Volterra VII ciclo di dottorato

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Origin of Planetary Satellites

• Angioletta Coradini

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Voyagers 1 and 2

• Voyagers 1 and 2 are currently at 90 and 75 AU,
  and receding at 3.5 and 3.1 AU/yr
• Pioneers 10 and 11 at 87 and 67 AU and receding
  at 2.6 and 2.5 AU/yr

Galileo

• More modern (launched 1989) but the high-gain
  antenna failed leaving ? Reducing drastically the
  data rate
• VE-Earth gravity assist
• Arrived at Jupiter in 1995 and deployed probe
  into Jupiters atmosphere
• Very complex series of fly-bys of all major
  Galilean satellites
• Deliberately crashed into Jupiter Sept 2003 Main
  source of results

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Cassini

• 6 tonnes, 2 billion, launched in 1997, planned
  from 1985
• Note the absence of scan platform and the
  reaction wheels
• Trajectory included Venus and Earth flybys, and
  will flyby Titan 44 times

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Family Portrait
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CHARACTERISTICS

• From medium size to very small sizes
• Icy/ Rocky composition
• Weak negative density gradients going from the
  inner to the outer regions of the system (
  Jupiter Case)
• Large variability both in terms of surface and
  internal evolution
• From largely differentiated bodies ( Europa,
  Ganymede) to completely undifferentiated (
  Callisto)
• Tidal heating and orbital evolution (Peale Annu.
  Rev. Astron. Astrophys. 1999 is a good reference)
• Role of volatiles (ammonia, methane)
• Size-related effects
• Different role and style of the craterization
• Impact crater populations and effects
• Presence or absence of volcanism
• Presence or absence of intrinsic magnetic fields

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Jupiter system Galilean Satellites

• Io
• Europa
• Callisto
• Ganymede

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Surface Structures
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Surface Characteristics

• Io Young surface, formed by ongoing volcanic
  activity.
• Europa Completely covered by crushed ice.
• Ganymede Dark (older) and bright (younger)
  areas, expansion indicated by the latter.
• Callisto Heavily cratered, old surface.

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Where are they?
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Saturn Observations
Small (lt500 km), inactive
Small, active
Medium, inactive
Medium, active
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Uranus Observations
Small, active
Small (lt500 km), inactive
Medium, inactive
Medium, active
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• Jup. Sat

Io

• Sat.

Europa
Ura.
X Charon
Char
Gan.
Callisto
Tit.
Epim
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Albedos

• Callisto and Uranian satellites are dark,
  Saturnian satellites bright (except parts of
  Iapetus)
• Albedo decreases with radial distance
• Uranian satellites are denser on average than
  Saturnian

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Composition

• Callisto has lower reflectivity and shallower
  absorption bands, indicating a higher non-ice
  component
• Ganymede and Callisto show slight differences
  between leading and trailing hemispheres
• Non-ice materials are probably hydrated minerals
  (clays)

Earth-based reflectance spectra, from Johnson, in
New Solar System
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Inferred Composition of Galilean Satellites
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Impact Cratering

• Main source of impact craters in outer solar
  system is comets
• Synchronously rotating satellite will be
  preferentially cratered on its leading hemisphere
  (think raindrops)
• So distribution of impact craters on surface can
  be used to test whether NSR has occurred
• Density of impact craters can be used to infer
  surface age
• Obtaining absolute surface ages requires the
  impact rate to be derived, from a combination of
  current and historical astronomical observations,
  and models. Uncertainties are currently large.
• Note that the impact rate will increase for
  satellites closer to the primary (effect of
  gravitational focusing)

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Cratering and Ages

• Cratering rate increases with decreasing distance
  to primary (grav. focusing), e.g. x2 at Rhea, x20
  at Mimas compared with Iapetus (Smith et al.
  Science 1982)
• Size of crater caused by particular object
  increases with decreasing distance to primary
• So observed crater density is a strong function
  of distance to primary as well as surface age
• This makes even relative cratering ages hard to
  determine and model-dependent, never mind
  absolute cratering ages (see Zahnle et al. Icarus
  2003)
• A consequence of gravitational focusing is that
  objects near the primary may have been disrupted
  once or several times by impacts (Mimas,
  Enceladus, Miranda, Ariel)

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Absolute Ages missing

• Uncertainties in absolute fluxes mean surface
  ages are very uncertain.
• Iapetus, Oberon, Titania and Umbriel are
  undoubtedly very old
• Mimas and Enceladus are at least slightly, and
  perhaps much, younger
• Parts of Miranda are very young
• Several satellites show a wide spectrum of ages
  (Enceladus, Rhea, Ariel)

From Zahnle et al. Icarus 2003
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General considerations

• Jupiter has 4 large (gt1500 km) moons, Saturn 1,
  and Uranus and Neptune none. Neptune appears to
  be moon-poor in general.
• All are synchronous, except Hyperion (chaotic)
• Densities are all close to 1 g/cc, suggesting
  mainly volatile ices (see next slide).
• Uranian satellites are denser than Saturnian ones
• Uranus satellite densities increase (roughly)
  with distance.
• Several of the periods are close to (or actually
  in) resonance e.g. Mimas-Tethys, Iapetus-Titan.
  May have had significant effects earlier in
  history.
• Uranian system has no resonances (at present day)
• Charon Pluto couple? Like the Moon??

