Chapter 11: Moons, Rings and Pluto

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Chapter 11 Moons,Rings and Pluto
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Ring and Satellite Systems

• General properties.
• Composition different from objects in the inner
  solar system
• Most contain dark, organic compounds mixed with
  ice and rock
• Dark materials imply small reflectivity
• Most satellites in regular orbits
• West-to-east direction
• In the plane of the planets equator
• Likely to have formed at about the same time as
  the planet
• Irregular satellites
• Retrograde (east-to-west), or else have orbits of
  high eccentricity, or high inclination
• Usually smaller satellites, located relatively
  far from their planet
• Formed subsequently or captured

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Jupiter's Moons

• 52 known satellites (as of about 2 years ago)
• Four large moons (Callisto, Ganymede, Europa, Io)
• Europa, Io are the size of our moon
• Ganymede, Callisto the largest are bigger
  than mercury
• Many smaller moons.
• Note moon and satellite mean the same thing in
  astronomy.

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Large "Galilean" Moons
Io

• Callisto, Ganymede, Europa, Io
• first seen by Galileo (the astronomer)
• studied by the Galileo space probe
• Hubble Space Telescope observations
• combined data has found important similarities to
  the terrestrial planets
• differences between moons mostly due to distance
  from Jupiter

Europa
Ganymede
Callisto
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Callisto Crater Packed

• Outermost of the Galilean satellites.
• 2 million km from Jupiter.
• Noontime surface temperature 130 K (140oC below
  freezing!)
• Diameter 4820 km same as Mercury
• Mass 1/3 Mercurys mass
• Not as dense as Mercury
• Composed largely of ice.
• Not fully differentiated.
• From details of gravitational pull on Galileo
  spacecraft.
• Surprising!
• Yet appears to be frozen
• Covered with craters
• Absence of interior forces geologically dead.

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Craters on Callisto
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Ganymede Largest, Most Varied

• Largest satellite
• Cratered, but less so than Callisto
• ¾ of the surface seem to have formed recently
• 1 billion years old rather than 4.4
• Differentiated
• Rock and metals sank to form a core about the
  size of the Moon
• Mantle crust floating above core.
• Magnetic Field
• partly molten interior

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Ganymede

• Ganymede is the largest moon in the solar system
• Diameter 5262 km
• Slightly larger than Mercury
• Ganymede should have little tidal heating due to
  its distance from Jupiter
• But, Ganymede shows evidence for surface
  alteration
• Ganymede must have had more geological activity
  in the past
• Did it once have a more eccentric orbit?

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Old Dark Terrain

• Old it is covered with craters
• Dark ice covered with dust from meteoroid impacts

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New Bright Terrain

• New due to fewer craters
• Bright due to fracturing of the icy surface

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Craters varying degradation

• Geologically active.
• Younger terrain result of tectonic and volcanic
  forces.
• Extensive Mountain ranges formed by compression
  of the crust.
• Some indication of large scale crustal movements.

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Why is Ganymede different from Callisto?

• Small difference in
• size.
• Internal heating.
• Gravity of Jupiter
• Ganymede close enough to Jupiter to have tidal
  force effects episodically heating the crust.

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Europa Ice-Covered Ocean
real color
enhanced color
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Europa

• Europas surface is covered with a layer of ice
• Under the ice is water or warm fluid ice
• Water flows up to the top continually resurfacing
  Europa
• Tidal heating produces the internal energy
• It also has differentiated Europa into an iron
  core, a rocky mantle and an icy crust
• Tidal flex may also crack the surface

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Evidence for Warm Oceans on Europa

• Europa has ice rafts where the surface has been
  broken up and reassembled
• Galileo has imaged faults where the ice has
  pulled apart and water as flowed up
• Europa also has smooth areas where water has
  flowed up and re-frozen
• On Earth simple life forms evolved under water at
  warm deep ocean vents
• Could something similar have happened on Europa?

