Transcript Lecture6

Chapter 6: Meteorites and
Meteoritics
• Estimating sizes, orbits
• Fate: breakup or impact
Review: 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 Matter
• Matter can coexist in different phases. At the triple
point, gas, solid and liquid coexist.
Phase diagram for water
States 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
Gases
Ideal Gas Law: relates pressure, density and
temperature
kT
P  nkT 
mH
Where n is the number density and  is the mass
density of the gas.  is the mean molecular weight.
m

mH
i.e. this is the average mass of a
free particle, in units of the mass
of hydrogen
• 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.
Solids
• 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.
Silicates
• 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.
Olivine
• simplest silicate
• SiO4 + Fe and/or
Mg
• Most dense silicate
• High melting point
(>1000 C)
Pyroxene
• chains of SiO4 + Fe,
Mg, Al, Ca…
Feldspars
• SiO2 + K, Al, Na, Ca…
• Least dense silicate
• Low melting point
(~600 C)
Rocks
• 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
Ices
• 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 Complex
Meteoritic 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).
Meteors
• 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 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 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.
πR2
vdt
R  a v
3 a v
dv


dt
M
4M R
2
2
2
(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 ρ , can solve for R, M
Example: 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 2  a v 2
3 a v 2
dv


dt
M
4M R
Meteoroid 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!
Ablation
 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 mmcm and leaves the interior very cool.
dM R 2  a v 3

dt
2Q
Example
Calculate the final mass of a meteor, assuming its final velocity is
much smaller than its initial velocity.
dM R 2  a v 3

dt
2Q
3 a v 2
dv

dt
4 M R
Meteors with high initial velocities will tend to be completely
destroyed; only the slowest survive.
Break
History 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
Classification
• 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.
Differentiation
• 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 meteorites
• 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
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.
Chondrules
• 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.
Chondrites
• 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 lecture
More meteors…
 Finish classification
 Ages
 Origins