Astronomy 330

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Astronomy 330

• Lecture 22

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Astronomy 330 Small Moons and Rings

• All of the outer, gas giant planets have many
  small bodies (satellites) in orbit around them in
  addition to the larger satellites we discussed
  previously,
• Further all the gas giants also have rings
  systems. The ring systems of Jupiter, Uranus and
  Neptune have only recently been detected.

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Astronomy 330 Moons of the Solar System (not all
shown!)
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID181
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Astronomy 330 Jupiters Rings from Galileo
http//pds-rings.seti.org/jupiter/galileo/PIA00657
.html
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Astronomy 330 Saturns Rings from Voyager
http//photojournal.jpl.nasa.gov/catalog/PIA01389
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Astronomy 330 Uranus and its Rings
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID136
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Astronomy 330 Neptunes Rings from Voyager
http//photojournal.jpl.nasa.gov/catalog/PIA01997
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Astronomy 330

• Divisions of out planets satellites
• Regular satellites - orbits are low eccentricity
  near the equatorial plane of their planet
• Irregular satellites - peculiar orbits, high
  eccentricity or high inclination relative to
  equatorial plane, or both
• Some satellites revolve in a retrograde fashion
  (e.g. Triton is irregular due to its retrograde
  orbit).

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Astronomy 330

• The irregular satellites have orbits resembling
  short period comets or asteroids while the
  regular satellites orbits resemble the planets
  revolving around the sun.
• It is most probable that the irregulars are
  captured satellites (i.e. bodies that just got
  too close to one of the planets and went into
  orbit around it).

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Astronomy 330 Orbits of Jupiters satellites
http//www.ifa.hawaii.edu/sheppard/satellites/orb
its.html
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Astronomy 330

• The orbits of the inner most satellites overlap
  with the outermost rings.
• The rings influence these satellites and vice
  versa.
• Because the regular satellites have the orbits
  that they do, they probably formed with planet in
  a sub nebula in an analogous way to the way the
  entire solar system formed.
• This sub nebula took the form of a disk of gas
  and dust in orbit around the forming planet, the
  satellites then formed in this disk.

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Astronomy 330 Saturns F ring and a small
satellite
http//photojournal.jpl.nasa.gov/catalog/PIA01951
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Astronomy 330

• The rings also can be considered regular
  objects in the way we just discussed and they may
  have formed from the sub-nebula as well (but new
  evidence which we will discuss counters this
  assertion).
• All the rings are within a distance of 2.5 times
  less than their planets radius.
• The inner planets do not have rings since the
  small particles that make up the rings undergo
  gravitational perturbations as well as
  interactions with the solar wind and its
  radiation will cause them to spiral into the
  planet.

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Astronomy 330

• Only the outer planets have the right conditions
  for rings to be present.
• It is unknown how the rings formed or how stable
  they are and how long they will last.

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Astronomy 330 History of discovery

• Galileo discovered Galilean satellites in 1610
  (They are???)
• Christian Huygens discovered Titan in 1655.
• Cassini, Herschel, and Nicholson also made
  discoveries of satellites.
• Voyagers discovered many small satellites.
• 3 for Jupiter
• 11 for Saturn
• 10 for Uranus
• 6 for Neptune

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Astronomy 330 The Jupiter System

• The four large, Galilean satellites range in size
  from a little smaller than the Moon to larger
  than Mercury.
• 4 small satellites orbit inside of Io.
• The rest are all distant irregulars which are
  probably captured comets or asteroidsrich in
  volatile compounds.
• The rings of Jupiter were also discovered by
  Voyagers in 1979.

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Astronomy 330 Jupiters rings

• Primary ring - 54,000 km from the planet and is
  about 5000 km widevery tenuous when compared to
  any other rings of other planetssurrounded by
  even more tenuous material called gossamer rings.
  A torus of material occurs inside the radius of
  the main ring.
• Particles in the rings are very small (on the
  order of 1 micron), like the particles in smoke.