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Io
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Io Volcano World

• Io is the most volcanically active world in the
  solar system
• Io has an elliptical orbit, so the tidal forces
  on it vary with time
• These changing forces squeeze and flex Io
  producing heat
• The hot interior produces massive volcanism
• The interior heat has also produced a
  differentiated interior
• Io has an iron core surrounded by a molten, rocky
  mantle

Jupiter
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Volcanism on Io

• Io has no impact craters
• They have been eradicated by lava
• Volcanoes produce plumes of material that extend
  up to 280 km above the surface
• The colors on Io come from sulfur (yellow, black,
  red) and from sulfur dioxide (SO2, white)
• Volcanoes can be very long lived
• Some have been observed for 20 years

Loki volcano erupts
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Pele Volcano
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Ios Plasma Torus

• Ios volcanoes put lots of ions into its orbit
• Ions are atoms that have lost an electron giving
  them a net electrical charge
• The ions are effected by Jupiters magnetic field
  producing a plasma torus
• As Jupiter rotates its changing magnetic field
  produces an electrical current through the torus
  and interior

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Saturns Moons and Rings

• 30 known satellites (before Cassini)
• Titan largest of Saturns satellites
• Almost as big as Ganymede
• Only satellite with substantial atmosphere
• Rings of Saturn

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Titan

• The second largest moon in the solar system
• Hint on the test, dont be fooled by the name!
• Only moon with a significant atmosphere
• Thick atmosphere makes the surface impossible to
  see
• Why does Titan have an atmosphere?
• Titan is large enough to have a strong
  gravitational field
• Titan is cold enough so that the gas in the
  atmosphere is slow moving

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Titan Cloud World

• Dense atmosphere
• pressure 1.5 times Earth's
• mostly nitrogen plus 6 argon and a few percent
  methane.
• trace amounts of organic compounds (i.e. ethane,
  hydrogen cyanide, carbon dioxide) and water
• water is formed when methane in Titan's upper
  atmosphere is exposed to sunlight.
• chemical activity despite low surface
  temperature, 94 K (-290 F).
• like the smog found over large cities, but much
  thicker.
• Conditions like Earth early in its history when
  life was first getting started.
• May have the necessary building blocks for life!

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Huygens Probe Lands on Titan
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Uranus System

• Ring and Satellite tilted at 98o just like the
  planet itself.
• 11 rings
• Composed of very dark particles
• Discovered 1977
• Narrow ribbons of material with broad gaps
• very different from the rings of Saturn
• 20 known satellites none really large

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Neptune System

• 8 known satellites
• 6 regular close to the planet
• 2 irregular farther out
• Triton (remember, Neptune carries a trident in
  mythology, Triton is Neptunes son)
• large moon in retrograde orbit,
• has an atmosphere,
• active volcanism,
• Bears some resemblance to Pluto

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Triton Ice World
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Triton Ice World

• orbit is retrograde
• didnt form with Neptune!
• Perhaps a captured Kuiper Belt object
• Capture may have shattered another Neptunian moon
• Capture scenario accounts for
• Triton's orbit
• unusual orbit of Nereid
• provides energy to melt and differentiate
  Triton's interior
• Historical connection to Pluto?
• similar bulk properties
• Pluto has eccentric Neptune-crossing orbit

Is Triton a captured comet? What about Pluto?
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• Ice volcanoes on Triton
• plume rising 8 km above the surface and extending
  140 km "downwind"
• Triton eruptions of volatile gases like nitrogen
  or methane driven by seasonal heating from the
  Sun.
• Earth, Venus, Mars rocky magma driven by
  internal heat.
• Io sulfur compounds driven by tidal
  interactions with Jupiter.

plume
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Triton's Atmosphere

• Tritons Atmosphere
• Triton has a very thin nitrogen atmosphere (1.6 X
  10-5 atmospheres of pressure)
• Triton is very cold (37 K) and thus nitrogen is
  mostly frozen on the surface
• A little bit of nitrogen evaporates to produce
  the atmosphere
• Motions of the atmosphere (wind) seem to effect
  the plumes

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Tenuous Clouds
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Summary Six Large Moons

• The six large moons of the gas giants resemble
  the terrestrial planets of the inner solar system
  
• They can have volcanoes, atmospheres, and
  evidence of resurfacing
• In general they are cold and have rocky interiors
  and icy exteriors
• Some produce internal energy through tidal
  heating
• Europa and Titan may have the conditions for life
  to exist

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Pluto
HST Picture

• Discovered through systematic search.
• At P. Lowell observatory in 1930.
• Named Pluto after the roman god of the underworld
  (also PL are the initials of Percival Lowell).
• Highest inclination to the ecliptic (17o).
• Largest eccentricity 0.248.
• Average distance 40 AU 5.9 billion km.
• Perihelion closer than Neptune
• Orbital period 248.6 earth years.
• Rotation 6.4 days on its side.
• Pluto's diameter 2240 km
• Largest satellite Charon
• Charons orbit is locked to Pluto Charon
  revolves and rotates in the same time as Pluto
  rotates.
• Also two smaller satellites found.

