Venus: Key Ideas: A Warm Up Exercise. Venus at a Glance -- Orbit. Venus at a Glance Planetary Data

Transcription

1 Venus A Warm Up Exercise Because Mercury has a high average density despite its relatively low mass, it is thought to a) Have a subsurface ocean b) Have a large iron core c) Be made largely of lead d) Be made of highly compressed rock e) Have lots of Hydrogen and Helium A Warm Up Exercise Because Mercury has a high average density despite its relatively low mass, it is thought to a) Have a subsurface ocean b) Have a large iron core c) Be made largely of lead d) Be made of highly compressed rock e) Have lots of Hydrogen and Helium Venus: Key Ideas: Venus is the second planet from the Sun Nearly the same size as the Earth Covered with opaque clouds Slow retrograde rotation Atmosphere: Hot, heavy CO 2 atmosphere Runaway Greenhouse effect Surface: mapped using radar Rolling plains, highlands & valleys Unique terrain features Venus at a Glance -- Orbit a = AU Period: days e = tilted by 3.39º to the ecliptic plane Most circular orbit among the planets. Venus at a Glance Planetary Data Radius: 6052 km (0.949 R Earth ) Mass: M Earth Rotation: days Retrograde! Axis Tilt: 177.4º Upside down! 1

2 Spacecraft Visits Flybys: Mariner & Pioneer satellites (1962) Landers: only USSR Venera 7 ( USSR) - first soft landing Atmospheric Probes: Pioneer Venus (US: 1978) Vega 1 & 2 balloon probes (USSR: 1985) Radar Mapping: Venera 15 & 16 (USSR: 1983) Magellan (US: ) Currently Venus Express (Europe: 2006 on) Veiled Venus Venus is completely covered by thick, opaque planet-wide clouds. First data about the surface came from radar signals bounced off Venus: Revealed the very slow, retrograde rotation of the planet. Radiometry revealed that the surface was very hot, upwards of 700 K! Retrograde Rotation Pioneer Venus Orbiter Image Venus has a slow, retrograde (east-to-west) rotation. Rotation Period is 243 Days This is surprisingly slow! A solar day takes about 117 days Possible Causes: Tidal interaction between Venus, Sun, & Earth with complex braking by the atmosphere. Massive glancing impact virtually de-spinning Venus and making it go slowly backwards. Venus Atmosphere I Composition: 96% Carbon Dioxide (CO 2 ) 3.5% N % Sulfur Dioxide (SO 2 ) <0.1% Water Vapor (bone dry!) Surface Pressure: 90 atmospheres Like ocean at a depth of ~1 km Surface Temperature: uniform 750 K (891º F) Venus Atmosphere II Despite the long solar days (177 days) the surface temperature is pretty uniform Enormous mass of the atmosphere stores heat well Rapid retrograde winds upper atmospheric winds circle planet in 4 days (350 km/hour) Powerful convection cells 2

3 Sulfuric Acid Clouds The clouds of Venus are not water vapor clouds like on Earth: Consist mostly of droplets of Sulfuric Acid (H 2 SO 4 ). Forms a thick layer between 48 and 58 km altitude. Lower atmosphere and surface are clear below the H 2 SO 4 cloud deck. Distortion from the hot, heavy atmosphere (Sulfuric acid) rain does not reach the surface evaporates on the way down The Old Picture Edgar Rice Burroughs ( ) as in the Tarzan books Japanese edition of Carson of Venus The Surface of Venus Venera 14, 1982 (lasted 60 minutes) A more modern artist s conception Venus in Color (Venera 13, 1982, 127 minutes) All landers were in regions away from the highlands rocks are basalts Runaway Greenhouse Effect Venus is so hot (750K) because of a runaway greenhouse effect: Hot, heavy CO 2 atmosphere, 100 heavier than the Earth. Heat trapping makes Venus +400 K hotter than it would be with no atmosphere. Water stays in vapor form, and is eventually broken into H 2 and O by UV photons. H 2 escapes, causing Venus to dry out. 3

4 Venus Unveiled The surface of Venus has been mapped using imaging radar. First global map was made in 1978 by the Pioneer Venus orbiter Venera 15 & 16 radar imagers (low resolution) Highest Resolution Map: Magellan Venus ( ) 98% of the surface at meter resolution Good enough to do geological assessment One of the last satellites launched with the shuttle Largely build from spare parts from other missions to save money! Magellan Radar Mapper ( ) The Result Ishtar Terra The Surface of Venus Terrain: ~85% rolling planes ~15% highland plateaus & mountain belts Highlands are concentrated into two regions: Ishtar Terra Aphrodite Terra Not ancient highlands like the Moon. Also see impact craters, volcanoes, and other geological features. Aphrodite Terra Venus Surface Topography Venus is pretty flat compared to the Earth 60% of the planet lies within 500 m of the average elevation 4

5 Volcanism & Geologic Activity Volcanoes are a common terrain feature: None in chains, suggesting no plate tectonics Pancake domes & coronae km across Some volcanoes may be active today, but only indirect evidence from changing concentrations of Sulfur Dioxide in the atmosphere Tectonics, but no plate tectonics: High temperatures makes the crustal rock soft. Upwelling material from the mantle. Downwelling causing compression. Sapas Mons (1.5 km high) Gula Mons Volcano & faults Impact Craters Only ~1000 impact craters are seen on Venus: Randomly scattered around the surface. None <3 km across (no meteors <30 m across) ~80% of the surface has been repaved in the last 500 Myr. Two competing ideas: Craters get filled in by volcanoes very quickly. Catastrophic volcanic repaving of the entire surface ~500 Myr ago. Mona Lisa crater (86 km) Impact Crater (Danilova) 5

6 Craters and the Age of the Surface Because there are so few craters, the surface must be fairly young about 500 million years This is about twice the average age of the Earth s surface Because the craters are distributed pretty uniformly, all Venus surface must have roughly the same age the entire surface must have been reprocessed about 500 million years ago It does not have one Magnetic Fields Probably because it is barely rotating, so nothing drives ordered convection in the core or because the core is solid. This is very different from Earth, where large pieces of very old crust (billions of years) have survived 6