Venus Atmosphere Puzzle, one man's struggle with atmospheric physics |
Venus Atmosphere Puzzle, one man's struggle with atmospheric physics |
Jun 5 2006, 12:15 PM
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Junior Member Group: Members Posts: 57 Joined: 13-February 06 From: Brisbane, Australia Member No.: 679 |
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? |
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Guest_DonPMitchell_* |
Jul 3 2006, 04:16 PM
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Guests |
Vega-1 and Vega-2 performed similar experiments. The LSA particle-size spectrometer and the ISAV-A optical aerosol analyzer were installed on the last two spacecrafts to land on Venus. The ISAV-A investigators concluded that the large particles were approximately spherical with a refractive index of 1.4 +/- 0.1.
The Vega instruments also measured sub-micron particles in considerable abundance, which were too small for the PV instrument to see. The Venera-9 and 10 nephelometers measured scattering at several angles, but that data was never as conclusive as the aerosol particle sensors on PV and Vega. |
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Jul 4 2006, 01:16 AM
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Junior Member Group: Members Posts: 57 Joined: 13-February 06 From: Brisbane, Australia Member No.: 679 |
Thanks Don & Ed
More data to cram into my brain. I'd really like to get my hands on the Venus International Reference Atmosphere, but VIRA isn't available even second hand on all the usual web-shops. I have found journal articles which give some values from VIRA, but they're kind of empty without the underlying rationale - the scale heights vary so much between levels I can't see any pattern except for large poly-nomials. Why? Tighter fit to the data I guess, but the underlying processes seem obscure. Will have to try and dredge up those Venus books. Adam |
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