Venus
Earth's Sister Planet

April 11, 2011, jeb

Outline Observing Venus from Earth General Properties Exploration of Venus Atmosphere Evolution of Venus' Atmosphere Venus' Surface Volcanism Today Venus' Magnetic Field and Internal Structure Venus' Basic Properties Venus' Golden Globe Awards Other sources on the Venus

Observing Venus from Earth

• Greatest elongation
  • Since Venus' orbit about the Sun lies within Earth's, it never separates from the Sun by more than 47^o, as viewed from the Earth
  
  
• appears above the horizon
  • before sunrise and after sunset.
  
  
• Extremely bright
  • (10 x brightest star) (reflectivity = 70%)
  • 3rd brightest object in the sky, next to the Sun and the Moon
  
  
• Very dense atmosphere and thick (opaque to visible light) clouds
  • From Earth, even with powerful telescopes, Venus appears featureless

Venus - General Properties

• 2nd Planet from the Sun
• semi-major axis ("radius") = 0.72 AU
  • period = 225 days
• Radius = 6052 km = 0.95 Rearth
• Mass = 4.9 x 10²⁴ kg = 0.82 M_Earth
• Density = 5240 kg/m³
  • Radius, mass, & density are Earth-like or "sister planet"
• Rotational Period = -243 days ( retrograde)
  • Solar day on Venus = 117 Earth days
  • By convention of astronomers, planets rotate from west to east
    Therefore, north latitudes lie below the ecliptic plan
  • Why retrograde -> collision?
• Atmosphere
  • very dense => high altitude clouds of sulfuric acid block surface
• 90x as massive as the Earth's atmosphere
  • 96.5% CO₂ (by volume)
  • few % Nitrogen (very little water)
• surface temperature = 730K (T_Earth=300K, T_Mercury=100-700K)
• Greenhouse Effect
  • temperature uniformity - poles, equator, day, night

Exploration of Venus

More than 20 spacecraft have visited, more than any other planet
• American Mariner 2 and Mariner 5 passed within 35,000 km in 1962 and 1967
• Mariner 10 grazed Venus at 6000 km in 1974
  • gravity assist on way to Mercury
• Soviet Venera 4 through 12 parachuted into the atmosphere between 1967 and 1978
  • Venera 7 was the first spacecraft to soft-land on Venus, in 1970
  • survived for 23 minutes of radio transmissions back to Earth
• Venera 15 and 16 orbiters send back detailed radar maps of the northern hemisphere in 1983
• Pioneer Venus - 1978
  • orbiter at 150 km
  • sent multiprobe into atmosphere resulting in detailed information
  • produced radar images of most of the surface
• Magellan - August 1990
  • produced spectacularly detailed surface images
  • resolution of about 120 m
• Messenger passed by Venus twice (2006 & 2007) on its way to Mercury
  • visible and near-infrared imaging data of the upper atmosphere of Venus
  • Ultraviolet and X-ray spectrometry of the upper atmosphere to characterize the composition
• Venus Express
  • European Space Agency mission
  • orbiting Venus since 2006
  • evidence for past oceans
  • confirms the presence of lightning on Venus, more common on Venus than on Earth
  • discovery that a huge double atmospheric vortex at the south pole
  • one of the most significant findings was signs Venus has been volcanically active in the last three million years -- suggesting the planet may still be geologically active

The Atmosphere of Venus

• Why is the atmosphere of Venus (Earth's sister) so different from Earth?
  • On Earth, CO₂ is absorbed in the oceans and rocks.
    • If the absorbed CO₂ on Earth were released into the atmosphere, 98% of the Earth's atmosphere would be CO₂
      and the atmospheric pressure would be 70 times what it is now.
  • -so, except for the O₂ and water, Earth's atmosphere would be similar to Venus' if the CO₂ has not gotten absorbed.
  • The oxygen on Earth is a product of life
  • The water on Venus has disappeared due to the extreme temperatures brought on by the runaway greenhouse effect
    • water vapor rose to high elevations and was broken down into oxygen and hydrogen by ultraviolet radiation
    • hydrogen, being light, escaped from Venus
    • oxygen combined with other atmospheric gases
    • water on Venus was lost forever

Summary of Atmospheric Components

Evolution of Venus' Atmosphere

• During secondary atmosphere formation temperature of Venus high
  • no oceans form (Water is broken down and H escapes) and gases are not absorbed
  • gases in atmosphere trap heat (greenhouse effect)
  • temperature rises even more
  • runaway greenhouse effect

Venus' Surface

• A couple of continental sized features shown on radar map of Venus
  • another version of radar map of Venus
  • similar radar map of Earth
  • Ishtar Terra (southern high latitudes)
    • Note - since Venus has retrograde rotation, south means near the top of our images
    • Lakshmi Planum - a plateau in Ishtar Terra
  • Aphrodite Terra (near the equator)
    • Ovda Regio - shows buckling surface and lava flooded regions
• Earth-like mountains
• No oceans
  • rolling plains (65%)
  • lowlands (27%)
  • modest highlands (8%) (continents)
• No plate tectonic activity
• Lava flows -> volcanic origin
• Craters - mostly of volcanic origin
  • Most common volcanoes are shield volcanoes
    • Two larger volcanoes are Sif Mons and Gula Mons
    • built up over long periods of time by succession of eruptions and lava flows
      • like Hawaiian Islands
    • characteristic caldera at summit
  • Lava domes were formed when lava oozed out of the surface, formed domes, and then withdrew
  • Largest volcanoes are the circular regions known as coronae
    • Aine, a corona south of Aphrodite Terra, 300 km across
    
    - some craters were formed by meteoritic impact
    
    
• The surface of Venus is rather young (less than 1 billion years) due to volcanic activity
  • Soviet Venera spacecraft have landed on Venus and sent back images
  • these probes can only operate for about one hour in the high temperatures on the surface
    • sharp edges on rocks imply they are young, and that there is ongoing surface activity

Volcanism today?

1. SO₂ levels above the thick cloud cover fluctuates frequently.
   • result from volcanic eruptions? don't know
2. Probes have observed suggestive radio emissions.
   • similar effects are initiated on Earth by erupting volcanoes
     • lightning discharges

Venus' Magnetic Field and Internal Structure

• Magnetic field - none detectable
  • this is consistent with slow rotation period of 243 days
• Model of internal structure is based on little hard data
  • Surface resembles surface of young Earth (at an age of about 1 billion years)
    • thin crust, volcanic activity,
    • internal convective processes not yet established
    • not yet developed plate tectonics
  • Why no development of plates?
    • Cooling slowly due to high surface temperature
      • crust remains too soft for Earth-style plates to develop
    • absence of water may also be important
      • no lubricating mechanism

Planet order from Sun Orbital period Semi-major axis Mass Radius Density Surface Gravity Escape speed Rotational Period Inclination Surface Magnetic Field Surface Temperature

2 0.62 years (225 Earth days) 0.72 AU 0.82 x Earth 0.95 x Earth 5240 kg/m3 0.91 x Earth 10.4 km/s -243 days 177.4° less than 0.001 x Earth 730 K

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Golden Globe Awards for Venus

Brightest Planet as viewed from the Earth Densest atmosphere of the Terrestrial planets Warmest surface temperature of the Terrestrial planets Slowest rotating planet Rotates backwards (compared to the trend of the Solar System)

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Other Sources on Venus

StarDate Guide to Venus
Welcome to the Planets - Venus
Views of the Solar System - Venus
The Nine Planets - Venus