Chapter 6: Meteorites and Chapter 6: Meteorites and MeteoriticsMeteoritics

•Estimating sizes, orbits•Fate: breakup or impact

Review: States of MatterReview: States of Matter

•Matter can be in different states, depending on how tightly bound the atoms are.

•Changes in phase require the breaking of a binding force

•For our purposes, we are mostly concerned with gases, solids and (to a lesser extent) liquids.

States of MatterStates of Matter

•Matter can coexist in different phases. At the triple point, gas, solid and liquid coexist.

Phase diagram for water

States of MatterStates of Matter

• The phase diagram for different elements tells us what phase they will be found in under given conditions.

• Knowing the triple point and critical point alone allow a rough estimate of the phase diagram.

Phase diagram for water Phase diagram for hydrogen

GasesGases

Ideal Gas Law: relates pressure, density and temperature

Hm

kTnkTP

Where n is the number density and is the mass density of the gas. is the mean molecular weight.

• Such an equation, relating pressure, density and temperature, is known as an equation of state.

• The equation of state for solids and liquids is generally much more complex and/or poorly known.

Hm

m

i.e. this is the average mass of a free particle, in units of the mass of hydrogen

SolidsSolids

•Minerals are substances that occur naturally and include no organic (animal or vegetable) compounds.

The most commonly occurring minerals are made of the most commonly occurring elements

In the inner SS these are dominantly O, Si, Mg, and Fe with lesser amounts of things like Na, Al, Ca, and Ni.

The minerals we find are vastly dominated by SiO4 – these are called silicates.

•Rocks are solids made of more than one mineral and the mix of minerals in rocks varies from one part of the SS to another and well as within a given body.

•Ices are solids whose composition consists of the abundant elements C,N,O in combination with H.

These compounds (water, carbon dioxide, methane, ammonia etc.) freeze at different temperatures; strictly speaking these are also minerals but are referred to as ices because of their low solidification temperatures.

Most common in the outer SS beyond ~3AU from the Sun.

SilicatesSilicates

Feldspars• SiO2 + K, Al, Na, Ca…

• Least dense silicate• Low melting point

(~600 C)

Pyroxene• chains of SiO4 + Fe,

Mg, Al, Ca…

Olivine• simplest silicate• SiO4 + Fe and/or

Mg• Most dense

silicate• High melting point

(>1000 C)

• The main silicate families are olivines, pyroxenes and feldspars. They are distinguished from each other by which elements are present and how complex are their crystalline structures.

RocksRocks

•Igneous Formed directly from cooling

of molten magma

•Sedimentary Deposition or cementing of

small particles

•Metamorphic rocks Originally formed as igneous

or sedimentary, but changed to a new form by high pressure, high temperature or addition of new chemicals

IcesIces

•solids that contain C,N,O – which are gaseous at T≥200K•CNO overall more abundant than Fe,Mg,Ca,Al,Na…

but Fe,Mg,…condense into grains at much higher T•ices are more abundant in outer SS objects – i.e. satellites,

comets, some asteroids, Kuiper Belt…•commonest ices: CH4 (methane), NH3 (ammonia), H2O (water)•vapourous “at the least excuse”

Core of Halley’s comet shown outgassing as Sun heats the ice

Meteoritic ComplexMeteoritic ComplexMeteoritic complex: interplanetary objects, such as dust particles,

asteroids and comets.

• Among terrestrial planets, the largest are ~30 km across• Among giant planets, the largest are at least ~500 km across• In the asteroid belt, between the terrestrials and giants, the

largest object is Ceres (1000 km).

MeteorsMeteors

•Occasionally one of these bodies intersects with Earth’s orbit, entering the atmosphere and burning up as a brief flash of light.

•This space debris is quickly slowed by frictional interaction with the atmosphere to speeds of ≤1km/s and the smallest ones burn up before reaching the surface. Some, normally the larger and more massive ones, will reach the Earth’s surface; they are called meteorites.

Meteorite FallsMeteorite Falls

• Supersonic velocities create sonic boom

• Larger objects are brighter• May leave dusty train of debris• Fall is so swift that only outer

~1mm is heated

•Fireball observed over Wales Sept 29 , 2003?

Maybe not, actually - but still a cool picture

•twisted meteor train – how?•non-spherical? non-uniform? “wobbling”?

