Download The Terrestrial Planets

The Terrestrial Planets
• In order of distance
from the Sun:
–
–
–
–
• In order of decreasing
mass:
Mercury
Venus
Earth
Mars
–
–
–
–
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Earth
Venus
Mars
Mercury
1
Mercury
• Smallest of the terrestrial planets (0.055
M⊕, only 4.4 times as massive as the
Moon).
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2
Mercury
• Difficult to observe,
since never more than
30º from the Sun
– led to early mistakes
about its rotation
• Can sometimes be
observed in transit
across the face of the
Sun (at inferior
conjunction)
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Transit of Mercury
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Rotation of Mercury
• Period of revolution: Prev = 88 days
• Period of rotation: Prot = 59 days
2
Prot = Prev
3
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Rotation of Mercury
• An example of commensurate periods.
– One period is a simple integer multiple (or
fraction of two integers) of the other.
– Commensurate periods arise from tidal friction.
– Moon is another example (Prot = Prev).
– This is also known as a resonance.
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Rotation of Mercury
• Mercury has two permanent tidal bulges
– Fairly eccentric orbit, e = 0.21.
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Rotation of Mercury
day 29
day 15
day 0
day 44
day 88
day 74
day 59
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Rotation of Mercury
• One bulge
always points
towards Sun at
perihelion
• One solar day =
2Prev = 176 days
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Mercury
• High mean density
– 5400 kg m-3,
uncompressed
– massive Fe core
(mantle lost in
collision?)
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Mercury
• Surface is similar to
lunar farside, except:
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Mercury
• Surface is similar to
lunar farside, except:
Scarp
– scarps, some 100’s of
kilometers long
• due to shrinkage of
planet by 1-3 km
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Mercury
• Surface is similar to
lunar farside, except:
– scarps, some 100’s of
kilometers long
• due to shrinkage of
planet by 1–3 km
– not saturated with
craters
• implies surface formed
later than the Moon’s
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Mercury
• Has very thin atmosphere
– High temperature (T ≈ 700 K) precludes a
permanent atmosphere
– Transient atmosphere of solar-wind helium
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Mercury
• Has a weak magnetic
field
– Strength 10 times
weaker than Earth’s,
but detectable (the Fe
core is important)
– Can deflect the solar
wind
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Venus
• Most brilliant object in the sky (except for
Sun and Moon).
• Easy to observe (greatest elongation ~47º).
• Very slow rotator
– Prot = 243 days (retrograde)
– no detectable magnetic field
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Venus
• Shrouded in heavy
high-altitude clouds
• In terms of mass,
radius, and density,
Venus is a near-twin
of the Earth
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Venus
• Atmosphere
– Very dense CO2 atmosphere
– Very high surface temperature (~750 K)
• without the dense atmosphere, the dayside temperature would
be about 460 K
• high temperature is due to the “Greenhouse Effect”
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Temperature (K)
19
The Greenhouse Effect
• A greenhouse is a glass structure that retains
heat
– Glass is transparent to optical wavelengths, so
visible sunlight heats greenhouse
– Greenhouse interior cools by re-radiating as a
blackbody at the inside temperature (typically
about 300 K)
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The Greenhouse Effect
• Glass is opaque in
the infrared, so heat
is trapped.
• Greenhouse heats
up until rate of
heating equals rate
of cooling.
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The Greenhouse Effect
• The Greenhouse
Effect works on Venus
because of the CO2
atmosphere
• CO2 is transparent to
optical light, but
strongly absorbs
infrared light.
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Why Are Venus and Earth So
Different?
• Earth has liquid water
surface (~75%).
– Water dissolves CO2 out of
atmosphere
– Formation of solid carbonates
(e.g., limestone = CaCO3)
• Venus is too hot for liquid
water
– CO2 doesn’t dissolve out
– Atmosphere is 96% CO2
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Why Are Venus and Earth So
Different?
• Earth’s atmosphere would be like the
atmosphere of Venus if the solid carbonates
were put back into the atmosphere.
• The atmosphere of Venus would be like that
of Earth if you could take the CO2 out of
the atmosphere.
