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Composition Trends in KBOs and Centaurs

Composition Trends in KBOs and Centaurs. C. M. Dalle Ore (SETI/NASA Ames) D. P. Cruikshank (NASA Ames) J.Emery (NASA Ames). Outline. Spectral modeling with the Shkuratov code: input, mixing strategies, goals and constraints.

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Composition Trends in KBOs and Centaurs

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  1. Composition Trends in KBOs and Centaurs C. M. Dalle Ore (SETI/NASA Ames) D. P. Cruikshank (NASA Ames) J.Emery (NASA Ames)

  2. Outline • Spectral modeling with the Shkuratov code: input, mixing strategies, goals and constraints. • Description of input materials and how they affect the resulting spectrum. • My best models, so far, for Quaoar, 1999TC36 and Varuna.

  3. Introduction In a previous study (Cruikshank and Dalle Ore Earth, Moon & Planets 92, 315, 2003)we found trends in KBOs compositions: minerals+organics(+H2O) New Spitzer data at 3.6 and 4.5m=>extended models for three objects from our previous sample: Quaoar, 1999TC36, and 20000Varuna. Comparison of new and old models confirms the need for “reddening” materials + darkening agent. Possible presence of methanol and/or water mixed in with organics.

  4. Modeling Goals Use plausible materials and plausible mixing schemes. Carry the same components for all data (when possible). Because of the lack of discrete spectral features in our data, the albedo information contained in the data (Stansberry et al., Ap.J. 643, 556, 2006) is essential in computing accurate abundances for each component. Find the best fitting model with the smallest possible number of components.

  5. Adopted Code Modeling of the observed reflectance was obtained by means of the Shkuratov radiative transfer code (Shkuratov et al. Icarus 137, 235, 1999). Input Parameters The Shkuratov code requires three input parameters: - relative abundances of each material - grain size information - optical constants for each material.

  6. Scattering Geometries and Mixing on Planetary Surfaces Spatial (areal) mixtures Adjacent small Particles of Different composition: Intimate Mixtures Inclusions in transparent particles Intra-Mixes, or Molecular Mixtures in individual particles

  7. Tholins are strongly colored • Tholins impart strong color to a planetary surface • The coloration is very effective when tholin inclusions occur in transparent ice particles, even when << 1% concentration • Tholins are complex Mixtures of C, H, and N

  8. Tholins (deposited on transparent substrates) made by H. Imanaka (Icarus 168, 344, 2004) by cold plasma irradiation of a flowing gaseous mixture of N2 : CH4 (9:1) at various pressures.

  9. Icy Components • Crystalline water ice or methanol were introduced for those objects that suggest or don't exclude in their observed reflectances the presence of these components. Wavelength

  10. Pxmg100 (Mg-rich pyroxene): sets albedo level for brighter objects. (Enstatite = Mg2Si2O6) Serpentine: sets albedo level for darker object with a convex curvature.Triton tholin, and Titan tholin: introduce a curvature in certain parts of the albedo spectrum. Triton tholin provides a steep monotonic red slope short of 2.5μm Titan tholin yields a convex slope whose curvature is centered around 1.4μm . Both also yield a quick rise at about 3m. Mineral and Organic Components

  11. Best Models for Quaoar Jewitt and Luu, Nature 432, 731, 2004 Intra-mixture of H2O and Triton tholin, necessary for steep slope. Pyx or Serpentine or ? darkens the water to yield the right albedo level. Tholins in right combination yield the red slope shape. Triton provides the fairly rapid rise after 3m.

  12. Best Models for 1999TC36: H2O based Dotto et al. Icarus 162, 408, 2003 Intra-mixture of H2O and Triton tholin, necessary for steep slope.Best fitted by serpentine at 1.5 and 2 m regions.Triton tholin brings the albedo up after 3 m, Titan tholin adds curvature in the 1 m region.

  13. Best Models for 1999TC36: CH3OH based Dotto et al. Icarus 162, 408, 2003 Intra-mixture of CH3OH and Triton tholin, necessary for steep slope. Best fitted by serpentine at 1.5 and 2 m regions.Triton tholin brings the albedo up after 3 m, Titan tholin adds curvature in the 1 m region. Fit is good at 3.6 and 4.5 m.

  14. Best Models for Varuna: H2O based Licandro et al. A&A 373, L29, 2001 Intra-mixture of H2O and Triton tholin, necessary for steep slope.Pyx is the better darkening agent based on the shape and albedo at 2.5 m. Pyx is too dark at 4.5 m. Serpentine is slightly better at 4.5 m, but drops at 2.4 m.

  15. Best Models for Varuna: CH3OH based Licandro et al. A&A 373, L29, 2001 Intra-mixture of CH3OH and Triton tholin, necessary for steep slope. Serpentine fits well the 2 m region, but drops at 2.4 m and is too dark hereafter.Pyx misses the 2 m feature. Fits well at 3.6 m, misses again at 4.5 m.

  16. Conclusions • We confirm the need of “reddening” materials to reproduce the red slope in the UV/vis part of the spectrum. Best results are achieved when the tholins are mixed molecularly into an ice. • Pyx, serpentine or yet another mineral are good choices as darkening agents of the albedo levels. • 4.5m data point can be challenging to fit: could a different organic material or mineral or ? yield a better fit?

  17. Data Sources • 1999TC36: Dotto et al. Icarus 162, 408, 2003. • Quaoar: Jewitt and Luu, Nature 432, 731, 2004. • 20000Varuna: Licandro et al. A&A 373, L29, 2001

  18. Sources of the Optical Constants • H2O: From Grundy from 0.96um on, spliced with: • 0.3 - 0.88 μm from S. Warren (Appl. Opt. 22, 1206, 1982) • 0.88 – 0.96μm from T. Roush (LPSC XXVII, 1107, 1996) • The adopted constants are for T=40K. • CH3OH: From Brown (?). T=90K. • Pmx100: A Mg-rich pyroxene from Dorschner et al. A&A 300, 503, 1995 (through Jena database) MgSiO3. • Triton tholin: Organic material from B. N. Khare et al. (BAAS 26, 1176, 1994) • Titan tholin: Organic material from B. N. Khare, C. Sagan(Icarus 60, 127, 1984) • Serpentine: From Roger Clark for lambda < 2.5 um and from Mooney & Knacke (1985, Icarus, v64, pp493-502) for lambda > 2.5 um.

  19. RF Power Supply 13.56 MHz Matching Networks Pressure gauge Pyrex Plate P Plasma Pump Substrates Water-cooled baseplate MFC T Gas A MFC Thermocouple gauge Gas B Tholin Production Experiments • Pressures : 0.26 hPa (0.2 torr), 1.6 hPa (1.2 torr) • CH4/N2 : 10/90 • RF Power: 100 W • Room Temperature

  20. Adopted Mixing Strategies Intimate mixtures are those in which the components are distributed in a salt and pepper fashion. Their reflectance is calculated from an average of the single-scattering albedo of each component. Intra-mixtures are those in which one component mixes with the other at the molecular level yielding a new component with new optical properties, calculated as a weighted mean of the original ones.

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