Reminders!Website: http://starsarestellar.blogspot.com/ Lectures 1-6 are available for download as study aids.Reading: You should have Chapters 1-5 read, andChapters 6 and 7 by the end of this weekend.Homework: Homework #2 is due this Wednesday at theBEGINNING of class.Discussion: Next week, we will be reviewing for themidterm.Midterm!!!The midterm is Thursday, June 11th in class. Don’t be late!The test will cover Chapters 1-8.It will consist of 40 to 50 questions and you will have until 12o’clock to complete it.The test is true-false and multiple choice. Make sure you have aSCAN-TRON 882 form and a #2 pencil. I will NOT have theseavailable in class.Bring your student ID.The test is closed book, closed notes, and no calculators.There will be a sheet of equations available.A practice midterm is available. Take the practice test for 1.5hours, and see how you do. If you have any questions, you canask about them in discussion next week.Terrestrial PlanetsToday’s Lecture:• Solar System Formation (chapter 5, pages 105-108)• Terrestrial Planets (chapter 6, pages 110-151)Earth and MoonMercury, Venus, and MarsThe Solar SystemSunTop view of thesolar system• All the planets orbitthe Sun in the samedirection.• Most of the planetsrotate in the samedirection too.• And so does the Sun.• Orbits are all nearly inthe same plane.Side view isa thin diskOrigin of the Solar System“Nebular Hypothesis” (Kant, Laplace, 18th century)• Before the Sun was born, there was a gravitationallycontracting, slowly spinning cloud of dust.• As it contracts due to gravity, is spins up to conserveangular momentum (figure skater effect)• Sun forms in the center, and the outer parts make adisk.• Planets form out of the disk.• Extra gas is blown away by the Sun’s winds.Terrestrial Planets• The inner planets: Mercury,Venus, Earth, and Mars• All have a smaller radius thanthe Earth• Not very massive (less thanEarth), but dense (>1 g/cm3,which is the density of water)• Close to the Sun and fewmoons• Rocky outer parts and ironcores.Earth: 3rd Rock from the Sun• Upper mantle and crust float on a soft, churning layer (aconvecting layer)• This causes continental drift (plate tectonics)• Mountains, volcanoes, and earthquakes all occur at the plateboundaries.The Earth (cont.)• Earth’s atmosphere: 79% N2, 20% O2 (so much oxygen is due tolife, photosynthesis).• The atmosphere is very thin < 100 km• Earth’s magnetic field looks like a giant bar magnet (dipole).Molten interior sustains a field which reverses every ~3 x 105 yrsTides: Differential forces from the Moon (mostly)• The force by the Moon on the Earth is greatest on the near sideand smallest on the far side.• This causes two high tides at each of these points.• There are TWO high tides per 24 hours!• What about the Sun? The Sun’s differential force is only 1/2 ofthe Moon’s because the Sun is so far away.Tides very high Tides not so highS, E, M aligned S, E, M perpendicular• In the past, the Moon rotated faster, but gradually lost its energydue to tidal friction.• Hence, the Moon’s rotation is now locked to its revolution: bothperiods are the same (“synchronous rotation”)• We always see the SAME SIDE OF THE MOON!The Sun and Moon and TidesNon-synchronous rotation Synchronous rotationWe see one side of our MoonIt’s orbit and rotation are in synchEarthMoonEarthMoonThe faces of the Moon• The Moon is heavilycratered due to impacts.The most heavily crateredparts are the oldest.• Lava flows producedmaria (“seas”) which areseen as dark regions• Very little erosion on theMoon: can get relative agesof features (craters onmaria must be youngerthan the maria)MariaMariaCratersCratersThe Moon’s Surface• Derive the absolute ages of rocks (since last molten)by radioactive dating (for example, Uranium) of Moonrocks brought back by astronauts (Apollo 11-17; 1969-1972)• Heavily cratered areas are 3.9-4.3 billion years old.• Maria areas are 3.1-3.9 billion years old.• There are few craters in the maria; thus, intensebombardment of the Moon happened very early afterSolar System formationFormation of the Moon• The Earth-Moon system is almost like a “doubleplanet” - the Moon is quite large compared to otherMoons that we know about.• The composition of the Moon is very similar (but notexactly so) to the Earth.• The Moon probably coalesced from debris ejected bya collision of a Mars-sized object with the Eath, earlyin the history of the Solar System.Mercury: The Innermost Planet• Difficult to study: alwaysnear the Sun in our sky• Sets or rises within 1 to 2hours of the Sun (nearhorizon so images blurred)• Rotation rate is knownfrom Doppler effect withradar.SunMercuryVenusEarthMercury: The Innermost Planet• Day and night cycle on Mercury= 176 days (88 days of sunshine, then 88 daysof darkness!)• Almost no atmosphereDaytime T = 430 °C, night T = -170 °C!!!• Mariner 10 flew past 3 times (1974/1975)• Heavily cratered like the Moon• No evidence for plate tectonics• MESSENGER satellite is currently studying Mercury.Found evidence for water in the atmosphere and aliquid (molten) core.Venus: The Evening(or Morning) Star• Easy to see in the eveningor early morning• Shrouded in highlyreflective clouds• NASA “Magellan”spacecraft (1990-1993);detailed radar maps. Sawcraters, volcanoes, andlarge plains.• Crust consists of only 1thick plate.SunMercuryVenusEarthVenus (cont.)• Thick atmosphere:90 times the Earth’s surface pressure!• 96% CO2 and less than 4% N2Earth 79% N2 and 20% O2Earth’ s CO2 is trapped in rocks and oceans• Temperature of surface is 480 degrees Celsius. It is thehottest planet, due to the “greenhouse effect.”• As CO2 increases on the Earth, we similarly get agreenhouse effect, contributing to global warmingVenus: the hottest planetSurface heats up due to visible light. The surface thenradiates in the infrared which is absorbed by theatmosphere (clouds) ---> temperature increases!atmosphereblocksinfraredMars: The Red Planet• Orange (reddish) due torust (iron oxide)• 2 tiny, irregular moons• Mars has polar ice caps(mostly CO2) that grow andshrink with the seasons• Thin atmosphere (1% ofEarth’s) 90% CO2.• Windy: major dust storms• T = -130 °C to 30 °C, butgenerally cold• No plate tectonicsSunMercuryVenusEarth MarsMars: Evidence for water• One of the first major discoveries: ancient riverbeds and flood plains! Water
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