuous F ring
Two
Uranian
shepherds
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– Not water ice?
– "Carbonaceous" material? Dark "organics"?
• Differences due to different ring origins?
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Next Lecture...
Summary
• The Trans-Neptunian Region
• All of the outer gas giant planets have ring systems.
• Saturn's rings are made up of billions of chunks of
dirty ice, each obeying Kepler’s laws.
• The rings of Jupiter
Jupiter, Uranus
Uranus, and Neptune are
composed of smaller (dust-sized) and much darker
particles of unknown composition.
• Rings occupy regions within the tidal stability
zones of their planets:
– Pluto:
• Discovery.
• Physical
Ph i l characteristics.
h
i i
• Charon.
– Other Kuiper Belt Objects.
• Reading: Chapter 12.3
– Ongoing impacts on small moons create ring particles
and explain why rings are common.
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