Lecture 1: Venus

A. P. Ingersoll

api@gps.caltech.edu

1. Three topics: (1) An ocean of air: The average pressure at the surface of Venus is92 bars. The Earth’s oceans spread across the planet would be ~300 bars. What
does this mean for the meteorology? (2) The runaway greenhouse – why is there so
little water on Venus today? (3) The super‐rotation – At the tropopause, theatmosphere spins 50 times faster than the solid planet. How is this maintained?
2. Basic facts about the Earth and Venus. We infer the size of the core from the bulkdensity. Other quantities have been measured. The similarities have lead to calling
Venus the Earth’s sister planet, but it would be a hellish place to live. [A. Ingersoll,Nature News and Views 450, 617‐618 (2007)]
3. The atmosphere is 96.5% CO2 and 3.5% N2 , with small amounts of Ar, CO, H2O,and O2. The total CO2 on Venus is only slightly greater than the CO2 locked up inlimestone on Earth. The total N2 on Venus is only slightly greater than the total N2on Earth. Water is totally different, suggesting it was lost. [Planetary Sciences, I. dePater and J Lissauer (2001)]
4. The clouds are made of sulfuric acid droplets about r = 1 micron in size. Theatmosphere and planet rotate opposite to the orbital motion ‐ clockwise as viewed
from above the solar system, which is opposite to the Earth. [NASA photo]
5. The surface is hot and dry, with very little evidence of erosion. [NASA]
6. Temperature mostly follows a dry adiabat, although there is a stable layer in the40‐50 km range. The sulfuric acid clouds block the escaping IR radiation, soconvection has to carry the heat through the clouds. This creates an adiabatic layerfrom 50‐60 km. Radiation to space occurs at ~65 km altitude. [Seiff, in Venus,
Hunten etal, Ed., U Arizona Press (1983)]
7. The clouds are near the 1 bar level on both planets. The atmosphere below theclouds is clear. Some sunlight reaches the ground. On Earth, no sunlight reaches thebottom of the oceans, which is a major difference in the dynamics. [A. Ingersoll,
Nature News and Views 450, 617‐618 (2007)]
8. Basic meteorology of Earth. Excess absorbed sunlight at the equator and excess
emitted longwave radiation at the poles requires poleward heat transport by theocean and atmosphere. The transfer of energy is not complete, because the poles arestill much colder than the equator. The emitted longwave is less at the poles because
they are colder.
9. The ocean and atmosphere play almost equal roles. The atmosphere is more
important at high latitudes.

10. The emitted longwave is almost flat at Venus, indicating that the poles arealmost as warm as the equator. The deep atmosphere of Venus is more effective
than the deep oceans of Earth because the deep atmosphere is stirred by convection,
since some sunlight reaches the surface. [F. W. Taylor in Venus, U Arizona Press(1983)]
11. Here is a one‐parameter model of the runaway greenhouse: The tropopause
temperature and the emitted longwave radiation increase together when theatmosphere is cold (left side of the curve). Then there is a maximum in the emitted
longwave flux, because the denominator increases faster than the numerator.
Further increase in the tropopause temperature puts more water vapor into thestratosphere, which blocks the emitted longwave radiation, and the flux decreases.
K is the absorption coefficient of water vapor and µ is the relative humidity. TheEarth emits about 240 W m‐2. If you moved Earth to the orbit of Venus it would haveto emit twice that amount, and it can’t do it as long as µ is close to 1.0. You have toboil the oceans to get µ to values less than 1.0. [Nakajima etal, J. Atmos. Sci, 49,
2256‐2266 (1992)]
12. Here’s a better theory: It’s not just the stratosphere that matters. If thetroposphere can’t deliver the IR radiation to the base of the stratosphere, then themaximum flux to space is even smaller. One estimate is 293 W m‐2 which is very
close to the Earth’s present value of 240 W m‐2 . [Nakajima etal]
13. Here’s an even better theory, taking into account all the non‐grey radiativetransfer and the moist and dry adiabatic lapse rates. The transition is at 293 W m‐2 .
Above this point, the temperature at the surface goes up to 1600 K and the pressure
goes up to greater than 100 bar. [Abe and Matsui, J. Atmos. Sci, 45, 3081‐3101(1988)]
14. The height of the tropopause goes up to 300 km, and water vapor is the main
constituent of the atmosphere. The tropopause is not much warmer than the current
tropopause of Earth, but the pressure of other gases like N2 is much less at thetropopause. These authors were thinking of early Earth when the solar constant S0
was less and there was an additional heat flux F0 due to impacts.
15. These are moist adiabats. The current Earth has water vapor/nitrogen ~ 10‐2 atthe surface and 10‐6 at the tropopause (lower curve). A water vapor atmosphere haswater vapor/nitrogen greater than 1.0 at all levels (upper curve). This is important,because nitrogen and oxygen act as a radiation shield that protects the water vapor
from ultraviolet light. Without this radiation shield, in a water vapor atmosphere,
the water will be turned into H and O. The H will escape into space, and the oxygen
will combine with C and Fe in the surface rocks. Pretty soon all the water will begone. [Ingersoll, J Atmos. Sci 26, 1191‐1198 (1969)]