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Surface of
Venus
• Mapped by radar
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Surface of
Venus
• Less surface
variation than on
Earth
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Surface of Venus
• Very large craters (small objects burn up in
dense atmosphere)
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Surface of Venus
• Large lava domes form where magma leaks
through surface cracks.
• Very large volcanoes form because of little
plate motion.
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Radar Construction of Maat Mons
(youngest surface feature)
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Mars
• Mars has superficial
similarities to Earth
– Equator inclined ~24º
to orbital plane.
• Mars has seasons, like
the Earth
– Length of solar day
~24h
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Mars
– Transparent
atmosphere with
clouds and dust storms
• But the surface pressure
of the Martian
atmosphere is 2% that
of the Earth, and is 95%
CO2
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Mars
Mars was long suspected
of harboring life,
perhaps even intelligent
life.
Early observers reported
apparent “canals” that
connect darker features.
Sketch of Mars by
Percival Lowell
(early 1900s).
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Surface of Mars
• Cratered surface, modified by wind erosion.
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Surface of Mars
• Surface composition:
– About 44% silicon dioxide (sand)
– About 19% iron oxide (rust)
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Surface of Mars
• Large volcanoes, such
as Olympus Mons
– enormous shield
volcano ⇒ little plate
motion
– little cratering ⇒ fairly
young surface
– sharp cliffs at edge ⇒
wind erosion
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Olympus Mons
• Olympus Mons is 600 km across and 25 km
high
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• Olympus
Mons is much
larger than
similar
features on
Earth.
• Similar
features on
Earth sink
into crust.
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Surface of Mars
• Evidence of water flows (floods)
– water probably now in a permafrost below
surface
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Surface of Mars
• Some water flows
must have been
fairly massive
(e.g., tear-drop
shaped islands in
plains).
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Surface of Mars
• Valles Marineris
(“Valley of the
Mariners”, or
“The Grand
Canyon of Mars”)
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Valles Marineris
• 5000 km long by 100 km wide, 6 km deep
– due to faulting and wind erosion (atmosphere is
denser in canyon)
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Valles Marineris
Landslide
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Martian Seasons
• Seasons on Mars are affected by the
planet’s large orbital eccentricity (e =
0.093).
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Martian Seasons
winter in North
Sun
Mars at perihelion
summer in South
Mars at aphelion
Sun
summer in North
winter in South
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Martian Seasons
• Seasons on Mars are affected by the
planet’s large orbital eccentricity (e =
0.093).
• Seasons are:
– moderated in the North
– enhanced in the South ⇒ strong surface winds
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Seasonal Variations in Southern
Polar Cap
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Martian Seasons
• Polar caps change with seasons
– change quickly, so must be thin
– in winter, cold enough to freeze CO2
• cap large, mostly CO2 frost
– in summer, CO2 melts
• cap small, probably mostly residual water
Edge of polar cap
shows multiple layers
At Martian atmospheric pressure, CO2 freezes at 150 K;
water freezes at 190 K.
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Dust Storms
• Strong surface winds
produce large-scale
dust storms that on
occasion cover the
entire planet.
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Martian Clouds
• Clouds are also
sometimes seen in the
atmosphere of Mars
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Condensation of the Atmosphere
• Frost can form in lowlying areas, just like
on Earth
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Satellites of Mars
• The Earth and Mars are the only terrestrial
planets with satellites.
• Mars has two very small satellites, which
are probably captured asteriods:
– Phobos (“Fear”)
• 20 × 23 × 28 km
– Deimos (“Terror”)
• 12 km (roughly spherical)
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Deimos
Phobos
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The Jovian Planets
• Jupiter, Saturn,
Uranus, and Neptune
have all been visited
by the Voyager space
probes.
