| Today: Finish Ch. 17, Terrestrial Planets | |
| —We will skip some slides | |
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 |