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Compositions/Formation

• Surface compositions mainly water ice (except
  for Io), plus contaminants (spectroscopy)
• Ios surface is silicates sulphur
• Interiors discussed in detail later, but
  roughly equal mix of water ice, silicates and
  iron
• How did they form?
• Presumably accreted while Jupiter was forming
• Lateral temperature gradient in nebula
• May have been earlier satellites that didnt
  survive
• Energy of accretion 0.6 Gms/Rs per unit mass
  2MJ/kg this is enough to heat ice through
  1000 K.
• Satellites subsequently evolved to their
  present-day positions

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Origin

• We expect that satellite formation be a natural
  byproduct of planet formation, given the
  multitude of satellites in our solar system.
• Two formation mechanisms are believed responsible
  for the majority of the large planetary
  satellites
• Impact
• Our own Moon is thought to have resulted from
  what was perhaps the largest impact of Earths
  accretion, and the so-called giant impact
  hypothesis is favored for its ability to explain
  the primary dynamical and physical attributes of
  the Earth-Moon system.
• co-formation
• The Galilean satellites are a key example of a
  satellite system that is believed to have
  co-formed with its parent planet, with satellites
  accumulating within a circumplanetary accretion
  disk that existed during the final stages of the
  planets own growth.

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Two extreme views for satellite origin
Num Mass Ratio Earth 1 0.012 I
Pluto 1 0.1 I Jupiter 4 2 x
10-4 Co-F Saturn 5 2.5 x 10-4 Co-F Uranus
4 10-4 Co-F or I Neptune 1 2 x 10-4
Bate et al.
I ? Impact
Co-I ? Co Formation
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Working Hypothesis Co-formation

• We consider a scenario in which the regular
  satellites form within circumplanetary accretion
  disks produced during the final stages of gas
  accretion (e.g.,Coradini et al1981, Magni and
  Coradini, 2003, Lubow et al. 1999 D'Angelo et
  al. 2002).
• For a given inflow rate M of gas and solids, a
  quasi steady-state circumplanetary gas disk is
  produced through a balance of the inflow supply
  and the disk's internal viscous evolution,
  assuming that the disk viscous spreading time is
  short compared to the timescale over which the
  inflow changes.

Minimum mass subnebulae for the Jupiter and
Saturn satellite systems (from Pollack
Consolmagno 1984).
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What is the mass Accretion rate?

• The accretion disk model assumes that the
  contraction of the planet to a scale of less than
  a few RJ occurred prior to the complete removal
  of the solar nebula.
• Late-inflowing gas containing sufficient angular
  momentum for centrifugal force balance at an
  orbit of 2030 RJ (e.g., Ruskol 1982), then leads
  to the formation of a circumplanetary disk
• Typical mass accretion rate can be

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Satellite a-disk

• Coradini et al. (1989) incorporated viscous
  evolution of an accretion disk formed via nebula
  mass inflow into circum-jovian orbit.
• We computed the steady-state disk conditions
  using the Lynden-Bell Pringle (1974) formalism.
  The disk was conceived to be highly convective
  with a strong turbulence viscosity parameter a
  0.1 in the inner satellite region, and
  

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The a-disk (contd)

• During the initial phase, solid accretion would
  be precluded by high temperatures or viscous
  turbulence in the disk, whereas in the latter,
  satellite accretion would proceed along the lines
  of the Lunine and Stevenson 82 model, i.e., in
  the absence of gas in flow.
• The assumption that satellite accretion would
  occur after mass inflow to Jupiter has ceased
  would require that

We disliked this result since is not clear how to
evaluate correctly the mass infall rate
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Difficulties

• Protosatellite disk of gas solids
• Current satellite masses disk solids 2 10-4
  Jupiter masses
• Required solar composition mass
• 100 MSAT 2 10-2MJ
• Standard approach protosatellite disk
  contained.02MJ
• Minimum mass sub-nebula (MMSN)
• ? Gas rich disk sGAS 105 g/cm2
• Basic difficulties MMSN disk is too hot,
  accretion too fast, satellite lifetimes against
  decay in short time due to friction with the gas

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Alternative model Slow-inflow accretion
diskCanup and Ward 2002

• Gas solids delivered during final stages of
  Jovian accretion 10-2MJ is minimum mass that
  was processed through satellite disk, but not
  necessarily in disk all at one time
• Gas maintains quasi steady-state solids accrete
  and buildup in disk with time
• Result prolonged satellite formation over gt105
  years in a cool, gas-starved disk
• Consistent with incompletely differentiated
  Callisto, icy outer satellites, satellite
  survival against viscous decay

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Implications of Canup-Ward model

• Regular satellites of giants formed during final
  slow accretion of gas and solids to planets
• Inward orbital migration of large satellites
  likely
• Differences in final satellites systems can
  result from similar conditions, depending of
  stopping inflow
• Galilean- like system with 4 large satellites at
  170,000 years.
• Galilean- like system with a single-like
  satellite 300,000 years.

