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ASTRONOMY 161 Introduction to Solar System Astronomy Class 19 Venus Friday, February 26 Venus: Basic characteristics Mass = 4.868×1024 kg (0.815 Earth) Radius = 6,052 km (0.949 Earth) Density = 5,204 kg/m³ Sidereal rotation period = -243.0185 d Albedo = 0.65 (Earth = 0.39) Average distance from Sun = 0.723 A.U. Venus: Key Concepts (1) The surface of Venus is hidden from us by clouds of sulfuric acid. (2) The atmosphere of Venus is hot because of a runaway greenhouse effect. (3) The surface of Venus shows volcanic activity but no plate tectonics. (4) The interior of Venus is similar to that of the Earth. Venus: Revolution and Rotation Orbital period = 225 days Rotation period = 243 days Rotation of Venus is retrograde. On Venus, the time from noon to noon = 117 days Venus is sometimes called Earth’s twin: Radius = 95% of Earth’s Mass = 82% of Earth’s Surface gravity = 90% of Earth’s (1) The surface of Venus is hidden from us by clouds of sulfuric acid. Venus has 100% cloud cover. Its rotation speed can be found from radar signals which pierce the clouds (the same method used for Mercury, using Doppler effect). The clouds of Venus, about 60 kilometers above the surface, are made of sulfuric acid. Acid rain evaporates in the hot air of Venus before it reaches the ground. (2) The atmosphere of Venus is hot because of a runaway greenhouse effect. Dense and high-pressure atmosphere: Air pressure at the surface of Venus = 90 times air pressure on Earth (2/3 of a ton per square inch). • • • • 96.5% carbon dioxide (CO2) 3.5% nitrogen (N2) 0.015% sulfur dioxide (SO2) almost no water vapor CO2 is a greenhouse gas: so Venus has a GARGANTUAN greenhouse effect. Surface temperature = 733 Kelvin (860 degrees Fahrenheit) Even hotter than Mercury! Temperatures drop very little during the long night, thanks to the thick atmospheric blanket. Escape velocity for Venus = 10.4 km/sec Venus was once cooler, and may have had seas. Volcanoes produced large amounts of H2O, CO2, S02 (all are greenhouse gases) Air temperature rose. H2O evaporated from seas; CO2, SO2 released from seas, rocks. RUNAWAY greenhouse effect! Venus has swift E-to-W winds at high altitudes, but no circular storms. Slow rotation means that the “coriolis effect” is tiny. (3) The surface of Venus shows volcanic activity but no plate tectonics. Because clouds are opaque, surface of Venus has been mapped using radar, by the spacecraft Magellan. Round-trip travel time gives us the distance: d=(t2-t1)/2c Most of Venus is covered with rolling plains, but there are two large highland regions: Ishtar Terra (north) ~ Australia Aphrodite Terra (equator) ~ Africa Venus: many volcanoes, few impact craters. More than 1600 major volcanoes. Estimated average age of crust = 500 million years (older than Earth, much younger than Mercury or Moon). Perspective view of Venusian volcanoes (Warning: vertical scale is exaggerated by a factor of 10): About 1000 large impact craters (more than Earth, much less than Moon). Perspective view of Venusian craters: Older craters have been obliterated by lava flows. Venus lava flows: On the Earth, the cold rigid lithosphere is broken into plates: On Venus, the hot plastic lithosphere does not break: No plate tectonics on Venus. Levels of Volcanic Activity Earth: extremely active Venus: mildly active Moon: volcanically dead Mercury: most likely volcanically dead We have evidence for convection in the asthenosphere of Venus. Wrinkled mountains are seen where the crust is compressed. Maxwell Mountains – the highest on Venus – consist of parallel ridges. More evidence for convection in the asthenosphere of Venus: Fractures (or “stretch marks”) are seen where the crust is pulled apart. Venus in extreme closeup: Image returned by Venera 13 spacecraft (1982). Venus Express First European (ESA) mission to Venus, in orbit since May 2006. First results from Venus Express: (4) The interior of Venus is similar to that of the Earth. Uncompressed density of Venus = uncompressed density of Earth = 4200 kg/m3. Venus probably has a metal core and rocky mantle, like the Earth. Few closing questions: 1) Name the major differences and similarities between Venus and the Earth. 2) How dense is the atmosphere of Venus? 3) Is there magnetic field on Venus? 4) What is the name of Venus’ moon? 5) Are there volcanoes on Venus? 6) Are there quakes on Venus?