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 12-1)

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 AU

• Day: – 243.01 days – 243.01 . Earth

• Year: 224.70 days 0.62 . Earth

Venus Data: Special Features• 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°Evening

• Greatest western elongation is ~ 47°Morning

Orbits of Venus & Earth

Venus’s Greatest Elongations

Eastern (Evening) Western (Morning)

1 November 2013 22 March 2014

6 June 2015 26 October 2015

12 January 2017 3 June 2017

17 August 2018 6 January 2019

24 March 2020 13 August 2020

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• Surface can be seen only using radar l’s

– Clouds encircle the planet in ~ 4 days• Best seen in UV 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 ~1.5 . 10–2 of all atmospheric

gases• Earth ~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 Earth-based observations– Late 1970’s 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

Volcanoes On Venus & Earth

VenusEarth0% oceans ~ 70% oceansGas enters atmosphere Gas absorbed by oceans

High CO2 & SO2 concentrations Low CO2 & SO2 concentrationsYellow sky Blue sky

Venus: A Mercator Projection

Venus: A Global Perspective

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

• Venus as seen from Earth– Very bright & excellent elongations– Distinct phases much like the Moon

• Perpetual cloud cover– Obscures the Venusian surface– Encircles the planet in only ~4 days– Radar needed to penetrate clouds

• Axial rotation– Retrograde, once in ~243 Earth days– Uranus & Pluto also retrograde

• 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– None of the classic evidence– Absence of oceans probably the cause– Evidence of “blob” tectonics

• Abundant pancake domes

• Spacecraft exploration of Venus– Russia– United States

Important Concepts