Astronomy 340 Fall 2005 29 September 2005 Class #8.

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Astronomy 340Fall 200529 September 2005Class #890 minutes of homework (for 6th graders, but you can extrapolate to college)15 minutes looking for assignment11 mins calling a friend for the assignment23 mins explaining to parents why the teacher is mean and just doesnt like children8 mins in the bathroom10 mins getting a snack7 mins checking the TV guide6 mins telling parents that the teacher never explained the homework10 mins sitting at the kitchen table waiting for Mom to do the assignmentReviewSurface composition of terrestrial planets dominated by silicates SiO2 (quartz), olivines, feldspars, etc.Tectonics important on the EarthSea-floor spreading, subduction zones, earthquakes, mountain chains, volcanic activityGeochronologyConsider: at t=0, N=N0t=, N = N0 D , where D is the number ofdaughter atoms after time, .So, N0 D = N0exp(-) = (1/)ln[1+(D/N)](D/N) can be easily measured.This is great as long as D only arises from radioactivedecay of N.Volcanic ActivityKey ingredient molten materialAccretion (primordial heat)Impact triggeredTidal heating/stretchingRadioactive decayLunar MareLunar mare resurfacing via someImpact that releases magma.Note low crater density.Volcanic ActivityKey ingredient molten materialAccretion (primordial heat)Impact triggeredTidal heating/stretchingRadioactive decay(magma)Olympus Mons Viking 1 Venus Tectonic Activity?Smrekar & Stefan 1997 Science 277, 1289Venus pastCrater 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 convectioncoronae are unique to Venusrising plumes of magma exert pressure on lithosphereless dense lithosphere deforms under pressure deformation of crust without tectonicsCoronae on Venus from Magellan radar imagingMartian Tectonic ActivityConnerney et al 99 Science 284 794Mars Global SurveyorDetected E-W linear magnetization in southern highlandsquasi-parallel linear features with alternating polarityNote: Earths global B-field is so much stronger it makes crustal sources hard to detectMartian Tectonic ActivityConnerney et al 99 Science 284 794Mars Global SurveyorDetected E-W linear magnetization in southern highlandsquasi-parallel linear features with alternating polarityNote: Earths global B-field is so much stronger it makes crustal sources hard to detectMars has no global field so crustal field must be remnant (frozen in time) from crystallizationMartian Crustal MagnetizationWorking modelCollection of strips 200 km wide, 30 km deepVariation in polarization every few 100 km3-5 reversals every 106 years (like seafloor spreading on Earth)Some evidence for plate tectonicsbut crust is rigid Earths crust appears to be the only one that participates in convectionImpacts and CrateringDominates surface properties of most rocky bodiesBack of the envelope calculation of the energy of an impactFormation of Impact CratersImpactor unperturbed by atmosphereImpact velocity ~ escape velocity (11 km s-1) tens of meters in diameterImpact velocity > speed of sound in rocks impact forms a shockPressures ~100 times stress levels of rock impact vaporizes rocksShock velocity ~10 km s-1 much faster than local sound speed so shock imparts kinetic energy into vaporized rockContact/CompressionProjectile stops 1-2 diameters into surface kinetic energy goes into shock wave tremendous pressures P ~ (1/2)0v2Peak shock pressures ~1000 kbar; pressure of vaporization ~600 kbarShock loses energyRadial dilution (1/r2)Heating/deformation of surface layerVelocity drops to local sound speed seismic wave transmitted through surfaceCan get melting at impact pointShock wave reflected back through projectile and it also gets vaporizedTotal time ~ few secondsExcavationShock 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)1/2Crater 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 impactLarge impacts multiring basins (Mars, Mercury, Moon)CratersCrater DensitySee Figure 5.31 in your book number of craters km-2 vs diameterSaturation equilibrium so many craters you just cant tell.Much of the lunar surfaceAlmost all of MercuryOnly Martian uplandsVenus, Earth not even close note cut-off on Venus distributionCalibrate with lunar surface rocks107 times more small craters (100m) as there are large craters (500-1000 km)Mercury South PoleLavinia Planum Impact CratersNote ejecta surrounding craterIts the size of Texas, Mr. President- from yet another bad movieComets small,rocky/icy things 10s of kmAsteroids small, rocky things a few to 10s of km the largest is the size of Texas (1000 km)100-300 NEAs knownClose encounters.Tunguska River in Siberia 30-50m meteroid exploded above ground flattened huge swath of forestYou make the catastropheNeed high velocitymax velocity ~ 70 km s-1 (combine Earths orbital velocity plus solar system escape velocity)Earth-asteroid encounters 25 km s-1Eart-comet encounters 60 km s-1Make it big.E ~ mv2 something 1000 km would wipe out the entire western hemisphere, but lets 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.