1	Astr 1050 Mon., Apr. 25, 2005 Today: Solar System Overview
2	Chapter 16: Origin of the Solar System Solar Nebula Hypothesis Context for Understanding Solar System Extrasolar Planets Dust Disks, Doppler Shifts, Transits and Eclipses Survey of the Solar System Terrestrial Planets Jovian Planets Other “Stuff” including apparent patterns with application to the nebular hypothesis
3	Patterns in Motion All planets orbit in almost the same plane (ecliptic, AKA Zodiac) Almost all motion is counterclockwise as seen from the north: All planets orbit in this direction Almost all planets spin in same direction with axes more-or-less perpendicular to ecliptic Regular moons (like our own moon) orbit in this direction, too Planets are regularly spaced steps increasing as we go outward
4	Solar Nebula Model Planets form from disk of gas surrounding the young sun Disk formation expected given angular momentum in collapsing cloud Naturally explains the regular (counterclockwise) motion Makes additional explicit predictions Should expect planets as a regular part of the star formation process Should see trends in composition with distance from sun Should see “fossil” evidence of early steps of planet formation
5	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 100 planets discovered since 1996 See http://exoplanets.org/ Tend to be big (³Jupiter) and very close to star (easier to see)
6	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 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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8	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
9	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
10	Growth of the Planetisimals Once a planetisimal reaches critical size gravity takes over
11	Evidence of Assembly Process? Craters
12	Craters evident on almost all small “planets”
13	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
14	Chapter 17: Terrestrial Planets Earth History, Interior, Crust, Atmosphere The Moon In particular origin Mercury Venus Mars Including water (and life ?)
15	“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, number of moons, etc.) but you should have some relative ideas about these (see “Data Files” in text).
16	Four Stages of Planetary Development
17	Timeline
18	The Active Earth Plate tectonics, volcanoes, etc., a lot of action!
19	Earth’s Atmosphere: Greenhouse Effect
20	The Moon and Mercury No atmosphere Cratering is evidence of final planet assembly – lots to be learned from craters
21	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)
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	Effects of late impacts
25	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
26	Venus
27	Expect Venus to be similar to Earth? (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 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 loss of its critical water Thick atmosphere and clouds block direct view so information from: Orbiting radar missions (Magellan in early 90’s) Russian landers (as in previous photo)
28	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
29	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
30	Volcanoes Sapas Mons Lava flows from central vents Flank eruptions Summit caldera Size: 250 miles diameter 1 mile high
31	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
32	Pancake Domes Pancake domes formed from very viscous lava
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34	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
35	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
36	Which planets can retain which gasses?
37	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 Water goes directly from solid phase to gas phase CO₂ (dry ice) acts 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”
38	Mars dust storm
39	Sand Dunes on Mars Spacecraft in Mars orbit Mars Global Explorer Mars Odyssey Even though atmosphere is thin, high winds can create dust storms
40	Water ice clouds
41	Ancient River Channels? (note channels older than some craters – by superposition)
42	Recent liquid water? (water seeping out of underground “aquifer” ?)
43	Layered Deposits
44	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 Missions making progress this semester: http://www.nasa.gov/vision/universe/solarsystem/mer_main.html Location of water critical to knowing where to search for possible past life
45	“Comparative Planetology” Think about how Venus, Earth, and Mars started out so similarly Think about what properties led to the very different environments today Think about how these issues may apply to the future of Earth, and even our prospects for terraforming (and there is a debate about whether we should terraform at all!).
46	Chapter 18: Worlds of the Outer Solar System Jupiter Condensation model Atmospheric winds Atmospheric chemistry Magnetic fields Other Jovian Planets (Saturn, Uranus, Neptune) will only cover major differences from Jupiter Satellites (i.e. Moons)
47	Jovian Planets
48	Ice+Rock Core H+He “Atmosphere”
49	Details of the atmosphere Mostly made of H, He Trace amounts of C, N, O, S CH₄ present as gas NH₃, NH₄SH, H₂O can condense in colder upper regions Þ clouds Colors from unknown trace chemicals Density of gas smoothly increases with depth till point where it is indistinguishable from liquid Þ no real “surface” At very high temperatures and pressures hydrogen becomes a “metal” and conducts electricity Þ generates magnetic field
50	Jupiter as seen by Cassini
51	Magnetic fields and trapped particles
52	Aurora on Jupiter
53	Comparison of Jovian Planets Variation in distance presumably ultimate causes other effects P: Kepler’s third law T: Falloff mostly just result of falling solar energy But Neptune hotter because more internal heat M: Clue to details of solar nebula mode Less material in outer solar system – or perhaps less efficient capture r: Should drop with mass because less compression Works for Saturn vs. Jupiter Increase for Uranus, Neptune indicates less H, He and more heavy material
