Download asteroids

Midterm material
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In class, Wednesday Oct 25.
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Please wait outside AT BACK until told to enter the room. Must write IN PEN.
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Non­programming calculators allowed (and required)
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No notes or hats. Formulae provided
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Covers all lectures before Oct 17, text material up to and including 83 (plus Chpt. 14 material covered in lecture on the Sun)
See course web page for EXTRA office hours.
Assignment 2 back in slots. Solutions posted. Last year's midterm cover pages posted.
Meteorites and Radiometric Chronology
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Nearly everything we know about absolute ages in planetary science comes from studies of meteorites and spacecraft samples.
Meteorites preserve our best record of the processes that occurred in the early Solar System.
Meteorite terminology
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Meteoroid: a small rock in space
Meteorite: The fragments (if any) that reach the ground once a meteoroid has passed through the Earth's atmosphere.
Meteor: (="a shooting star"). Visible light emitted when meteoroid passes through the Earth's atmosphere. Big ones are called 'bolides'
Meteorite discovery circumstances
1)
Falls: Meteor observed, meteorite immediately recovered.
➔ Know when the meteorite fell.
2)
Finds: Meteorite just found on the ground and realized to be meteorite.
➔ Have no idea how long meteorite has been on Earth (but can be measured).
Where do meteorites come from?
There were 4 historical ideas:
1)
Pieces of demolished rocky bodies in the asteroid belt
2)
Pieces of comets
3)
Pieces knocked off the Moon
4)
Pieces of other planets (Mars?)
Where do meteorites come from?
Facts
1)
Most meteorites are from the asteroid belt.
2)
~0.1% certainly from the Moon (anorthosites, impact breccias)
3)
~0.1% are from a large object with a Mars­like atmosphere (=> Mars)
4)
Some could be from comets
Evidence for asteroidal origin
1)
Reflectance spectra of many meteorites (taken in lab) resemble those of some asteroids. 2)
Orbits of 4 recovered meteorites have been measured: ➔ Pribram, Lost City, Innisfree
➔ All had perihelion <1 AU, aphelion in the asteroid belt. Reflectance spectra of asteroids
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Telescopic reflectance spectra of asteroids show a variety of types. Reflectance spectra of minerals
The minerals
that asteroids
are made of show similar types of features when
measured in a lab on Earth
Canadian MORP project
❑ (M)eteorite (O)bservation and (R)ecovery (P)roject
❑ Monitored skies of central prairies for about a decade
Canadian MORP project
❑ Monitored millions of square kilometers
Canadian MORP project
❑ Triangulation allowed orbit computation
Meteorite orbit determination
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If meteor is well­
observed, a recovered fall can give the orbit of the meteoroid.
Note the black 'fusion crust'
<­­ Innisfree
Orbits of three well­ known falls
Meteorite types 92.8%
----1.5%
5.7%
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100.0%
Stones:
Chondrites
Achondrites
Stony-Irons
Irons
Of falls
85.7%
7.1%
Meteorite paradox
Most of the meteorites people bring to scientists are irons!
Why?
A) Stronger irons resist breakup in the atmosphere
B) Iron meteorites survive longer on Earth's surface
C) People more likely to pick up an iron meteorite
D)All of the above Parent bodies
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Meteorites are fragments of some 'parent body'
Meteoroid lived most of Solar System history in this bigger body until collision released millions of meteoroids.
Parent body types
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'Primitive' or 'Undifferentiated'
Most chondrites come from this type
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'Processed' or 'differentiated'
Achondrites, stony­
irons, irons (going deeper into body)
Stony meteorites
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Two types:
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ACHONDRITES
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From differentiated parent body
CHONDRITES
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Primitive parent body
Chondrites contain chondrules
Chondrules: mm­sized spheres of silicate
Heated/cooled very fast! ( 'frozen' droplets of melt). ­NOT found in Earth or Moon rocks
Stony­Irons
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Interface between metal core and rocky crust of a differentiated parent body
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Nickel­iron metal embedded in silicate
Irons
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Cores of differentiated parent bodies
Irons
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An iron meteorite has been found on Mars!
Found by martian rovers
­ About 30 cm across
Not a surprise...but cool!
Differentiated asteroids cooled slowly
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The Ni­Fe in iron meteorites is very pure : settled to core
Because they cooled slowly can see growth of iron grains: Widmanstätten Pattern
Bigger bodies cool slower. Measurements imply: PB~100­200 km
Meteorite iron is VERY pure!
❑ Before 2000 B.C. meteorites were best source of iron
❑ A kris dagger (Thailand)
The importance of chondrites
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Their chemistry and mineralogy can be studied in the laboratory
Provide a test for the Lewis condensation model
In fact: can tell the (P,T) at which they condensed
Recall: Except for INERT GASES and some VOLATILES, chondrites have solar compositions.
Except for some elements, chondrites have an isotopic pattern identical to the Sun's atmosphere
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Chondrites lack noble gases (He, Ar, Ne) because they don't get incorporated into dust that sediments from the nebula
H does not get extracted as efficiently either
Li is destroyed in Sun
C,N,O trapped in nebular gas molecules and didn't produce dust
Asteroid belt composition
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Reflectance spectra of asteroids are sorted into various 'classes' (S,C, P,D)
The abundance of these classes varies with distance to the Sun
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Preserves T structure of Solar Nebula!
Some of these classes are linked with certain types of meteorites
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S asteroids: Ordinary chondrites?
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C asteroids: Cabonaceous chondrites
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M asteroids: Irons?
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Reflectance spectra of the main belt show a zonal structure: different classes dominate at different distances.
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Links with meteorites show more volatile­rich asteroids at greater distances.
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C­type asteroids have spectra
that look like water­rich carbonaceous chondrites
Belt preserves T structure of solar nebula!