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KBO Thermal Evolution

KBO Thermal Evolution. A. Coradini, M.T. Capria and M.C. De Sanctis. 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

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KBO Thermal Evolution

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  1. KBO Thermal Evolution A. Coradini, M.T. Capria and M.C. De Sanctis KBO Workshop Catania 3-7 Luglio 2006

  2. 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 KBO Workshop Catania 3-7 Luglio 2006

  3. ORIGIN and implications for the initial conditions KBO Workshop Catania 3-7 Luglio 2006

  4. 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, 1999;Willacy and Langer, 2000; Aikawa et al., 2002) KBO Workshop Catania 3-7 Luglio 2006

  5. 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 ofK, 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) KBO Workshop Catania 3-7 Luglio 2006

  6. 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 T< 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 KBO Workshop Catania 3-7 Luglio 2006

  7. 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) KBO Workshop Catania 3-7 Luglio 2006

  8. 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.

  9. 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 KBO Workshop Catania 3-7 Luglio 2006

  10. 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 KBO Workshop Catania 3-7 Luglio 2006

  11. 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 KBO Workshop Catania 3-7 Luglio 2006

  12. Radioactive elements KBO Workshop Catania 3-7 Luglio 2006

  13. 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) KBO Workshop Catania 3-7 Luglio 2006

  14. KBO Workshop Catania 3-7 Luglio 2006

  15. InitialConditions • 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 KBO Workshop Catania 3-7 Luglio 2006

  16. THE MODEL KBO Workshop Catania 3-7 Luglio 2006

  17. 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! KBO Workshop Catania 3-7 Luglio 2006

  18. 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 (>10 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. Volatile Rich vs Volatile poor Objects KBO Workshop Catania 3-7 Luglio 2006

  19. A Noradioactive elements 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) KBO Workshop Catania 3-7 Luglio 2006

  20. Composition Profile KBO Workshop Catania 3-7 Luglio 2006

  21. 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 KBO Workshop Catania 3-7 Luglio 2006

  22. 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  KBO Workshop Catania 3-7 Luglio 2006

  23. “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 KBO Workshop Catania 3-7 Luglio 2006

  24. Is this body in convection? 1 Two possible definition of critical Raleigh number, in presence (2) or absence (1) of radioactive elements 2 2 Ra > 1000 Convection can take place only if radioactive element are present Assuming H= 4.3x10-3 and η0 = 1014 Pa s Ra2 ~ 1500-3000  possible convection KBO Workshop Catania 3-7 Luglio 2006

  25. 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. KBO Workshop Catania 3-7 Luglio 2006

  26. 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 KBO Workshop Catania 3-7 Luglio 2006

  27. A comparison Undiff.layer U Con.region Depleted core enriched layer Und.Core Crust Undiff. layer enriched layer Low 26 Al High 26 Al KBO Workshop Catania 3-7 Luglio 2006

  28. Orbital Evolution KBO Workshop Catania 3-7 Luglio 2006

  29. Multistage Capture Coradini et al. 1997 KBO Workshop Catania 3-7 Luglio 2006

  30. 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. KBO Workshop Catania 3-7 Luglio 2006

  31. 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 don’t 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 KBO Workshop Catania 3-7 Luglio 2006

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