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EPS 102 lecture 9

EPS 102 lecture 9 - EPS 102 Lecture 9 Tuesday February 17th...

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EPS 102 Lecture 9 Tuesday February 17 th , 2009 It is likely the Earth was violent hot and completely molten when it formed, and has been gradually cooling ever since. This is based on modeling. Bigger picture: a molecular cloud has dust, will eventually collapse and it is spinning by conservation of angular momentum. The dust and gas is going to get hot as it collapses. The collapse in the perpendicular direction is easy, but the collapse in the direction perpendicular to the rotational axis is not, therefore disks are formed. We think that the planets formed from this disk material. between the time that you start accumulating a star, and the disk blows up is very fast= 10 million years; everything happens very quickly. We need to have these things happen fast. We can construct likely conditions inside this disk. Our protostar is accumulating material and maybe it hasn’t completely turned on yet but gas and dust is accumulating onto it. This gas and dust is orbiting around the star. We can estimate the pressure, density, and temperature. These things will decrease though. We expect things to be hot but as we move away we expect things to be cooler. Temperatures in the disk could have gotten up to 2000 degrees. We think that the temperatures were high enough to vaporize the dust. The fine dust flakes that never got vaporized have unusual isotopic evidence. The disk that produced our star probably had many supernova explosions. Once it gets vaporized, everything gets mixed together and homogenized. The unusual things stand out as anomalies. We can characterize the amount of material in the disk because we are going to build planets out of them. You take a column of material of the disk, or a square meter; and ask how much mass will be in that column. Maybe 1000 grams per square centimeter, or a kilogram per square centimeter. The dust makes up about 1%, and that is driven by the composition of the sun. the bunch of other stuff besides H and He represents 1% of the mass. We will predict how to build our planets out of this disk. There are at least 2 stages in which we put it together. 1) how do we take that gus and dust and accrete it into larger objects? Electrostatic effects, clumping solid flakes together. At one point, those effects don’t become important anymore. How do we go from cm size to bigger objects so that gravity can take over? We have a non gravitational effect when we start building and then we move over to gravity. An early idea: wouldn’t it be great if you were in your disk about 1AU from the sun, and the snowflakes are condensing out of the dust, you are getting Si, Ca, etc… if it turned out that the disk was gravitationally unstable. If it simply started to collapse on itself so that planetessimals can build up. If your disk has enough mass in it, it can go unstable; it depends on the sound speed (how fast sound passes through the disk), and it depends on how fast it is rotating around the sun. we are rotating around the sun once a year. A disk 1AU from the sun will also rotate once per year. We can find that we don’t have enough
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