Transcript The surface composition of Ceres: Using new IRTF spectral

The surface composition of Ceres:
Using new IRTF spectral measurements
Andrew Rivkin (JHU/APL)
Eric Volquardsen (UH/IRTF)
Beth Clark (Ithaca College)
Icarus, press.
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)
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.
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Atmospheric transmission vs.
wavelength (bottom), compared to
meteorite spectra (top)
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.
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)
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 ironrich clays (also found in CI meteorites)
suggest a relatively oxidized
precursor material for Ceres.
Objects with Ceres-type bands
(squares) appear in restricted parts
of the asteroid belt, compared to
Pallas-type bands (stars)
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.