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Multiple and Fast: The Accretion of
Ordinary Chondrite Parent Bodies
P. Vernazza (LAM), B. Zanda (MNHN), R. Binzel (MIT), T. Hiroi
(Brown University), F. E. DeMeo (MIT), M. Birlan (IMCCE)
Asteroid Facts:
•  Most are in the main
asteroid belt (between
Mars and Jupiter).
•  Largest is Ceres,
discovered 1801
(~ 950 km across).
•  More than 300,000
asteroids known.
•  Estimated one-million
main-belt asteroids
larger than 1 km.
(FB) Gaspra (12 km)
S
S
S
(FB) Ida (56 km)
(RV) Eros (33 km)
Dactyl (1 km)
(RV) Itokawa (500 m)
S
(FB) Mathilde (66 km)
C
Xc
(FB) Lutetia (100 km)
(RV) Vesta (520 km)
(FB) Steins (5 km)
Xe
V
How do planetary systems form and evolve? Asteroids = key population to
progress on the following questions:   The role of planetary
migrations in the evolution   The origin of water on a planet
(Earth)   The formation and evolution of
planetesimals Vesta seen by NASA/DAWN Extra-­solar systems offer only snapshots of their
architecture at a given time Planetary systems with at least 3 planets (Lovis et al. 2011) Evidence of Migrations Deciphering the History of the Solar System
Solar nebula
Proto disk
Planetesimals: asteroids comets TNOs tell us about: Planetesimals
The migration of dust prior to the accretion process Solar System
The primordial chemical composition from which planets once accreted The migration of bodies during the formation and evolution of the Solar System Radial mixing in the protoplanetary disk
Evidence in comets from IR spectra
Evidence in asteroids
(e.g. Hale-Bopp,Crovisier et al. 1997, Brucato et al. 1999)
(chondritic meteorites)
4x5.5 mm
Crystalline and
amorphous
olivine
Chondrules
formed at
T>1600 ºC
Matrix
formed at
T<200 ºC
The migration of bodies during the formation and evolution of the Solar System Predicted migrations from dynamical simulations Oort
Cloud
Jupiter, Saturn Uranus, Neptune The diversity of asteroids is diagnostic and a
consequence of planetary migrations 2 spectral classes ~5 spectral classes 24 spectral classes Asteroids = Condensed version of the primordial Solar System Constraints on the Formation of
Ordinary Chondrite Parent Bodies
Meteorite fall statistics
Stony-Irons
Irons
Achondrites
Carbonaceous
Chondrites
Ordinary
Chondrites
80%
Ordinary Chondrite meteorites
•  3 groups: H (~35%), L (~37%) and LL (~8%)
•  Constraints on the formation and early evolution of the
Solar System :
–  migration processes in the disk
–  the post-accretional heating events
–  the collisional events that have occurred since 4.6 Gyrs
Ordinary Chondrites can’t tell us
the whole story though
• 
• 
• 
• 
• 
Formation location of the different classes of OCs?
Initial average size of their parent bodies?
Amplitude of the bias in our collections?
How many parent bodies for a given meteorite class ?
Level of radial mixing experienced by their parent
bodies after their formation?
•  Their accretion timescale?
S-type asteroids: the parent bodies of
Ordinary Chondrites ?
S-type asteroid
Space
Weathering
Ordinary Chondrite
[Gaffey et al., 1993]
The Hayabusa mission has ended
a 40 years long debate !
S-type asteroids are the parent bodies
of Ordinary chondrites !
Nakamura
et al. 2011,
Science
Location of S-type asteroids
Input Data
Meteorites
Main BeltAsteroids
(laboratory measurements)
(ground-based observations)
•  >100 ordinary chondrite spectra
(0.4-2.5 µm)
250°C
900°C
Expected
structure via
26Al & 60Fe
heating
•  93 S-type asteroid spectra
(0.4-2.5 µm)
Survey almost complete
down to H=8.5 (D~60km)
Compositional Modeling
Compositional model of Shkuratov et al. (1999).
Olivine (Ol), Orthopyroxene (Opx), Clinopyroxene (Cpx)
+ data
+ data
11 Parthenope
◊ model
◊ model
Olivine: 77%
Olivine: 77%
Orthopyroxene: 23%
Orthopyroxene: 23%
Grain Size: 17
Grain Size: 17
Model predicted the
recently confirmed
link (Nakamura et
al. 2011) between
25143 Itokawa and
LL chondrites
(Vernazza et al.
2008)
Asteroid slopes accounted for using weathering model of Brunetto et al. (2006).
Composition of OCs
Two compositional groups
Multiple parent bodies for a given
meteorite class
Implications:
early ring like structures in the disk?
Lyra & Kuchner
(2013) in Nature
Asteroid surfaces as exposed interiors
Implications:
Present Surface = Primordial surface & interior
Coherent with Vesta’s
surface composition
(as seen from Dawn)
‘Fast’ accretion of the H parent bodies
I
Paradox: Paucity of ordinary L
chondrite parent bodies
Fall
statistics
Implications:
Few objects do contribute to the meteorite flux !
O’Brien &
Greenberg 2005
Coherent with cosmic ray
exposure ages which imply
that a few collisional events
dominate the meteorite flux
Common origin for 2/3 of L chondrites
~2/3 of OLC meteorites (25% of all meteorites !)
were heavily-shocked and degassed with
39Ar-40Ar ages 470±5 My (Korochantseva et al.
2007)
=> common origin
=> their parent body suffered a major impact
~470 Ma ago and catastrophically disrupted.
Limestone beds in the Thorsberg
quarry in southern Sweden where
the fossil meteorites were
deposited in the Ordovician ~470
Myr ago.
Strikingly, the timing of the shock event coincides
with the stratigraphic age (467 ± 2) of the midOrdovician strata where abundant fossil OLC
meteorites were found (Schmitz et al. 2003)
I
II
II
Size sorting of chondrules as
the origin of ‘reversed’
formation locations ?
I
Gradient envisioned by meteoricists
Conclusion
Planetesimal formation must occur from a ring of
homogenized chondrule populations: otherwise, how
would we get compositional clones?
The production of compositional clones is a natural
outcome of planetesimal formation: Parent bodies of
meteorites are, as a matter of fact, not always unique.
The formation process of the H chondrite parent bodies
(and by extension, most planetesimals) must have been
‘fast’