Meteor sizesMeteor sizes

•Consider a meteoroid (radius R, density ρM, mass M=4/3 (πR3ρM) entering the atmosphere (density ρa) at some speed v

the meteor must clear a path through the atmosphere to move forward – push air out of the way.

vdt

πR2

R

v

M

vR

dt

dv

M

aa

4

3 222

(Typically this relation gives results which are correct to about a factor of 2.)

• Observe v, dv/dt; know ρa vs. altitude

=> this gives R2/M

• if we assume a value for ρM, can solve for R, M

Example: meteor sizesExample: meteor sizes

From film footage of a fireball you estimate a meteor’s initial velocity to be 20 km/s. When it disappears from view, 3 s later, it is traveling at a velocity of about 10 km/s.

Assume it is traveling roughly horizontally, at an altitude where the air density is 5x10-5 kg/m3. Estimate its mass.

R

v

M

vR

dt

dv

M

aa

4

3 222

Meteoroid BreakupMeteoroid Breakup

EK~0.1v2(km/s) eV/atom

interaction with atmosphere, with sufficient EK, means meteoroid is destroyed

energy to detach 1 atom ~ 2-3eV if v~10km/s -> EK ~10 eV – more than

enough!

AblationAblation The slowing of the meteor also

means an energy loss which results in heating of both the meteor and the air around it.

ablation efficiency is higher for lower ρa

can relate ΔEK and mass loss to meteor luminosity

so L→ M (photometric mass) Although the fall through the

atmosphere heats a meteor, this heating penetrates only the outer mm-cm and leaves the interior very cool.

Q

vR

dt

dM a

2

32

ExampleExample

Calculate the final mass of a meteor, assuming its final velocity is much smaller than its initial velocity.

Q

vR

dt

dM a

2

32

R

v

dt

dv

M

a

4

3 2

Meteors with high initial velocities will tend to be completely destroyed; only the slowest survive.

BreakBreak

History of Meteorite StudiesHistory of Meteorite Studies

•Read interesting history in textbook, p. 131

•“Stones from the sky” observed by ancient Chinese, Greeks, Romans

•Only became accepted in early 1800s•Huge collections of meteorites found in Antarctic ice fields in

1969 Falling on ice less damaging then on rock Easier to find (contrast with surrounding terrain) Less subject to erosion

ClassificationClassification

• Some meteorites have clearly been altered by heat pressure, mechanical shock, or other processes

Sometimes the heating is gentle, so they are only slightly altered Others have completely melted, allowing the separation of heavy and

light materials: differentiation• Differentiation forms the main basis of classification• Many are breccias: formed of broken fragments cemented together

Breccias and fractures suggest violent collisions

(a) A stony meteorite often has a dark crust, created when its surface is melted by the tremendous heat generated during passage through the atmosphere. (b) Iron meteorites usually contain some nickel as well. Most show characteristic crystalline patterns when their surfaces are cut, polished, and etched with acid.

DifferentiationDifferentiation

•How does differentiation occur? Imagine heating a solid mixture of primordial material so that it

melts High density metals such as iron and nickel, along with

elements with a chemical affinity for iron (e.g., Co, Ni, Ru, Rh, Pd, Os Ir, and Pt) drain to the centre to form a metal core

Lighter, silica-associated elements float to the surface to form stony mantle.

Collisions then produce fragments with different compositions. Later collisions can weld fragments together, forming breccias.

Stone meteoritesStone meteorites

Ordinary chondrites are the most common type of stone meteorite,

Carbonaceous (C) chondrites are some of the most complex of all meteorites.

Enstatite (E) chondrites are a rare and unusual type of meteorite.

Achondrite meteorites are very similar in appearance to terrestrial igneous rocks.

• Stony meteorites are the most common (95%)• Include the least differentiated meteorites• Most numerous are the chondrites: a class that contain

small spherules called chondrules

ChondrulesChondrules

•Glassy spherules embedded in meteorites•Composed primarily of olivine and pyroxene, with moderately

high melting temperatures.•Form during rapid cooling of droplets of molten material

•Formation? Possibly formed in

shock waves within original solar nebula

Or during impacts on surfaces of planetesimals.

ChondritesChondrites

•Chondritic meteors have nearly solar abundances of elements – except for volatiles

the oldest SS samples

formed from early solar nebula

most are unchanged since formation

Next lectureNext lecture

More meteors… Finish classification Ages Origins