Chondrite Meteorite types, 4 October 2013 !Meteorites, like comets, are made of the original materials of the solar system 1.that were compressed together but never heated (or heated only enough to melt ice, but not rocks or tars). !Other objects in the solar system which we will discuss later were heated A.and changed.!Meteoroids/Meteors/Meteorites/"Falling stars":!2.Meteors appear when a particle in space collides with the earth's A.atmosphere: !Meteors look like spots of light streaking through the sky for a few a.seconds. !This is why their colloquial name is "falling star" !1.Especially bright meteors are called fireballs or bolides, but these are 2.not technical distinctions.!These particles come from comets, or possibly parts of the solar nebula: !b.When a comet's ices are melted by the sun, it leaves unmelted dust 1.from its coma and dust tail behind. !The Earth sometimes runs into this trail of dust along a comet's orbit, 2.resulting in streaks of light from the dust's collision with earth's atmosphere. !You can see meteors half a dozen times per hour on any clear, dark c.night. !The meteors you see here (which are not part of a predicted meteor 1.shower) are called "sporadic meteors."!Sporadic meteors are from dust trails so diffused that they could not 2.be traced to the original comet that made them. !During the peak of the best meteor showers, you can see one meteor d.per minute. !These meteors are from the dust trails of comets that have come by 1.more recently.!The meteors of these showers appear to radiate (diverge from) a 2.single point in the sky. You can trace the paths of all the meteors back to this one point. !This point is called the "radiant."!A.In reality, the dust is in parallels orbits as it hits the earth, but our B.perspective makes it look as if it radiates. !A similar perspective effect can be seen when looking down a a.long hall of doors. The hall appears to radiate from the opposite end of the hall, but it is actually all straight. !Notable Meteor showers: !3.Perseid meteor shower: !A.This shower occurs every second week of August for several a.weeks.!During this shower, Earth runs into a stream of dust left behind b.by comet 1862 #3 (comet Swift-Tuttle). !The radiant of this shower is the constellation Perseus (hence c.the name "perseid").!This shower peaks at 50 meteors an hour. !d.Leonid meteor shower: !B.This shower occurs every November.!1.During this shower, Earth runs into a stream of dust left 2.behind by comet 1866 #1 (comet Tempel-Tuttle). !The radiant of this shower is the constellation Leo (hence 3.the name "leonid").!The meteors of the Leonid shower are more rapid than 4.other meteor showers, and only occasionally abundant. !On rare occasions, the Leonids are the biggest meteor A.storms.!This usually happens every 30 years, but not dependably.!B.In 1833, there was a famously large meteor shower, a.during which "the stars fell like rain."!The 1966 Leonid shower was also intense, but there b.was nothing special about the 1899 meteor shower. ! There probably won't be a good Leonid shower for a c.century!Other major annual meteor showers are the Quadratids in January C.and the Geminids in December. !The timing of these showers is well established, but the number a.of meteors are variable. !The timing is very narrow–"these showers usually last for only a b.couple hours.!The weather during these showers is very lousy. !c.There are other comets coming which may leave dust that will D.result in a meteor storm.!What falling stars are called depends on where they are: !B.In space and invisible to us, they are called meteoroids. !a.Meteoroids are are the size of a grain of sand and usually black, so 1.they are impossible to see from miles away. !Falling stars can be meteoroids for up to 4.5 billion years (since the 2.beginning of the solar system). !In the Earth's air, they are called "meteors"!b.When a meteoroid hits earth's atmosphere, it heats up a pocket of air 1.around itself, causing the air to glow until the particle is vaporized (or falls to earth).!A meteoroid is a meteor for a couple of seconds. !2. On the ground, they are called "meteorites. "!c.The few meteors that are not completely vaporized and fall to earth 1.are now known as meteorites forever. !This system of naming is not ideal:!d.The word "meteoroid" is by definition unobservable, because if you 1.can observe a meteoroid in space, you would call it an asteroid. !As technology has allowed for smaller and smaller asteroids to be A.found, the boundary between meteoroid and asteroid has changed radically.! The smallest object detectable in space shrank from 100 a.kilometers across in the 1600s, to 6-8 meters across with today's telescopes. !The objects have changed from meteoroids to asteroids in this B.century, not because they are different in any fundamental way, but because our technology is better. !A classification system that does not rely on the nature of an object, 2.but rather our technology, is junk.!Meteorites, as fragments left over from comets, are specimens of what is C.going on in the solar system in different places and in different circumstances. !This is the reason why this course focuses more on meteorites than the a.textbook. !Though this course goes into a lot of meteorite types, it is still only a 1.fraction of all the known types. !Chondrites: !b.Chondrites are meteorites made of several different kinds of rock 1.crammed together at high pressure, but never at a very high temperature. !Astronomers know that the temperature was low because a A.number of the ingredients in Chondrites would vaporize at high temperatures. !Any meteorite with chondrules in it never got hotter than about a.1100 degrees celsius, which would melt the chondrules.!This process where an object is compressed without ever 1.heating is called cold compression. !This is puzzling, because temperature and pressure tend to be b.high and low at the same time. !Any student who solves this problem to the satisfaction of c.experts in the specialty, gets an instant A for the entire course, regardless of anything else.!The dominant ingredients in a Chondrite are little round rocks called 2."chondrules" ("little round thing" in greek). These are suspended in a matrix of rock that sometimes contains carbon and calcium. !Each chondrule was separately flash melted and then flash frozen. !A.They must have melted in less that one second, been held a.together by surface tension in the neutral
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