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

[ngunn]

Hi Messenger. You found my post confusing??!!

I don't think graal and I are in disagreement at all, just thinking through the same thing in different ways, which I for one have found illuminating.

The Titan case is very different from Venus because after decreasing to the tropopause the temperature rises strongly but erratically with height. No simple model will do in this case, neither isothermal nor adiabatic. One would have to integrate by the computer equivalent of counting squares under the graph. To make things still more complicated I wouldn't be surprised if the temperature profile varies quite a bit with latitude, season, cryovolcanism and precipitation processes, maybe also interaction with Saturn's magnetosphere. But we have wandered quite a long way from Venus now and should really be in 'Titan's atmosphere and weather'.

[remcook]

just a note: Titan's temperature does vary significant with latitude, but not with longitude. at the north, the lower stratosphere gets very cold (at least 20 degrees colder than the equator) and the higher stratosphere gets very warm.

[qraal]

Hi All

Well I decided to do a numerical experiment on a model Venus atmosphere and sum up 100 metre high blocks of atmosphere, computing Cp, p, rho, T, m, and energy with each step. I looked up the behaviour of CO2's heat capacity, Cp, over the temperature range I was interested in and discovered it varied dramatically - from 0.735 KJ/kg.K at 200 K to 1.148 KJ/kg.K at 750 K. Quite unlike the behaviour of air for which Cp is constant over the terrestrial temperature range. I could fit the behaviour of Cp with temperature well enough with a quadratic over the range 275 K - 750 K, and a linear fit between 275-200 K.

I baulked at an analytical solution, so I chucked the equations of state into an Excel spread-sheet and summed it up, from 735 K and 92 bar to 0.17 bar at 198 K. What I found was the mass is exactly Po/g, which was quite a "surprise" considering my first doubts. The total atmospheric heat energy is 0.706 teraJoules, which would radiate away to space in about 138 years at a constant 231 K effective temperature - if you could shade Venus. Of course at 304 K the lower reaches would start condensing as liquid, adding extra latent heat.

My final task is to work out the average temperature.

Adam

[qraal]

Hi again

And the average temperature is 630 K.

Hmmm... that was an interesting exercise. One of these days I will have to do it with smaller increments in a C program and calculate a new value for gravity at each step. But I can't imagine it will be much different in final outcome.

Of course the really thorough approach is to do a General Circulation Model of the whole planet... whew!

My curiosity was first sparked by Paul Birch's paper "Terraforming Venus" quickly, which used a figure of 630 K for Venus' average temperature - how did he arrive at that figure? He also used 0.84 KJ/kg.K as Venus' heat capacity - the average I got is more like 1.075 KJ/kg.K. His figures were fine for a rough estimate, but the problem had me intrigued, so I did my first naive analytical solution and quickly got very puzzled by the discrepancy between our results.

Now it seems the numerical approach has verified his estimate, more or less.

My rough model has a lot of room for improvement, but it's kind of fun in a nerdy way ;-)

Adam

[ngunn]

Hmmm... that was an interesting exercise. One of these days I will have to do it with smaller increments in a C program and calculate a new value for gravity at each step. But I can't imagine it will be much different in final outcome.

Yes, someone's still watching this space! Nice to have your updates.

From the quote above I was wondering what you did do about g in your latest approximation? You're bound to get a mass per unit area of Po/g if you use the same g throughout, regardless of how the other parameters vary. The only thing the temperature and pressure do is control the mean height of an air molecule, hence the mean g it experiences and therefore its mean weight. So if you allowed for a decrease of g with height you should have got a higher mass . . .

