Composition Trends in KBOs and Centaurs

1
Composition Trends in KBOs and Centaurs

• C. M. Dalle Ore
• (SETI/NASA Ames)
• D. P. Cruikshank
• (NASA Ames)
• J.Emery
• (NASA Ames)

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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.

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Introduction

• In a previous study (Cruikshank and Dalle Ore
  Earth, Moon Planets 92, 315, 2003) we found
  trends in KBOs compositions mineralsorganics(H2
  O)
• New Spitzer data at 3.6 and 4.5?mgtextended
  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.

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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.
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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.
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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
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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 ltlt 1 concentration

• Tholins are complex Mixtures of C, H, and N

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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 (91) at various pressures.
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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
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Mineral and Organic Components
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.
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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.
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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.
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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.
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Best Models for Varuna H2O based
Licandro et al. AA 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.
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Best Models for Varuna CH3OH based
Licandro et al. AA 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.
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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?

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Data Sources

• 1999TC36 Dotto et al. Icarus 162, 408, 2003.
• Quaoar Jewitt and Luu, Nature 432, 731, 2004.
• 20000Varuna Licandro et al. AA 373, L29, 2001

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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 0.96 µm from T. Roush (LPSC XXVII, 1107,
  1996)
• The adopted constants are for T40K.
• CH3OH From Brown (?). T90K.
• Pmx100 A Mg-rich pyroxene from Dorschner et al.
  AA 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 lt 2.5 um
  and from Mooney Knacke (1985, Icarus, v64,
  pp493-502) for lambda gt 2.5 um.

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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

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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.