Asteroids and Meteorites

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Asteroids and Meteorites
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In this lecture

• Asteroids
• Sizes, locations
• Dynamics
• Families
• Compositional gradient
• Taxonomic types
• Meteroites
• Connection to asteroids
• Timescales

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• Size distribution can be approximated by a power
  law
• Largest ones dominate mass
• Smallest ones dominate surface area

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• Whats an Asteroid
• Chiron
• Asteroid reservoirs
• Main belt
• 2.0 - 3.5 AU
• Thousand of objects - 3x1021 kg
• About half the mass in the largest 4 bodies
• Ceres, Vesta, Pallas, Hygiea
• Jupiters Lagrange populations
• Hilda Asteroids 32 resonances
• Trojans at L4,L5 points
• Near Earth objects
• Atens
• Apollos
• Amors

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Near Earth Objects

• Amors
• Apollos
• Atens
• Short dynamical lifetimes 10 Myr
• Most get ejected from the solar system
• or fall into the Sun
• or impact a terrestrial planet

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Trojans and Hildas

• Trojans (and Greeks)
• Occupy L4, L5 Lagrange points
• About 2000 discovered to date
• L4 population twice that of L5
• Objects can execute horseshoe orbits

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Shape of the Main Belt

• Inner edge
• Precession resonance with Saturn (v6)
• Asteroid and Saturns orbits precess
• Resonance at 2.2AU when i0
• Resonance at gt2.2AU when igt0
• Outer edge
• Jupiters early inward motion
• Conservation of momentum with ejected bodies
• 21 resonance moves with Jupiter
• Clears material

V6
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Kirkwood Gaps

• Where period is simple fraction of Jupiters
• period
• Semi-major axis from Keplers law
• e.g. P ?PJupiter then a2.5 AU
• Gaps exist only in parameter space
• Dont think this looks like Saturns rings!
• Particles stray into these orbits
• Collisions
• Yarkovsky effect
• Afternoon/morning temperature differences
• Photons carry momentum hv/c
• Small (but constant) thrust
• Prograde rotators speed up, spiral out
• Retrograde rotators slow down, spiral in
• Seasonal temperature variation can affect slow
  rotators more

Morning-cold
Afternoon-warm
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Asteroid Spins

• Bodies rapidly realign their axis so that moment
  of inertia is maximized
• Rotation induces elastic stresses in body ? ?2
  R2
• Timescale to reorient spin axis

• Bigger asteroids have spin periods of 8-12 hours
• Periods lt 2hrs- body will break up
• Shortest period known
• 11 minutes
• 1998ky26 15m across
• Smaller asteroids have much higher spins
• More affected by impacts
• YORP effect

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YORP Effect

• Yarkovsky-O'Keefe-Radzievskii-Paddack
• Extension of Yarkovsky effect for rough surfaces
• Anomalous number of rapid rotators
• Alignment of family spin axis (Koronis family)
• More radiation off steeper slopes
• Small asymmetries in slope distribution lead to
  torques
• Torque can spin bodies up or down
• Torques can change obliquity
• Torque sensitive to shape, and a R2 and a 1/a2

Taylor et al., 2007
Vokrouhlický et al., 2007
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Dynamical Families

• Orbital elements vary with time
• Calculate proper elements
• Run simulation for several Myr
• Take average values of a, i, e
• Groups (Hirayama families) probably from recent
  break up of larger body

Piotr Deuar, Data from the AstDyn site
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Zappala et al., 1995

• Clustering analysis
• Hierarchical clustering method
• Wavelet analysis method
• Nomenclature
• Families - Distinct grouping
• Clans - Several families beginning to overlap
• Tribes - Hard to distinguish from background

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Thomas et al., 1997
Example Asteroid Family - Vestoids

• Vesta and the Vestoids
• Dynamical and compositional family

Piotr Deuar, Data from the AstDyn site
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Taxonomic Types

• Asteroids are characterized by the albedo and
  color

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• Compositional zoning in the asteroid belt
• Objects with a lt 2.7 AU anhydrous silicates
• Dominated by S-type asteroids
• Differentiation, metallic cores implies melting
  of silicates
• Objects with 2.7 AU lt a lt 3.4 AU hydrated
  silicates
• Dominated by C-type asteroids
• Clay minerals implies melting of ice
• Objects with a gt 3.4 AU no hydration (ice never
  melted)
• Primitive (unprocessed) objects
• Objects too small for magma oceans
• Radioactive heating
• Induction and shock heating

Grimm and McSween, 1993
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• Combining radiogenic heating with accretional
  heating

• Accretional heat and radiogenic heat
• Heating governed by body size
• and Initial concentration of Al26
• Radiogenic heat alone

Merk et al., 2002
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Meteorites the best samples

• Samples of primitive bodies - Chondrites
• Contain metal rich chondrules
• Metal-rich spheres that were heated above 1500K
  and then cooled very fast
• Matrix material primative material solar
  composition
• Thermal alteration
• Aquous alteration

• X-wind theory (from Shu et al.)
• Particles get ejected towards the star along
  magnetic field lines
• Dust agglomerates melt, volatiles are removed
• Molten droplets of metal rich material are
  funneled toward the bipolar outflow
• Rain out across the disk
• Travel through space above the disk results in
  quick cooling
• These cooled droplets form the chondrules and get
  incorporated into the forming asteroids

Shu et al., Science 1997.
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• Samples of differentiated bodies
• Crustal samples
• HED samples show evidence of surface volcanism
• Mantle samples
• E.g. Pallasite (core mantle boundary)
• Core samples
• Irons

Willamete meteorite - iron
Pallasite
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• Correspondence between Asteroids and Meteorites
• Determined spectrally doesnt always work

• Certain types of asteroidal material is over
  represented
• Planetary samples also exist

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• Asteroid surfaces exhibit space weathering
• C-types not very much
• S-types a lot (still not as much as the Moon)
• Weathering works faster on some surface
  compositions

• Asteroids are collisional fragments
• Collisions in the asteroid belt usually cause
  fragmantation
• Smaller asteroids (in general) are the result of
  more recent collisions less weathered
• Material around impact craters is also fresher

Ida (and Dactyl) Galileo mission

• S-type conundrum
• S-Type asteroids are the most common asteroid
• Ordinary chondrites are the most numerous
  meteorites
• Parent bodies couldnt be identified, but
• Galileo flyby of S-type asteroids showed surface
  color has less red patches
• NEAR mission Eros showed similar elemental
  composition to chondrites

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• Meteorite summary

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Timescales

• Multiple timescale measured
• Crystalization
• Long-lived radioisotopes
• Excavation
• Cosmic ray
• Earth-dwelling time
• C14 or cosmic ray products decay

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Summary

• Asteroids
• Sizes, locations
• Dynamics
• Families
• Compositional gradient
• Taxonomic types
• Meteroites
• Connection to asteroids
• Timescales