ASTRONOMY 340
FALL 2007
4 October 2007
Class #10
Review
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Venus resurfacing event(s) 300-500 Myr ago
Just how do you determine the age of a planetary
surface?
Impacts
Mars
Martian Crustal Magnetization
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Working model
Collection of strips 200 km wide, 30 km deep
 Variation in polarization every few 100 km
 3-5 reversals every 106 years (like seafloor spreading
on Earth)
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Some evidence for plate tectonics…but crust is rigid

Earth’s crust appears to be the only one that
participates in convection
Formation of Impact Craters
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Impactor unperturbed by atmosphere
Impact velocity ~ escape velocity (11 km s-1)
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tens of meters in diameter
Impact velocity > speed of sound in rocks  impact
forms a shock
~100 times stress levels of rock  impact
vaporizes rocks
 Pressures
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Shock velocity ~10 km s-1  much faster than local
sound speed so shock imparts kinetic energy into
vaporized rock
Contact/Compression
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Projectile stops 1-2 diameters into surface  kinetic energy goes into
shock wave  tremendous pressures  P ~ (1/2)ρ0v2
 Peak shock pressures ~1000 kbar; pressure of vaporization ~600 kbar
Shock loses energy
 Radial dilution (1/r2)
 Heating/deformation of surface layer
 Velocity drops to local sound speed – seismic wave transmitted through
surface
Can get melting at impact point
Shock wave reflected back through projectile and it also gets
vaporized
Total time ~ few seconds
Excavation
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Shock wave imparts kinetic energy into vaporized debris
 excavation of both projectile and impact zone
(defined as radius at which shock velocty ~ sound speed
(meters per second)
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Timescale is just a dynamical/crossing time (t = (D/g)1/2
Crater size? D goes as E1/3  empirically, it looks like ~
10 times diameter of projectile (but see equation 5.26b).
Can get secondary craters from debris blown out by initial
impact
Large impacts  multiring basins (Mars, Mercury, Moon)
Craters
35m
2m
4yr
1km
50m
1600yr
7km
350m
51,000yr
10km
500m
105 yr
200km
10km
150 Myr
Small
Earthquake
Barringer
Meteor
Crater
9.6 mag
earthquake
Sweden
Largest
craters/KT
impactor
Crater Density
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See Figure 5.31 in your book  number of craters km-2
vs diameter
Saturation equilibrium – so many craters you just can’t
tell….
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Much of the lunar surface
Almost all of Mercury
Only Martian uplands
Venus, Earth not even close  note cut-off on Venus’ distribution
Calibrate with lunar surface rocks
107 times more small craters (100m) as there are large
craters (500-1000 km)
Mercury
South Pole
Lavinia Planum Impact Craters
Note ejecta surrounding crater
“It’s the size of Texas, Mr. President”
- from yet another bad movie
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Comets – small,rocky/icy things  10s of km
Asteroids – small, rocky things  a few to 10s of
km  the largest is the size of Texas (1000 km)
 100-300
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NEAs known
Close encounters….
River in Siberia  30-50m meteroid
exploded above ground  flattened huge swath of
forest
 Tunguska
You make the catastrophe…
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Need high velocity
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Make it big….
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max velocity ~ 70 km s-1 (combine Earth’s orbital velocity plus
solar system escape velocity)
Earth-asteroid encounters  25 km s-1
Eart-comet encounters  60 km s-1
E ~ mv2  something 1000 km would wipe out the entire western
hemisphere, but let’s be realistic and go for ~10m (1021 J) or ~1
km (1023 J)
One impact imparts more energy in a few seconds than
the Earth releases in a year via volcanism etc.
Surface Composition
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Reflection spectroscopy. (remember radiative transfer!)
What is the surface made of? Rocks, mostly igneous
Minerals = solid chemical compounds with specific atomic
structure
Common Minerals
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Silicates
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Si is produced via He-burning in stellar interiors, released via SNe.
O is produced in massive and intermediate mass stars
Si, O bind easily  SiO4, SiO3 bind with lots of other things (Mg, Al, Fe)
and form a solid at high temperature
SiO2 = quartz
(Fe,Mg)2SiO4 = olivine (most common)
CaAl2Si2O8 = feldspar  60% of surface rocks on Earth
Various Oxides

Fe2O3 = hematite  generally formed from a reaction between Fe, O,
and H2O  has been found in Martian samples
Common Minerals
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Silicon
 3rd
most abundant element (after O, Fe)
 Cosmically as abundant as Fe, Mg
 Less abundant than C,N,O
 Chemically between metals and non-metals
 Can survive as solid in interstellar/circumstellar
environment
Common Minerals
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Silicon
3rd most abundant element (after O, Fe)
 Cosmically as abundant as Fe, Mg
 Less abundant than C,N,O
 Chemically between metals and non-metals
 Can survive as solid in interstellar/circumstellar environment
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Silicates
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“lithophiles” = silicates and things that tend to attach
themselves to silicates  low density minerals, reside in the
crust
Common Minerals
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Silicon
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Silicates
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3rd most abundant element (after O, Fe)
Cosmically as abundant as Fe, Mg
Less abundant than C,N,O
Chemically between metals and non-metals
Can survive as solid in interstellar/circumstellar environment
“lithophiles” = silicates and things that tend to attach themselves to
silicates  low density minerals, reside in the crust
Igneous rocks
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40-75% SiO2
O:Si ratio is high at high temperature crystallization and you get more
olivine; low at low T and you get more quartz
Common Minerals
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Silicon
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Silicates
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“lithophiles” = silicates and things that tend to attach themselves to silicates 
low density minerals, reside in the crust
Igneous rocks
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3rd most abundant element (after O, Fe)
Cosmically as abundant as Fe, Mg
Less abundant than C,N,O
Chemically between metals and non-metals
Can survive as solid in interstellar/circumstellar environment
40-75% SiO2
O:Si ratio is high at high temperature crystallization and you get more olivine;
low at low T and you get more quartz
Differentiation  absence of “siderophiles” in crust is evidence of
differentiation
Tectonics  What Separates the Earth
from Others
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Convection  means of transporting heat  driven
by internal heat (radioactive decay?)
Crustal plates are cold upper lid on convective cells
 “subsolidus” convection in mantle (3000 km thick)
Consequences
Volcanic activity, mountain chains
 Mid-ocean ridges
 Continental drift, earthquakes
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Tectonics
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1st evidence  mapping magnetic field in Indian Ocean floor
 detection of distinct linear features interpreted as “sea-floor
spreading”
Puzzle-piece like nature of continents
Youngest rocks near mid-ocean ridges
Earth Topographic Map
Dating: Radionuclide
Chronometry
• Processes ( Most Important Cases)
• 40K 
40Ar
t ½ = 1.4 x 109 yrs
• 87Rb  87Sr
t ½ = 6 x1010 yrs
• 238 U  206 Pb
t ½ = 5 x 109 yrs
• Lunar Results
• Oldest Highland Anorthosite tsolid. = 4.2 x 109 yrs
• Youngest Mare Basalts tsolid. = 3.1 x 109 yrs
• Terrestrial Results
Radioactive Decay
• Consider a number density, n, of atoms
• Which decays at an average rate, l
dn = - n λ dt
n
= e-λt
n0
• The solution to which is:
• Now n = ½ n0 at time t = t1/2 so exp  λ t 1 2  = 2
• Or, τ1 2 = ln 2  0.693
λ
l
l
 0.693 t 
n

• And finally:  exp  
n0
 t1 2 
0.693
t1 2