I’m wondering if Venus could have lost carbon in the form of methane evaporating into space from the upper atmosphere. methane CH4 has a molecular weight of just 16, half O2′s 32 and about 1/3 of CO2′s 44. Lighter molecules travel faster at the same temperature and thus are more likely to reach escape velocity. CO2+2(H2O) -> CH4 + 2O2. If this is the case, wonder what happened to the oxygen?

I’m attempting to do that very calculation including sedimentary rocks on Earth and I’m only getting a value of 18 bar for Earth, not 60. The estimates for the amount of limestone/dolomite in sedimentary rocks seems to vary widely depending upon the source of the information so it’s almost impossible to get an accurate number.

It all depends on the insolation which in turn depends primarily on the distance from the star, albedo of the planet, and luminosity of the star. In the case of Earth with a 243-day retrograde rotation the sun would appear to move in the sky 2.47 degrees per 24 hours (west to east). If you consider the subsolar point the sun will be within 20 degrees of zenith for 389 hours (16.2 days). The area near the subsolar point is going to get very hot with no chance to cool off at night.

Let’s compute how much energy that point receives. Assume the solar constant is 1366 watts/m^2 and the albedo is 30%. Assume the average elevation angle is 80 degrees. Then average solar power received over 389 hours is 1366 * 0.7 * sin(80) = 924 watts/m^2. Assume that the same energy is re-radiated back into space assuming a blackbody emitter. Then temperature of emission is (924 / 5.67E-8)^0.25 = 357 deg K = 84 deg C. This is only 16 deg C less than the boiling point of water. The evaporation of water will be tremendous causing huge quantities of water vapor to be released into the atmosphere. The temperature only has to rise 16 deg C before the oceans really start to boil in earnest. So a slow rotation is not a good thing for a terrestrial planet.

I think the key difference is the slow rotation. If you took the Earth and slowed it down so that the rotation period is 243 days retrograde then the sun would rise in the west and set in the east. The time between consecutive sunrises would be 146 days (3,504 hours) which means that typically the sun would be above the horizon for 73 days (1,752 hours). When exposed to the sun for 73 days straight the oceans of the earth would boil emitting huge quantities of water vapor into the atmosphere. Since water vapor is a greenhouse gas it would cause a run-away greenhouse effect with all the water in the oceans boiling away eventually. The final surface temperature would probably be at least 300 deg C if not more so all life would be extinguished. I don’t know if climatologists have actually run this model or not, but it would be interesting to see what the final results are for it.

In any case, even a factor of 5-7 less atmosphere than Venus would result in a pretty unpleasant atmosphere…

Venus:
http://en.wikipedia.org/wiki/Atmosphere_of_Venus
Total mass = 4.8E20 kg
Mass of CO2 = 4.63E20 kg

Earth:
Carbonaceous rocks (limestone, dolomite):
Total mass = 1.8E20 kg (max)
Equivalent CO2 mass = 7.9E19 kg (max)

CO2 dissolved in seawater:
http://www.eoearth.org/article/Carbon_cycle
Total amount of carbon = 40,000 billion metric tons = 4.0E16 kg
CO2 mass equivalent = 1.5E17 kg
(This is a factor of 90 less than the value of 1.4E19 kg which was cited by someone earlier – a URL for that number would be much appreciated)

Total CO2 on Earth: 7.9E19 kg (or 9.3E19 kg if the 1.4E19 kg for dissolved CO2 is correct)

This is still a factor of 5.8 (or perhaps as little as 5.0) less than the number for Venus. So I’m not sure the premise that Earth and Venus started out identical is valid.

Some other interesting stuff on the Wikipedia entry for Venus is as follows:

“Another intriguing piece of evidence comes from measurements of sulfur dioxide concentrations in the atmosphere, which were found to drop by a factor of 10 between 1978 and 1986. This may imply that the levels had earlier been boosted by a large volcanic eruption.”

“The atmosphere of Venus is in state of a vigorous circulation and super-rotation. The whole atmosphere circles the planet in just four days (super-rotation), which is a short time compared with the sideral rotational period of 243 days. The winds supporting super-rotation blow as fast as 100 m/s.”

I don’t know how the atmosphere can travel around the planet in just 4 days when the rotation period is 243 days. That makes no sense to me at all.

Bear in mind that limestone is only one of the two main carbonate rocks, the other being dolomite… from here, we have that carbonates (not just limestones) make up to 10-15% of sedimentary rocks, which is a factor of 2-3 more than Tom Marking used…

The American market re-cut of that film turned it into a cartoon. It has since been released in its original form as “Der Schweigende Stern” (The Silent Star) which is based on Stanislaw Lem’s first novel “The Astronauts.” For those who have only seen the butchered King Features release, the original movie is a revelation. It was shot in full 16:9 format 70mm Commie-color (I forget what the eastern bloc’s version of Technicolor was called, probably Agfacolor). The opening credits are completely different with some very avant-garde music. The whole thing feels like a grownup’s version of the film. It comes only in German with English subtitles.

