The surface composition of Ceres: Using new IRTF spectral measurements

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The surface composition of CeresUsing new IRTF
spectral measurements

• Andrew Rivkin (JHU/APL)
• Eric Volquardsen (UH/IRTF)
• Beth Clark (Ithaca College)

Icarus, press.
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Ceres

• Ceres is the largest object in the asteroid
  belt, and a target of the Dawn mission. Recent
  work suggests Ceres is a complex object with a
  rocky core over an icy mantle, perhaps with a
  primitive crust, and the prospect for profound
  chemical evolution having occurred over its
  history.
• Despite 30 years of observations,
  interpretations of its surface composition still
  disagree on the presence / absence of water ice,
  ammonium (NH4), and whether any of the
  meteorites in our collections are good analogs
  for the composition of Ceres.

HST image of Ceres (J. Parker PI)
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Observations

• The spectrum of Ceres contains few diagnostic
  absorption features at visible wavelengths, so
  the 3-?m region, where OH and H2O produce
  diagnostic absorptions in minerals, is critical
  for understanding the composition of Ceres.
• Due to its superior atmospheric transmission,
  Mauna Kea is the only northern hemisphere site
  where the 3-?m wavelength region can be regularly
  observed, and the IRTF is one of the few
  telescopes in the world with the instrumentation
  necessary for this spectroscopic observation.

Atmospheric transmission vs. wavelength (bottom),
compared to meteorite spectra (top)
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Modeling

• We modeled the 2-4 ?m spectrum of Ceres
  averaged over an entire night using a Hapke
  theory based mixing code. Models with 5
  carbonates (solid lines), an amount similar to
  what is seen in CI meteorites, are a much better
  fit than models with no carbonates (dashed).
• When considered in combination with archival KAO
  data leads to preference for iron-rich clays over
  ammonium-rich clays. Furthermore, water ice
  appears to be excluded at a meaningful level over
  a large fraction of Ceres surface.

Ceres spectra (black and red) compared to CM
meteorite (green) and model spectra (other lines)
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Implications

• Along with Earth and Mars, Ceres is only the
  third solar system object with evidence for
  carbonates, supporting models of Ceres chemical
  evolution that predict release of CO2 during
  aqueous alteration. The preference for iron-rich
  clays (also found in CI meteorites) suggest a
  relatively oxidized precursor material for Ceres.
• Other objects are also observed (with the
  IRTF) to have Ceres-like band shapes, suggesting
  the possible use of 3-?m band shapes to map out
  oxidation state of the early solar system.

Objects with Ceres-type bands (squares) appear in
restricted parts of the asteroid belt, compared
to Pallas-type bands (stars)