An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30
Venus is the second planet from the Sun
- Nearly the same size as the Earth
- Covered with opaque clouds
- Slow retrograde rotation
- Hot, heavy CO2 atmosphere
- Runaway Greenhouse effect
Surface: mapped using Radar
- Rolling plains, highlands & valleys
- Unique terrain features
Venus at a Glance
- Orbital Data
- Semi-Major Axis: a = 0.723 AU
- Orbital Period: 224.7 days
- Eccentricity: e = 0.007 (Most circular orbit of the planets)
- Inclination: i = 3.39°
- Planetary Data
- Radius: R = 6052 km (0.949 REarth)
- Mass: M = 0.815 MEarth
- Rotation: 243.02 days Retrograde!
- Axis Tilt: 177.4°, Upside down!
- Mariner & Pioneer satellites (1962)
- Venera 7 (1970 - USSR) - first soft landing
- Atmospheric Probes:
- Pioneer Venus (US: 1978)
- Vega 1 & 2 balloon probes (USSR: 1985)
- Pioneer Venus Orbiter (US: 1978)
- Venera 15 & 16 (USSR: 1983)
- Magellan (US: 1990-1994) - most detailed radar mapping
- Mars Express (ESA: 2005-present)
The ESA Venus
Express spacecraft, is currently in orbit around Venus studying its
Venus is completely covered by a thick planet-wide layer of opaque
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!
Venus has a slow, retrograde (east-to-west) rotation.
- Rotation Period is 243 Days
- This is unusually Slow
- 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.
- 96% Carbon Dioxide (CO2)
- 3.5% N2
- 0.15% Sulfur Dioxide (SO2)
- <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)
Sulfuric Acid Clouds
The clouds of Venus are not water vapor clouds like on Earth:
- Consist mostly of droplets of Sulfuric Acid (H2SO4).
- Forms a thick layer between 48 and 58 km altitude.
Lower atmosphere and surface are clear below the H2SO4
- Get optical distortion of the horizon from the hot, heavy atmosphere.
Runaway Greenhouse Effect
Venus is so hot (750K) because of a runaway greenhouse effect:
The result is that Venus is extremely dry today with very little
- Hot, heavy CO2 atmosphere, 100x heavier than
- Heat trapping make Venus about +500K hotter than it would
be with no atmosphere.
- Water stays in vapor form, and is eventually broken into
H2 and O by UV photons.
- H2 escapes into space and is lost.
The surface of Venus has been mapped using imaging radar.
- First global map was made in 1978 by the Pioneer Venus
- Venera 15 & 16 radar imagers (low resolution)
Highest Resolution Map:
- Magellan Venus (1990-1994)
- 98% of the surface at 120-300 meter resolution
- Good enough to do geological assessment
The Surface of Venus
- ~85% rolling planes
- ~15% highland plateaus & mountain belts
Highlands are concentrated into two regions:
These are not ancient highlands like the Moon & Mercury.
- Ishtar Terra
- Aphrodite Terra
Also identified impact craters, volcanoes, and other geological
Volcanism & Geologic Activity
Volcanoes are a common terrain feature:
- None in chains, suggesting no plate tectonics
- Pancake domes & coronae 100-200 km across
- Are some volcanoes active today??
Tectonics, but not plate tectonics:
- High temperatures makes the crustal rock soft.
- Upwelling material from the mantle.
- Downwelling causing compression.
Only ~1000 Impact craters are seen on Venus:
- Randomly scattered around the surface.
- None <3km across (meaning no impacts by meteors <30m across,
smaller meteors probably burn up in the heavy atmosphere before the
reach the ground)
- ~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
Venus & Earth
Volcanic & tectonic repaving:
- Earth: on-going process
- Venus: most repaving occurred ~500 Myr ago
Tectonic activity is different:
- Earth: lateral recycling from sliding motions of
- Venus: vertical recycling via upwelling &
This difference may be related to lack of water in Venus' crust,
which makes it more pliable than the Earth's crust.
Atmospheres reflect differing histories:
Why the differences, given that they had the same starting point
(volcanic outgassing of CO2 & H2O)?
- Earth: Warm, light, moist N2 & O2
atmosphere with liquid of water.
- Venus: Hot, heavy, dry CO2 atmosphere, and no
water in any form.
- Venus experienced a runaway greenhouse effect due to greater solar
heating than the Earth.
- This drove away all of its water, so there were no oceans in which
CO2 chemistry could occur to lock up CO2 into
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Updated: 2007 November 4
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