Venus Atmosphere Puzzle, one man's struggle with atmospheric physics Options

[qraal]

Hi All

This might seem like a really dumb question, but what's the mass of the Cytherean atmosphere per unit area?

At first pass I thought it was easy - same as for an isothermal atmosphere, Po/g, where Po is surface pressure and g is surface gravity. Simple. Except Venus doesn't come close to approximating an isothermal atmosphere. From a graph in Mark Bullock's PhD thesis (Hi Mark if you're visiting) I pulled the figures for Po and To as 92 bar and 735 K, while the left-side of the temperature curve was 250 K at 0.1 bar and 63 km. At about 210 K the temperature drop with altitude stops, then slowly rises into the Cytherean stratosphere.

Ok. My atmospheric physics is pretty limited - I 'modelled' that lapse rate pressure curve as a power law:

P/Po = (T/To)^n

and likewise for density, d/do = (T/To)^n.

Temperature, T, as a function of altitude, Z, I computed as T(Z) = To*(1-Z/(n.Zo)).

Zo = (k.T/m.g), where k is Boltzmann's constant and m is the molecular mass of the atmosphere.

These equations I then integrated between 210 K and 0.033 bar, 70 km, and 735 K and 92 bar, zero altitude.

The resulting equation is m = (n/(n+1))*(do.Zo)*(1 - (T/To))^(n+1) - a bit of simple algebra and the Gas equation shows that do.Zo = Po/g.

Thus the mass is lower than for a simple isothermal atmosphere by roughly (n/(n+1)). In this case n = 6.33, higher than the dry adiabat for CO2 which gives n = 4.45.

Now an adiabatic or polytropic atmosphere is an idealisation, but it seems odd to me that whenever Venus' atmospheric mass is discussed people always use the higher isothermal value. Have I missed something important in the physics, or is Venus's atmospheric mass just 86.4% of the usually quoted value?

[Guest_DonPMitchell_*]

Planetary circulation is fascinating. Rotating patterns of vortices are not unusual.

[attachment=6553:attachment]

This old Tiros-9 photo of the Earth shows a number of large cyclonic and anti-cyclonic weather patterns extending cross the north and south temperate zones. You get a sort of standing-wave pattern like that, with about six of these giant vortices moving around the Earth in the north and south. Called Rossby Waves or planetary waves.

Around the equator, where the change in angular momentum is less, you just have simple hadley-cell circulation, tilted at an angle by coriolis force.

On Jupiter you see many bands of this stuff. A trade-wind band around teh equator like Earth, then some Rossby waves in the temperate bands, and then nearer the poles, the powerful coriolic forces lead to chaotic circulation.

[Bob Shaw]

Planetary circulation is fascinating. Rotating patterns of vortices are not unusual.

This old Tiros-9 photo of the Earth shows a number of large cyclonic and anti-cyclonic weather patterns extending cross the north and south temperate zones. You get a sort of standing-wave pattern like that, with about six of these giant vortices moving around the Earth in the north and south. Called Rossby Waves or planetary waves.

Around the equator, where the change in angular momentum is less, you just have simple hadley-cell circulation, tilted at an angle by coriolis force.

On Jupiter you see many bands of this stuff. A trade-wind band around teh equator like Earth, then some Rossby waves in the temperate bands, and then nearer the poles, the powerful coriolic forces lead to chaotic circulation.

Don:

Remember the early National Geographic paintings of our world as seen from space, back in the mid 1950s? They tended to show Earth with belts, just like Jupiter!

Bob Shaw