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Voyager
Trajectories
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The Jovian Planets
• Jupiter, Saturn, Uranus, and Neptune are all
massive bodies
– formed in outer part of pre-solar nebula where
ices condense
– growth by accretion and coalescence
– reached large enough mass (> 15 M⊕) to attract
and retain H and He, the most abundant
elements
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The Jovian Planets
• Jovian planets are
fluid bodies
– supported by balance
between pressure and
gravity ⇒ Hydrostatic
equilibrium
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Jovian Interiors
Central Pressure
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Interiors of Jovian Planets
• Jupiter and Saturn are massive enough to
have metallic hydrogen zones
– electrons not bound to individual atoms
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Phase Diagram for Hydrogen
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Radius of Jovian Planets
• Increasing the mass of
a Jovian planet mostly
results in compression
to higher density.
• Despite their gross
differences in mass,
the Jovian planets
have similar sizes.
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Interiors of Jovian Planets
• Jovian planets have internal heat sources
– Jovian planets radiate more energy than they
absorb from the Sun.
– Heat generated by contraction is slowly
radiated away.
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Jovian Planets
Stored Energy
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Atmospheres of Jovian Planets
• Appearance of atmosphere depends on
temperatures at highest levels.
– Jupiter
– Saturn
Methane, frozen ammonia crystals
(in Saturn, the NH3 is deeper and
harder to see, giving it a uniformly
hazy appearance)
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Jupiter
Galilean
satellite
Ganymede
Great Red
Spot
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Saturn
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Atmospheres of Jovian Planets
• Uranus
• Neptune
Methane (CH4) only; NH3 completely
frozen out.
Uranus and Neptune have a greenish-blue appearance
because methane absorbs red light.
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The Outer Jovian Planets
Uranus
Neptune
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Atmospheres of Jovian Planets
• Rapid differential rotation of Jovian planets
stretches clouds into bands.
Earth
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Jupiter
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Rotation of Jupiter
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Atmospheres of Jovian Planets
• The different colored
bands in Jupiter’s
atmosphere show
convective motion.
– Zones: Higher, cooler,
lighter colors
– Belts: Lower, warmer,
darker colors
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Jupiter’s Zones and Belts
Zones
Belts
Infrared (5μm)
Visible
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Jupiter’s “Great Red
Spot”
• Large cyclonic storm,
first discovered by
Cassini (1665).
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Uranus: The Largest Retrograde
Rotator
• Rotation axis
inclined by 98°
• Consequence:
Solar day at poles
is ½Prev = 48
years.
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Magnetic Fields
• All Jovian
planets have
strong
magnetic fields.
– massive
– rapid rotators
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Jupiter’s Magnetic Field
Magnetosphere
traps charged
particles from
volcanic eruptions
on Io.
Strong magnetic
field causes
a strong bow
shock in the
solar wind.
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The Magnetic Field of Uranus
The magnetic axis of
Uranus is highly
inclined relative to
the rotation axis.
This is not understood.
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Jovian Magnetic Fields
• As with Earth, interaction with high-energy
solar particles can lead to aurorae.
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Radio Emission from Jovian
Planets
Cyclotron Emission
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Jovian Satellites
• Galilean satellites
probably formed like
miniature Solar
System.
• Smaller Jovian
satellites are
probably captured
asteroids.
– Note that the outer
four satellites have
high eccentricity,
retrograde orbits.
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Planetary Satellites
• A few planets have satellites that are
terrestrial-planet size.
• Many satellites are small, irregular objects.
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Maximum Size of Irregular
Objects
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The Galilean Satellites
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Galilean Satellites
Satellite
Io
Distance (in Density
-3
Jupiter radii) (g cm )
5.9
3.6
Europa
9.4
3.0
Ganymede
15.0
1.9
Callisto
26.4
1.9
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Formation of Jupiter
• The trend in
density mimics the
trend seen in the
Solar System.
• Jupiter formed a
localized “warm
region” in which
the condensation
of ices was
suppressed.
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Galilean Satellites
• Io
– Most geologically
active object in the
Solar System.
– 100 m of new surface
every million years.
– No cratering.
– Energy derived from
tidal forces.
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Volcanic
Eruption on
Io
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Io Surface Changes Over Five Months
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Galilean Satellites
• Ganymede and Calllisto: icy surfaces with
embedded hydrocarbons.