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Astronomy 330 Jupiters ring system
http//photojournal.jpl.nasa.gov/catalog/PIA03001
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Astronomy 330

• The particles in the rings are composed of dark,
  silicate dust and come from small impacts and
  energetic particle impacts (from the
  magnetosphere) on the two smallest inner
  satellites.
• The rings diffuse over time since each particle
  acquires a small electric charge due to there
  small size and their interactions with the
  magnetosphere.
• The ring material is constantly replenished by
  impacts with the small moons.

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Astronomy 330 The Saturn System

• The largest, regular ring system.
• Titan is the size of Ganymede and Callisto.
• 6 satellites with diameters of 400-1,500 km.
  (Jupiter has no satellites of this size).
• The rest of Saturns satellites are irregular and
  small and dark (with the exception of Iapetus).

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Astronomy 330 Saturns complicated rings
http//photojournal.jpl.nasa.gov/catalog/PIA02275
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Astronomy 330 Saturns Moon Iapetus from Cassini
http//photojournal.jpl.nasa.gov/catalog/PIA06166
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Astronomy 330 Mimas, Dione, Rhea, and Saturns
rings
http//photojournal.jpl.nasa.gov/catalog/PIA06642
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Astronomy 330 Uranus and Neptune Systems

• Uranus has 15 irregular satellites (same as
  Saturn). They orbit in the equatorial plane of
  Uranus and these orbits have the high inclination
  of Uranuss axis of rotation.
• The five largest size satellites of Uranus are
  about the same size as the medium sized
  satellites of Saturn.

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Astronomy 330 Small satellites and a ring of
Uranus
http//photojournal.jpl.nasa.gov/catalog/PIA00368
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Astronomy 330

• Neptune has 2 systems of satellites.
• 6 are in regular orbits and 2 are highly
  irregular.
• Triton is large and in a retrograde orbit.
• Nereid has the highest eccentricity of any
  satellite (0.75).
• The regular satellites are all close to Neptune,
  just as the inner satellites of Uranus.

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Astronomy 330 Neptunes Rings and Nereid
http//photojournal.jpl.nasa.gov/targetFamily/Nept
une?subselectTarget3ANereid3A
http//photojournal.jpl.nasa.gov/catalog/PIA02224
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Astronomy 330

• Both Uranus and Neptune have substantial rings,
  but they are not like those of Saturn.
• Their rings are narrow and dark, not broad and
  bright.
• These rings are much less massive than Saturns
  and they are narrow with wide gaps between them.
  The rings of Saturn are broad with narrow gaps
  between them.

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Astronomy 330 Overview of the small satellites

• Most of these satellites have not been studied up
  close and in detail.
• We will focus our discussion on a few chosen
  ones.
• Reflectivity (albedo) is one way of studying
  these satellites and the reflectivity of Saturns
  satellites is found to be very high on average.

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Astronomy 330

• Apparently these satellites are covered with ice.
  But comets are mostly ice and they have dark
  coatings of dust. Also, these satellites are
  heavily cratered. Therefore they must have had
  ice deposited on their surfaces early in their
  lives and they have not changed much since then.
• So something different must have happened in the
  lives of Saturns satellites when compared with
  those of Jupiter, Uranus and Neptune.

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Astronomy 330 Saturns medium moons

• Medium size moons, the rings, and a dozen small
  satellites form a regular system around Saturn.
• Rhea, Dione, Tethys and Mimas are medium sized
  moons of Saturn which are all similar to one
  another and well behaved.
• They are all of roughly the same density and bulk
  composition.
• Enceladus and Iapetus (to be discussed later) are
  different and have unique properties.

