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Module 3 Notes.docx

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Department
Astronomy
Course
AS101
Professor
Shohini Ghose
Semester
Fall

Description
Module 3 Notes Comparative Planetology – seeking to understand the similarities and the differences between and among the planets Solar Nebular Theory – main theory of formation of our solar system - imagines that some cataclysmic event initiated the collapse of a nebula that caused material falling inward to some centre converting gravitational potential energy into kinetic energy making the centre, or core, hotter and hotter Terrestrial Planets – four inner planets - Mercury, Venus, Earth and Mars - small, dense, rocky worlds with little or no atmosphere Jovian Planets – four outer planets – Jupiter, Saturn, Uranus, and Neptune - large, low-density worlds with thick atmospheres and liquid or ice interiors Planetary Characteristics:  all planets orbit the Sun in the same direction – counter clockwise (ccw)(as viewed from above North Pole)  all orbits lie in nearly the same plane  almost all planets have nearly circular orbits (Mercury is a minor exception)  most planets rotate ccw (Venus and Uranus are exceptions) including the Sun  most moons orbit their planet in same direction as the planet's rotation and orbit in their planet's equatorial plane Mercury and Venus have no moons; Earth has one and Mars has two very small asteroid-like moons. The Jovian planets, by contrast, have many. Jupiter is listed as having 6 but it actually has over 60; Saturn has almost as many and Uranus and Neptune have 40 between them. All Jovian planets have ring systems. Saturn’s rings are made of ice particles. The rings of Jupiter, Uranus and Neptune are made of dark rocky particles. Terrestrial planets have no rings. Asteroids lie primarily between Mars and Jupiter and a fairly broad belt in the same plane as planetary orbits. Comets follow either elliptical orbits or parabolic/hyperbolic orbits passing close to the Sun once. They are made largely of ices mixed with rocky dust, no bigger than a few km across. They come from two major sources – the Kuiper belt (a doughnut shaped region starting around Neptune and extending out into space) and the Oort Cloud (a spherical region completely surrounding the solar system and extending out some 50,000 AU). As a comet gets close to the Sun it generates a coma (an atmosphere of escaping gases and dust) around its nucleus and two tails: a plasma tail of ionized gas swept away by the solar wind, and a dust tail of small solid particles created by the escaping atmosphere (escapes from the comet because of its weak gravity). When we see a comet in the sky we don’t actually see its core but rather the lengthy beautiful tail emanating from the core. This tail always points away from the Sun. During each pass of the Sun, comets lose material through sublimation and tail formation. Meteoroids, Meteors and Meteorites are found around Earth. Meteors are actually small bits of rock and/or metal falling into Earth’s atmosphere that heat up due to friction with the air. We see them as “shooting stars” (of course, they are not stars at all). A meteoroid is what the rocky object is called before it hits the atmosphere and becomes a meteor. If the meteoroid is massive enough to have any part of it left before it hits the Earth’s surface it then becomes a meteorite. Kuiper Belt – begins at about orbit of Neptune and extends out to about 100 AU - this doughnut-shaped belt lies mainly in the planetary or ecliptic plane Oort Cloud – a spherical cloud surrounding solar system, centred on Sun, and comets from this region come into solar system from all directions; extends from the outer part of the Kuiper belt to about halfway to nearest stars Half-Life – the time it takes for half of the atoms to decay in a radioactive element - it appears that our solar system formed about 4.6 billion years ago Summary Planet Orbital Radius (AU) Mercury 0.39 Venus 0.72 Earth 1.0
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