(4) Meteorites: Remnants of Creation

• Meteoroid: small piece of debris in space• Meteor: space debris heated by friction as it plunges into

the Earth’s atmosphere• Meteorite: Space debris that has reached the ground

Two basic properties of the Solar System

• Chemical composition. Fractionation: chemical sorting that results, for example, in different planets being composed of different materials

• All planets have nearly circular orbits that lie roughly in the same plane, and all revolve in the same direction around the sun.

Formation and Condensation of the Solar Nebula

• Stars form out of clouds of molecular gas & dust• Collapse occurs when the gas is dense enough to

collapse under its own weight• Central parts of collapsing cloud become heated, & the

shrinking nebulae begin to spin faster

• Results

Angular Momentum = Mass x Velocity x RadiusExample: ice skater pulling in their arms as they spin

→ center becomes star→ spinning disk ultimately gives rise to planets

Star-Forming Region in the Orion Constellation

Disk Evolution• Temperature gradient develops in the disk

• Grains, whose composition depends on the local temperature, begin to condense

→ outer disk cools→ inner disk is heated by proto-Sun

→ stick together initially, building up planetesimals→ planetesimals attract each other gravitionally (a process called accretion)→ Protoplanets form, sweeping up grains in their path→ As protoplanets grow in size, fragmentationbecomes important for the production of meteoroids & asteroids (as well as for heating the interior of the planets)

Artist’s conception of collapsing stellar disk

An example of a disk: β Pictoris

Another Example

Central starhas been blocked by a Coronagraph

Circumstellar disks (optical)

More Examples

Solar System formation time-line

(Solar System Formation – Video)

The Importance of Meteorites

• Remnants from the formation of the solar system• Provide clues to their Parent Bodies• Easily accessible

Classification

1) Iron: nearly pure metallic (nickel-iron) of high density (7 g / cm3)

2) Stones: like terrestrial rocks (silicates, carbon compounds, water)

3) Stony Irons: mixture of 1) & 2)

Classification (cont)

• Primitive meteorites: meteorites for which little chemical evolution has occurred since the formation of the solar system (stones)

• Differentiated (or igneous) meteorites: meteorites that have experienced significant chemical chance since their formation (i.e., solidified from molten state; iron/stony irons)

• Another term commonly used: Breccias: fragmented and recemented rocks.

Locating Meteorites

Qu: Where is a good place to find meteorites?An: The Antarctic

Why?1) Relatively easy to find in ice2) The ice protects the meteorite from additional

weathering

Name of the Meteorite = Location it is found

Age-Dating Meteorites• Solidification Age: Time since the material became

solid

• Gas Retention Age: A measure of the age of a rock, defined in terms of its ability to retain radioactive argon(which is the daughter product of potassium)

Half-Life• Half-Life: Given a quantity of material, the half-life is the

time which half the material will have decayed into the daughter product

• Radioactive Decay

• The Decay Rates

U-238 (92p+,146n) → Pb-206 (82p+,124n) + (10p+,22n)K-40 (19p+,21n) → Ar-40 (18p+,22n)

U-238 → 4.5 billion yearsK-40 → 1.25 billion years

Examples -

Radioactive decay of Potassium-40 to Argon-40

Radioactive Decay• Mathematically, this is written as

N = N0 e –Lt

L is a constant that can be derived as follows,

N = 0.5 N0 = N0 e –L thl

L =

t = ln

thl

ln 2

thl

ln 2NN0

Present amount

Initial amount of Parent product

I.e, at time = half-lifetime

Age of rock

Fraction ofParent productleft

Summary of Meteorites• Age-dating of meteorites puts the solar system formation

age at 4.5 billion years ago• The parent bodies of meteorites appear to be primarily

asteroids (with diameters of a few 100 km), though some clearly come from the moon and Mars.

• Reconstruction of the orbits of a few meteorites indicates that they may have originated in the asteroid belt

→ Little chemical processing of primitive meteorites→ Crystal patterns of differentiated meteorites → “basalt” meteorites from lunar surfaces (3.3 – 3.9 billion years old→ SNC meteorites which contain material associated with very recent volcanism (1.4 billion years ago – i.e., from Mars)