CH5 - Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson...

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Unformatted text preview: Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson & McMillan, Astronomy Chapter 5 The Earth and Moon The Earth and Moon The The Earth and Moon together exhibit most of the features seen in planets and moons generally moons This lecture will explore the features in a general way that can be applied to other planets and moons planets Gravity at close range: Gravity Tides The Earth and Moon are close The strength of gravity changes noticeably from one side of each body to the other to This is true for all large objects in space This This distorts the Earth and the Moon This Tides Tides Difference in Gravity Force Difference from Centre to Edge due to from Something far away GMm GMm ∆F = − 2 R ( R + rm ) 2 R + 1 rm ≈ R 2rm ( R + 1 rm ) = GMm 2 2 2 2 Assume 2 2 2 R + rm ≈ R 2 R ( R + rm ) 2rm ≈ GMm 3 Tidal Force R m rm R M Tides Tides Tides distort the solid Earth little solid The water on the Earth moves in water on response to the difference in gravity due to the location of the Moon due There are two high tides and two low tides per day tides Tides vs. Total Force Tides Gravitational Forces equal and opposite Tidal Forces are not GMm F= Gravitational Force 2 R 2rm Ftide = GMm 3 Tidal Force R Solar & Lunar Solar Tides The Sun is very far away but much heavier: Solar tide = ½ the lunar the tide tide (Solar gravity = 200 x lunar Solar gravity) gravity) Solar and Lunar tides may work together or against each other against Tidal Locking Tidal The Tidal bulge on Earth precedes the Moon’s position Moon The Moon’s gravity pulls the tidal bulge The The Earth’s spin is slowing The Earth’s day is getting longer Earth day The moon is moving out The Tidal Locking Tidal The Earth is heavier – the tide on the the Moon due to Earth is larger Moon The Moon lost its spin long ago The The Moon’s near face is locked The near pointing at the Earth: Synchronous Orbit Orbit Earth vs. Moon Earth The Earth is a very active planet: Volcanos, Plate Tectonics, Volcanos Plate Earthquakes, Hydrosphere, Atmosphere, Magnetism Atmosphere, The Moon is dead: None of the above The Earth vs. Moon Earth The Earth and Moon are similar: The Composition (type of rocks) at surface Composition Same location in Solar System Same The Earth and Moon probably formed together – what went wrong for the what Moon? Active Features of Earth of Molten Interior and thin crust: Volcanos, Plate tectonics, Earthquakes Volcanos Hydrosphere: Oceans, lakes, rivers Hydrosphere: Atmosphere: Air, Weather Atmosphere: Magnetosphere: Strong Magnetic Field Magnetosphere: Earth vs. Moon: Earth Overall Structure Earth’s Interior: Earth Seismology Earthquakes send waves travelling through the Earth Earth Different waves travel differently in liquid and solid to let us infer the structure structure Interior of Earth Interior The Earth has a hot liquid interior interior Earth has a thin crust of material on the surface material This hot interior generates a large number of Earth’s active large active features: Volcanos, Volcanos Earthquakes, Plate Tectonics, Magnetism Tectonics, Why is the Earth hot inside? Why Heat of formation: Colliding Planetesimals Planetesimals Radioactivity: Most important Radioactivity: Rock is a good insulator – the heat has the stayed around for a long time stayed The Moon is smaller – it has cooled Molten Earth Molten The Earth was once molten The Differentiation: In a fluid Differentiation In state heavy material sinks to the centre the Core: Nickel/Iron 5-12 g/cc Core: Mantle: Rock 3-4 g/cc Mantle: Moon: Rock 3.3 g/cc Moon: Earth’s Surface: Earth Continental Drift The thin solid Crust floats on the liquid mantle in plates liquid The plates move because of The Convection in the hot mantle Convection is a way to move heat from the hotter interior to the outer parts parts Continental Drift Continental Constant geological activity Constant New surface material New The Lunar Surface The The Moon has no significant interior heat to drive volcanoes, earthquakes, etc… currently etc The Lunar Mare are basalt lava flows from long ago lava The lunar highlands are lighter rock lighter The Lunar Surface The The Moon’s surface is has The surface been geologically inactive for 4 billion years for The main evolution since then has been through meteorite impacts meteorite Biggest Crater Biggest in Solar System in Craters over time Craters The Moon and Earth had similar impact histories impact Earth craters disappear rapidly due to wind/water due Gas Gas Atmospheres Having an atmosphere depends on two factors: 1. How much gas you start with 2. How easily the atmospheric gases escape The Moon’s escape velocity iis low enough that s escape warm gases would escape steadily warm Hydrospheres: Liquids Hydrospheres The Earth’s atmosphere has enough The atmosphere pressure to have liquid water at the surface surface On the Moon liquid water would evaporate to gas and escape evaporate Moon rocks are bone dry Ice crystals may lurk in permanent shadows near the lunar poles shadows Earth’s Atmosphere Earth Very thin: Very 99% of it lower than 99% 30 km 30 Several zones based on temperature on Earth’s Atmosphere Earth Atmosphere The troposphere The The Earth’s surface is warm The The heat can’t radiate away The radiate efficiently – the gas (air) the moves instead: Convection Convection Convection drives weather and clouds form at the top of the troposphere (lowest layer) layer) The Greenhouse Effect The Clouds and Carbon Dioxide prevent infrared leaving Earth efficiently Earth No greenhouse effect: No Earth at –23 C Earth Moderate greenhouse effect: Moderate Earth at 17C Earth’s Earth Upper Atmosphere Upper The Ozone layer is where UV is absorbed Ozone = O3 Ozone This makes it warmer there there Earth’s Earth Ionosphere: Nearly in space Ionosphere: The uppermost atmosphere is exposed to radiation and particles from the Sun and space and Atoms get ionized Atoms Aurora (Northern Lights) are light emitted by ionized particles by Earth’s Earth Magnetosphere Magnetosphere Moving conductors produce magnetic fields (e.g. electric motors) fields Earth’s lliquid metal core moves due to Earth iquid convection convection Earth has a strong magnetic field Earth The Moon has no magnetic field The Magnetosphere Magnetosphere The Solar Wind is ionized atoms from the Sun (dangerous) the The Solar Wind is deflected by the magnetosphere magnetosphere The Solar Wind sweeps out the magnetosphere into a long tail magnetosphere Making the Moon Making Evidence: Evidence: Rocks are similar to Earth’s Mantle Rocks Very Old Very Theories: Co-evolution, Capture, Impact Co Moon Formation Theories Moon Co-evolution Formed together in the Solar System Formed Problem: Why no iron core in the Moon like Earths? like Moon Formation Theories Moon Capture Capture a passing planetesimal from planetesimal from somewhere else somewhere Unlikely Unlikely Problem: Doesn’t explain why the Moon Problem: explain is very similar to rocks in Earth’s mantle is Moon Formation Theories Moon Impact Something large struck the Earth just after Earth formed just Can carve off some rocky mantle and leave the iron core behind and Works in computer simulations: Mars sized object hits early Earth Mars Favoured theory Favoured Earth vs. Moon Difference Earth Both have rocky dense material, but… The Earth is more massive: Retains heat: Hot interior Retains Volcanoes, Plate Tectonics, Earthquakes: Active Geology, Magnetic Field Active Holds onto an Atmosphere Holds Air, Liquid Water, Weather, Greenhouse Effect: Warmer surface, Erosion Effect: ...
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