Rings and Natural Satellites

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Rings and Natural Satellites
2
Planetary rings
3
Saturns rings

• Main structures A and B rings, separated by the
  Cassini Division (21 resonance with satellite
  Mimas)
• The outer part of the A ring hosts the Encke
  Division, which is cleared by satellite Pan
• The C and D rings are broad, faint structures
  interior to the B ring (D ring unobservable from
  Earth)
• The E ring is very wide and diffuse, fed by
  volcanic ejecta from satellite Enceladus
• The F and G rings are very narrow the F ring is
  shepherded by satellites Prometheus and Pandora

4
Co-orbiting satellites

• An object B orbiting very close to another object
  A about the same planet in nearly circular orbits
  performs a horseshoe orbit due to the mutual
  gravitational attraction
• Example Saturns co-orbiting satellites Janus
  and Epimetheus

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Janus and Epimetheus
Janus, 193x173x137 km
Epimetheus, 135x108x105 km
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Fine structure of the rings

• All the major ring components exhibit a fine
  pattern of radial density variation with rather
  high contrast, giving them the appearance of a
  gramophone record

Voyager 2 false-color picture of Saturns rings
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Apparent repulsion

• - a small particle B orbiting near a larger
  object A experiences a hyperbolic deflection when
  passing near A.
• - This leads to loss or gain of angular
  momentum, causing the orbit of B to be repelled
  from A

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Gap clearing shepherding
Satellite Pan orbiting inside the Encke Division
Satellites Prometheus and Pandora orbit on the
inner resp. outer side of the F ring
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Jupiters rings

• Even the Main Ring is very faint
• All rings are strongly forward scattering and
  consist of very small particles
• The Halo is inside the main ring, and the two
  Gossamer rings are outside
• All the inner satellites are connected to the
  ring structures

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Jupiters main ring in forward scattering
Voyager picture taken in the direction of the Sun
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Uranus rings inner moons

• The rings were discovered during a stellar
  occultation in 1977
• They are dark and narrow, situated mostly rather
  close together
• The outermost rings are connected with the system
  of small, inner satellites

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Neptunes rings inner moons

• Data mainly from stellar occultations and Voyager
  2 imaging
• Main rings LeVerrier and Adams broader features
  in between Galle, Arago and Lassell
• 5 satellites orbit inside the Adams ring 3
  inside the LeVerrier ring

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Neptunes ring arcs

• Stellar occultation measurements indicated
  asymmetric ring features
• Voyager 2 pictures revealed arcs (clumps of
  material) in the Adams ring Fraternité, Egalité,
  Liberté

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The Roche limit

• Repulsive, tidal acceleration
• Mutual attraction
• FtFg?

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Rings and Roche limits

• Jupiter the RL is in the Gossamer region
• Saturn the RL is in the A-B ring region
• Uranus the RL is outside the ? ring, in the
  region of the outer rings
• Neptune the RL is near the Adams ring
• Indication collisional shattering of small,
  inner moons and dispersion of material inside the
  RL may have caused, and still be causing the rings

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Planetary satellite systems

• The terrestrial planets have few satellites,
  while the giant planets have a multitude
• In some respects the giant planet satellite
  systems resemble the Solar System in miniature,
  but each system is highly unique
• The giant planet satellites may be arranged in
  three broad categories corresponding to an inner,
  a central and an outer zone with respect to the
  planet

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Giant planet satellites

• The inner satellites are always small and have
  equatorial, circular orbits
• (regular orbits)
• The central zone contains all the large,
  classical satellites, and in the
• case of Saturn also some small ones. All except
  Neptunes have regular
• orbits
• All the outer satellites are irregular (high
  inclinations to the equator) and
• small nearly all are recent discoveries

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Origin of the satellites

• The inner, small satellites orbit within or near
  the Roche Limit and ring system. They appear to
  be eroded remnants of tidal disruption or
  collisional fragmentation
• The central, regular satellites were formed by
  solid accretion in a circumplanetary gas/dust
  disk that may have been the result of gas capture
  from the solar nebula
• The outer, irregular satellites have orbits that
  are perturbed by the Sun more than by the
  equatorial flattening of the planet they were
  captured a long time ago

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Physical data (Jovian)
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Jupiters Galilean satellites

• Discovered by Galileo in 1610
• Europa is slightly smaller than the Moon
  Callisto and Ganymede are larger than Mercury
• Io has a rocky composition Europa is mostly
  rocky Ganymede and Callisto are 50 rock and 50
  ice
• Tidal heating effects are important for Io and
  Europa

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Tidal heating of satellites

• The tidal force from the planet raises bulges on
  the planet-facing and planet-opposing sides of
  the satellite
• The orbits of Io and Europa around Jupiter are
  eccentric due to mutual gravitational forces of
  the 421 resonance Io-Europa-Ganymede triplet
• The orbital eccentricity causes flexing of the
  satellite due to (1) varying distance from
  Jupiter (2) varying angular velocity while the
  rotational velocity is constant

