1	Astr 1050 Fri., Dec. 6, 2002 Today: Astronomy Articles for Extra Credit Finish Ch. 16, The Origin of the Solar System Start Ch. 17, Terrestrial Planets—will skip some slides
2	The Material Available Inner solar system dominated by silicate rocks SiO₂ (quartz) Mg₂SiO₄ Fe₂SiO₄ (olivine) etc. Outer solar system dominated by H₂, He, ice (H₂O)
3	Reason for Jovian Planets Because you cannot condense O by itself (but only in compounds also containing Si, Mg, Fe), you don’t have much material available for making terrestrial planets. You are limited by the low abundance of Si, Mg, Fe: Terrestrial planets are relatively small Once solid H₂O becomes available you have lots more material Starting at Jupiter you can make a big enough core from solid H₂O that you can gravitationally hold onto the H and He gas
4	Growth of the Planetisimals Once a planetisimal reaches critical size gravity takes over
5	Growth and Differentiation of Planets Planet forms from homogeneous mix of material Planet heats up “Heat of formation” (i.e. energy from gravity) Heat from radioactive decay of U, etc. Dense material (Fe) sinks to center Certain “siderophile” elements (like Ni) Other “lithophile” elements remain behind Homogeneous model too simple Final collisions can be big: Little planetesimals first form bigger ones, then bigger ones collide to form yet bigger ones Moon may be result of impact of Mars size body as Earth formed (more later) First material to condense might separate out early
6	Heterogeneous Growth of Planets Fe is among the first materials to condense as nebula cools Might form iron cores before lower temperature materials condenses Has implications for separation of lower temperature “siderophiles” during later differentiation
7	Evidence of Assembly Process? Craters
8	Craters evident on almost all small “planets”
9	Even larger planets typically have some
10	Clearing of the Nebula Radiation pressure (pressure of light) Will see present day effects in comets 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).
14	Four Stages of Planetary Development
15	Timeline
16	Earth’s Interior
17	The Active Earth Plate techtonics, volcanoes, etc.
18	Earth’s Atmosphere: Greenhouse Effect
19	The Moon and Mercury No atmosphere Cratering is evidence of final planet assembly – lots to be learned from craters
20	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 of 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)
21	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
22	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
23	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
24	Superposition (way to get relative ages) Newer features are superposed on top of older ones Large impact forms basin Basin floods with lava Additional impacts occur in mare lava Over time both crater rate and volcanic activity are declining Craters less because debris swept up Volcanism less because moon cooling
25	Why do lava flows come out in mare basins? Mare basins are the lowest areas of the planet The crust beneath them is badly fractured by the impacts When do the lavas come out? Superposition only gives relative ages Can use crater counts to estimate absolute ages – but need to know crater rates Apollo missions provided samples from which we have radioactive decay ages
26	Problems with the Condensation Model: Why is the moon so different than the earth?
27	Effects of late impacts
28	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
29	Relative size of core in Mercury
30	Venus
31	Expect Venus to be similar to Earth (but it isn’t) Venus only slightly closer to sun, so expect about same initial composition Venus only slightly smaller than Earth, so expect about same heat flow Venus atmosphere is dramatically different Very thick CO₂ atmosphere Virtually no water in atmosphere or or on surface Venus shows relatively recent volcanic activity, but no plate tectonics Both probably related to its slightly closer position to the sun which caused lost of its critical water Thick atmosphere and clouds block direct view so information from: Orbiting radar missions (Magellan in early 90’s) Russian landers
32	Why does Venus have much more CO₂ in atmosphere than Earth? Amount of CO₂ in atmosphere on Venus roughly equal to amount of CO₂ in limestone on Earth With no oceans, don’t have a way to get CO₂ out of atmosphere and back into rocks Runaway effect, because high T causes faster loss of water to space. If H₂O gets into upper atmosphere it is broken down into O, H by UV sunlight H is so light it escapes to space On Earth cooler T traps H₂O in lower atmosphere (it condenses if it gets to high) Location closer to the sun pushed Venus “over the edge” compared to Earth
33	Surface Relief of Venus from Radar Venus does show evidence of “recent” volcanism It does not show linear ridges, trenches, or rigid plates In a few spots there are weak hints of this – but clearly different
34	Volcanoes Sapas Mons Lava flows from central vents Flank eruptions Summit caldera Size: 250 miles diameter 1 mile high
35	Lava Channels Large! 100’s of miles long 1.2 miles wide High Venus temperatures may allow very long flows Composition could also be different
36	Pancake Domes Pancake domes formed from very viscous lava
37	“Ticks” Domes which have partially collapsed?
