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Some small areas may be much more recent no ancient

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Unformatted text preview: : Magellan Mission Venus: Magellan Mission US radar orbiter mapping mission. Launch from Space Shuttle, May 1989 Mapped Venus Sept. 1990 ­ Oct. 1994: “Imaged” 98% of Venus' surface with a resolution of ~ 1 km. Measured surface topography to ~100 m vertical resolution. Venus: Magellan Results • Nearly 1000 impact craters > 1 km in size. Crater density implies young ~500 MY age. Some small areas may be much more recent. No "ancient" surface. • Much evidence of tectonism, but no clear evidence of plate tectonics like on Earth. • Abundant evidence of past volcanism: volcanoes and vast low-lying lava plains. • Some evidence for erosion: dunes, wind streaks, lava channels. Radar return depends on surface roughness and dielectric properties. •Smooth areas (like basalt flows) have low radar reflectivity, whereas crater ejecta or one side of fractures reflect radar back to the spacecraft. •Highlands at altitudes >2.4 km have high radar emissivity, indicating presence of semi­ conducting materials with high dielectric constants (iron sulfides or metallic frosts formed from volcanic exhalations?). Impact Craters Limited number and wide distribution of craters used to argue for global volcanic resurfacing at ~500 million years ago, but resurfacing could have been spread over a billion years instead. Heat loss (mostly by conduction) is slower than on Earth, so periodically the crust may founder and sink into the overheated mantle (repeatedly?). Small size (3 km) cutoff of craters is due to atmospheric disruption of smaller impactors. Ejecta extend out ~2.5 crater radii, farther than predicted from ballistic emplacement in the dense vensusian atmosphere. Dickinson Crater (D ~70 km) Three Craters (largest: D~50 km) Golubkina Crater (D ~ 30 km) Venus: Tectonics Venus: Tectonics Numerous patterns of ridges and fractures, including mountain belts and rift valleys. E...
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