| 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 | |
| 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 110 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, “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 | ||
| 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 | ||
| 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 | ||
| 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? | ||
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) | ||
| 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 |
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| 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 |
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| 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 |