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Some key open issues for the Canup and Ward 2002
Model

• 1) Character of late inflow onto Jupiter/Saturn?
• Flow dynamics within Hill sphere
• Specific angular momentum on inflow
• Dust/Ice Ratio unknown
• 2) Disk viscosity magnitude character?
• Turbulence due to inflow (e.g., Cassen Moosman)
• Torques from growing satellites (e.g., Goodman
  Rafikov)
• General turbulence associated with Keplerian
  disks (e.g., Klahr Bodenheimer)

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Hydro-dynamical model

• The planet accretion has been treated assuming an
  annular region to mimic the planet feeding zone.
• This region is centered on the Sun and has a
  thickness comparable with the height of the
  protosolar nebula at the same distance.

• In the case treated here, at the Jupiter
  distance, the annular region has a thickness of
  about 10 a.u.
• The reference system is rigidly rotating with an
  angular velocity equal to the Keplerian one.

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Area where the gas has prograde motion ?Accretion
disk
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• The region that we can call disk is deeply
  imbedded in the feeding zone and much smaller
  than the Hill sphere.
• Moreover is characterized by the fact that the
  gas motion from being prograde becomes retrograde
  in the planet reference system.
• In Figure are depicted the regions where the gas
  is in Keplerian motion in different simulations
  characterized

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• Pressure and temperature profile of the planet
  for model J4.
• The planet is characterized by the presence of a
  large inner convective zone divided by a
  radiative region from the external convecting
  layer.

• The two profiles correspond to the planet s
  masses 0.1and 0.3 Jupiter masses.
• The external envelope of the planet covers a very
  large region of more than 700 RJ

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• Final stage the atmosphere of the planet largely
  cover the formation region of planetary
  satellites for models JF3-JF1.
• Model JF4 exhibits a different behaviour, since
  the planet atmosphere covers only the present
  position of Io.

The abrupt changes in slope are due to opacity
variations
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During the slow contraction phase the planet
recedes from the region where presently satellite
are located. The formation region of Callisto is
in 100.000 years emptied, but 10 millions of
years arent sufficient to clear up the formation
region of Io. The planet luminosity evolves from
values ranging from about 1.1 10-6 1.1 10-5
-after 10.000 years of evolution to about 1.1
10-6 after about 107 .
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Radius!!!

• Protoplanet radius and external radius of the
  protosatellitary disk versus accretion timescale
  for the growing planet in its final evolution
  phases.
• At right are plotted the distances of the
  Galilean satellites

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The ?-disk
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Accretion time of an object by collision in the
disk
Gravitational Focusing
Orbital decay time
The orbital decay timescale of an object due to
gas drag within a disk with sound speed c.
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Small Particles have shorter decay time than
accreted particles
Decay Time 104 years
In the disk accreted particles can survive to
the viscous drift and to the disk dissipation!
Decay Time 106 years
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Particles surviving in the disk, during the final
stage, can increase their mass more efficiently
having larger relative velocities.
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For long lasting disks the particles accretion is
driven by gas interactions
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Only at Callisto distance ice is always present
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Only at Callisto distance ice is always present
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The role of external planetesimals

• N-body simulation of dynamical evolution of
  15,000 objects under their reciprocal
  gravitational interactions in the gravitational
  field of Jupiter.
• The planetesimals entering in a zone close to the
  planet as far as 10 of the Hill lobe are
  captured
• An initial gap 2 au has been assumend

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The planetesimals accretion (no gas) 100,000
years evolution 15,000 planetesimals of 1016g

• 31 Planetesimals Captured
• 718 Planetesimals Expelled

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The planetesimals accretion (gas viscosity) in
150,000 years .The gas-planetesimals velocity is
computed by considering the gas flux in the
tube in which it moves.
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Balance of captured and Ejected planetesimals
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Planetary System detectability
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What happened to the Saturn System?
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Working Hypothesis impact

• The origin of the Moon is one of the oldest and
  most studied problems in planetary science.
• The Moons lack of a large iron core together
  with planet accretion model predictions that
  large impacts would be common ? Hartmann and
  Davis (1975) Model
• An impact with the Earth could have ejected
  iron-depleted mantle material into orbit from
  which the Moon then formed.
• An independent and contemporaneous investigation
  by Cameron and Ward (1976) recognized that the
  oblique impact of a roughly Mars-sized planet
  could
• account for the rapid initial terrestrial
  rotation rate implied by the current angular
  momentum of the Earth Moon system,
• vaporization might provide a physical mechanism
  to emplace material into bound orbit.

