Astronomy 340Fall 2005

29 September 2005

Class #8

90 minutes of homework (for 6th graders, but you can extrapolate to college…) 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

Surface composition of terrestrial planets dominated by silicates SiO2 (quartz), olivines, feldspars, etc.

Tectonics – important on the Earth Sea-floor spreading, subduction zones,

earthquakes, mountain chains, volcanic activity

Geochronology

Consider: at t=0, N=N0

t=τ, Nτ = N0 – Dτ , where Dτ is the number of“daughter” atoms after time, τ.

So, N0 – Dτ = N0exp(-λτ) τ = (1/λ)ln[1+(Dτ/Nτ)]

(Dτ/Nτ) can be easily measured.

This is great as long as D only arises from radioactivedecay of N.

Volcanic Activity

Key ingredient molten material Accretion (primordial heat)

Impact triggered Tidal heating/stretching Radioactive decay

Lunar Mare

Lunar mare – resurfacing via someImpact that releases magma.

Note low crater density.

Volcanic Activity

Key ingredient molten material Accretion (primordial heat)

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

H2O, CO2, SO2 (recall that Io has SO2 or S2 gas)

Olympus Mons Viking 1

Venus’ Tectonic Activity?Smrekar & Stefan 1997 Science 277, 1289

Venus’ past 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?

Catastrophic loss of H2O from mantle? no convection “coronae” are unique to Venus

rising plumes of magma exert pressure on lithosphere less dense lithosphere deforms under pressure deformation of crust without tectonics

Coronae on Venus – from Magellan radar imaging

Martian Tectonic ActivityConnerney et al ’99 Science 284 794

Mars Global Surveyor 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

Martian Tectonic ActivityConnerney et al ’99 Science 284 794

Mars Global Surveyor 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 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

Impacts and Cratering

Dominates surface properties of most rocky bodies

“Back of the envelope” calculation of the energy of an impact…

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

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

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 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)

Craters35m 2m 4yr Small

Earthquake

1km 50m 1600yr Barringer Meteor Crater

7km 350m 51,000yr 9.6 mag earthquake

10km 500m 105 yr Sweden

200km 10km 150 Myr Largest craters/KT impactor

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 (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 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 NEAs known

Close encounters…. Tunguska River in Siberia 30-50m meteroid

exploded above ground flattened huge swath of forest

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.