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Astronomy 330 Rhea
http//photojournal.jpl.nasa.gov/catalog/PIA06575
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Astronomy 330 Dione
http//photojournal.jpl.nasa.gov/catalog/PIA06162
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Astronomy 330 Tethys
http//photojournal.jpl.nasa.gov/catalog/PIA01397
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Astronomy 330 Mimas
http//photojournal.jpl.nasa.gov/catalog/PIA06582
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Astronomy 330

• This system of moons and rings around Saturn is
  quite different than the system surrounding
  Jupiter. This may be due to Saturns smaller
  mass, the lower temperature in the sub-nebula out
  of which the system formed, or other unknown
  processes.

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Astronomy 330 Rhea

• Rhea is the largest satellite in this class of
  Saturns satellitesdiameters 1,530 km (half as
  big as Europa, but 50 larger than the asteroid
  Ceres).
• Density of 1.3 g/cm3 and is lower than any other
  solid body in the solar system.
• This low density is due to the fact that Rhea is
  less compressed than other bodies, but is still
  composed of the same stuff as the other
  satellites of the outer planets.
• Rhea is half water ice and half rock (silicate
  minerals and metals)like the galilean
  satellites.

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Astronomy 330

• Rhea has a high reflectivity (60) and its IR
  spectrum is dominated by absorption bands of
  water.
• It is heavily cratered, so it has not been
  resurfaced as is the case for Europa.
• The craters also look much like those on the
  Moon.
• The crater density on Rhea is about 1000 10 km
  craters per 1 million km2. This is as high as
  the lunar highlands.

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Astronomy 330 Closeup of Rhea
http//photojournal.jpl.nasa.gov/catalog/PIA02240
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Astronomy 330

• There is no evidence of geologic activity which
  would erase craters.
• Dione, Tethys and Mimas display many of the same
  characteristics just described for Rhea.
• They all have surfaces of nearly pure water ice,
  similar bulk compositions and many craters.
• Since these satellites are close to Saturn,
  Saturns gravity will have an effect.

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Astronomy 330

• Comets and other debris will be attracted into
  the vicinity of Saturn and result in many impacts
  on these satellites.
• Further, the closer the satellite to the planet,
  the impacts we would expect. This is what is
  seen form Mimas, the closest satellite in this
  class to Saturn.

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Astronomy 330 Enceladus and Iapetus

• Enceladus appears to have remain volcanically
  active even though it is very small in size.
• It has its own ring, the E ring of Saturn is
  closely associated with Enceladus.
• Enceladus is almost 100 reflective. This results
  in a low surface temperature of 55K.
• Iapetus orbits in the outer part of the Saturn
  system and has one light side and one dark side.

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Astronomy 330 Enceldus
http//photojournal.jpl.nasa.gov/catalog/PIA06187
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Astronomy 330

• Enceladus seems to be the source for the E ring
  around Saturn.
• The E ring is faint and thin and is composed of a
  cloud of small, icy particles and fills much of
  the space between the orbits of Mimas, Enceladus,
  and Tethys.
• The max. brightness of the E ring occurs near the
  orbit of Enceladus and these ring particles
  cannot remain near Saturn for very long due to
  radiation pressure effects. Therefore these
  particles must be replenished or the ring has
  only formed recently.

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Astronomy 330

• Enceladus is the likely source for these icy
  particles on the E ring.
• Also, these icy particles could be a source which
  is coating the other bright medium sized
  satellites of Saturn.
• Over much of the surface of Enceladus the impact
  carters have been erased.
• These smooth areas are probably no more than a
  few hundred million years old.
• Smooth area also show ridges and flow marks.

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Astronomy 330 Ridges on Enceladus
http//photojournal.jpl.nasa.gov/catalog/PIA06191
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Astronomy 330

• This is evidence for volcanism of water or
  cryovolcanism.
• Cryovolcanism is just like volcanism on earth
  except that it involves flows of partially melted
  ice, not silicates.
• Water seems to what caused these features on
  Enceladus, but other materials with lower melting
  points might be important on satellites of Uranus
  and Neptune.

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Astronomy 330

• What is the source of heat for this cryovolcanism
  on Enceladus?
• Enceladus is less active than Io, but it is also
  smaller and it is composed of a large fraction of
  ice.
• It still is not clear whether tidal heating plays
  a role on Enceladus as on Io since Enceladus does
  not have a high eccentricity and keeps the same
  face towards Saturn.

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Astronomy 330 Iapetus

• A very dark leading hemisphere and a light,
  bright trailing hemisphere
• Iapetus keeps the same towards the planet Saturn
  so the brightness of Iapetus when viewed
  telescopically changes dramatically.
• The bright side is water ice with 50
  reflectivity
• The dark side is reddish-black is probably
  covered with carbon compoundsonly 3
  reflectivity.

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Astronomy 330 Light and Dark on Iapetus
http//photojournal.jpl.nasa.gov/catalog/PIA06170
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Astronomy 330

• The dark martial is consistent with a mixture of
  nitrogen-rich organic material mixed with carbon
  and icebut its exact composition is unknown
• Similar dark material is common in outer solar
  system on some asteroids and on some comets.
• Also, the dark material the make up Uranuss and
  Neptunes rings and coats their inner satellites
  has an equally low reflectivity (but is not red).
  Could be different combinations of the materials
  listed above.

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Astronomy 330 More views of Iapetus (Cassini)
http//photojournal.jpl.nasa.gov/catalog/PIA06145
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Astronomy 330

• Voyager measured the mass of Iapetus and its
  density is similar to the other icy satellites.
• Iapetus is of similar composition to the other
  icy satellites and the dark marterial is likely a
  coating which came from somewhere else.
• It is unknown how this material got on the
  surface of Iapetus, but the most likely
  explanation is by an impact which deposited the
  material there.

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Astronomy 330 Miranda, a satellite of Uranus

• Uranus has no satellite with a diameter larger
  than 2000 km.
• It has 5 medium sized satellitesMiranda, Ariel,
  Umbriel, Titania, and Oberon.
• The sizes of the satellites ar similar to those
  of the inner satellites of Saturn.
• Their densities are also similar1.3 - 1.6 g/cm3
  which suggests a slightly higher concentration of
  silicates and metals than ices.

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Astronomy 330 Ariel
http//photojournal.jpl.nasa.gov/catalog/PIA00041
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Astronomy 330 Miranda
http//photojournal.jpl.nasa.gov/catalog/PIA02217
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Astronomy 330 Oberon
http//photojournal.jpl.nasa.gov/catalog/PIA00034
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Astronomy 330 Titania
http//photojournal.jpl.nasa.gov/catalog/PIA01979
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Astronomy 330 Umbriel
http//photojournal.jpl.nasa.gov/target/Umbriel
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Astronomy 330

• The surface composition of Uranuss satellites
  also resembles that for the Saturnian inner
  satelliteswater ice but with a reflectivity of
  20 - 30 (dirty ice).
• It was thought Miranda would be dull, but Voyager
  discovered it wasnt!
• Miranda was the only such satellite to get a
  close flyby of Voyager and this was only be
  chance. It was simply in a good position for the
  path Voyager needed to take to get it to Neptune.

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Astronomy 330

• Mirandas diameter
• Miranda has a surface extensively modified by
  internal processes.
• It shows valley systems of 50 km across and 10 km
  deepindicates tectonic activity.
• Oval and trapezoidal mountains cover half of the
  surface.
• Craters are softened by overlying material in
  some areas.
• A cliff observed which is 10 - 15 km high.

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Astronomy 330

• Why is Miranda active at all? One wouldnt
  expect this since it is small and must have
  cooled since its formation.
• One suggestion A HUGE impact shattered Miranda
  into several large pieces which then came back
  together.
• Or, Miranda was too small to complete
  differentiation and internal mixing before it
  cooled.

