Pluto, Rings, 2 October 2013!Pluto: !1.(1930) Discovered by Clyde Tombaugh during a hunt for a planet beyond A.Neptune at Lowell observatory. !Characteristics: !B.It is about 40 times farther from the sun than the earth.!a.It was one of the faintest things distinguishable at its discovery, and b.images of it are still vague and blurry.!There is a probe which should arrive there in 2015 to get good 1.images of the surface. !There is an ongoing argument(~30 years) about whether Pluto is a planet:!C.There is no known difference between this object and a comet.!a.Like a comet, Pluto has ices on its surface, a coma (atmosphere of 1.boiled off ices), and an elliptical/inclined orbit.!The reasons that Pluto is still called a planet are that it was called a b.planet at first, and that it would be the only planet discovered by Americans rather than Europeans. !The real question is whether the word "planet" means anything to c.science: !There is no consistent system for classifying objects as planets in 1.space: !The standards for what is called a planet have changed many times A.over the eras. !Any system that uses the same label for Mercury and Jupiter (which 2.we now know are radically different) is a meaningless system.!To include Pluto (whose characteristics are that of a comet), a A.system must have even less meaning. !The word "planet" should probably be used only by historical 3.astronomers and historians. !Rings: The four planets that definitely have rings are the gas giants Jupiter, 1.Uranus, Saturn, and Neptune. Earth may have had a ring. !Jupiter's rings: black dust!A.They were discovered in the seventies by a passing space craft, surprising a.everyone. !Even those who thought Jupiter would have a ring were surprised at 1.the true nature of it. !The rings of Jupiter are made of thin scatterings of tiny black dust b.particles. !They are only visible when backlit by the sun, during which they 1.appear as a faint ring of light. !This phenomenon is known to physics as "forward scattering":!A.All we need to know it that this depends on the size of the light a.waves of the sun and the smallness of the particles in Jupiter's ring.!A lot of these particles continuously spiral into Jupiter, so the ring is 2.decreasing all the time. !Uranus's rings: black dust!B.In the seventies, the rings of Uranus were discovered by astronomers a.observing the planet as it occulted a star. !occulted = passed in front of!1.The astronomers were trying to use the star as a tracer to see what 2.gases were in the upper atmosphere of Neptune. !If you continuously take the spectrum of a star as a planet moves in A.front of it, you can see what gasses are in the planet's atmosphere by how the spectrum changes. !The star blinked out before the planet got to it, and then again once 3.the planet was gone. !Now, the Hubble telescope can provide pictures of these rings.!4.Uranus's rings are also composed of particles of black dust, only visible b.when back lit.!Neptune's rings: black dust!C.Segments of rings around Neptune were discovered in the mid eighties a.by occultation from the earth. !These segments of rings turned out to be thick spots in complete 1.rings. !Neptune's rings are also composed of particles of black dust, only 2.visible while back lit. !Saturn's rings: gaudy snow and black dust!D.Background: !a.The rings of Saturn were first noticed by Galileo, although from his 1.early telescope he could only tell that they were ring-shaped, and went around the planet without touching it anywhere.!In the mid 1800s, James Clerk Maxwell figured out that that the ring 2.was composed of billions of individual particles. A solid ring would break itself apart and fall into the planet.!General Nature: !b.Saturn's rings are made up of bright snow flakes and snow balls, with a 1.thin scattering of black dust particles. !The rings of saturn are made of dozens of tiny little ringlets, most of 2.which are slightly spiral.!Gaps in the rings are caused by the gravitational influences of Saturn's 3.moons. !Saturn's rings are not exactly symmetrical, and change over time.!4.We can see Saturn's rings head on twice every thirty years, but most 5.of the time we see some tilt. !Oddities in Saturn's rings that no one really understands: !c.In a picture of a ringlet of Saturn, there was a sequence of stripes 1.across the ring in the spacing of an arithmetic progression. !There are spokes made of dust which appear on Saturn's rings. These 2.spokes are sharpest when they are in the part of the rings just emerging from the shadow of Saturn's disk. As the ring particles orbit Saturn for a few hours, the spokes smear out. !This phenomenon was discovered from earth, but not paid A.attention to until it was imaged by the voyager mission. !Astronomers hoped the Cassini spacecraft orbiting Saturn would B.explain these spokes, but it has not after a number of years. !Any student who solves this problem to the satisfaction of experts C.in the specialty, gets an instant A for the entire course, regardless of anything else.!In the almost empty gaps of Saturn's rings there are irregular ringlets 3.that can vary in thickness and diameter over their circumferences. !Saturn's outer ring has kinks (only sometimes).!4.On the first voyager mission, the outer ring of Saturn unexpectedly A.had kinks. This seemed impossible to theoreticians, but they worked out a reason why the outer ring had to have kinks. Unfortunately for them, Saturn's rings had unkinked themselves by the next voyager mission. !Any student who solves this problem to the satisfaction of experts B.in the specialty, gets an instant A for the entire course, regardless of anything else.!Evidence of possible rings around Earth: !E.There are tiny, spherical structures in the mud under the sea which are a.only at the equator. ! These could be the remnants of a ring around the earth that decayed. !b.There is also evidence of a ring around asteroid 532 Herculina, but F.evidence is very uncertain. !There is strengthening evidence that planet rings are not original with planets, 2.but rather the wreckage of comets that got too close:!The contents of rings systems are identical with the contents of comets. !A.Comets are so fragile that they will crumble into dust if exposed to a B.planet's gravitational field. !There is no evidence that rings can last for more than a million years.!C.The planets that have rings are also the most likely planets to be hit by D.comets from the Kuiper belt and
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