| Today: Astronomy Articles for Extra Credit | |
| Finish Ch. 16, The Origin of the Solar System | |
| Start Ch. 17, Terrestrial Planets—will skip some slides | |
| Inner solar system dominated by silicate rocks | ||
| SiO2 (quartz) Mg2SiO4 Fe2SiO4 (olivine) etc. | ||
| Outer solar system dominated by H2, He, ice (H2O) | ||
| 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 |
| Once a planetisimal reaches critical size gravity takes over |
Growth and Differentiation of Planets
| Planet forms from homogeneous mix of material | ||||
| Planet heats up | ||||
| “Heat of formation” (i.e. energy from gravity) |
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| 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 | ||||
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 |
Evidence of Assembly Process? Craters
Craters evident on almost all small “planets”
Even larger planets typically have some
| 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 | ||
| 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). |
Four Stages of Planetary Development
| Plate techtonics, volcanoes, etc. |
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 Ţ 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 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) | ||
| 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 = ˝ 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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Superposition
(way to get relative ages)
| Newer features are superposed on top of older ones |
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| 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 | ||
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 | ||
Problems with the
Condensation Model:
Why is the moon so different than the earth?
| 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 |
Relative size of core in Mercury
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 |
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| Thick atmosphere and clouds block direct view so information from: | ||
| Orbiting radar missions (Magellan in early 90’s) | ||
| Russian landers | ||
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 |
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| 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 | ||
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 | ||
| Sapas Mons | ||
| Lava flows from central vents | ||
| Flank eruptions | ||
| Summit caldera | ||
| Size: | ||
| 250 miles diameter | ||
| 1 mile high | ||
| Large! | ||
| 100’s of miles long | ||
| 1.2 miles wide | ||
| High Venus temperatures may allow very long flows | ||
| Composition could also be different | ||
| Pancake domes formed from very viscous lava |
| Domes which have partially collapsed? |
| Corona possibly due to upward moving
plume of hot mantle which bow up surface, then spreads out and cools (as in a “lava lamp”) |
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 |
Mars and the Pattern of
Geologic Activity
and Atmospheric Loss
| Expect intermediate geologic activity based on size | |||
| RMars = 0.53 REarth RMoon = 0.27 REarth | |||
| 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 | |||
| Compare velocity of gas atoms (Vgas) to planet’s escape velocity Vesc | ||
| 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 ´ Vavg gas < Vesc to keep atmosphere for 4.5 billion years. |
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| In above equations R = planet radius, M
= planet mass, T = planet temperature, m = mass of atom or molecule, k and G are physical constants |
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| Big planets have larger larger Vesc (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 Vgas so hold atmospheres better | ||
| Saturn’s moon Titan will hold an atmosphere better than our moon | ||
| Heavier gasses have lower Vgas so are retained better than light ones | ||
| CO2 or O2 retained better than He, H2, 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. |
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Which planets can retain which gasses?
| 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” | |||
| Two spacecraft now in Mars orbit | ||
| Mars Global Explorer | ||
| Mars Odyssey | ||
| Even though atmosphere is thin, high winds can create dust storms | ||
Ancient River
Channels?
(note channels older than some craters – by superposition)
Recent liquid water?
(water seeping out of underground “aquifer” ?)
| 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 |
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| Location of water critical to knowing where to search for possible past life |