This picture illustrates the tidal lag of a
planet that rotates faster than the orbital
motion of the satellite
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Ios volcanism

• Ios tidal heating causes a constant volcanism
• tidal heat is too large to be removed by
  conduction or solid-state convection ? melting of
  the subsurface and volcanic eruptions
• over 200 volcanic calderas, generally over 20 km
  in size
• volcanic flows hundreds of km long indicate low
  viscosity similar to terrestrial basalt lavas
• resurfacing rate estimated to 1-10 cm/year
• all geologic features related to volcanism no
  impact craters

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Europa

• H2O crust 100 km thick the lower part is
  certainly liquid
• Weak magnetic field, induced by a conducting
  liquid (salty water?) moving in Jupiters
  magnetic field
• Very bright surface
• Extremely flat, topography lt 300 m few impact
  craters indicate young surface (10-100 Myr)
• The ice is broken up into plates 30 km in size
  lateral movements have occurred
• Some evidence of geyser- or volcanic-like
  activity along ridges active resurfacing?

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Physical data (Saturnian)
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Enceladus

• Very bright, icy object
• Smooth surface with few impact craters
• Hot spot near the south pole
• Geyser-like activity due to tidally induced
  cryovolcanism
• Source of the material in Saturns E ring

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Saturns E ring
This Cassini picture shows part of the E ring,
and the bright spot is Enceladus, from where the
material originates
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Titan

• Visual appearance from a distance orange,
  featureless
• Dense atmosphere ps1.5 bar, N2 and minor CH4
• Optically opaque, dense upper layer of
  photochemical smog hydrocarbons, nitriles
• Aerosols precipitate out of the gas as 0.2-1 ?m
  particles, accumulate into larger aggregates and
  fall to the surface

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Titans photochemistry

• Solar uv and particle radiation dissociate N2
  molecules at gt1000 km height
• N atoms react with methane, producing H (escaping
  into space), HCN, hydrocarbons and C-N compounds
• These react further, producing stable species
  that sink into lower layers, eventually
  precipitating onto the surface
• This is a sink of methane (minor atmospheric
  constituent), which needs to be resupplied from
  the surface of Titan

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Results from Huygens landing on Titan

• Geologically young surface
• evidence of flow around islands
• deposits and rocks of water ice
• drainage channels which may have been created by
  methane springs
• Landing occurred in liquid-saturated mud
• No extensive liquid methane-rich hydrocarbon
  ocean at the surface, but lakes occur
• Possible cryovolcanism releases methane into the
  atmosphere

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Hyperion

• 34 mean motion resonance with Titan
• Composition dirty ice
• Very porous
• Deep, sharp-edged craters make it look like a
  sponge
• Chaotic rotation ? uniform surface

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Iapetus

• Extreme albedo contrast between the leading and
  trailing hemispheres
• Equatorial ridge of about 10 km height,
  encircling the object
• Icy composition with very dark material powdered
  on the surface
• Impact craters are abundant

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Phoebe

• The largest irregular satellite (210 km
  diameter)
• Imaged by the Cassini probe intensively cratered
• Spectra show abundant water ice, hydrous
  minerals, CO2, organics, nitriles, cyanide
  compounds
• Composition similar to comets density of 1.6
  g/cm3 indicates compact object like Pluto and
  Charon

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Physical data (Uranian, Neptunian)
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Miranda

• Very complex despite its small size
• some areas are very old and heavily cratered
• other regions are endogenic and crater poor,
  consisting of white and dark bands and highly
  fractured scarps and ridges
• likely origin tidal heating, differentiation,
  impact disruption and reassembly into a rubble
  pile

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Triton

• Somewhat smaller than the Moon, extremely cold
• Tenuous atmosphere of N2 with trace CH4
• Very bright surface made of N2 and CH4 ice with
  trace NH3
• Trailing-leading hemispheric dichotomy
• Cryo-volcanoes of liquid N2 in polar regions with
  constant insolation carry particles into the
  atmosphere

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Irregular satellites (1)

• Orbits are contained within the Hill radius
• Moderate to high eccentricities
• Separation into prograde and retrograde classes
• Groupings are evident mostly for jovian satellites

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Irregular satellites (2)

• Similar colours for members of the same dynamical
  group indicate an origin by collisional
  fragmentation
• Capture models a temporary capture is made
  permanent by dissipative forces
• - Increase of the planetary mass by accretion
• - Gas drag through a planetary envelope or
  circumplanetary disk
• - Collision or close encounter with another
  satellite
• - Dynamical friction from a huge number of small
  objects orbiting in the vicinity

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Collisional captures

• Triton
• Somewhat smaller than Europa but larger than
  Pluto
• Comparable to other large satellites with respect
  to distance from the planet
• Orbit is circular but retrograde!
• Collisional capture also expelled Nereid into its
  highly elliptic orbit, and ejected other original
  satellites
• Irregular satellites may also be collisionally
  captured, but their parents were smaller and may
  have been fragmented