Transcript Terrestrial Planets: Surfaces

ASTRONOMY 340
FALL 2007
27 September 2007
Class #8
90 minutes of homework (for 6th graders, but
you can extrapolate to college…)
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15 minutes looking for assignment
11 mins calling a friend for the assignment
23 mins explaining to parents why the teacher is mean
and just doesn’t like children
8 mins in the bathroom
10 mins getting a snack
7 mins checking the TV guide
6 mins telling parents that the teacher never explained
the homework
10 mins sitting at the kitchen table waiting for Mom to do
the assignment
Review
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CO molecule – Rayleigh-Jeans approximation 
substitute temperature for intensity in radiative
transfer eqn.
Tb = (λ2/2k)Bλ
 Tb(s) = Tb(0)e-τ(s)+T(1-e-τ(s))
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Planetary Surfaces
Processes at work: impact, weathering, atmosphere, geology
(tectonics/volcanic
 Mercury: heavily cratered, no tectonics
 Venus: global resurfacing 300 Myr ago, no tectonics
 Mars: water, older volcanoes
 Earth: tectonics, water, weather
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Volcanic Activity
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Key ingredient  molten material
 Accretion
 Impact
 Tidal
(primordial heat)
triggered
heating/stretching
 Radioactive decay
Lunar mare – resurfacing via some
Impact that releases magma.
Lunar Mare
Note low crater density.
Volcanic Activity
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Key ingredient  molten material
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Accretion (primordial heat)
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Impact triggered
Tidal heating/stretching
Radioactive decay
ρ(magma)<ρ(rock)  magma rises through “plumes” 
volcanoes sit atop plumes  same physics on any
planet/moon
Magma acts to resurface
Volcanic composition
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H2O, CO2, SO2 (recall that Io has SO2 or S2 gas)
Venus’ Tectonic Activity?
Smrekar & Stefan 1997 Science 277, 1289
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Venus’ past
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Crater distribution is even & young  no resurfacing over past
300-500 Myr (Price & Supper 1994 Nature 372 756)
No global ridge system and a lack of significant upwellings
(Solomon et al. Science 252 297)
Why such a big difference compared with Earth?
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Catastrophic loss of H2O from mantle?  no convection
“coronae” are unique to Venus
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rising plumes of magma exert pressure on lithosphere
less dense lithosphere deforms under pressure
deformation of crust without tectonics
Martian Tectonic Activity
Connerney et al ’99 Science 284 794
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Mars Global Surveyor
Detected E-W linear magnetization in southern
highlands
 “quasi-parallel linear features with alternating
polarity”
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Note: Earth’s global B-field is so much stronger it
makes crustal sources hard to detect
Martian Tectonic Activity
Connerney et al ’99 Science 284 794
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Mars Global Surveyor
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Detected E-W linear magnetization in southern highlands
“quasi-parallel linear features with alternating polarity”
Note: Earth’s global B-field is so much stronger it makes
crustal sources hard to detect
Mars has no global field so crustal field must be remnant
(“frozen in time”) from crystallization
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
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Earth’s crust appears to be the only one that
participates in convection
Impacts and Cratering
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Dominates surface properties of most rocky bodies
“Back of the envelope” calculation of the energy of
an impact…
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 time 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.