[enahs]

I was discussing something with a friend, and now I am contemplating something.

The pressure at the surface of Venus is ~93+ times that of Earth.

Using a simple model of earths atmosphere as 80% N₂ and 20% 0₂, giving an average molecular weight of our Atmosphere is ~ 38.4 g/mol.

Venus can be modeled as 96.5% CO₂ and and 3.5% SO₂, giving an average molecular weight of ~ 47.7 g.mol.

Also, the average force of gravity of Venus is 9.4% less then earth, countering the extra mass some.

Then how does Venus have a pressure at the surface more then some 90 times that of Earth?

The mean temperature for Earth is 287K and Venus is 735K; it is only a mean of ~2.6 times as hot as earth.

How does one reconcile the pressure? Using the ideal gas equation (and even though CO₂ is a poor ideal gas, it is not that poor), increasing the temperature by a factor of 2.6 will only increase the pressure by a factor of 2.6. The volume difference is only ~10% as well. After the increase in temperature the and the decrease in volume the pressure is still ~90 times that of Earth. This means using the ideal gas law, the atmosphere of Venus would have to contain 90 times more mols of atmosphere, which is not the case, it in fact has roughly 10% less then Earth.


If we represent the mass of the gas by a column of the same height. However, because the force of gravity is ~9.5 less then that on Venus, the average molecular weight on Venus can be represented as 42.9 g/Mol, not the 47.7 g/mol as found here on Earth. Put another way, on average one mol of Venuses atmosphere only has 4.5g more mass as compared to Earth (or it weighs roughly 11.7% more then earths mass). Again if you used this method and do the calculations, you are still of by a factor of more then 90!


Convection and various atmosphere phenomenon will produce variations in the pressure, such as those found in and around hurricanes and tornado's here on Earth. However Venus does not have an "active" atmosphere as Earths.


Even if we assume our data has an margin of error of 200% and the error in treating carbon dioxide as a ideal gas is a factor of 200%, that still does not even come close to explaining the pressure differences.


I know I am treating this in a simplistic manner, and atmosphere science is quite complicated and you actually have to account of the pressure and such as function of the altitude due to changes in temperature. I just do not see how making these simple assumptions can lead to an error of over 90 times.


The only major difference I can think of between Earth and Venus is that Venus has no magnetic field. As a result it is at the complete whims of the suns solar winds and solar flares. I do not know how that could increase the pressure 90+ times, though.


I did some Google searching and could not find a good explanation or justification.

Anybody have any ideas, thoughts, comments?

[Borek]

Good question, No idea, only random thoughts so far.

How thick is Venus atmosphere? CO₂ is heavier than both N₂ and O₂, thus its molecules are slower at the same temperature, and they need to reach escape velocity to leave gravitational field - so the atmosphere is thicker than ours. To calculate pressure at the surface we shoud find mass of the gas over the surface S (integral of the gas density, found using by barometric formula) - that'll give a force, F/S gives pressure.

What is dependence between surface temperature and pressure? Heating gas at the surface will make atmosphere inflate, most likely PV=nRT is completely of no use here.

[r7t3n5]

P (surface) = P (h) + dgh  ,where d = density, g is gravity, h is height of atmosphere, P (surface) is the surface pressure, P(h) is the pressure at the height h above the surface.  The height of Venus' atmosphere is much higher than Earth.  This equation is not really correct because the density of the atomsphere changes with height, but the point is that the surface pressure depends on the height of the column above. 

[enahs]

Ohh, I am glad I saw this labeled as new. I had no idea that Venuses atmosphere was so high. It is over twice that of the Earths. I thought they where about the same. Thank you, that does help a lot!