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Large .. But how large??

• Since Cameron and Benz (1991), progressively
  larger impactors relative to the targets were
  considered in an effort to increase the yield of
  orbiting material, with Cameron (2000, 2001)
  considering collisions that all involved
  impactors containing 30 of the total colliding
  mass.
• The type of impact favored by those works
  involved an impactor with roughly twice the mass
  of Mars and an impact angular momentum close to
  that of the current EarthMoon system, LEM, but
  with a total mass (impactor plus target) of only
  MT 0.65M?. In this so-called early-Earth
  scenario, the Earth is only partially accreted
  when the Moon forms and must subsequently gain
  0.35M?, with the later growth involving
  sufficiently small and numerous impacts so that
  the system angular momentum is not drastically
  altered.

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Key Constrains ( Canup 2004)

• The lunar forming impact is constrained by basic
  properties of the EarthMoon system
• The system angular momentum, LEM 3.5 1041
  g-cm2/sec,
• the masses of the Earth and Moon (ML 7.35
  1025 g 0.0123M?), and
• The observed degree of lunar iron depletion.

Smooth particle hydrodynamics, or SPH
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Example of Lunar Forming Impact

• SPH (e.g., Benz et al. 1986)
• Good way to evaluate state equations
• A sophisticated, semi-analytic equation of state
  known as ANEOS (Thompson and Lauson, 1972) has
  been utilized by previous giant impact studies
  (Benz et al., 1989 Cameron and Benz, 1991
  Cameron, 1997, 2000, 2001)
• ANEOS takes into account of the entropy and
  energy required for vaporization of molecular
  species (such as mantle rock, modified to take
  into account of the for molecular vapor
• Targets ? e. g. contain 30 iron and 70
  silicate (forsterite/dunite) by mass,
  differentiated into a core-mantle prior to the
  impact. We create objects in one of two ways.
• Collisionally generate the objects by colliding
  an iron projectile into a dunite target to
  produce a self-equilibrated and
  self-differentiated object.

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Proto-Earth containing 0.89M?.
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• Time series from an N 60,000 particle
  simulation.
• Color scales with particle temperature, with red
  indicating particles with temperatures exceeding
  6444 K.

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• Peak particle temperatures experienced during
  impact color scales with temperature in degrees K
  with red for T gt9000 K
• same as (a), close-up on impactor
• mapping of final particle states yellowgreen
  particles end up in the orbiting disk, red escape
  the system, and blue end up in the protoearth
• same as (c), close-up on impactor
• mapping of final particle states onto time step
  shown in Fig. 2b, same color scale as (c) and
  (d)
• instantaneous particle temperatures within a
  4000-km slice centered on the z 0 plane for the
  time step shown in (e).

The vectors are proportional to the particle
velocity magnitude
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Post-impact state of proto-earth and disk

• (a) Temperatures within a 2000-km thick slice
  through the protoearth, parallel with and
  centered on the equatorial plane of the planet
• (b) same slice as shown in (a) but color scales
  with the source object of the material, with red
  particles originating from the impactor and blue
  from the target
• (c) same slice as in (a), but color scales with
  material type, with iron particles in red and
  dunite particles in blue
• (d) the entire proto-earth and disk, with color
  scaling with material type (iron vs. dunite) as
  in (c).

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An impact formation of Pluto-Charon?
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Pluto-Charon and Plutinos

• Alan Stern, Robin Canup, and Daniel Durda have
  found clues that some KBOs in neighboring orbits
  to Pluto may, in fact, be debris created in the
  Pluto-Charon forming event.
• The evidence found by the SwRI team linking some
  KBOs called "Plutinos" to Pluto-Charon comes in
  three forms.
• First, there is a close orbital similarity
  between some KBOs and Pluto that is consistent
  with the expected distribution of debris from the
  Pluto-Charon formation event. Second, the colors
  of Pluto and some KBOs, and Charon and other
  KBOs, suggest similar surface compositions.
• Third, the apparent size distribution of the
  objects that suggest themselves as potential
  shards of the Pluto-Charon forming collision is
  similar to both laboratory results from studies
  of catastrophic collisions and asteroid belt
  families known to result from collisions

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..possible conclusion

• The formation of satellites can take place in
  different conditions and through different
  mechanisms
• We have reviewed two main mechanisms, but they
  can variously overlap depending on the boundary
  conditions
• It is important try to correlate the process of
  satellite formation to the local situation
  generated by central body formation