| Today: Finish Ch. 15, Cosmology | |
| Start Solar System | |
| Flatness Problem – why so close to a critical universe? | ||
| Horizon Problem – why is background all same T? | ||
| SOLVED BY AN “INFLATIONARY UNIVERSE” | ||
| “Grand Unified Theories” of combined Gravity/Weak/Electric/Nuclear forces predict very rapid expansion at very early time: “inflation” | ||
| When inflation ends, all matter moving away with v=vescape (flat universe – curvature forced to zero) | ||
| Also solves horizon problem – everything was in causal contact | ||
Implications of Slowing Expansion Rate
| Our calculation of age T=1/Ho = 13.6 billion years assumed constant rate | ||
| Gravity should slow the expansion rate over time | ||
| If density is high enough, expansion should turn around | ||
| If expansion was faster in past, it took less time to get to present size | ||
| For “Flat” universe T = 2/3 * (1/Ho) = 9.3 billion years | ||
| contradiction with other ages if T is too small | ||
Is the expansion rate slowing?
| Look “into the past” to see if expansion rate was faster in early history. | ||
| To “look into the past” look very far away: | ||
| Find “Ho” for very distant objects, compare that to “Ho” for closer objects | ||
| Remember – we found Ho by plotting velocity (vr) vs. distance | ||
| We found velocity vr from the red shift (z) | ||
| We found distance by measuring apparent
magnitude (mv) of known brightness objects |
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| We can test for changing Ho by measuring mv vs. z | ||
Measuring deceleration using supernovae
| Plot of mv vs. z is really a plot of distance vs. velocity | ||
| If faint (Þdistant Þearlier)
objects show slightly higher z than expected from extrapolation based on nearby (present day) objects, then expansion rate was faster in the past and has been decelerating |
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| Surprise results from 1998 indeed do suggest accelerating expansion | ||
| May be due to “cosmological constant” proposed by Einstein | ||
| AKA “Dark energy” or “Quintessence” | ||
| General Relativity allows a repulsive term | ||
| Einstein proposed it to allow “steady state” universe | ||
| He decided it wasn’t needed after Hubble Law discovered | ||
| Is the acceleration right? | ||
| Could it be observational effect – dust dims distant supernova? | ||
| Could it be evolution effect – supernova were fainter in the past? | ||
| So far the results seem to stand up | ||
| Still being determined: 1) density, 2) cosmological constant | ||
| With cosmological constant included, can have a “flat universe” even with acceleration. | ||
| Given “repulsion” need to use relativistic “geometrical” definition of flatness, not the escape argument one given earlier. | ||
| Energy (and equivalent mass) from cosmological constant may provide density needed to produce flat universe. | ||
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 | ||
| 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 Galilean satellites and our own moon) orbit in this direction too | |||
| Planets are regularly spaced | |||
| steps increasing as we go outward | |||
| 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 | ||
| 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) |
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| 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) | ||
| 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 |
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| Asteroids – mostly between orbits of Mars and Jupiter | ||
| Comets – mostly in outermost part of solar system | ||
| Meteorites – material which falls to earth | ||
| 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) |
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| 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) |
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| 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 | |||
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 | ||
| Once a planetisimal reaches critical size gravity takes over |
Evidence of Assembly Process? Craters
Craters evident on almost all small “planets”
| Radiation pressure (pressure of light) | ||
| See present day effects in comets | ||
| Solar Wind | ||
| Strong solar winds from young T Tauri stars | ||
| 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 | ||
Chapter 17: Terrestrial Planets
| Earth | ||
| History, Interior, Crust, Atmosphere | ||
| The Moon | ||
| In particular origin | ||
| Mercury | ||
| Venus | ||
| Mars | ||
| Including water (and life ?) | ||
| 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!!! |
Four Stages of Planetary Development
Earth’s Atmosphere: Greenhouse Effect
| No atmosphere | |
| Cratering is evidence of final planet assembly – lots to be learned from craters |
| Judge age of surface by amount of
craters: more craters Þ more ancient surface (for some objects, have radioactive age dates) |
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| 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) | ||
| Crater caused by the explosion | ||
| Impactor is melted, perhaps
vaporized by the kinetic energy released |
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| 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 |
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Examples of craters on the moon
| Images on line at The Lunar and Planetary Institute: http://www.lpi.usra.edu/expmoon/lunar_missions.html |
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| Detailed record of Apollo work
at: http://www.hq.nasa.gov/office/pao/History/alsj/frame.html |
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| 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 |