Prelude to Dawn: Vesta and its Relationship to the Vestoids Tom Burbine Mount Holyoke College tburbi

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Prelude to DawnVesta and its Relationship to
the VestoidsTom BurbineMount Holyoke
Collegetburbine_at_mtholyoke.edu
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Number of Collaborators

• Paul Buchanan
• Rick Binzel (MIT)
• Tim McCoy (Smithsonian)

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• The DAWN spacecraft will hopefully be launched in
  2007 to spectrally and chemically map the
  surface of 4 Vesta.
• Arrive at Vesta in 2011

Zellner et al. (1997)
http//www.jpl.nasa.gov/images/dawn/dawn_vesta_cer
es-browse.jpg
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• My cost 20,000
• Dawns cost 450,000,000

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Why do people study meteorites and asteroids?

• Meteorites can be studied in detail in the
  laboratory
• Isotopic ratios
• Ages
• Minerals
• Elemental abundances
• Asteroids can currently only be studied by remote
  sensing
• Asteroids give you locations where things formed

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Rocks give you War and Peace, but the first 100
pages are missing. You know how it comes out, but
youre missing the formative stages.
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If you can link meteorites with asteroids?

• You can possibly map the surface and interior
  of an asteroid if you can get high-resolution
  spectra of the surface
• You can possibly determine what are the isotopic,
  mineralogical, and chemical gradients in the
  solar nebula

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Meteorites

• HEDs howardites, eucrites, and diogenites
• basaltic meteorites that appear genetically
  related to each other
• Eucrites pigeonite and plagioclase
• Diogenites orthopyroxene
• Howardites mixture of eucritic and diogenitic
  material
• Mesosiderites stony-iron meteorites that are a
  mixture of HED-like material and metallic iron

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eucrite
diogenite
howardite
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eucrite 7 cm long
howardite 1.9 cm long
diogenite 13 cm long
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mesosiderite
NWA 2932
http//i1.ebayimg.com/02/i/06/df/a1/3f_1.JPG
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http//www4.nau.edu/meteorite/Meteorite/Images/Ves
taStratigraphy.jpg
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Eucrites have band positions at longer
wavelengths than howardites Howardites have band
positions at longer wavelengths than diogenites
eucrite (Bouvante)
Band II
Band I
How light is reflected from a sample.
howardite (EET 87503)
Normalized Reflectance
visible
near-infrared
diogenite (LAP 91900)
Wavelength (µm)
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Absorption features due to the presence of Fe2
Splitting due to the application of a
non-spherical electrostatic field from
surrounding atoms
3d
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Electrons

• Electrons can absorb photons at specific energies
  to go from one energy level to another
• The energies of these photons correspond to
  visible and near-infrared wavelengths

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Ca and Mg

• Ca and Mg do not have incompletely filled 3d
  orbitals
• Do not have absorption features in the visible
  and near-infrared
• Pure enstatite has no absorption features
• But their presence affects the position of the
  absorption features due to Fe

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Ibitira
(Weichert et al. 2004)
Greenwood et al. (2005)
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Yamaguchi et al. (2002)
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eucrite fractionation line
Greenwood et al. (2006)
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What do the Meteorites tell us?

• Most HEDs appear to come from the same parent
  body
• 4 Vesta?
• A few HEDs have distinctly different oxygen
  isotopic values, implying that they come from
  different parent bodies
• Iron meteorites imply the formation of a large
  number of differentiated bodies
• Mesosiderites have indistinguishable oxygen
  isotopes from most HEDs
• Did an iron asteroid impact Vesta and form
  mesosiderites?

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Asteroids

• 4 Vesta - 500 km diameter asteroid
• reflectance spectrum similar to the HEDs
• Vesta family asteroids with similar orbital
  elements to Vesta
• Over 4,500 known members
• Vestoids asteroids with reflectance spectra
  (visible and/or near-infrared) similar to HEDs
• found inside and outside the Vesta family and
  among the near-Earth asteroids
• Usually have estimated diameters less than 10 km

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V-type 4 Vesta
Normalized Reflectance
Band I
Band II
howardite (EET 87503) (Hiroi et al. 1995)
Wavelength (µm)
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Vesta and the HEDs

• Vesta seems to be the best spectral match to a
  group of meteorites

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Most asteroids do not match
25143 Itokawa (Binzel et al., 2001)
Greenwell Springs (LL4)
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D
A
T
Classes Defined By David Tholen (1984)
X
S
B
Q
C
F
R
G
V
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25143 Itokawa Picture from Hayabusa Spacecraft
535 294 209 m
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Vestoid
V-type 1929 Kollaa
Normalized Reflectance
eucrite (Bouvante) (Burbine et al. 2001)
Wavelength (µm)
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Normalized Reflectance
Wavelength (µm)
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31 resonance
1459 Magnya
1468 Zomba
21238 1995 WV7
4731 Monicagrady (not a Vestoid)
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31 resonance
1459 Magnya
1468 Zomba
Fragments of Vesta?
21238 1995 WV7
minimum
?
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What do the Asteroids tell us?

