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Transcript
PHYS 185 Chapter 5 Highlights 1. Definition of a planet a. Who: The International Astronomical Society (IAU) governs the definition and naming of planets. Individual members are professional astronomers from all over the world, at the Ph.D. level or beyond, and active in professional research and education in astronomy. b. Why: There was no formal definition! Then came the discovery of other large objects orbiting the sun, out beyond Pluto: i. Quaoar (“kwah-oh-are”) in 2002 - 1/2 size of Pluto; kicked off the whole debate about a planet really was ii. Eris in 2003 – larger than Pluto and has a moon Dysnomia. So you can’t define a planet based on it having a moon, unless you make Eris a planet. iii. Sedna in 2004 - 3X further out than Pluto, bigger than Quaoar iv. more than 1000 objects found that are at least 1/2 the size of Pluto, some of them with moons. Are these planets? Kuiper belt objects? c. Official definition voted on Aug 24, 2006: A "planet"1 is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. 1 The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. d. Pluto was designated as a “dwarf planet”. In June 2008, the IAU established a sub-category of dwarf planets called “plutoids” – based on size, brightness, and having an orbit beyond Neptune. Total to date is four: Pluto, Eris, Haumea, and Makemake. 2. Mercury a. Has the most extreme temperature changes in the Solar System i. Very hot days, since close to the sun ii. Very cold nights, since very slow rotation (nights are long) and very little atmosphere (not much to hold heat in) 3. Venus a. Intense cloud cover prevented any pictures of surface for a long time i. Clouds are made of sulfuric acid! ii. High pressures (~90 atm, equivalent to swimming 2700 feet underwater on Earth) crushed early probes b. Good model for studying Greenhouse Effect i. CO2 in atmosphere lets sunlight in (transparent to visible wavelengths) but traps heat in (opaque to infrared wavelengths, “heat” = IR wavelengths) ii. Has the hottest temperatures of the Solar System: average temperature hotter than Mercury’s high temperatures 4. Mars a. Olympus Mons – largest volcano in the Solar System b. Mars has two moons: Phobos (“Fear”) and Deimos (“Panic”), that look like potatoes – indicates that their origin was that they were likely captured (irregular space debris wandering by and caught my Mars gravity) c. Spirit and Opportunity – rovers on Mars collecting data. 5. Earth a. atmosphere mostly of nitrogen, with some carbon dioxide (CO2) and oxygen b. abundant life c. etc… see chapter 4 for details 6. Jupiter a. Big ball of gas, mostly H & He, surrounding rocky core b. Deep inside has liquid metallic H (pressures large enough to squeeze H gas into a liquid form). “Metal” means electrons are free to roam from atom to atom. Moving charged particles create magnetic fields, so Jupiter has a very large, strong magnetic field. If Jupiter’s magnetic field were visible it would fill an area in our sky about 16x the size of a full moon! c. 4 Galilean moons, visible with small telescope: i. Io - orbits in about 1.8 days! Lots of active volcanoes due to strong tidal forces since so close to a large planet (near the Roche limit; see Saturn) ii. Europa - may have water – may be a place where life can be found iii. Ganymede - largest moon in the solar system iv. Callisto – bears scars of huge asteroid impact d. Latest count = 63 moons! 7. Saturn a. Rings are chunks of ice and ice coated rock, from dust to 10 meter size b. Rings are 89,000 km wide, but only 2 km thick (that’s very thin!) c. Put all the ring material together and it would make a moon only 250 km in diameter. d. “Roche Limit” = closest to a planet a moon can orbit without being ripped apart by tidal forces. If planet and moon have same density, Roche limit is about 2.4X the radius of a planet. (Note: Io is getting darned close to its Roche limit around Jupiter – tidal forces aren’t ripping it apart, but that’s what causes the volcanic activity.) e. Origin of rings, some current models i. Was a moon that strayed inside of the Roche limit ii. Was a moon destroyed by a collision near the Roche limit iii. Was material left over from the formation of the planet, but since inside the Roche limit it could never get together to form a moon 8. Uranus a. Discovered 1781 by Herschel – was charting faint stars and noticed this one moved against the background of stars b. Appears blue-green due to a methane atmosphere – methane absorbs long wavelength light (red), leaving short wavelengths (blue) to reflect c. Appears “tipped over” – axis of rotation is over 90° to ecliptic plane – maybe a violent collision in the early days of the solar system?? 9. Neptune a. Most moons are irregular = probably captured. Triton shows retrograde motion = orbits around the planet backwards = probably captured. Triton is slowly spiraling inward so will eventually get ripped apart by tidal forces. b. Discovery was based on applying mathematics and Newton’s Laws of Motion i. Uranus’ orbit didn’t seem quite right ii. 1840’s, mathematicians Adams & Leverrier independently calculated where another planet should be, beyond Uranus, to affect Uranus’ orbit iii. 1846, Astronomer Galle found Neptune, right where the mathematicians said it should be, on the first try! 10. Pluto a. Discovered 1930 by Tombaugh, really by luck. It was thought that there were discrepancies in Neptune’s orbit, so people tried to do calculations to find another planet. There really were NOT major discrepancies, but it got people out looking for a new planet. Of course, now it’s not a planet! b. Pluto is made up of ice & rock c. Charon, discovered 1978. It’s relatively close to same size as Pluto, they are close = Pluto-Charon are 1/20th of the distance from Earth to Moon. d. Pluto and Charon orbit around a point between them, their center of mass e. Current debate: Is Charon a moon? Are Pluto & Charon a binary system?