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Astronomy 330 Miranda closeup
http//photojournal.jpl.nasa.gov/catalog/PIA01354
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Astronomy 330 The Small Inner Satellites of
Saturn

• Are all icy and bright objects
• All have unique orbits.
• 3 innermost small satellites are embedded in the
  rings of Saturn.
• The next 3 farther out are at the edge of the
  rings system and interact with them.
• Others are at the same distance from Saturn as
  Dione and Tethysan example of what is know as
  Lagrange orbits.

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Astronomy 330

• There is also a class of satellites known as
  co-orbital satellites.
• Their names are Janus and Epimtheus.
• Their orbits differ in radius by only 50 km and
  the satellites occasionally lap each other
  (about 1 every 4 years).
• When this happens these satellites actually
  exchange orbits due to their gravitational
  interaction and they do not collide, as you might
  expect.

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Astronomy 330 Janus from Cassini
http//photojournal.jpl.nasa.gov/catalog/PIA06613
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Astronomy 330

• The co-orbital satellites are elongated and
  irregular, perhaps they were once a single body
  that was split into two by a collision.
• All the small inner satellites have this
  irregular appearance as well and might be the
  result of large impacts.
• The rings of Saturn may also have been formed
  through collisions and the creation of debris.

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Astronomy 330 The rings of Saturn

• First detected by Galileo (the human not the
  spacecraft!) in 1610.
• Huygens recognized them as rings in 1659.
• Cassini discovered that there were at least 2
  rings in 1675the Cassini division.
• The rings are composed of millions of tiny
  objectsat the inner edge of the rings it takes
  5.6 hrs to travel around the planet and at the
  outer edge, 14.2 hrs.

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Astronomy 330

• IR observations in the 1970s revealed that the
  rings are composed mostly of water ice.
• Radar bounced off the rings showed that the size
  of the particles in them was about 10-100 cms but
  the smallest are like grains of sands and the
  largest are boulder sized.
• Most of the ring particles are highly reflective
  (50-60 reflectivity).
• Some particles are darkerperhaps organic
  compounds or silicates.

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Astronomy 330

• Each ring particles follows an almost perfectly
  circular path (if it didnt it would collide with
  other particles).
• Collisions of this sort have the effect of making
  the orbits circular and to bring them all into
  one plane. Think something similar might have
  happened to make the solar system itself a planar
  system?

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Astronomy 330

• The rings are broad and thin.
• The inner most ring visible from Earth starts at
  7000 km about the surface of Saturn and extends
  for 70,000 km.
• The rings would just fit inside the orbit of the
  Moon if Saturn was where Earth is.
• The thickness of the rings is no more than 20 m!
• 3 rings are visible from Earth, the A, B, and C
  rings.

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Astronomy 330

• The F ring was discovered by Pioneer in 1979.
• Voyagers discovered that, in fact, these rings
  are themselves subdivided into tens of thousands
  of sub rings or ringlets
• The ringlets are not separated by gaps but are
  really just density enhancements of the rings
  particles.

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Astronomy 330

• The are only a few true gaps in the rings.
• The D ring is the innermost set of rings and was
  discovered by Voyager.
• The C ring is next farthest out and starts at
  7000 km from Saturns surface.
• There are two gaps in the C rings and within one
  gap is an eccentric, narrow ring. The ecentric
  ring also is or a different composition as we can
  tell by it different color compared to the other
  rings.

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Astronomy 330 The Ring System of Saturn
A
B
Cassini Division
C
F
D not visible
http//photojournal.jpl.nasa.gov/catalog/PIA02269
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Astronomy 330

• There are two other such eccentric rings at
  Saturn and they show up only in gaps. The rings
  Uranus and Neptune are also eccentric.
• At 32,000 km from the surface of Saturn the rings
  particles become very dense and the ringlets
  become more complex.
• The B ring starts here, which is the brightest
  part of the ring system.
• The A and B rings also contain most of the mass
  of the ring system.

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Astronomy 330

• The B ring extends to 57,000 km.
• In the B ring the particles are closely packed so
  that that ring is opaquelight doesnt get
  through.
• Here, the particles are also larger, of size 10
  of cms to meters in diameter
• There are no gaps in the B ring.