• Most Vestoids appear to be fragments of Vesta
• What is their mineralogy?
• Some Vestoids are relatively far from Vesta
• Do they have different mineralogies?

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My Work

• Our group (headed by Rick Binzel at MIT) has
  near-infrared spectra of 17 main-belt
  Vestoids
• I am trying to determine the mineralogy of these
  objects
• Use HEDs as a guide

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Deriving Formulas for Determining Pyroxene
Mineralogies

• Used high-resolution spectra of 13 HEDs (Hiroi)
  and high-quality analyses of the same HEDs
  (primarily from Buchanan)
• Band I center fitted Band I minimum with
  continuum slope removed
• Band II minimum fitted Band II minimum (not
  sure what continuum slope is over this wavelength
  region)
• average pyroxene mineralogy average of a number
  of grains

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Ca-content Mg-content Fe-content
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Takeda (1997)
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R2 0.9253
diogenites
eucrites
howardite
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En
En
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Wo
Wo
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Fs
Fs
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Mg
Mg
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Gaffey et al. (2002) also has formulas

• Fs (5) 268.2Band II center (µm) - 483.7
  (Wolt11)
• Fs (5) 57.5Band II center (µm) - 72.7
  (Wo11-30 Fslt25 excluded)
• Fs (4) -12.9Band II center (µm) 45.9
  (Wo30-45)
• Fs (5) -118.0Band II center (µm) 278.5
  (Wogt45)
• Wo (3) 347.9Band I center (µm) - 313.6
  (Fslt10 Wo?5-35 excluded)
• Wo (3) 456.2Band I center (µm) - 416.9
  (Fs10-25 Wo?10-25 excluded)
• Wo (4) 418.9Band I center (µm) - 380.9
  (Fs25-50)

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With all these formulas

• Should be able to predict the mineralogy of
  Vestoids
• One complication
• HED band positions move to shorter wavelengths as
  the temperature decreases
• Meteorites are measured at room temperature (300
  K)
• Vestoids have temperatures of 160-200 K
• Band I moves 0.00 to -0.01 µm for asteroid
  temperatures
• Band II moves -0.02 to -0.03 µm for asteroid
  temperatures

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Some Preliminary Fits of Some Vestoids

• Most of the fits were done using polynomials to
  fit Band I and Band II, respectively
• I am checking to see how much the calculated band
  position depends on the fitting routine
• Did not divide out the continuum slope to
  determine the Band I center for the asteroids
• some asteroids did not have visible spectra
• Error bars are just estimates
• I need to fit with a variety of methods and
  determine uncertainties
• Did not calculate temperatures for the asteroids
  (just made the same estimated temperature
  correction for all the asteroid band positions)

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eucrites
3155
2851
howardite
21238
diogenites
(minima for asteroids)
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21238 1995 WV7
1929 Kollaa
Wavelength (µm)
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Mesosiderites
Pinnaroo metal
Estherville
Vaca Muerta
Normalized Reflectance
Lamont
Pinnaroo
0.5
1.0
1.5
2.0
2.5
Wavelength (µm)
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added mesosiderites
Lamont
Estherville
(minima for asteroids)
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1468 Zomba
1459 Magnya
(average of the results from the Band I and Band
II formulas)
21238 1995 WV7
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1468 Zomba
1459 Magnya
2851 Harbin
21238 1995 WV7
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1468 Zomba
1459 Magnya
3155 Lee
2851 Harbin
21238 1995 WV7
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1468 Zomba
1459 Magnya
2851 Harbin
21238 1995 WV7
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21238 1995 WV7
diogenites
2851 Harbin
3155 Lee
1459 Magnya
1468 Zomba
eucrites
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Conclusions

• Most of the Vestoids have inferred compositions
  consistent with eucrites/howardites
• The Vestoids that do not appear to be related
  to Vesta have a range of compositions
• Further work needs to be done to see how well we
  can map Vesta using these fragments

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Any questions?