ASTR 20 Lecture Notes - Lecture 11: Uranium-238, Subduction, Cyanobacteria
Document Summary
Small bodies formed from dust and gas eddies. Protoplanets 9 or 10 formed from planetesimals accretion. Planets formed by combining protoplanets swept up by gravitational attraction. Left over debris craters newly formed planets. Present distribution of major elements and u, th, he and ar in the earth"s atmosphere, crust and in seawater. Earth is internally heated mostly by decay of uranium 238 and potassium 40. Mantle holds sio2, fe, mg, ca, al. Cross section of earth showing in a rudimentary way the relation of the upper mantle to subduction zones and midocean ridges. The early atmosphere would have been similar to the sun. Mainly hydrogen and helium, but this atmosphere was lost quickly for 2. The gravity of the modest size earth was not strong enough to prevent such light gases from escaping to space. It appears that around 30-100 million years after the earth"s formation, it was struck by a large object the size of mars.
