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?

[Phil Stooke]

There's some great Venus atmosphere stuff (and some on Mars) from a presentation here:

http://www.mrc.uidaho.edu/entryws/full/pro...e_detailed.html

Check out the zipped powerpoint presentation I-4.1 on the Venera missions.

Phil

[Guest_DonPMitchell_*]

There's some great Venus atmosphere stuff (and some on Mars) from a presentation here:

http://www.mrc.uidaho.edu/entryws/full/pro...e_detailed.html

Check out the zipped powerpoint presentation I-4.1 on the Venera missions.

Phil

Kerzhanovich was in charge of the doppler shift studies in the Venera missions, and now works at JPL. He was famous for his arguments with mission planners, to land the Venera capsules further off center, so he could get better horizontal wind measurements. Astronomers thought the atmosphere was superrotating, from UV cloud studies, but there was a long history of people misjudging the rotation of Venus, and not complete certainty that the UV features were some kind of atmospheric wave phenomenon. So Kerzhanovich's dopper measurements were the first real proof of superrotating zonal winds on Venus.

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I see he liked my annotated version of the Venera-7 doppler data (grin). But the best analysis of that data was made by John Ainsorth at Goddard SFC. The Soviets really didn't want to talk about the parachut failures too much, but Ainsorth did some pretty detailed simulations and paints a fairly hair-raising picture of the probe's descent. After complete parachute failure, the probe hit the surface of Venus at 38 miles per hour, bouncing on impact and then coming to rest at a 50-degree tilt which severely lowered its radio signal to Earth. Russian engineers later used the doppler data and the fact that Venera-7 was not destroyed on impact, to calculate properties of the surface soil.

One man's parachute failure is another man's penetrometer!