1	Astr 1050 Wed., Dec. 4, 2002 Today: Chapter 16, The Origin of the Solar System (We’ll just hit the highlights for Ch. 16-19.)
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) 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 Galilean satellites and our own moon) orbit in this direction too Planets are regularly spaced steps increasing as we go outward
4	Spacing of Planets Regular spacing of planets on a logarithmic scale Each orbit is ~75% larger than the previous one Need to include the asteroids as a “planet”
5	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
6	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)
7	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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9	Production of the Elements H, He made in the big bang Elements up to Fe generated by fusion in stars expelled back into interstellar medium from red giants and supernova Elements heavier than Fe require energy to make: neutron capture side effect in fusion chains s process in supernova explosions r process p⁺ capture occasionally important p process
10	Timing of the Assembly Process? Over time inside ⁸⁷Rb one n®p⁺ so ⁸⁷Rb®⁸⁷Sr Use relative amounts of Rb, Sr to determine age of rocks Half life: Time it takes ½ of “parent” to decay to “daughter” Other unstable elements: ⁴⁰K²³⁵U²³⁸U Ages of old surfaces and meteorites: ~4.5 billion years
11	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
12	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
13	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)
14	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
15	Growth of the Planetisimals Once a planetisimal reaches critical size gravity takes over
16	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
17	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
18	Evidence of Assembly Process? Craters
19	Craters evident on almost all small “planets”
20	Even larger planets typically have some
21	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
22	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?