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Lecture 16

AS101 Lecture 16

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Patrick Mc Graw

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Lecture 16 4/10/2013 12:33:00 PM The Terrestrial Planets Review: Solar Nebula Model:  Solar system formed from a collapsing cloud.  Spin of the solar system (and all the objects in it) comes from the spin of the original cloud.  Spin is also responsible for the flatness of most of the solar system in the plane of the ecliptic. (Note: this is the same process that gives the Milky Way galaxy its mostly flat shape.)  Chemical composition and size of planets changes with distance from the sun.  This has to do with changes in temperature with distance from the sun: farther away = colder, and therefore more solids available for forming planets.  One important boundary was the ice line: the distance at which temperatures were low enough to solidify water.  Process of planet formation would have stopped soon after the sun “turned on” and drive away most of the remaining gas + dust. Review: two main ways of detecting extrasolar planets:  Doppler method: o Doppler method:  Detect the slight wobble of the star as the planet's gravity pulls on it. o To detect the wobble, look for regularly recurring changes in the star's radial velocity (motion toward or away from us) using the Doppler effect. o This allows us to figure out the orbital period (from the period of the wobble) and the mass of the planet (from the size of the wobble.)  Transit method: o Transit method:  Detect recurring changes in the star's brightness as the planet moves in front of it. o This allows us to find the planet's orbital period and its size (from the amount of light it blocks.) o Transit only works if the planet's orbit happens to be almost edge-on to our line of sight. Revirew: Extrasolar Planets:  Detection of planets is easiest when the planets are massive and close to the star (so that their gravitational pull is strong and their orbital period is short).  Therefore the planets we have found so far are probably a very biased sample: lots of “hot Jupiters.”  If our understanding of how solar systems form is correct, these should be rare because large planets can normally only form farther away. Learning about earth:  Eratosthenes figured out the size a couple of millennia ago.  We can also find the mass. How?  We know the orbital period and average distance of the Moon from the Earth...  So Newton's version of Kepler's third law allows us to calculate the mass. Newtons Generalization of Keplers 3 rdlaw: 2 P = 4π a3 G M   Where M is the mass of the sun (or other central object)  This can be used to figure out the mass of the Sun.  Similarly, the orbital periods and distance of Earth's or other planets' moons can be used to figure out the planet's mass.  Fine print: Actually, the M in this equation should be the combined mass of the two objects, but the planets' masses are much smaller than the sun's. Learning about earth:  Then we can divide mass by volume to get density. What is the earth made of?  We can get samples of rocks from near the surface...  We can also divide the mass by the volume to get the density, which gives us at least some clues to what the planet as a whole is made of. (This is true for any planet or other object.)  How else can we learn what's inside? Some ways of learning about Earth’s interior:  Seismology: When an earthquake happens, vibrations travel through the Earth in the form of s waves and p waves  Both types of waves get refracted as they travel through different materials.  s waves cannot travel through liquids at all--- so the fact that s waves do not travel through the middle of the Earth provides evidence that part of the core is liquid.  Earth's magnetic field also provides indirect evidence that part of the core is liquid metal:  combination of convection and the Earth's rotation creates the magnetic field.  Features of the surface due to plate tectonics also provide indirect evidence of what is happening underneath. Mantle:  a layer below the top crust, not quite liquid but soft enough to flow slowly. Learning about Earths interior: other clues from the surface:  Volcanic activity.  Height of mountains tells us something about how thick the crust is and what is supporting it (mountains can only reach a certain maximum height before their weight causes them to start sinking.) Earth and its neighbours:  Some of our understanding of other planets comes from comparing and contrasting their features with Earth.  What are some things we learn? Comparative Planetology:
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