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Gravity of these massive planets drew in H, He gases‣Where did asteroids and comets come from? ‣They are leftover planetesimals, according to the nebular theory‣When did the planets form? ‣Radiometric dating indicates that planets formed 4.6 billion years agoWhat are terrestrial planets like on the inside? ‣Core, mantle, crust structure, ‣Denser material is found deeper inside‣What processes shape planetary surfaces? ‣Cratering, volcanism, tectonics, erosion‣What determines a world’s level of geological activity? ‣Interior heat drives geological activity, ‣Big planets cool off slower than small ones, ‣Radioactive decay is currently main heat source‣How does a planet’s surface reveal its geological age? ‣Amount of cratering tells us how old the surface is. The Moon is coverered with a gently rolling layer of powdery soil with scattered rocks that is called the regolith. Two types of regions on the Moon: ‣Highlands, ‣Lighter-colored hilly regions, ‣Covered in craters; ‣Maria, ‣Darker, smooth, lowlying plains, ‣Few craters, ‣Cover about 17% of the Moon’s surface.Southern hemisphere of mars has older surface. ‣What geological processes shaped our Moon? ‣Early cratering still present, ‣Maria resulted from volcanism‣What geological processes shaped Mercury? ‣Cratering and volcanism similar to Moon, ‣Tectonic features indicate early shrinkage‣What are the major geological features of Venus? ‣Venus has few craters - young surface, ‣Dominated by volcanism, ‣Also has tectonics but little or no erosion‣What are the major geological features of Mars? ‣Differences in cratering across surface, ‣Giant shield volcanoes, ‣Evidence of tectonic activity‣What geological evidence tells us that water once flowed on Mars? ‣Features that look like dry riverbeds, ‣Some craters appear to be eroded, ‣Rovers have found rocks that appear to have formed in water. No greenhouse effect earths average temp below freezing. Venus, earth, mars have significant atmospheres. What is Venus like today? ‣Venus has an extremely thick CO2 atmosphere, ‣Hottest planetary surface in the solar system ‣How did Venus get so hot? ‣Thick CO2 atmosphere leads to a huge greenhouse effect‣Why did Earth’s atmosphere end up so different? ‣Temperatures just right for oceans of water, ‣Oceans keep most CO2 out of atmosphereJupiter & Saturn – gas giants; Composition: mostly hydrogen & helium, ‣Atmospheres ~1000 km thick, ‣Liquid H interiors, ‣High pressures, ‣Liquid metal at very high pressures, ‣“Rocky” cores, ‣Rock, “ices”, metal etc.., ‣Temps > 20,000º C!Uranus & Neptune – ice giants; Atmosphere: H, He, and traces of methane, ‣Methane absorbs red light, giving planets their bluish colors, ‣