Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 2:30

Lecture 34:

Venus Unveiled

Key Ideas:

• Nearly the same size as the Earth
• Covered with opaque clouds
• Slow retrograde rotation

Atmosphere:

• 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 R_Earth)

Mass: M = 0.815 M_Earth

Rotation: 243.02 days Retrograde!

Axis Tilt: 177.4°, Upside down!

Spacecraft Visits

Flybys:

Mariner & Pioneer satellites (1962)

Landers:

Venera 7 (1970 - USSR) - first soft landing

Atmospheric Probes:

Pioneer Venus (US: 1978)

Vega 1 & 2 balloon probes (USSR: 1985)

Orbiters:

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 atmosphere.

Veiled Venus

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

• Rotation Period is 243 Days
• This is unusually 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.

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 ocean at a depth of ~1 km

Surface Temperature: uniform 750 K (891º F)

Sulfuric Acid Clouds

• 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 cloud deck.

• Get optical distortion of the horizon from the hot, heavy atmosphere.

Runaway Greenhouse Effect

• Hot, heavy CO₂ atmosphere, 100x heavier than the Earth.
• 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.

Venus Unveiled

• First global map was made in 1978 by the Pioneer Venus orbiter
• 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:

• Ishtar Terra
• Aphrodite Terra

Also identified impact craters, volcanoes, and other geological features.

Volcanism & Geologic Activity

• 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.

Impact Craters

• 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 Myr ago.

Venus & Earth

• Earth: on-going process
• Venus: most repaving occurred ~500 Myr ago

Tectonic activity is different:

• Earth: lateral recycling from sliding motions of tectonic plates.
• Venus: vertical recycling via upwelling & downwelling.

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:

• 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 crustal rocks.