ASTRONOMY 340 FALL September 2007 Class #8

90 minutes of homework (for 6 th 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.  T b = ( λ 2 /2k)B λ  T b (s) = T b (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

Review  Surface composition of terrestrial planets dominated by silicates  SiO 2 (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=N 0 t=τ, N τ = N 0 – D τ, where D τ is the number of “daughter” atoms after time, τ. So, N 0 – D τ = N 0 exp(-λτ)  τ = (1/λ)ln[1+(D τ /N τ )] (D τ /N τ ) can be easily measured. This is great as long as D only arises from radioactive decay of N.

Volcanic Activity  Key ingredient  molten material  Accretion (primordial heat) Impact triggered  Tidal heating/stretching  Radioactive decay

Lunar Mare Lunar mare – resurfacing via some Impact 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  H 2 O, CO 2, SO 2 (recall that Io has SO 2 or S 2 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 Myr (Price & Supper 1994 Nature )  No global ridge system and a lack of significant upwellings (Solomon et al. Science )  Why such a big difference compared with Earth?  Catastrophic loss of H 2 O 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 Activity Connerney et al ’99 Science  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 Activity Connerney et al ’99 Science  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 10 6 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) ρ 0 v 2  Peak shock pressures ~1000 kbar; pressure of vaporization ~600 kbar  Shock loses energy  Radial dilution (1/r 2 )  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 E 1/3  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 35m2m4yrSmall Earthquake 1km50m1600yrBarringer Meteor Crater 7km350m51,000yr9.6 mag earthquake 10km500m10 5 yrSweden 200km10km150 MyrLargest 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  10 7 times more small craters (100m) as there are large craters ( 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)  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 ~ mv 2  something 1000 km would wipe out the entire western hemisphere, but let’s be realistic and go for ~10m (10 21 J) or ~1 km (10 23 J)  One impact imparts more energy in a few seconds than the Earth releases in a year via volcanism etc.