Callisto
Ganymede
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Europa
• Europa is similar to
Ganymede and
Callisto, but with
fracture patterns
and flooding.
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Europa
• Suspected of having
a subsurface water
ocean.
• This is currently
regarded as the
most promising site
to search for life
elsewhere in our
own Solar System.
An icy crater on Europa
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Titan
• Saturn’s moon Titan is the
only satellite in the Solar
System with an
atmosphere.
• Probably has liquid N2
surface
• Atmospheric composition:
– 99% N2
– 1% CH4
“smoggy”
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Planetary Rings
• Rings are very thin.
– Saturn’s rings are less than 100 m thick, and
slightly corrugated (rather than completely flat).
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Planetary Rings
• Saturn’s rings disappear once every 14.5 years when
Earth crosses the orbital plane of rings.
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Planetary Rings and the Roche Limit
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Planetary Rings
• All Jovian planets have equatorial rings
inside the Roche limit.
• Reflected sunlight spectrum shows that the
rings are made of tiny particles in Keplerian
orbits:
Vorbital ∝ r
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−1 / 2
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Spectrograph
slit position
Red shift
Spectral
line
Blue shift
Solid-body
rotation
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Keplerian orbits V ∝ r -1/2
102
Origin of the Rings
•
Rings might represent:
1) unformed bodies
2) broken-up (tidally disrupted) bodies
•
Current evidence favors second
explanation because rings are not expected
to be long-lived phenomenon.
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Shepherd
Satellites
• Shepherd satellites are
small satellites within
the ring system that
keep the satellites
from diffusing away
from their original
orbits.
Rings of Uranus
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Ring Destruction
• Large particles will undergo collisions that
grind them down.
• Small particles are removed by solar wind
and radiation pressure.
• Particles slowly diffuse away from their
initial orbits.
– This process can be controlled and avoided by
shepherd satellites.
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How Shepherd Satellites Work
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Detailed Structure of Saturn’s
Rings
Cassini division
Encke division
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Ring Structure
• Ring structure can be very detailed.
• Structure is due to perturbations by
satellites.
– Particles in Cassini division in 2:1 resonance
with Mimas, the largest of the inner satellites of
Saturn.
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Perturbing Ring Orbits
• A particle in resonance
with a satellite is
perturbed repeatedly at
the same place in its
orbit, gradually
making the orbit more
eccentric.
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Perturbing Ring Orbits
• Particles in non-circular orbits are most
likely to undergo collisions, which will tend
to circularize their orbits at a new location.
• By this process, particles in resonant orbits
are gradually removed.
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Sizes of Ring Particles
• Sizes are determined by scattering
characteristics
– Large particles are seen in backscattering
(essentially reflection).
– Small particles (compared to wavelengths of
light) are seen in forward scattering.
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Sizes of Particles in Saturn’s Rings
• Sizes of particles in
Saturn’s ring range
from mm to several
meters (house-size).
• Composition seems to
be mostly water ice
(highly reflective).
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Sizes of Ring Particles
• Uranus also has
large particles
(centimeter to
meter size), but
they are hard to see
because they are so
dark.
– Reflect only about
5% of visible light,
similar to coal!
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Jovian Planet Rings in
Forward Scattering
Except for Saturn, Jovian rings
are best seen in forward
scattering.
Neptune
Jupiter
Uranus
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Ring Particles
• Forward scattering
shows that much of
the ring material is
microscopic dust.
• Except for Saturn,
rings are dark.
– Mostly rocky or ice
with embedded
hydrocarbons.
Uranus in forward scattering
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Spokes in Saturn’s Rings
• Persistent radial features called spokes are
sometimes seen in Saturn’s rings.
• These do not rotate in a Keplerian fashion.
• These are probably due to microscopic dust
particles that are electrically charged and
trapped by Saturn’s magnetic field.
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Spokes in Saturn’s Rings
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Origin of Spoke Material
• Origin of dust in spokes is unclear, but
thought to be due to meteoroids.
• Dust in spokes suggests that Saturn’s rings
are young; prolonged accumulation of dust
would make them dark.
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