KBO Workshop

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KBO Thermal Evolution

• A. Coradini, M.T. Capria and M.C. De Sanctis

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Outline of the talk

• Origin ? implications on composition, formation
  temperatures, amount of radiogenic elements
• Thermal evolution ? the model (s)
• Importance of initial conditions
• Importance of radiogenic elements
• Importance of orbital evolution
• Final structures ? implication for comets

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• ORIGIN and implications for the initial conditions

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The Disk chemistry? composition (1)

• For many years focus was placed on thermochemical
  models as predictors of the gaseous composition,
  and these models have relevance in the high
  pressure, 10-6 bar, (inner) regions of the
  nebula (e.g., Fegley, 1999).
• However, for most of the disk mass, the observed
  chemistry appears to be in disequilibrium and
  quite similar to that seen in dense regions of
  the interstellar medium (ISM) that are directly
  exposed to radiation (Aikawa and Herbst,
  1999Willacy and Langer, 2000 Aikawa et al.,
  2002)

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Models Outer Solar System Chemistry (2)

• At r 100 AU, the disk can be divided into
  three layers
• the photon dominated region (PDR),
• the warm molecular layer,
• and the midplane freeze-out layer.

• The disk is irradiated by UV radiation from the
  central star and interstellar radiation field
  that ionize and dissociate molecules and atoms
  in the surface layer.
• In the midplane the temperature is mostly lower
  than the freeze-out temperature of CO 20 K.
  Heavy-element species are significantly depleted
  onto grains.
• At intermediate heights, the temperature is
  several 10s of K, and the density is
  sufficiently high ( 106 cm-3) to ensure the
  existence of molecules even if the UV radiation
  is not completely attenuated by the upper layer
  Here water is still frozen onto grains, trapping
  much of the oxygen in the solid state. Thus, the
  warm CO-rich gas layers will have C/O 1,
  leading to a rich and extensive carbon-based
  chemistry ( From Bergin et al, PPV 2006)

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Is the nebular chemistry preserved? (3)

• We now have observational and theoretical
  evidence for active chemical zones thus it is
  likely that the most volatile species, which are
  frozen on grains in the infalling material (e.g.,
  CO, N2) do undergo significant processing.
• For the least volatile molecules, as water ice,
  sublimation and gas-phase alteration is less
  likely, unless there is significant radial mixing
  from the warmer inner disk to colder outer
  regions. (Bergin et al.2006)

• Ices in comets condensed Tlt 100K
• Amorphous form can trap other gases, but also
  high volatility ices can be present
• Amounts depend on formation distance
• Release of gases
• Very low temperature for CO and high volatile
  ices (30 K)
• 120-137K amorphous ? crystalline phase
• Sublimation starts 160-180K

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Do comet composition reflect the one of ISM?

• Abundance seem consistent taking into account of
  the large errors and local variability

From Meeck 2006, quoting Irvine et al 2000
(PPIV) Mumma et al (2005) Ehrenfreund et al.,
Bockelee-Morvanet al. (2004)
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Information from comets is the nebular isotopic
composition preserved?

• Aikawa and Herbst (1999) calculated molecular
  abundances and D/H ratios in a fluid parcel
  accreting from a core to the disk, and then from
  the outer disk radius to the comet-forming region
  (30 AU), showing that ratios such as DCN/HCN
  depend on the ionization rate in the disk, and
  can decrease from 0.01 to 0.002 due to chemical
  reactions during migration within the disk.
• HDO and DCN abundances are consistent with
  ion-molecule reactions at T 30 K.

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The planetesimal of outer solar system ? structure

• The composition of these bodies shall reflect the
  PSD composition, so these object should by reach
  in water ice, carbon compounds ammonia..
• The structure could reflect the most important
  processes leading to the formation of specific
  bodies
• Gravitational instability in the original disk
• Almost homogeneous large planetesimals, inside
  which the original structure should be
  re-organized due to self-compaction.

• Direct accumulation from smaller bodies
• Rubble pile objects formed by smaller and
  heterogeneous blocks
• Combination of these two mechanisms can lead to a
  very large variability
• Porosity should be present

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Possible Temperature range to be considered as
initial condition

• Ortho/Para ratio ? T 30 K
• Spin temperature ? Tspin 30 K
• 0.01 CO/H2O 0.2 ? T 30 - 50 K
• HDO and DCN ? T 30 K

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The amount of radioactive elements

• The comets and consequently Kuiper belt object
  contain a measurable amount of dust? that
  following the pioneering idea of Whipple and
  Stefanic (1966) can contain a certain amount of
  radioactive elements

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Radioactive elements
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Short lived Radioactive elements

• 26Al is the first radioactive nucleus ever
  detected in the Galaxy through its
  characteristic gamma-ray line signature, at 1.8
  MeV (Mahoney et al. 1982).
• Taking into account its short lifetime (1 Myr),
  its detection convincingly demonstrates that
  nucleosynthesis is still active in the Milky Way
  (Clayton 1984).
• The detected flux corresponds to 2 M? of 26Al
  currently present in the ISM (and produced per
  Myr, assuming a steady state situation).
• Link between 26Al? 60 Fe shall be established
• Other sources of 26Al can be found ( e.g.
  Wolf-Rayet stars (N. Prantzos, 2006)
• Because of the growing evidence that the
  short-lived radionuclides in the protoplanetary
  disk came from multiple sources, we cannot a
  priori assume homogeneous distribution for any
  such radionuclide ( Gournelle, 2006)