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Astronomy 330 B and C rings
http//photojournal.jpl.nasa.gov/catalog/PIA01531
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Astronomy 330

• The Cassini division separated the A and B rings
  and is about 3,500 km wide.
• The Cassini division is not empty! There are
  several ringlets here, one of which is an
  eccentric ring.
• Also, one small satellite orbits inside the
  Cassini division.
• The A ring begins as 61,000 km.
• The A ring is between the B ring and the C ring.

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Astronomy 330 A ring
Shepherd Satellite
Cassini Division Not empty!
Encke Division
http//photojournal.jpl.nasa.gov/catalog/PIA01988
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Astronomy 330

• The Encke division is a gap that occurs in the A
  ring and it contains two, dicontinuous, kinky
  ringlets, and one small satellite.
• The ringlets are about 20 km wide.
• The A ring ends at 96,000 km. and ends abruptly.
• The F ring occurs 4000 km farther out than the A
  ring and is a bright, isolated ribbon of
  marterial that varies in width from 30 to 500 km.
  and is also an eccentric ring. It appears
  braidedconstructed of several different strands.

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Astronomy 330

• Two small satellites orbit the F ring on either
  side.
• The E and G rings occur even farther out and are
  very tenous.
• Remember the E rings seems to orginate from
  Enceladus.
• The G ring is composed of very small particles.

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Astronomy 330 The rings of Uranus and Neptune

• The rings of Uranus were discovered by making
  occultation observations of a star passing behind
  the planet (1977).
• The rings of Uranus have the ability to block
  light from passing through, but they do not
  reflect well and so are not easily visible.
• The rings of Uranus are narrow and dark.
• The rings of Uranus are different from those
  around Saturn.
• First they are very narrow, whereas Saturns are
  broad.

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Astronomy

• Second, the Saturn ring particles are bright and
  composed of icethe particles of Uranuss rings
  are dark and composed of carbonaceous material.
• The rings were first photographed by Voyager.
• There are 10 major rings of Uranus and are very
  circular and very narrow (no more than 10 km
  wide).
• The particles within a ring are very close
  together and are very opaque as a result.

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Astronomy 330

• The Epsilon ring contains most of the mass of the
  Uranus ring system and is also peculiar in that
  it is eccentric and variable in width. It is
  similar to the F ring of Saturn. The Eta ring is
  also irregularSome gravitational perturbation
  must be affecting the orbits of the rings
  particles to create these effects.

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Astronomy 330 The Rings of Uranus against the
background of the planet
Episilon
http//photojournal.jpl.nasa.gov/catalog/PIA01985
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Astronomy 330 Neptunes Rings

• The rings of Neptune seem to be denser arcs
  embedded in very tenuous and thin rings.
• Three rings were discovered by Voyager 2 and
  named Galle, Leverier, and Adams.

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Astronomy 330

• As the rings are studied in more detail by
  spacecraft, more and more complex detail has
  emerged.
• Voyager also showed that the rings of Saturn
  change with time (on the order of hours).
• These changes are examples of spiral density
  waves (like in galaxies). Wave that follow a
  spiral pattern and occur in flat spinning disks
  in which individual particles interact
  gravitationally.
• Many of the patterns and changes in the rings can
  be attributed to small, nearby satellites.

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Astronomy 330 Density Waves
http//photojournal.jpl.nasa.gov/catalog/PIA02275
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Astronomy 330

• A ring, by itself, has the tendency to spread out
  over time. Something must be holding the rings
  in place.
• This force must be gravitational.
• The outer edge of the A ring is in a 6/7
  resonance with the moons Janus and Epimethus,
  which keeps the ring particles near the edge
  trapped at this distance.
• The small satellites Prometheus and Pandora
  affect the F ringthey orbit on either side of
  this ring and are called shepherd satellites.

