Lecture 12: Venus Atmosphere and Surface

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Lecture 12 Venus Atmosphere and Surface
Meteo 466
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Venus Physical data

• Sidereal rotation period -243.01 Earth days
  (retrograde)
• Orbital period 224.7 Earth days
• Venus day 116.75 Earth days
• Synodic period 583.92 Earth days ( 5.001 times
  Venus daylength!)
• This means that Venus always shows the same side
  to Earth when the planets are in conjunction
• Is Venus rotation in some kind of resonance with
  Earth?

Data from Wikkipedia
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Venus as seen by Magellan

• Image made using
• synthetic aperture
• radar (SAR)

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Venus topography (from Magellan radar altimeter)

• 60 of surface within 500 m of mean elevation
• Highest elevation (Maxwell Montes) 12 km above
  mean

Beatty and Chaikin, The New Solar System, Ed. 4 ,
Fig. 8.9 (1999)
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Earth as seen from Seasat(radar altimetry)

• Seafloor (70 of surface) 4 km below sea
  level
• Continents (30 of surface) 1 km above sea
  level (average)

Beatty Chaikin, The New Solar System, ed. 2, p.
71
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Sapas Mons (from Magellan)

• A large shield volcano, vertically exaggerated
• Combination of radar altimetry and reflectivity

Beatty and Chaikin, The New Solar System, Ed. 4,
Fig. 8.13 (1999)
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Venus impact craters

• All observed craters are gt3 km in diameter
• Why do you think this is true?

• Howe (center) 37 km diameter
• Danilova (upper left) 48 km
• Aglaonice (upper right) 63 km

Beatty and Chaikin, The New Solar System, Ed. 4,
Fig. 8.13 (1999)
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Equal-area projection showing 842 impact craters
Simple cylindrical projection

• Furthermore, impact craters
• are randomly distributed over
• Venus surface
• What does this imply?

G.G. Schaber et al., JGR 97, 13257 (1992)
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VenusNo plate tectonics!

• Age of entire surface is 0.5-1 b.y
• Episodic cycle of volcanism
• Surface is static for long time periods
• Heat from radioactive decay builds up in Venus
  interior
• Widespread melting and volcanism removes the heat
  and resurfaces the planet
• Then, the cycle repeats..

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Venus temperature profile
Prinn and Fegley, Ann. Rev. Earth Planet. Sci.
(1987)
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Venus cloud particles
70
np
?ext
ml
Altitude (km)
45
0
Number density, ?ext, mass loading
Knollenberg and Hunten, Science (1979)
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Retrograde zonal wind velocity vs. altitude
Prinn and Fegley, Ann. Rev. Earth Planet. Sci.
(1987)
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Venus Lower atmosphere composition
SO3
H2SO4
OCS
H2O
CO
Mills, Esposito, and Yung, in Exploring Venus as
a Terrestrial Planet, Geophysical Monograph
Series 176, AGU (2007)
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Sulfur and chlorine photochemistry

• Sulfur and chlorine are both orders of magnitude
  more abundant in Venus atmosphere than in
  Earths
• Venus Earth
• SO2 150 ppm 0.1 ppb
• Total Cl 0.5 ppm 50 ppt (natural)
• 1 ppb (perturbed)

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Sulfur photochemistry in Earths atmosphere
Prinn and Fegley, Ann. Rev. Earth Planet. Sci.
(1987)
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Sulfur photochemistry in Venus atmosphere
Prinn and Fegley, Ann. Rev. Earth Planet. Sci.
(1987)
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Sulfur photochemistry

• SO2 is photolyzed above the cloud tops
• SO2 h? ? SO O
• This can be followed by
• SO SO ? SO2 S
• Then, get sulfur chain formation
• S S M ? S2 M
• S S2 M ? S3 M
• S2 S2 M ? S4 M
• S4 S4 M ? S8 M
• Sulfur allotropes (Sn) and sulfanes (HSn) are the
  most probable UV absorbers in the Venus clouds

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Atmosphere-surface interactions

• It has been proposed (Bullock and Grinspoon, JGR
  101, 7521, 1996) that CO2 and SO2 are in
  thermodynamic equilibrium with the surface
• CaCO3 SiO2 ? CaSiO3 CO2
• calcite quartz wollastonite
• SO2 CaCO3 ? CaSO4 CO
• anhydrite
• 3 FeS2 2 CO 4 CO2 ? 6 COS Fe3O4
• pyrite magnetite

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Atmosphere-surface interactions

• If this is correct, then the present Venus
  atmosphere is
• unstable
• -- Higher Ts leads to higher pCO2, which in
  turn leads to higher Ts
• (positive feedback loop
• -- Lower Ts leads to lower pCO2, which leads
  to still lower Ts

Bullock and Grinspoon (1996)
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Atmosphere-surface interactions
Partial pressure
Surface temperature

• According to Bullock and Grinspoon, CO2, SO2 and
  surface
• temperature should be steadily declining

Bullock and Grinspoon (1996)
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Atmosphere-surface interactions

• 90 bars of CO2 is equivalent to a layer of
  carbonate rock
• 1 km thick
• Requires all the Ca from 10 km of Venus surface

Bullock and Grinspoon (1996)
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Atmosphere-surface interactions (cont.)

• Is this consistent with what we see?
• Probably not More likely, atmosphere-surface
  interactions are slow ? Venus atmosphere just
  serves as a big collector for CO2
• SO2 interactions with the surface seem more
  probable, but there is a problem
• SO2 CaCO3 ? CaSO4 CO
• quartz anhydrite
• Equilibrium pSO2 for this reaction is only about
  1 of observed pSO2 (1.5?10-2 bar)

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Pyrite buffered SO2

• This gives a predicted pSO2 that is close to
  that
• observed
• 150 ppm of SO2 is equivalent to a layer of
  pyrite
• only 5 cm thick
• This might also explain the high radar
  reflectivity
• of the tops of Venus mountains

Hashimoto and Abe, Planet. Sp. Sci. 53, 839848
(2005)