Charon
Pluto
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Pluto Basics

• Not visited by spacecraft,
• very faint,
• observation requires best telescopes
• Diameter 2190 km (60 of the Moon)
• Density 2.1 g/cm3
• Mixture of rocky material and water ice
• Similar to Triton (Neptune)
• Highly reflective surface
• frozen methane, carbon monoxide, nitrogen
• Surface temperature 50 K/ 60 K
• Tenuous atmosphere.

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Quaoar new planet?

• orbit more circular than Pluto's
• closer to the ecliptic 7.9 degree inclination
  compared to Pluto's 17 degrees.
• diameter 1280 km vs. Pluto's 2240 km
• possibly Pluto and Quaoar are both Kuiper belt
  objects

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Pluto's Orbit
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11.3.4 The Nature of Pluto

• Pluto is not like the Terrestrial or Jovian
  planets.
• Pluto, Quaoar, Xena, Charon, and possibly Triton,
  are examples of Kuiper belt objects.

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Rings

• All four giant planets have rings
• Each ring is a system of billions of small
  particles (moonlets).
• Each ring displays complicated structure related
  to the interaction between the rings and
  satellites.
• The four ring systems are quite different.

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Ring Basics

• Saturn
• Made up of icy particles spread out into several
  vast, flat rings, with a great deal of fine
  structure.
• Neptune/Uranus
• Made up of dark particles, confined to a few
  narrow rings, with broad empty gaps.
• Jupiter
• Rings are transient dust bands, constantly
  renewed by erosion of dust grains from small
  satellites

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What causes Rings?

• Ring collection of vast numbers of particles
• Each particle obeys Keplers laws.
• Inner particles revolve faster
• Ring does not rotate as a solid body.
• Better to consider the revolution of individual
  moonlets.
• Particles within the ring are close to one
  another.
• Exert mutual gravitational influence, even
  collide in low speed collisions.
• Gives rise to waves that move across the rings.
• Two basic theories
• Breakup theory, remains of a shattered satellite.
• Make up of particles that did not fuse into a
  single body

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Ring Causes Continued

• Gravitation of the planet
• Tidal forces for orbits close to the planet, can
  tear bodies apart, or inhibit loose particles to
  come together.
• Rings of Saturn, Uranus are close to the planet
• Breakup a satellite, or a passing comet may
  have come too close and torn apart under tidal
  forces, or through some collision.
• It is believed that some of the rings are young,
  and must therefore be the result of a breakup.

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Rings of Saturn

• Many rings and sub-rings A, B, C
• B-Ring Brightest, most closely packed particles
• A/C-rings translucent.
• Total mass of B that of icy satellite 250 km in
  a diameter.
• A B separated by a wide gap called Cassini
  division.
• Rings are broad and very thin.
• Main ring 70000 km, thickness 20 m.
• Composed of water ice.
• Particles range from grains the size of sand up
  to house-sized boulders
• A handful of narrow rings 100 km, in addition
  to the main rings.

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Rings of Uranus and Neptune

• Narrow and black
• Almost invisible from Earth
• Nine rings discovered (1977) during observation
  of a star occultation
• First seen by Voyager (1986)
• Outermost and most massive called Epsilon
• 100 km wide, 100 m thick, 51000 km from the
  planet.
• Other rings much smaller 10 km wide.
• Particles are very dark black carbon and
  hydrocarbon compounds.
• Rings of Neptune are similar but even more
  tenuous.

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Satellite-Ring Interactions

• Rings have intricate structure as discovered by
  Voyager.
• Structures due to mainly gravitational effects of
  satellites.
• Without satellites, the rings would be flat and
  featureless.
• There could even be no rings at all
• Gaps in Saturn A-ring result from gravitational
  resonances with smaller inner satellites. (Mimas)

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• Uranus' moon Miranda innermost and smallest of
  the five major satellites, just 480 kilometers
  (about 300 miles) in diameter.
• Two major strikingly different types of terrain
• old, heavily cratered, rolling terrain with
  relatively uniform reflectivity.
• young, complex terrain characterized by sets of
  bright and dark bands, scarps and ridges (ovoid
  regions at right and left and the distinctive
  chevron feature below and right of center).
• likely due to upwelling of partially melted ices

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Discussion Question

• Astronomers wish to search for life in the ocean
  believed to lie beneath the ice of Europa. How
  should we approach this exploration to avoid
  possible cross contamination of Earth and Europa
  with organisms (DNA) from each other?