astronomy 340 fall 2007 27 september 2007 class #8

ASTRONOMY 340FALL 200727 September 2007

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

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))

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

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)

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

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.