Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 33: Veiled 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 CO₂ atmosphere; Runaway greenhouse effect
Surface:: Mapped with Radar; Rolling plains, highlands, and valleys; Unique terrain features

Veiled Venus

Venus is completely covered by thick clouds
First data came from radar bounced off Venus: Found Venus has a very slow, retrograde rotation; Showed that the surface was very hot - greater than 700 K!

Spacecraft Visits

Flybys: Mariner and Pioneer satellites (1962)
Landers: Mostly USSR: Venera 7 (1970) - first soft landing
Atmospheric Probes:: Pioneer Venus (1978, US); Vega 1 and 2 balloon probes (1985, USSR)
Radar Mapping:: Venera 15 and 16 (1983, USSR); Magellan (1990-1994, US)

Retrograde Rotation

Venus has a slow, retrograde (east-to-west) rotation: Rotation period is 243 days; This is surprisingly slow!
Possible Causes:: Tidal interaction between Venus, Sun, and Earth with complex braking by the atmosphere; Massive glancing impact virtually de-spinning Venus and making it go slowly backwards

Venus' Atmosphere

Composition:: 96% Carbon Dioxide (CO₂); 3.5% Nitrogen (N₂); 0.15% Sulfer Dioxide (SO₂); less than 0.1% Water Vapor (H₂O) - very dry!
Surface Pressure: 90 atmospheres: Like the ocean at a depth of ~1 km
Surface Temperature: uniform 750 K (891 F)

Surfuric Acid Clouds

The clouds of Venus are not water vapor clouds like on Earth:: Mostly droplets of Sulfuric Acid (H₂SO₄); Form a thick layer between 48 and 58 km altitude
Lower atmosphere and surface are clear below the H₂SO₄ clouds: Get optical distortion of the horizon by the hot, heavy atmosphere

Runaway Greenhouse Effect

Venus is so hot (750 K) because of a runaway greenhouse effect:: Hot, heavy CO₂ atmosphere; Heat trapping makes Venus 500 K hotter than it would be with no atmosphere (compare with 35 K for Earth); Water stays as vapor (no rain) and gets broken into H₂ and O by UV photons; H₂ escapes into space
This makes Venus extremely hot and dry today

Surface of Venus

Terrain:: ~85% rolling plains; ~15% highland plateaus and mountain ranges
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

Volcanism and Geologic Activity

Volcanoes are a common terrain feature: None in chains, suggesting no plate tectonics; Pancake domes and coronae 100-200 km across; Are some volcanoes active today?
Tectonics, but not plate tectonics: High temperatures makes the crustal rock soft; Upwelling of material from the mantle; Downwelling causing compression

Impact Craters

Only ~1000 impact craters are seen on Venus:: Randomly scattered around the surface; None less than 3 km across (no meteors less than 30 meters across); ~80% of the surface has been repaved in the last 500 Myr
Two competing ideas:: Craters get very quickly filled in by volcanism; Catastrophic volcanic repaving of the entire surface ~500 Myr ago

Venus and Earth

Volcanic and tectonic repaving: Earth: on-going process:; Venus: most repaving occurred ~500 Myr ago
Tectonic activity is different:: Earth: lateral recycling by sliding motions of tectonic plates; Venus: vertical recycling via upwelling and downwelling; May be related to lack of water in Venus' crust

Atmospheres of Earth and Venus

Earth: Warm, light, moist, nitrogen and oxygen atmosphere
Venus: Hot, heavy, very dry carbon dioxide atmosphere
Why so different?: Same starting point (volcanic outgassing); Venus had a runaway greenhouse effect (more solar heating) and drove off its water; No oceans means no CO₂ chemistry to lock CO₂ into crustal rocks

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.