[The Messenger]

[quote name='qraal' date='Jun 23 2006, 06:42 AM' post='59569']
Hi All

Well I decided to do a numerical experiment on a model Venus atmosphere and sum up 100 metre high blocks of atmosphere, computing Cp, p, rho, T, m, and energy with each step. I looked up the behaviour of CO2's heat capacity, Cp, over the temperature range I was interested in and discovered it varied dramatically - from 0.735 KJ/kg.K at 200 K to 1.148 KJ/kg.K at 750 K.
[/quote]
That is good news - now we know that as the average Earth temperature increases, the additional CO2 will act as a heat sink and the glaciers will come back

Seriously, this was good analytical work, eliminating a gross inconsistency and demonstrating that there is almost order in the universe...

[/quote]

[qraal]

Hi Messenger & ngunn

Hey thanks for the nice comments and useful gedankenexperiments. More update as follows...

I've 'discovered' that the US NIST has the proper fit equation for Cp of CO2 online - I was close as it's almost a quadratic. I just did a rough fit from a table of values, but I was pretty close.

Now I need to work out a simplified equation for 'g' for a future C version of the model. Of course I could just use the good odd inverse square law, but why make it easy on me and hard on the number cruncher? Gravity varies almost linearly with altitude in the range we're discussing. The less maths functions I need to call from the C library the better.

Also I've managed to find a discussion of cloud formation with data on Venus's sulphuric acid clouds, so I'll be able to add that little refinement to the higher altitude computations.

Also I did an Earth version of the model based on the International Standard Atmosphere's lapse rates and altitudes, all the way to the Stratopause's ceiling at 51 km. The match is pretty good and the mass of the air column is exactly as expected, Po/g. The ISA uses 'geopotential altitude' so I didn't need to compute 'g'.

Now all I need is to figure out the heat transfer equations and I can model the atmosphere temporally too. I've got a copy of Mark Bullock's thesis and he covers that in some detail, probably more detail than my poor laptop can number crunch in a meaningful time.

Adam

[RNeuhaus]

Venus' Double Vortex Confirmed in New Animation

A huge "double-eye" atmospheric vortex has been confirmed to exist at the South Pole of the planet Venus.

http://www.space.com/scienceastronomy/0606...nus_vortex.html



There are two vortex in the South Pole. That is odd!!

Scientists think the vortexes are created by a combination of a natural cycling of hot air in the planet's atmosphere and high velocity, westward-blowing winds that take only four days circle the planet. It is still unclear, however, why there are two vortexes at each pole.

Rodolfo

[Guest_DonPMitchell_*]

At last, some pictures from VEX. These are fascinating.

[attachment=6431:attachment] [attachment=6432:attachment]

Here are the best previous images of the double vortex, by Pioneer Venus, and the Venera-15 IR spectrometer.

[qraal]

Thanks Don

Man that's so bizarre.

Venus is a surprising planet - not 'dead' at all, except in a restricted sense, and I'm still hanging out for sulphur munching bugs in the clouds.

There's a surprising amount of energy locked up in that 4 day rotation.

Adam

At last, some pictures from VEX. These are fascinating.

(snipped pix)

Here are the best previous images of the double vortex, by Pioneer Venus, and the Venera-15 IR spectrometer.

[qraal]

Hi All

Updated the gravity - now it's a linear approximation good out to about 200 km. Considering the quintic equations I used working out Cp(T) for N2 and CO2 the computational improvement is slight, but the mental exercise was worthwhile. Also I had made a basic error working out the enthalpy of the gas mix for each cell, so I've corrected that too. The total energy, using 0 K as the reference point, is about 588 GJ/sq.m.

Having done all that I've been trying to educate myself on cloud condensation physics and getting a headache in the process. Does anyone here know if the cloud layers have been measured in their vertical extent and optical depth? I'm pretty sure I've read some such data, but a good reference on the topic would be handy. The article I have is more theoretical than factual and gave me a few hints on how to modify the model, but now that I'm tweaking it I'm dissatisfied with a 'fudge factor' and want some real understanding.

