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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 7: What Happened on Venus?

Key Ideas

Venus appears to satisfy many of the requirements for habitability
--Equilibrium temperature would support liquid water
--Nearly the same size as the Earth
--Has an atmosphere
--But slow retrograde rotation -- no magnetic field
The surface of Venus is ~750 million years old
--No large-scale plate tectonics
--Entire surface repaved by large-scale volcanic activity?
Venus has a hot, heavy CO2 atmosphere with very little water.
The surface temperature of Venus is 743 K (878 F)!
What went wrong?
--No carbon dioxide sink
--Probably lost all of its water in a 'runaway greenhouse'
Venus has implications for the range of habitability

Venus and the Criteria for Habitability

Properties of Venus
a = 0.723 AU
P = 224.7 days
Radius = 0.949 Earth Radii
Mass = 0.815 Earth Masses

Equilibrium Temperature of Venus

a=0.723 AU --> T~327K, 54 C, 130 F

T=100-120 F for reasonable albedos
Other Indicators of Habitability
Cooling Time 95% of Earth's
Escape Speed 93% of Earth's
Thermal velocity 108% of Earth's

Venus seems to satisfy all the physical requirements for a habitable planet

Venus Unveiled

Venus has been visited by several spacecraft:
Flybys: Mariner & Pioneer satellites (1962)
--Venera 7 (1970 - USSR) - first soft landing
Atmospheric Probes:
--Pioneer Venus (US: 1978)
--Vega 1 & 2 balloon probes (USSR: 1985)
--Venera 15 & 16 (USSR: 1983)
--Magellan (US: 1990-1994)
--Venus Express (ESA: 2005-present)

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, ?700 K!

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, & Earth with complex braking by the atmosphere.
--Massive glancing impact virtually de-spinning Venus and making it go slowly backwards.
No magnetic field

The Surface of Venus

--~85% rolling plains
--~15% highland plateaus & mountain belts
Highlands are concentrated into two regions:
--Ishtar Terra
--Aphrodite Terra
Also see impact craters, volcanoes, and other geological features.

Volcanism & Geologic Activity
Volcanoes are a common terrain feature:
--None in chains, suggesting no plate tectonics
--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 < 3 km across (no meteors <30m 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 versus Earth
Volcanic & 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 & downwelling.
Lack of plate tectonics on Venus?
--May be related to lack of water in Venus' crust

Hellish Venus

Venus' Atmosphere
--96% Carbon Dioxide (CO2)
--3.5% N2
--0.15% Sulfur Dioxide (SO2)
--<0.1% Water Vapor (bone dry!)
Surface Pressure: 90 atmospheres
--Like the ocean at a depth of ~1 km!
Surface Temperature: uniform 750 K (891 F)

Sulfuric 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 & 58 km altitude

Why Venus Is So Hot
Venus is so hot (750K) because of the greenhouse effect:

Hot, heavy CO2 atmosphere

Heat trapping makes Venus 500K hotter than it would be with no atmosphere.

Venus has very little water
Because Venus is so hot, water stays as a vapor
Water vapor gets broken into H2 and O by UV photons.
H2 escapes into space
Water is constantly removed
This makes Venus extremely dry today!

Atmospheres of Earth & Venus
Warm, light, moist, N2 & O2 atmosphere

Hot, heavy, very dry CO2 atmosphere

Why so different?

What Went Wrong on Venus?

Volatiles from outer solar system

Early Atmospheres Similar
Volatiles from outer solar system

Nearly the same amount of CO2 on Earth & Venus
On Earth, CO2 is locked up on rocks
On Venus, CO2 is in the atmosphere

Likely had the same amount of water
Early Atmospheres Similar
Deuterium fraction much higher than on the Earth
Runaway Greenhouse Effect
Slightly closer distance leads to a runaway greenhouse effect.

What happened on Venus has implications for habitability of other planets.

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

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