EPS 102 lecture 10 - EPS 102 Lecture 10 Thursday February...

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EPS 102 Lecture 10 Thursday February 19 th , 2009 We used to think the Earth was put together by little km sized bodies clinking together over long periods of time and that it wasn’t hot when it started. But now we know that it formed quickly with very large impacts that made it pretty hot and had other consequences. We want to see if there is evidence to support these ideas. One way to see is the moon. It is our most familiar astronomical neighbor. The moon is a really small body and in a geological sense cooled down quickly and became geologically dead. It is a good record of our solar system’s history. If you look at the moon, it’s surface is divided into two areas. There are very light highland areas and darker maria which are the consequences of giant impacts. It’s composition (the moon’s) based on samples we brought back = pretty much similar to the Earth’s mantle. We know what it’s mass is, radius, etc. . we can figure out the average density of the moon. The Earth is about 5.5g/cm^3, but when we uncompress it it would have a density of about 4.3 g/cm^3 ; it is denser due to the pressure of the Earth. The moon has magnetized rocks on it’s surface and an iron core. maybe it used to have a dynamo early in it’s history. But it has a very small core so that would be hard to happen. Why is it that the moon came to be with such a small iron core? we can look at the surface of the moon more and see the distinctions between both areas: the highlands are very ancient, and heavily cratered. These are very old rocks; younger rocks are maria which are made of basalts (these come out of the mid ocean ridges on Earth). It has fewer craters on it, and it is younger than the highlands. Quite a bit younger is a relative word: young means late in the first billion years of the moon’s formation. Maria are circular. A lot of stuff on the moon is homogeneous: like orthoclase, plagioclase feldspars, etc. . A lot of rocks recovered from the highlands are broken up, partially melted indicating a tumultuous early history. In some cases these can have dates that are younger because they were re melted due to impacts. The more younger rocks reflect that they were re-melted by subsequent impacts. Mantle melt produces basalt, and something similar must have happened on the moon. Moon rocks are 100 million years younger than the oldest rocks. We also know that the surface is covered by regolith which is produced by meteorites hitting the moon: it is a fine soil. In maria, regolith can be a couple of meters thick. On highlands, they can be about 100m thick. It is a soil produced by tiny meteorites. The moon has almost no atmosphere. on Earth, meteorites come through the atmosphere and burn up before they make it to the surface. On the moon, it doesn’t matter what size the meteorite is; it will hit it. Samples were brought back of regolith (these are also made of little glassy bits). Because the moon has no atmosphere, there is no limit to how small these meteorites can
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This note was uploaded on 02/04/2010 for the course EPS 102 taught by Professor Staff during the Spring '08 term at University of California, Berkeley.

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EPS 102 lecture 10 - EPS 102 Lecture 10 Thursday February...

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