Chilling Venus's atmosphere out, as a precursor to terraforming, was the main problem I wanted to investigate when I began this exercise, but as I've progressed the fun has come from comparing it against the data. Some very odd things would happen on a darkened Venus - CO2 snow would probably fall (and sublime on the way down) before CO2 rain did, for example. I've been imagining a scenario in which hydrogen was crashed into Venus to burn the CO2 into carbon and water, via the Bosch reaction, then wondering what condensing water would do to a surface made of carbon-deprived oxides of calcium and the like. Would a mad upheaval of the regolith ensue, rapidly forming carbonates? Kind of like 'cooking' limestone to make lime, but in reverse.

Adam

[edstrick]

The soviet venus descent probes measured light levels and then (later missions) visible and near-IR spectra. They also took cloud samples and got composition data. They may also have carried "nephelometers" as well. Quite a bit of data.

The american Pioneer venus probes carried nephelometers on the small probes and more sophisticated cloud particle size spectrometer and infrared radiometer on the large probe. There's a rather vast amount of information for a rather small number of atmospheric descents.

The most global data on clouds is from radio occultation temperature/pressure profiles down to some 35 km altitude, and occultation data from Magellan provides cloud absorption profiles.

The starting place for all current studying of Venus is the Univ. Arizona Press book "VENUS" from the early 90's.

[qraal]

Hi ed

Thanks for the heads up on that reference, which I'll have to track down.

Adam

[edstrick]

The Univ of Arizona has had a method of producing "Weighty Tomes" for some decades.

They announce a conference on a subject, recently held was "Protostars and Planets V". With the aid of experts in the field, they pick a set of topics and the authors who are most expert in those topics. "You have just been volunteered to write a summary article on what we know about the vertical cloud structure of Venus' atmosphere, and we have volunteered the Russian Expert, Academician Boris, to be your co-author on this paper."

It's a sort of "you can't refuse" honor.

Other reseachers are invited (open invitation) to present papers at the meeting. The review papers <first versions ready at the time of the meeting> and selected other papers are published in a thousand or so page volume with a title like "Venus" or "Protostars and Planets V" at what used to be and probably still is a remarkably low price for such an academic "tome".

These books become landmarks and mileposts in the field. For studying pre-Magellan Venus studies, the "Venus" volume is 1,000% essential. You START there and work your way through relevant articles and the relevant papers referernced in those articles.

[Guest_DonPMitchell_*]

The U of Arizona books are essential. There are actually two Venus books, and you want them both (Venus and Venus II). Also check out their Mars book.

There have been many descent probes on Venus. The first really detailed information came from Venera-9 and 10 in 1975, which had nephelometers, spetrometers, and various other instruments. The three-layer cloud structure was discovered then, as well as the first photos of the surface.

Venera-11, 12 and the Pioneer probes arrived in 1978. The Venera probes contained more sophisticated spectrometers than the 1975 versions, and also mass spectrometers, gas chromatography and x-ray fluorescence spectrometers. The latter provided the first real data on the composition of the cloud material, including the discovery of Iron Chloride as a component. The PV large probe had a particle-size device which provided unique data. There has been much debate about the PV results, which some believe show a distinct large particle size that may be crystals -- the so-called mode 3 controversy.

Venera descent probes relayed data through the high-gain connection of the main spacecraft, so they were able to send almost 100 times as much data as the Pioneer probes, including large numbers of mass spectra and optical spectra as they descended.

The last word has been from the Vega descent probes. They were particularly geared to study the clouds and try to answer questions raised by the Pioneer particle-size spectrometer. Two different particle analyzers were on the Vega probes, as well as a more sophisticated gas chromatography experiment, which discovered the profile of chlorine, sulphur and phosphorus abundance as a function of altitude. The clouds are a lot more complex than just sulphuric acid droplets, particularly the lower layers.

The big names in Venusian atmospheric chemistry are Vladimir Krasnopolsky, the late Vasily Moroz and Larry Esposito. They have a joint paper in Venus II, and Krasnopolsky has a good book on the chemistry of Mars and Venus.