Meteorites: Remnants of Creation

Solar Nebula: flattened rotating disk, hotter toward center

Solid grains form only in proper temperature environment

Only refractory elements remain solid to form planets, moons, and asteroids near center

Fractionation: lighter (volatile) elements escape in hot, close to center, environment

Chemical reactions forming specific minerals and rocks occur only in certain range of distances from center Þ Chemical condensation sequence - Chemically sorted with distance Þ differences in composition among planets and other objects

Consistent with uncompressed density - decrease away from center

Sequence: condensation Þ solid grains Þ planetesimals - accretion Þ protoplanets - collisions and impacts Þ fragmentation, especially smaller bodies

T-Tauri winds (strong solar wind), just before main sequence, clears residual dust

Gravitational interaction ejects remainder

Meteorite Types: Irons Stones Stony - Irons

Fragments of parent bodies - comets and asteroids

Irons: pure nickel - iron, not oxides

Stones: like terrestrial rocks

Stony - Irons: mixture

Primitive (chondrites): chemically like parent body, e.g., carbonaceous chondrite

Differentiated or igneous: solidified from molten state - fragments of differentiated parent body

Breccias: cemented pieces from impact processes on parent body - relatively small and airless - asteroids

Dating methodologies

Radioactive decay: isotopes emit gamma, electrons, alpha particles

Þ change to other isotopes of same and other elements

Parent Þ daughter

Half - life: 1/2 of parent changes to daughter

Use parent elements with half-lives like ages of rocks studied

Solidification Age: time daughter product accumulated unmixed with parent, i.e., since rock solidified

Original parent / daughter ratios determined using different mineral grains and different isotopes of same element

Parent Daughter Half‑life (billions of yrs.)

Samarium (Sm‑147) neodymium (Nd‑143) 106

Rubidium (Rb‑87) strontium (Sr‑87) 48.8

Thorium (Th‑232) lead (Pb‑208) 14.0

Uranium (U‑238) lead (Pb‑206) 4.47

Potassium (K‑40) argon (Ar‑40) 1.31

Solidification ages (billions of years) of primitive meteorite groups

H group chondrites 4.50 ± 0.04 by L group chondrites 4.43 ± 0.05 by

LL group chondrites 4.51 ~ 0.03 by E group chondrites 4.45 ± 0.03 by

Gas retention age:

Uses the amount of radiogenic argon from the decay of potassium-40,

Þ time since meteorite was last at temperature at which gas leakage could occur

Primitive Meteorites

Chondrites - contain chondrules, solidified droplets

Carbonaceous chondrites - rich in carbon, contain volatiles, water soluble compounds, amino acids and nucleic acids, L & R aminos - most primitive

Interplanetary dust - mostly cometary origin, collected at high altitudes

Differentiated Meteorites

Irons: Mostly pure nickel-iron. From cores of differentiated asteroids 4.4 - 4.5 billion years old Þ parent bodies differentiated early

Stony basaltic: from crust of differentiated body. Lighter lava rose to top

Eucretes: from parent body asteroid Vesta - expelled from impacts

SNC's: from Mars

Sources - parent bodies: Mainly differentiated asteroids

Comets for some chondrites

Asteroids

Discovery - Bode's Law

The Titius-Bode Law Þ spacing of the planets in the Solar System

Bode predicted planet between Mars and Jupiter Þ asteroid belt. Begin with the sequence:

N =

0

3

6

12

24

48

96

192

384

(N+ 4)/10 =

0.4

0.7

1.0

1.6

2.8

5.2

10.0

19.6

38.8

Body	Actual distance (A.U.)	Bode's Law <A.U.)< td>
Mercury	0.39	0.4
Venus	0.72	0.7
Earth	1.00	1.0
Mars	1.52	1.6
?		2.8
Jupiter	5.20	5.2
Saturn	9.54	10.0
Uranus	19.19	19.6

Þ 1801- Ceres, 1^st asteroid discovered Þ Discovery of Asteroid belt

Ceres largest - 1 \ 2 total mass Total mass = 1/2000 Earth, 1/20 Moon

Size ~ 1/D² , D = relative Diameter: If 2 asteroids at 500 km, how many at 100 km? Þ 2 x 1/ 0.2² = 2 x 25 = 50 asteroids

