1. Astronomy 340 Fall 2007
4 October 2007
Class #10

2. Review Venus resurfacing event(s) 300-500 Myr ago
Just how do you determine the age of a planetary surface?
Impacts
Mars

3. Martian Crustal Magnetization
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)
Some evidence for plate tectonics…but crust is rigid 
Earth’s crust appears to be the only one that participates in convection

4. Formation of Impact Craters
Impactor unperturbed by atmosphere
Impact velocity ~ escape velocity (11 km s-1)
 tens of meters in diameter
Impact velocity > speed of sound in rocks  impact forms a shock
Pressures ~100 times stress levels of rock  impact vaporizes rocks
Shock velocity ~10 km s-1  much faster than local sound speed so shock imparts kinetic energy into vaporized rock

5. Contact/Compression
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

6. Excavation
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)
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)

7. Craters 35m 2m 4yr Small Earthquake 1km 50m 1600yr
Barringer Meteor Crater
7km
350m
51,000yr
9.6 mag earthquake
10km
500m
105 yr
Sweden
200km
150 Myr
Largest craters/KT impactor

8. Crater Density
See Figure 5.31 in your book  number of craters km-2 vs diameter
Saturation equilibrium – so many craters you just can’t tell….
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 ( km)

9. Mercury South Pole

10. Lavinia Planum Impact Craters
Note ejecta surrounding crater

12. “It’s the size of Texas, Mr. President” - from yet another bad movie
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)
NEAs known
Close encounters….
Tunguska River in Siberia  30-50m meteroid exploded above ground  flattened huge swath of forest

13. You make the catastrophe…
Need high velocity
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
Make it big….
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.

14. Surface Composition
Reflection spectroscopy. (remember radiative transfer!)
What is the surface made of? Rocks, mostly igneous
Minerals = solid chemical compounds with specific atomic structure

15. Common Minerals Silicates Various Oxides
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

16. Common Minerals 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

17. Common Minerals Silicon Silicates
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
Silicates
“lithophiles” = silicates and things that tend to attach themselves to silicates  low density minerals, reside in the crust

18. Common Minerals Silicon Silicates Igneous rocks
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
Silicates
“lithophiles” = silicates and things that tend to attach themselves to silicates  low density minerals, reside in the crust
Igneous rocks
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

19. Common Minerals Silicon Silicates Igneous rocks
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
Silicates
“lithophiles” = silicates and things that tend to attach themselves to silicates  low density minerals, reside in the crust
Igneous rocks
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

20. Tectonics  What Separates the Earth from Others
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

21. Tectonics
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

22. Earth Topographic Map

23. 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

24. Radioactive Decay Consider a number density, n, of atoms
Which decays at an average rate, l
The solution to which is:
Now n = ½ n0 at time t = t1/2 so
Or,
And finally: