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Transcript
Solar System • Sun, 8 planets, hundred moons, thousand.dwarf.planets million asteroids, billion comets etc. Comparative Planetology • Compares planets and other solar system bodies to help understand how they formed and evolved • Diameters: 10 times Earth=Jupiter & 10Jupiters=Sun • Distances: Sun-Earth=8light-minutes & Sun-Jupiter=1Hour Solar System Looks Like a Disk • As the planet’s distance from the sun increases; circumference of orbit increases & velocity decreases so period of orbit increases • Planets revolve in orbits that are tilted less than 7° to sun’s equator • Planets ALL orbit in same direction • As Sun rotates Obliquity of Rotation Axes • • • • • Most planets rotate counter-clockwise Most planets rotation axis is perpendicular to ecliptic plane Sun’s rotation axis tilted by 7° But why is Uranus tilted ~90°? Why does Venus rotate retrograde=clockwise? Two Kinds of Planets • Planets are relatively isolated • Terrestrial planets- 4 inner planets • Jovian planets – 4 outer planets Terrestrial Planets • • • • • • Mercury, Venus, Earth, Moon, Mars, Located closer together near sun Smaller, denser, rocky Solid surface & Weak magnetic field Little or no Hydrogen and Helium Few moons, no rings Jovian Planets • • • • • Jupiter, Saturn, Uranus, Neptune Widely spaced in outer solar system Larger, gaseous, containing Hydrogen & Helium like Sun Dense atmosphere with no surface & Strong magnetic field Many satellites, ring systems Average Density of Planets • Density is mass divided by volume – How heavy per volume • Terrestrial Planets more dense than Jovian gas giants • Problem is moon is not as dense (3.4gm/cm3) as Earth (5.5) Interplanetary Matter • Planets have cleared their orbits, but there’s debris leftover • Comet & Asteroid dust ~40,000 tons/year falls to Earth • Seen as Zodiacal Light Asteroids = Minor Planet • • • • Rocky, smaller than the moon; larger than 100meters diam. Not enough gravity to pull them into spherical shape Ceres~1000km diam; Vesta~500km; 15>250; ~million >1km Movie of Eros Asteroid Belt • Asteroids, Near Earth Objects, Comets • Millions of asteroids generally orbit between Mars and Jupiter • Over 444,080 with good orbits • Named by discoverer • 1 named after UVic and 14 others named after UVic people Kuiper Belt Region • More than 1000 icy objects like Pluto in Kuiper Belt @ 30-50?AU Comet: Hale-Bopp 1997 • Dirty snowball; leftover from formation of solar system • Nucleus 1-10km • Ices melt/steam when close to sun • Forms million km long tail Oort Cloud • Trillions of comets in orbits of ~a light year • 1000 times further than Kuiper belt Meteorites • Peekskill meteorite or fireball • 1992 New York; Bright as full moon • Meteorite on ground, meteor in air and meteoroid in space • Pinhead sized rocks burn up ~80km up • Some leftover from formation of Solar System Radioactive Dating • • • • • • Clocks start at time of solidifying and reset by melting 238U radioactive decays to 234Th → 206Pb With half-life of 4.5billion years We have rock samples of the Earth, Moon, Mars, meteorites All date to maximum of ~4.6 Billion years Time of formation of Solar System Spacecraft Missions Show New Worlds • Mercury– 1 flyby& 1 orbiter • Venus–many flyby, 4.orbiters • Mars – many orbiters, 4.landers & 4 rovers • Jupiter, Saturn, etc. – 4 flyby & 2 orbiters Gravity Assist • Throws ball on parked car • Throws ball on moving car • Throws ball on orbiting car Pale Blue Dot • Voyager 1 looks back and takes a picture of the Earth • Gives us perspective What are the characteristic properties of the solar system that a theory of its formation needs to explain: a) Planets isolated in space=cleared orbit b) Disk shape of solar system- small orbit inclination; prograde circular motion; same tilt&direction of rotation axes (almost) c) Jovian/Terrestrial planets: low/high density, huge/small atmospheres, fast/slower rotation rates, many/few moons & rings and Space Debris – icy comets, rocky asteroids, meteoroids, Kuiper Belt d) Earth, Moon, Mars, Meteorites, Sun; all 4.6 billion years old e) All of the above Trans-Neptunian Objects • Objects which orbit the sun beyond Neptune Impact Crater Formation • • • • • Impactor has velocity 10 times rifle bullet Releases energy 10 times equal mass of dynamite Impactor vaporized when temperature reaches millions K Shock wave forms shocked quartz found only in impacts Rebound can launch rocks without destroying them Rocky Surfaces Saturated with Craters • Planets formed from many meteorite impacts • Circular craters due to explosion caused by impact • Solar System “Full”? Erosion of Craters • Craters hidden by: 1. Volcanoes – lava flows 2. Continental drift 3. Erosion by atmosphere 4. Oceans Younger Terrain = Fewer Craters • The larger the terrestrial world; the more internal heat it will retain = higher temperature in core • The higher temperature; more geologically active so more craters are covered = fewer craters showing • Older terrains = more craters Comet Impacts on Jupiter • • • • Painting shows Jul94 impacts of Comet Shoemaker-Levy 9 HST photo from 20Jul09 Comets deposit ice Heating planet 03Jun10 Jupiter Impact • Last June Jupiter became a little more massive • Two amateur astronomers recorded the impact • 2009 impact left scar • Notice missing SE belt Planetary Magnetic Fields • Jovian planets plus Earth have magnetic fields • Various directions and strengths and offsets Planetary Dynamo • Conducting, convecting/circulating, rotating core generates magnetic field • Earth: Iron core; Jupiter: liquid metallic hydrogen • Uranus, Neptune: water with ions Magnetometer • Measures the strength & direction of Magnetic field • Can probe the interior of the “planet” Elemental Abundances • Element determined by number of protons=atomic number • Neutrons determine Isotope • Elemental abundances on Earth • Not the same as in stars, Jupiter, Saturn Elements Forged In Star’s Cores • You are made of star dust • Stars have died so you may live Chondrules • Many meteorites contain small glassy inclusions • Formed by rapid melting and solidified in hours • Chondrules date from formation of Solar System Outgassing • Water, carbon dioxide, sulfur dioxide, nitrogen released by volcanoes • Forms atmosphere Angular Momentum Problem • The protosun rotates rapidly and has huge convection cells so it generates a strong magnetic field • The magnetic field tries to accelerate the disk and solar wind, slowing rotation by magnetic braking • Observations of stars with known ages in Hyades etc. show that the older stars rotate more slowly Late Heavy Bombardment • Late Heavy Bombardment second surge in impacts • When Jupiter, Saturn, Uranus, Neptune change orbits scattering planetesimals? Debris Disk • After the dense disks that form planets • Cold disks of dust are left • Made from asteroid collisions & comet’s tails Comet NEAT 29Jan & 2Feb03 • • • • Dirty snowball or maybe icy mud ball is better Nucleus ~ 10km Tail ~1,000,000km to 1 astronomical unit Very eccentric elliptical orbit Atmospheric Escape • • • • More massive planets have higher Escape Speed Hotter planets have faster moving air molecules Heavier molecules move more slowly Atmosphere can leak into space over eons of time