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Initial Conditions

• The body is initially homogeneous and uniformly
  porous, composed of ices and dust.
• The most important ices are H2O, CO2 and CO, in
  various proportions the most abundant molecule
  is water, while CO2 and CO are secondary
  volatile species that have been observed in the
  coma of comets . Ices can be also trapped in the
  amorphous ice
• CO can be used as representative of the very
  volatile species and the CO2 as representative of
  the moderate volatile species these molecules
  are among the most abundant with respect to
  water, being CO up to 20 relative to water and
  CO2 up to 10.
• The dust is embedded in the ice matrix, and can
  be lost by the body, due to the ice sublimation

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THE MODEL
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Relevant physics for comets inherited models

• Heat diffusion in the nucleus ?
• Crystallization of amorphous H2O ice ?
• Release of gases trapped in amorphous H2O ice
• Sublimation/recondensation of volatiles in the
  nucleus ?
• Fracture of the nucleus due to gas internal
  pressure
• Gas diffusion in the nucleus ?
• Dust release and mantle formation ?
• Heath sources ? Solar heating and radioactive
  decay
• Obviously these models are more suitable for
  comet-like objects!

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Volatile Rich vs Volatile poor Objects

• In the last years we have developed several
  models on KBO based essentially on an extension
  of our previous models to larger and cooler
  objects.
• Obviously this is one of the possible approaches
  to perform this kind of simulations.
• The reason to apply comet-type model derives
  also from the observation that that comet-like
  objects can be active also at large distances
  (gt10 AU) from the Sun.

• Kuiper Belt objects, can develop a coma made by
  very volatile elements, such as CO.
• This activity can be either triggered by the
  small input of solar radiation or triggered by
  the presence of long life radioactive elements.
• In what follows will discuss the comparison of
  comet-like objects with denser ond less volatile
  rich objects, that can be the result of local
  accretion through smaller planetesimals.

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KBO thermal evolution Low density bodies

• After several million of year, depending on the
  amount and kind of radioisotopes in the models,
  the CO reach a quasi-stationary level.
• In the figure the stationary profile is
  obtained not considering 26Al in 1 107 years.
• The 3 models differs in the radioactive elements
  content (model 0 no radioactivity)

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Composition Profile
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Thermal models using outer solar system
satellites inheritance? Phoebe like object

• The internal density assumed will be much higher
  then the one needed to explain the cometary
  activity, also according with recent measurements

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Relevant physics for high density models

• Heat diffusion in the nucleus ?
• Ri-arrangment of the internal structure due to
  hydrostatic pressure?
• Heat sources ?
• Impact heating during accretion?
• Radiogenic heating ?

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High Density Bodies

• This body in about 10 5 years the central
  temperature has increased from 20 to 80 K due to
  the decay of Al 26.
• As expected, the higher amount of refractory
  material, the higher thermal conductivity

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Is this body in convection?
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Two possible definition of critical Raleigh
number, in presence (2) or absence (1) of
radioactive elements
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2
Ra gt 1000 Convection can take place only if
radioactive element are present
Assuming H 4.3x10-3 and ?0 1014 Pa s Ra2
1500-3000 ? possible convection
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KBO with moderate content of 26 Al

• The KBOs can be strongly volatile depleted
  objects, at least in their upper layers).
• The KBOs are also very differentiated a typical
  result is that interlaced layers of CO- depleted
  and CO-enriched are found, particularly when the
  icy bodies are considered.
• If this result is confirmed, the evolution of
  KBO injected in hotter parts of the Solar System
  will be characterized by outburst of volatiles,
  when the enriched layers reach sublimation
  temperature.
• Finally an undifferentiated core can survive,
  depending on the size and radiogenic element
  content of the body.

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KBO with high content of 26 Al

• In this case we have a strong heating of the
  interior surmounted by a lower temperature layer.
  
• Only the external layers can be still enriched in
  volatile
• Therefore the two classes of models behave in a
  completely different way ? In this second case
  the amount of volatiles is much lower and can be
  further affected by the dynamical evolution

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A comparison
Undiff.layer
U
Con.region
Depleted core
enriched layer
Und.Core
Crust
Undiff. layer
enriched layer
Low 26 Al
High 26 Al
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Orbital Evolution
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Multistage Capture
Coradini et al. 1997
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What us the effect orbital evolution ?

• The general behavior of the body when it arrives
  on its final orbit does not change substantially
  provided that the amorphous ice level is not
  reached by the thermal heat wave, as for models T
  and V.
• The stratigraphy, however, is substantially
  different from what it is expected, because
  amorphous ice is preserved, as are other
  volatiles.
• Considering the coupling between thermal and
  dynamical evolution, that the final stratigraphy
  of these objects is such that the external layers
  protect the internal ones, thus preserving
  pristine composition
• High volatility ices, if present as gases are
  lost also at great orbital distances, but they
  can also survive as trapped volatiles also close
  to the surface.

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Conclusions

• Great variability in the comets is expected,
  given the many possibilities of global and local
  differentiation
• The interplay between the effects of solar
  heating and internal heating in bodies with
  different initial conditions can bring to
  completely different situations however thermal
  processes occur close to surface, deeper layers
  thermally isolated
• The amount of short lived radioactive elements is
  a key parameter, and we dont think that the
  chondritic amount can be applied to KBO,
  otherwise the presence volatiles and the
  cometary activity cannot be explained.
• However, since a background of these elements
  is always present, the effects of limited
  quantities of them should possibly be important