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11b. Cloud-Covered Venus
• The Venusian atmosphere
• Venus has slow retrograde rotation
• Venus has a hot dense atmosphere
• Volcanic eruptions form Venusian clouds
• Climatic evolution on Venus
• Venus shows no evidence of plate tectonics
Venus Data (Table 11-2)
Mercury-Venus-Mars Relative Sizes
Venus Data: Numbers
• Diameter: 12,104. km
0.949 . Earth
• Mass:
4.9 . 1024 kg
0.815 . Earth
• Density:
5.24 . water
0.953 . Earth
• Orbit:
1.1 . 108 km
0.72
• Day:
• Year:
AU
– 243.01 days
– 243.01
.
Earth
224.70 days
0.62
.
Earth
Venus Data: Special Features
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•
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Venus is the second planet from the Sun
Venus is the second largest terrestrial planet
Venus has many active volcanoes
Venus is almost a twin of the Earth except …
– Venus has ~ 93 times Earth’s atmosphere
– Venus’ atmosphere is ~ 96% CO2
– Venus is perpetually cloud covered
– Venus’ average surface temperature is ~ 480°C
• Venus’ surface can be “seen” only with radar
• Venus is very easy to observe from Earth
– Venus is seen as much as 47° away from the Sun
– Venus goes through phases much like the Moon
Venus Phases & Angular Diameters
http://www.spacestationinfo.com/images/venus-phase1.gif
Relative Sizes of Terrestrial Planets
http://upload.wikimedia.org/wikipedia/commons/d/dd/4_Terrestrial_Planets_Size_Comp_True_Color.png
Venus As Seen From Earth
• Venus is outshone only by the Sun & Moon
– Venus is very close to the Sun
• Venus is ~ 0.7 AU from the Sun
– Venus is very close to the Earth
• Venus is ~ 0.3 AU from the Earth at inferior conjunction
– Venus is very large
• Venus is ~ 95% the diameter of Earth
– Venus has an albedo of ~ 0.59
• Venus is perpetually cloud-covered
• Venus has large elongations
– The Venusian orbit is nearly circular
• Greatest eastern elongation is ~ 47°
• Greatest western elongation is ~ 47°
Evening
Morning
Orbits of Venus & Earth
Venus’s Greatest Elongations
Eastern (Evening)
Western (Morning)
27 March 2012
15 August 2012
1 November 2013
22 March 2014
6 June 2015
26 October 2015
12 January 2017
3 June 2017
17 August 2018
6 January 2019
Venus Elongation Explorer
Venus’s Atmosphere: A First Look
• Venus is perpetually cloud-covered
– This makes Venus extremely bright
– Cloud
details are best seen with ultraviolet l’s
– Surface details are only seen with
radar
l’s
• Earth-based imaging systems
• Magellan
orbital mission
• The Venusian atmosphere is extremely dense
– About 93 times more than Earth
Venus Seen In Ultraviolet Light
Venus’s Slow Retrograde Rotation
• Observational difficulties
– Perpetual cloud cover obscures the surface
• Only seen in
radar
l’s
– Clouds encircle the planet in ~ 4 days
• Best seen in ultraviolet l’s
• Successful observations
– Doppler shift analyses in the early 1960’s
• Transmit one precise l
• Receive a slightly spread out range of l’s
– One edge of Venus is moving toward Earth
– One edge of Venus is moving away from Earth
– Results
• Venus rotates on its axis in a retrograde direction
– Uranus & Pluto also exhibit retrograde axial rotation
• Venus’s day is ~ 243 Earth days long
Prograde & Retrograde Rotation
Venus’s a Hot, Dense Atmosphere
• Insolation
[Incoming solar radiation]
– Venus averages ~ 0.72 AU from the Sun
• 1 / 0.722 = 1 / 0.52 = ~ 1.93 > sunlight than Earth
– Venus would be hotter even w/Earth’s atmosphere
• Venusian environment
– Intense sunlight evaporated Venus’s oceans
– Volcanic gases directly enter Venus’s atmosphere
• Most of Earth’s volcanic gases dissolve in ocean water
– CO2 is extremely common in volcanic eruptions
• CO2 is an excellent absorber of infrared [heat] radiation
• An important comparison
– Venus: 96.5% CO2 increases temperature ~ 400°C
– Earth: 0.04% CO2 increases temperature ~ 36°C
Volcanic Eruptions Produce Clouds
• Atmospheric sulfur compounds
– Fractional amounts
• Venus
• Earth
~1.5 . 10–2 of all atmospheric gases
~1.0 . 10–9 of all atmospheric gases
– Venus has ~ 93 times more atmosphere than Earth
• Venus’s air has ~ 1.35 109 x more sulfur than Earth’s air
• Probable cause
– Like CO2, sulfur is common in volcanic eruptions
• No oceans to absorb this sulfur
• Instances of increased Venusian sulfur levels
– Late 1950’s
– Late 1970’s
Earth-based observations
Pioneer Venus Orbiter
Venusian Cloud Layers
Venera 13 Images Venus’s Surface
Venusian rocks appear orange because of cloud colors.