38	Corona and a possible model Corona possibly due to upward moving plume of hot mantle which bow up surface, then spreads out and cools (as in a “lava lamp”)
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40	Lots of Martian Science Fiction Best, most recent and scientifically accurate is probably Kim Stanley Robinson’s series: Red Mars, Blue Mars, Green Mars Terraforming/colonization of Mars
41	Mars and the Pattern of Geologic Activity and Atmospheric Loss Expect intermediate geologic activity based on size R_Mars = 0.53 R_EarthR_Moon = 0.27 R_Earth Earth still active but lunar mare volcanism ended ~3 billion years ago Expect intermediate atmospheric loss Smaller size will make atmospheric escape easier Cooler temperature (farther from sun) will make astmospheric escape harder In some ways Mars is most “Earth-like” planet Has polar caps Has weather patterns Had (in past) running water May have had conditions necessary for development of life
42	Why some atmospheres are lost Compare velocity of gas atoms (V_gas) to planet’s escape velocity V_esc If any significant # of atoms have escape speed atmosphere will eventually be lost In a gas the atoms have a range of velocities, with a few atoms having up to about 10 ´ the average velocity, so we need 10 ´ V_{avg gas} < V_esc to keep atmosphere for 4.5 billion years. In above equations R = planet radius, M = planet mass, T = planet temperature, m = mass of atom or molecule, k and G are physical constants Big planets have larger larger V_esc (i.e. larger M/RµR³/R) so hold atmospheres better Earth would retain an atmosphere better than Mercury or the Moon Cold planets have lower V_gas so hold atmospheres better Saturn’s moon Titan will hold an atmosphere better than our moon Heavier gasses have lower V_gas so are retained better than light ones CO₂ or O₂ retained better than He, H_2, or H Even with “heavy” gasses like we H₂O we need to worry about loss of H if solar UV breaks H₂O apart. That is what happens on Venus.
43	Which planets can retain which gasses?
44	Mars atmosphere today Pressure is only ~1% of Earth’s Composition: 95% CO₂ 3% N₂ 2% Ar Water: Pressure too low for liquid water to exist Boiling point drops with pressure Freezing point doesn’t change much with pressure Eventually boiling point reaches freezing point Water goes directly from solid phase to gas phase CO₂ (dry ice) is like this even at terrestrial atmospheric pressure Water seen in atmosphere Water seen in polar caps Evidence of running water in past Carbon dioxide (CO₂) Gets cold enough for even this to freeze at polar caps Unusual meteorology, as atmosphere moves from one pole to other each “year”
45	Mars dust storm
46	Sand Dunes on Mars Two spacecraft now in Mars orbit Mars Global Explorer Mars Odyssey Even though atmosphere is thin, high winds can create dust storms
47	Water ice clouds
48	Ancient River Channels? (note channels older than some craters – by superposition)
49	Recent liquid water? (water seeping out of underground “aquifer” ?)
50	Layered Deposits
51	Where is the water today? Much may have escaped to space Some is locked up in N Polar Cap Much could be stored in subsurface ice (permafrost) Mars Global Observer and Mars Odyssey studying these issues now Location of water critical to knowing where to search for possible past life