16. Evidence that this might have happened is found in the heavy isotope ofhydrogen – deuterium. Venus is off scale. The lighter H escapes faster, leaving the Dbehind, and the D/H ratio increases with time. The current D/H is 150 times theEarth’s value, which is higher than anything else in the solar system. [de Pater andLissauer, Planetary Sciences, Cambridge University Press, 2001]
17. Venus Express found that the D/H is 300 times the Earth’s value in the upper
atmosphere, indicating that this fractionation process is operating today. [Bertaux,Nature 450, 646‐649 (2007) ]18. Here’s another picture of the sulfuric acid clouds. Now we talk about thecirculation of the atmosphere.
19. The wind profile. Remember they are blowing to the west. Let’s use the words
prograde and retrograde for winds blowing in the same or opposite to the planet’s
rotation. The Earth’s tropospheric jet streams have a speed of 40 m s‐1, which is also
the speed of a typhoon. The jet streams on Earth are prograde. [Schubert, in Venus,
Hunten, Ed., University of Arzona Press, 1983]
20. There is some confusion about east and west. The winds are prograde; they blowin the same direction as the spin of the planet, which is opposite to the spin of theEarth. These are winds at cloud top level, measured by tracking clouds in series ofimages. The meridional wind at cloud top level is from the equator to the pole inboth hemispheres. [Gierasch etal, in Venus II, Bougher etal, Eds. U Arizona, 1997]
21. Buoyancy frequency (Brunt frequency) squared = N2 . Notice the stable region
from 40‐50 km. Neutral stability (convection) above and below. [Gierasch etal]
22. This model reproduces the zonal wind with maximum speed of 100 m s‐1.
[Yamamoto and Takahashi, J. Atmos. Sci 60, 561‐574 (2003)]
23. Rising motion at the equator, sinking motion poleward of 35 degrees. Poleward
flow (not shown) agrees with cloud tracked meridional winds. This mean
meridional circulation (MMC) brings air with high angular momentum (AM) per unit
mass up at the equator and sends air with low AM per unit mass down at the poles.
[Figs. 22‐26 are from Yamamoto and Takahashi]
24. Since the air is moving poleward in each hemisphere, it should conserve angularmomentum, but it doesn’t. Instead, it loses angular momentum to the eddies, whichtransport it toward the equator. You can see this in the tilt of the velocity vectors.
Air moving toward the equator (left side) carries eastward momentum, and air
moving away from the equator carries westward momentum. The net effect is toadd eastward momentum to the equator and remove it from higher latitudes.
This is the Gierash mechanism for maintaining the super‐rotation. The mean
meridional circulation brings it up from below and the eddies bring it in from thehigher latitudes.

25. This shows the same thing. The eddies make u’v’ < 0 in the north and u’v’ > 0 inthe south.
26. This shows u’w’ < 0, which means the vertical transport of AM is in the wrong
direction to maintain the zonal flow. The u’w’ term is downward, but not as large as
the upward transport by the MMC.
27. An alternate view. These are tides driven by absorption of sunlight in the clouds ,60‐65 km altitude. The slope of the phase lines means the tidal wave is has phasevelocity upward and to the left (group velocity is downward and to the left). The jetis to the right (prograde) and the sun is moving to the left (retrograde). This means
the tides are accelerating the flow in the clouds, which is the peak of the jet andhelping to maintain the jet speed. [Tagaki and Matsuda, Geophys. Res. Lett. 33,
L13102 (2006)]
28. The magnitude of the acceleration by this method is comparable to that by theGierasch mechanism.
29. A question: Is there something analogous to the Quasi Biennial Oscillation(QBO)? Notice the descending pattern of easterlies and westerlies with a ~26 monthperiod. Figure is from [Holton, Introduction to Dynamics Meteorology, 2004]
30. Cartoon of a theory of the QBO. Both easterly and westerly waves (± c)propagate up from the troposphere. The easterly waves (c <0) carry easterly
momentum, and the westerly waves carry westerly momentum. Both waves dump
their momentum when they propagate into a region that is moving as fast as thewave ‐ a critical level. This brings down the easterly parts and the westerly parts ofthe zonal jets. The pattern descends. The 26 month period depends on how much
momentum is carried by the waves. If the waves are energetic, the period is shorter.
[G. Holton, Introduction to Dynamic Meteorology (2004)]
31. Phase diagram for Rossby waves, Rossby‐gravity waves, and pure gravity waves.
They are all trapped at the equator. The Kelvin wave moves the fastest, but it can
only propagate to the east. The RGW and the KW carry westward and eastward
momentum, respectively. [A Gill, Atmosphere‐Ocean Dynamics, Academic Press,
1983]
32. One cycle of a Kelvin wave. The solid lines are pressure. The arrows are the windvectors. Figures 32‐35 are from [Holton]
33. Same for the RGW [Holton]
34. It’s the tilt of the phase lines that matters. The eastward tilt with height means
the Kelvin wave carries eastward momentum upward.

35. The RGW tilts to the west. Here it is not the tilt, but it is the northward transportof heat, which tends to accelerate the jet.