The same picture corrected to remove atmospheric colors.
1 March 1982
Venusian Atmospheric Circulation
Venusian Climatic Evolution
• Proto-atmospheres
– Venus & Earth were probably remarkably similar
• Countless volcanic eruptions provided H2O, CO2 & SO2
• Proto-Sun
– Infant Sun produced only ~70% of today’s energy
• All stars gradually increase their energy output
• Climatic evolution
– Infant Venus was cool enough to have liquid water
• Single-celled life forms may have evolved on Venus
– Juvenile Venus became too hot to have oceans
• The same fate faces Earth in ~ 1 billion years
Venus Shows No Plate Tectonics
• Expected signs
– Globe-circling volcanic mountain chains
– Extensive sets of transform faults
– Extremely long subduction trenches
• Observed signs
– No elongated volcanic mountain chains
• Substantial evidence of hot-spot volcanoes
– No confirmed transform faults
– No confirmed subduction trenches
• Probable cause
– No oceans to affect subduction zone activity
• Venusian lithosphere is too hot & soft to sustain forces
• Subducted water promotes lower temperature melting
• “Flake [Blob] tectonics”
Pancake domes
Mantle Convection: Earth & Venus
Volcanoes On Venus & Earth
Venus
0% oceans
Gas enters atmosphere
High CO2 & SO2 concentrations
Yellow sky
Earth
~ 70% oceans
Gas absorbed by oceans
Low CO2 & SO2 concentrations
Blue sky
Venus: A Mercator Projection
Venus: A Global Perspective
Venus: A 3-D Elevation Model
Magellan: Venus False-Color Terrain [HD]
Venus Fly-Over
Venus Fly-Over
Volcanic Activity On Venus & Earth
Aine Corona With Pancake Domes
Solar System’s Longest Channel
Theia Mons (Earth Radar Image)
Maat Mons (Vertical Exag. = 22.5)
Spacecraft Exploration of Venus
• Russia
– Venera missions
• 10 of 16 spacecraft successfully arrived at Venus
• Venera 4 entered the atmosphere on 18 October 1967
• Venera 7 soft-landed on 15 December 1970
• Venera 12 operated ≥110 minutes on 21 December 1978
• United States
– Voyager missions
• Primarily orbiters with low-resolution radar images
– Magellan mission
• High-resolution radar images of almost the entire surface
Important Concepts
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Venus as seen from Earth
•
– Very bright & excellent elongations
– Distinct phases much like the Moon
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•
Axial rotation
– Retrograde, once in ~243 Earth days
– Uranus & Pluto also retrograde
•
– None of the classic evidence
– Absence of oceans probably the cause
– Evidence of “blob” tectonics
Perpetual cloud cover
– Obscures the Venusian surface
– Encircles the planet in only ~4 days
– Radar needed to penetrate clouds
The Venusian atmosphere
– Basic properties
• Dominance of CO2 & SO2
• High temperature & pressure
• Apparent lack of liquid water
– Evolution
• Initially much like Earth’s atmosphere
• Solar radiation increased ~30%
No plate tectonics on Venus
• Abundant pancake domes
•
Spacecraft exploration of Venus
– Russia
– United States