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Astronomy 330 Prometheus and Rings
http//photojournal.jpl.nasa.gov/catalog/PIA06481
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Astronomy 330

• The shepherd satellites keep the ring particles
  in a tight orbit and their influence also makes
  the F ring braided.
• There are shepherd satellites (called Cordelia
  and Ophelia) for the Epsilon ring of Uranus as
  well, but no others have been found for the
  other, thin rings of Uranus.

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Astronomy 330

• There are also satellites embedded in the rings
  of Saturn and there may be other, unseen ones as
  well.
• Unseen satellites are thought to be responsible
  for the gaps in Saturns rings.
• The Cassini division shows scalloped edges. This
  is evidence of the gravitational influence of an
  unseen, embedded satellite.
• From the the size of these scallops it has been
  estimated that these unseen satellites are less
  than 15 km in size.

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Astronomy 330

• One such satellite was found by Voyager, called
  Pan, in the Encke division.
• Such embedded satellites may also be responsible
  for the structure of eccentric rings.
• So, embedded satellites can produce clear zones
  or they can produce a narrow ring, eccentric
  ring.

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Astronomy 330

• The larger satellites also exert resonant
  gravitational effects on the rings of Saturn,
  Uranus and Neptune.
• The Cassini division is caused by Mimas. The
  inner edge is in a 2/1 resonance (inner edge
  orbits 2 times for 1 orbit of Mimas).
• Some of the spiral density waves are due to such
  resonances as well.

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Astronomy 330 Prometheus, Pandora, Pan and Atlas
http//photojournal.jpl.nasa.gov/catalog/PIA06084
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Astronomy 330 Origin of the Rings

• Two basic theories
• Breakup theory - rings are a shattered satellite
• Non-breakup theory - rings are junk left over
  which could not form into a larger satellite

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Astronomy 330 Tidal stability limit

• Remember, tides are very sensitive to distance.
• They distort satellites. If the satellite where
  to get to closer the tides would increase until
  it puller the satellite into pieces (we even saw
  this with SL 9).
• The tidal stability limit is the distance from a
  planet that satellite can be and just remain
  bound together. Within this distance the
  satellite is ripped apart. This is also called
  the Roche distance after Eouard Roche.

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Astronomy 330

• The Roche limit depends on
• The density of the satellite
• The internal strength of the satellite
• The Roche distance is usually calculated as if
  the satellite had no intrinsic strength.
• This can also be thought of as the distance
  within which objects cannot come together due to
  their mutual gravitational attraction.
• The major rings of the giant planets are within
  the Roche limits of those planets.
• The G and E rings are outside the Roche distance.

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Astronomy 330

• So the rings, being composed of many individual
  particles, cannot gravitationally coalesce to
  form larger satellites inside the Roche limit.

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Astronomy 330 The Breakup Theory

• We know that in the past the rate of collisions
  was higher.
• Second, we discussed that the rate of collisions
  is even higher near a planet is higher due to the
  gravitational focusing effect of the planet.
• So, Saturn may have had one of two small
  satellites in orbits near the planet that were
  smashed to tiny bits. This debris could be what
  formed the rings, the shepherding satellites and
  also the other irregular inner satellites.

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• The debris, if it was inside the Roche limit,
  would not be able to reform itself.
• The total mass of Saturns B ring has been
  estimated to be about 1018 kg and the mass of
  Uranuss rings is about 1000 times less and
  Neptunes are unknown.
• 1018 could construct an icy satellite of about
  250 km in diameterabout the size of Janus.

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• Calculations also show that if the rings are
  composed of small particles they should not last
  more than 100 million years. The particles would
  be eroded by meteoritic impacts and sputtering in
  the magnetosphere.
• So, either the rings are young or they are
  replenished (by the breakup of the larger, km
  size pieces therein..note km size pieces have
  not been directly detected).

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Astronomy 330 Reading

• Read Chapter 16 of Morrison and Owen