1	Astr 1050 Fri., Apr. 23, 2004 Today: Extra Credit Articles Continue with the Solar System Start Ch 17., Terrestrial Planets Recall: Nice webpage your classmate provided http://www.nationalgeographic.com/solarsystem/splash.html
2	Extra-Solar Planets Hard to see faint planet right next to very bright star Two indirect techniques available (Like a binary star system but where 2nd “star” has extremely low mass) Watch for Doppler “wobble” in position/spectrum of star Watch for “transit” of planet which slightly dims light from star About 110 planets discovered since 1996 See http://exoplanets.org/ Tend to be big (³Jupiter) and very close to star (easier to see)
3	Characteristics of “Planets” Two types of planets Terrestrial Planets: small, rocky material, inner solar system Jovian Planets: large, H, He gas, outer solar system Small left-over material, “debris”, that provides “fossil” record of early conditions Asteroids – mostly between orbits of Mars and Jupiter Comets – mostly in outermost part of solar system Meteorites – material which falls to earth
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5	Patterns in Composition Terrestrial Planets Relatively small Made primarily of rocky material: Si, O, Fe, Mg perhaps with Fe cores (Note – for earth H₂O is only a very small fraction of the total) Jovian Planets Relatively large Atmospheres made of H₂, He, with traces of CH₄, NH₃, H₂O, ... Surrounded by satellites covered with frozen H₂O Within terrestrial planets inner ones tend to have higher densities (when corrected for compression due to gravity) Planet Density Uncompressed Density (gm/cm³) (gm/cm³) Mercury 5.44 5.30 Venus 5.24 3.96 Earth 5.50 4.07 Mars 3.94 3.73 (Moon) 3.36 3.40
6	Equilibrium Condensation Model Start with material of solar composition material (H, He, C, N, O, Ne, Mg, Si, S, Fe ...) Material starts out hot enough that everything is a gas May not be exactly true but is simplest starting point As gas cools, different chemicals condense First high temperature chemicals, then intermediate ones, then ices Solids begin to stick together or accrete snowflakes Þ snowballs (“Velcro Effect”) Once large enough gravity pulls solids together into planetesimals planetesimals grow with size At some point wind from sun expels all the gas from the system Only the solid planetesimals remain to build planets Composition depends on temperature at that point (in time and space) Gas can only remain if trapped in the gravity of a large enough planet
7	Growth of the Planetisimals Once a planetisimal reaches critical size gravity takes over
8	Evidence of Assembly Process? Craters
9	Craters evident on almost all small “planets”
10	Clearing of the Nebula Radiation pressure (pressure of light) See present day effects in comets Most important effect Solar Wind Strong solar winds from young T Tauri stars Will see present day effects in comets Sweeping up of debris into planets Late Heavy Bombardment Ejection of material by near misses with planets Like “gravity assist maneuvers” with spacecraft Origin of the comets
11	Patterns and Predictions Why do different planets have different levels of geologic activity? Why do different planets have different atmospheres? What are ages of old “unaltered” planetary surfaces? Should be similar, and agree roughly with age of Sun Does composition of asteroids match predictions? Lower temperature than Mars region: Hydrated silicates, etc. What types of minerals do we see in meteorites? What types of ices and minerals do we see in comets?
12	Chapter 17: Terrestrial Planets Earth History, Interior, Crust, Atmosphere The Moon In particular origin Mercury Venus Mars Including water (and life ?)
13	“Comparative Planetology” Basis for comparisons is Earth Properties of Earth Similarities and differences with Mars and Venus help us understand Earth better (e.g., life, greenhouse effect, etc.) Won’t spend much class time on basic properties (size, gravity, orbital period, length of day, etc.) but you should have some relative ideas about these (see “Data Files” in text). There will be a few exam questions!!!
14	Four Stages of Planetary Development
15	Earth’s Atmosphere: Greenhouse Effect
16	The Moon and Mercury No atmosphere Cratering is evidence of final planet assembly – lots to be learned from craters
17	Patterns in Geologic Activity Judge age of surface by amount of craters: more craters Þ more ancient surface (for some objects, have radioactive age dates) Moon “dead” after about 1 billion years Mercury “dead” early in its lifetime Mars active through ~1/2 of its lifetime Venus active till “recent” times Earth still active Big objects cool off slower Amount of heat (stored or generated) proportional to Volume ( so R³) Rate of heat loss proportional (roughly) to Surface Area (so R²) Heat/(Unit Area) µ R³/R² = R so activity roughly proportional to R Same reason that big things taken out of oven cool slower than small things (cake cools slower than cookies)
18	What is a crater? Must think of them as caused by very large explosions from release of kinetic energy of impactor Like a mortar shell – it isn’t the size of the shell which matters, its how much energy you get out of the explosion DO NOT think of them as just holes drilled into surface – think EXPLOSION Kinetic Energy E = ½ m v² v is roughly escape speed of earth m = mass = volume * density (Consider a 1 km asteroid) E This is ~4500 ´ the size of the largest (~50 Mt) hydrogen bombs ever built and this is for a relatively small size asteroid
19	Formation of an impact crater Crater caused by the explosion Impactor is melted, perhaps vaporized by the kinetic energy released Temporary “transient” crater is round Gravity causes walls to slump inward forming “terraces” Movement of material inward from all sides (trying to fill in the hole) may push up central peak in the middle. Final crater is typically ~10 times the size of the impactor
20	Examples of craters on the moon Images on line at The Lunar and Planetary Institute: http://www.lpi.usra.edu/expmoon/lunar_missions.html Detailed record of Apollo work at: http://www.hq.nasa.gov/office/pao/History/alsj/frame.html
21	Effects of late impacts
22	Moon: Giant Impact Hypothesis Explains lack of large iron core Explains lack of “volatile” elements Explains why moon looks a lot like earth’s mantle, minus the volatiles Explains large angular momentum in the earth-moon system
23	Venus