500 km

Size and Reflectivity - Methodologies

Previously and still mainly too small to image directly from Earth, so…

Occultations

Time of passage in front of star from different locations: V x t = length

Reflection & Emission

1. Compare reflected sunlight to emitted IR Þ reflectivity (albedo)

2. Total light measured = (light / M²) _{(at Earth from asteroid)} x albedo x cross section Area

Þ cross sectional Area = Total light / [(light / M²) _{(at Earth from asteroid)} x albedo]

Asteroid Composition

Reflection Spectra used to identify major minerals on surface:

Absorption at certain wavelengths different for different minerals

Compare to meteorites

Resonances:

Kirkwood gaps in asteroid belt

Distances correspond to fractions of Jupiter's orbital period

Number of asteroid orbits

Asteroid Families

Similar orbits, reflectivities, and spectra Þfragments of same parent asteroid

1/2 of all are members of families

10% of all in Koronos, Eos, and Themis families

Asteroid Compositional Classes

C -type: dark, like carbonaceous chondrites

S - type: lighter, silicate or stony

M - type: metallic

Vesta: a special case

Vesta, a Basaltic (Volcanic) Asteroid: the Eucrite Parent Body?

Scientific Logic:

· Eucrites match spectrum of Vesta closely but not exactly

· Eucrites spectra not all exactly the same either

· Vesta rotates, not uniform over surface Þ

Eucrites expelled from different parts of lava (basaltic) surface?

· Chemistry of eucrites reveal composition of mantle beneath crust of parent body

· But no meteorites found with that composition - should be more mantle meteorite samples than crust (eucrite) meteorite samples if parent body had broken up Þ eucrite parent body still intact

Þ eucrites chipped from volcanic crust by impacts

· Parent body intact + no other asteroid has basaltic surface like Vesta

Þ Vesta is parent body

· Conclusion reinforced by: only a large asteroid, like Vesta, would be heated enough to generate basalt surface

· Then, meteoritic evidence Þ dates:

solidification age of lava flows = 4.5 billion yrs

gas retention age of impact releasing meteorites = 3 billion yrs

· But, did eucrites come directly from Vesta or from intermediate (larger fragments) objects?

· Three small Vesta-like asteroids discovered - orbits indicate could be large fragments from Vesta - possibly eucrites came directly from these

·

Questions

· Could Vesta have sustained impact great enough for such large fragments?

· Why did only Vesta, not Ceres and Pallas, volcanically active?

Distant Asteroids (or Old Comets)

Near - Earth Asteroids

Apollo first - 1948 Þ Sometimes called Apollo asteroids

· Orbits unstable

· 1/3 will hit Earth eventually - once every 100 million years

· Most from asteroid collisions ~ several / million years

· Some are old comets with volatiles exhausted, e.g., Chiron

· Impacts Þ extinction events - major influence on biological evolution

Remote Sensing:

· Appear as streaks on photos due to orbital motion

· Imaged directly by Viking ( Phobos and Deimos - Mars) and Galileo (Gaspra, Ida and Dactyl)

· Recent images from Keck telescopes

· Radar images Þ surface images of Toutatis

and

Castalia

Computation: Collision Frequency

Volume in asteroid belt = 10²⁵ km³

Number of asteroids = 10⁵

Volume available per asteroid: 10²⁵ km³ / 10⁵ = 10²⁰ km³ / asteroid

Average spacing between asteroids = cube root 10²⁰ km³ = 5 x 10⁶ km

Typical velocity relative to other asteroids = 4 km/sec

Avg. diameter of 10 km Þ 100 km² cross section

Volume swept by each asteroid = 4 km/sec x 100 km² = 400 km³ / sec

Volume / asteroid / year = (400 km³ / sec) x 3 x 10⁷ sec / year = 10¹⁰ km³ / year

Volume available per asteroid / Volume swept by each asteroid / year =

# years / collision / asteroid

= 10²⁰ km³ / asteroid / 10¹⁰ km³ / year = once every 10¹⁰ years / collision / asteroid

Years between collision for any asteroid =

10¹⁰ year / collision/asteroid / 10⁵ asteroids = 10⁵ years/ collision Þ fragments and dust