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Lecture 23 Movie Night 2 Monday, April 19 SW119 from 7:00~9:30 How do surface building processes apply to the Terrestrial Planets? Vulcanism Impact cratering Erosion Tectonics erosion cratering vulcanism tectonics Small rocky planets Cold, solid interiors Tectonic & volcanic activity only during first billion years Many ancient craters No erosion Little outgassing Atmospheres lost due to weak gravity (Moon, Mercury) Mercury Geologically dead Vulcanism when young (small lava plains, smaller crater density than lunar highlands) Days / Nights last 3 months – very hot / cold Mercury Tectonics when young Very high cliffs extending for hundreds of kilometers, formed as folds as planet cooled and contracted Moon Highlands (light colored areas): heavily cratered about four billion years old Mare Dark colored areas: giant impact basins flooded with lava lightly cratered about 3.2-3.9 billion years old (younger than highlands) (Earth, Venus) Large rocky planets Warm, convecting interiors Ongoing tectonic & volcanic activity Most ancient craters erased Considerable outgassing Atmospheres retained Erosion exists Venus Fully cloud covered – surface not visible Surface features on Venus have been investigated using radar from orbiting spacecraft young surface mostly gentle rolling plains. evidence of volcanism. Erosion & tectonics few impact craters tectonic activity (folded grooves shown here) Mars Marsrovers.jpl.nasa.gov erosion cratering vulcanism tectonics fewer craters in northern hemisphere heavily cratered southern hemisphere giant volcanoes Olympus Mons dry stream beds, once had running water Considerable evidence of water flows – younger terrain heavily cratered southern hemisphere Older terrain Tectonics & erosion Erosion around edge of Olympus Mons Conclusion: part of surface young, part old Summary: Surface Properties Cratered terrain ubiquitous in the solar system cratering due to planetesimals left over from formation of solar system smallest planets (Moon, Mercury) heavily cratered middle sized planet (Mars) has both heavily cratered and also relatively uncratered regions and extinct volcanos largest planets (Venus, Earth) have few craters and active volcanos Why? More impact craters implies an older surface Rate of cratering declined rapidly after planet formation Many craters remain where there is no tectonic or volcanic activity Why do some planets have old surfaces and some young surfaces? The level of tectonic and volcanic activity Thickness of lithosphere controls extent of volcanic activity. Thin lithosphere -> hot rocks from interior break through surface Thick lithosphere -> hot rocks from interior can't break through surface.