It was made in 1959 as a response to films like “Forbidden Planet” to showcase the production abilities of the Soviet countries and their Communist states (it was a joint East German/Polish production).

I have a chapter on it in “Spaceship Handbook” that I’m going to have to re-write for the second edition and base it on the real film. All I had available at the time I wrote it originally was the US version. The “Cosmostrator” (Kosmocrator) is one of the coolest looking spaceships ever to grace the silver screen. Really difficult to model, though.

- Jack

PS – It was also given the MST3K treatment in their second season.

Even including the CO2 dissolved in the Earth’s oceans Venus still has a factor of ten more CO2 than Earth does. The reason might have to do with the varying chemical composition in the original solar nebula based on distance.

Here there’s also Carbon dissolved in seawater, about 1.4e19kg CO2 equivalent, total that plus limestone is ~4e19kg, or 8.7% of the Venusian atmospheric total. Still a nice warm blanky, I guess.

Hmm. I’d heard the same with comparative atmospheres. Wonder why we’d start with so much less atmospheric CO2, and so much more H2O?
A quick read suggests to me more impacts after Neptune moved out and scattered the iceballs. That would’ve made for a pretty sky full of very large, young, disintegrating comets, at least up until one of them killed you.

That’s the point I got interested in.

Yes, you can. Jupiter’s moon Io is a perfect example. It’s the most volcanically active body in the solar system but it lacks plate tectonics. Mars is another example. It has the solar system’s highest volcano, Olympus Mons, with an altitude of 27 km but there is no evidence of plate tectonics.

Venus is somewhat ambiguous. There is evidence of a massive resurfacing event which took place 300 – 500 million years ago but no evidence of gradual plate tectonics operating currently. But Venus is chock full of volcanoes. There are 167 giant volcanoes on Venus with bases over 100 km across. There is just 1 such feature on Earth (Hawaii).

BA, you forgot to include ‘nearly’ in your description.

********************************************************
Venus calculation:

http://en.wikipedia.org/wiki/Venus
Venusian atmosphere: surface pressure = 9.3 million pascals
surface temperature = 735 deg K

Modified ideal gas law: rho = M*P / (R*T)
rho is density (kg/m^3), M is molecular weight (0.044 kg per mole), and R is ideal gas constant (8.314 joules per deg K per mole).

So surface density = 66.96 kg per m^3
96.5 percent of atmosphere is CO2 so CO2 surface density = 64.6
kg per m^3

http://en.wikipedia.org/wiki/Atmosphere_of_Venus
atmospheric scale height of Venus is height at which density falls to factor of exp(-1) = 36.8% of surface density. Using table for pressure as a substitute the approximate scale height is 15 km.

Total CO2 on Venus in a vertical column = scale height * surface density = 969,000 kg per m^2

********************************************************
Earth calculation:

Calcium carbonate: molecular weight = 0.100 kg per mole
(multiply by 0.44 to get equivalent mass in CO2)

http://www.chem1.com/acad/webtext/geochem/04txt.html
5 percent of sedimentary rock is limestone

http://www.ufrsd.net/staffwww/stefanl/Geology/sedrock/index.htm
5 percent of earth’s crust is sedimentary rock

http://www.utdallas.edu/~msweet/oc-unit2.html
Earth’s crust is 0.4% of its total mass

Using a mass for the earth of 5.97E24 kg and surface area of 5.10E14 square meters:
Mass of earth’s crust = 2.39E22 kg
Mass of sedimentary rock = 1.2E21 kg
Mass of limestone = 6.0E19 kg
Equivalent CO2 mass of limestone = 2.6E19 kg

Equivalent CO2 mass of limestone per square meter =
51000 kg per m^2

The value for Venus is 19 times higher than this amount. So even though there are uncertainties in some of these percentages it still seems like Venus has much more CO2 than Earth even if you account for the CO2 on Earth tied up as calcium carbonate.

People have posted lots of interesting ideas. We touch on Carl Sagan’s suggestion of cloud seeding the upper atmosphere of Venus with algae, which now, with a better understanding of the atmosphere has been shown to not work. There are however some very interesting ideas from various posters before the thread was taken over several times by the “Floating Cities” folks. (Which to me is a dead end idea.)

However, by trying to solve the staggering problem of how to hypothetically transform Venus into another Earth through a long series of baby steps, we can learn an awful lot about the nature of Venus known and unknown so far. The new findings Phil writes about could add even more questions and ideas.

http://www.bautforum.com/life-space/59971-making-venus-livable.html