54	Saturn as seen by the Hubble Space Telescope
55	The Roche Limit When can tides tear a moon apart? As a planetary body get close to another object, tidal forces distort the body more and more. Remember, Earth raises tides on the Moon just like it raises tides on the Earth If the distortion gets large enough, the moon will be pulled apart Happens at “Roche Limit” when moon is ~2.44 ´ radius of planet away At that point, tidal force pulling up on surface of moon is greater than moon’s gravity pulling down Only matters for objects held together by gravity Astronaut in orbit will not be pulled apart Is held together by much stronger chemical forces Astronaut standing on the outside of the shuttle, hoping the shuttle’s gravity would hold her there, will be pulled away from the shuttle
56	Rings are individual particles all orbiting separately Each particle – dust to golf ball to boulder size – is really a separate moon on its own orbit Orbit with Keplerian velocities: high in close, slow farther out Nearby relative velocities are low – so particles just gently bump into each other – slowly grinding themselves up Structure in rings largely caused by gravity of moons
57	Comparison of Rings All within Roche limit Details controlled by Resonances and Shepard Satellites
58	Jupiter as a miniature solar system Four large moons (Io, Europa, Ganymede, Callisto) Regular (equatorial, circular) orbits Pattern of changing density and composition with distance Inner two (Io, Europa) mostly rocky Outer two (Ganymede, Callisto) more icy
59	Io, Europa break rules about activity Io most volcanically active body in solar system Europa shows new icy surface with few craters
60	Tidal heating explains activity Large tides from Jupiter flex satellites Friction from flexing heats interiors Important for Io, Europa, some other outer solar system satellites
61	Possible H₂O ocean on Europa Tidal heating may keep H₂O liquid under ice cover Perhaps a location where life could evolve “Europa Orbiter” Mission being planned to determine if ocean exists
62	Callisto not active
63	Comparison of Satellites
64	Titan Largest moon of Saturn Has thick atmosphere Pressure ~ 1 earth atmosphere Mostly N₂, some CH₄ Gas held because of low T UV acting on CH₄ Þ smog Ethane produced – Lakes? Can “see” surface only in IR Cassini dropped a probe in early 2005. “Code of the Lifemaker” by James P. Hogan, good sf
65	Chapter 19: Meteorites, Asteroids, Comets Small bodies are not geologically active They provide “fossil” record of early solar system Asteroids Mostly from region between Mars and Jupiter Left over small debris from accretion, never assembled into a large planet Meteorites come mostly from asteroids Comets “Stored” on large elliptical orbits beyond planets Thought to be “planetesimals” from Jovian planet region, almost ejected from solar system in its early history Meteorites provide only samples besides Apollo With sample in hand, can perform very detailed analysis: detailed chemistry; radioisotope age; other isotope info
66	Asteroids Most located between Mars and Jupiter Largest is Ceres 1/3 diameter of moon Most much smaller >8,000 known Total mass << Earth A few make it to earth source of the meteorites
67	The larger asteroids
68	Are Asteroids Primitive? Ida (56 km diam.) and its moon Dactyl (1.5 km diam.) Colors have been “stretched” to show subtle differences Imaged by Galileo on its way out to Jupiter
69	Phobos & Deimos: Two “misplaced” asteroids? Phobos and Diemos are small (~25 km and ~15 km diam.) moons of Mars Look like captured asteroids rather than moons formed in place Are “C” class – i.e. dark “Carbonaceous” type “asteroids”
70	Types of Meteorites Three main kinds of meteorites Carbonaceous chondrites: Most primitive material – dark because of C Stones Similar to igneous rocks Irons Metallic iron – with peculiarities
71	Meteors vs. Meteorites Meteor is seen as streak in sky Meteorite is a rock on the ground Meteoroid is a rock in space Meteor showers (related to comet orbits) rarely produce meteorites Apparently most comet debris is small and doesn’t survive reentry Meteorites can be “finds” or “falls” For a fall – descent actually observed and sometimes orbit computed Most have orbits with aphelion in asteroid belt
72	Large Meteor over the Tetons (1972)
73	The Leonids 2001 APOD site: Picture by Chen Huang-Ming
74	Meteor Showers and Comets Meteor showers caused by large amount of small debris spread out along comet orbits Almost none makes it to the ground – no meteorites Occur each year as earth passes through orbit of comet Appears to come from “radiant point” in sky Leonids: Mid November
75	Comets: Hale-Bopp in April 1997
76	Comet characteristics Most on long elliptical orbits Short period comets – go to outer solar system “Jupiter family” still ~ in plane of ecliptic “Halley family” are highly inclined to ecliptic Longer period ones go out thousands of AU Most of these are highly inclined to ecliptic Become active only in inner solar system Made of volatile ices and dust Sun heats and vaporizes ice, releasing dust “Dirty snowball” model
77	Comet structure Gas sublimates from nucleus Dense coma surrounds nucleus Ion tail is ionized gas points directly away from sun shows emission spectrum ions swept up in solar wind Dust tail curves slightly outward from orbit shows reflected sunlight solar radiation pressure gently pushes dust out of orbit
78	Hale-Bopp clearly shows components
79	Where do comets come from? Long period comets: The Oort Cloud Most (original) orbits have aphelions of >1000 AU Need ~6 trillion comets out there to produce number seen in here Total mass of 38 M_Earth Passing stars deflect comets in from the cloud
80	Importance of comets Evidence of solar nebula Source of H₂O and CO₂ for earth Impacts continue Impacts on Earth Extinction of the dinosaurs SL-9 impact on Jupiter
81	Pluto and Charon