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Berkeley ASTRON 10 - 12. Comets

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Brownlee particles, Comets; 30 September - 2 October 2013. !In the solar system, there are some extremely primitive things: !1.For example: !A.Brownlee particles: small object less than a millimeter across that haven't a.changed since the solar nebula was a nebula. !Research on these objects was done by Don Brownlee. !1.Comets: the coldest, most primitive objects!2.Comets linger at the farthest parts of the solar system in the sphere-A.shaped Oort cloud and the disk-shaped Kuiper belt.!There is a strong suspicion, but no proof that the Kuiper belt is the a.innermost part of the Oort cloud.!Comet nuclei in the Kuiper belt and Oort cloud are difficult to detect b.because they are small (about the size of a city), pitch black (reflectivity of 4%), and 1,000 times farther from the sun than the earth is. !All we know about these comets is extrapolations from the few 1.comets who were brought farther into the solar system. !Song: Awake O Ye Comets.!2.When comets get closer to the sun, they become visible on earth as bright B.objects in the sky with long tails behind them. !The reason for this change in appearance is the ultraviolet light of the a.sun heating up the surface of the comet, vaporizing its ices and tars.!The cloud of vapor around a comet that results from this process is 1.called a "coma."!The sun's ultraviolet light also ionizes a comet's gas, resulting in a tail of b.blue gas in the direction away from the sun (regardless of the direction of the comet). !The ionized gas from the comets reacts to magnetic fields from the 1.sun. !Every time the boundary of a magnetic field crosses the nucleus of a 2.comet, the comets ion trail breaks off and new one with opposite polarity forms. !Comets also have a yellow tail of dust which is broad and slightly curved. c.They appear as they do because the dust is heavier and more varied than the ions. !The closer to the sun a comet is, the longer its tail. !d.Sometimes the tail has vertical stripes, probably from puffs of dust that e.are stirred when the comet spins (although no one can prove comets spin yet). !This phenomena was once mistaken for a comet with six tails!1.Since astronomers have been able to visit comet nuclei with spacecraft, C.they have learned that comet nuclei are odd-shaped black cores composed of patches of different materials. !Astronomers discovered that comet nuclei were black and odd shaped a.by sending a space craft to the nucleus of Halley's comet. !This comet, which last appeared in 1986, is the first comet whose 1.return was predicted. !Before then, everyone thought that the nucleus of a comet had the 2.composition of a "dirty snowball."!For example, people in 1858 thought that the nucleus of Donatis' A.comet (a black shape amidst the comet's fountains of gas and dust) was just the shadow of the nucleus, when it was actually the nucleus itself. !All of the other comet nuclei visited by spacecraft have also had black 3.cores. !In all the comets visited by spacecraft that could make measurements, b.the ingredients of the comet are patchy rather that mixed in. (undifferentiated). !In the comet Hale-Bopp, spectroscopes show lots of ammonia in one 1.area, and lots of methane in another.!Astronomers interpret this to mean that there are pockets of A.ammonia and methane ice on different areas of the comet. When each of these pockets are turned toward the sun and heated, they release the gases within them. !This idea is very consistent with our current understanding that comets c.have not changed, or have hardly changed since the beginning of the solar system four and a half billion years ago. !Everything else in the solar system has been heated, and therefore 1.changed.! Since comets must have been put together from the solar nebula, you 2.can tell what materials the solar nebula was composed of and the form those materials were in (clumps). !We have learned this from visiting many different comets with d.spacecrafts.!Comets are almost always named for their discoverer, so both amateurs D.and real astronomers are interested in finding comets. !From visiting comets, asteroids, and the moon with space, astronomers have 3.noticed similarities in composition. !There are at least a dozen, maybe twenty objects designated as asteroids A.that are actually comets. !For example, 2060 Chiron was designated as an asteroid, but it is actually a.a comet. !Some moons are also probably comets captured by a planets orbit: !B.There is no known difference between the two moons of Mars (Phobos a.and Deimos) and outgassed comet nuclei. !Astronomers don't know if the comets were outgassed and then 1.captured by mars, or captured by mars and then outgassed. !Spacecraft sent to these moons discovered that these comets have icy 2.cores, but no ice on their surfaces. It is possible that if the moons broke open they could become comets again. !Saturn's moons Pheobe and Hyperion are probably captured comets. !b.Neptune's outermost moon Nereid is probably a comet. !c.The best voyager images of it are too pixelated to prove or disprove 1.this.


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Berkeley ASTRON 10 - 12. Comets

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