Chilling Venus

An old SF dream, independently invented by Olaf Stapledon and Jack Williamson, is the idea of terraforming. Venus has long been viewed as a suitable target. When CO2 was first found in huge amounts in the 1920s, Stapledon imagined giant electrolysis stations converting the atmosphere. In the 1940s the “dry formaldehyde” Venus was a popular model and became the setting of Poul Anderson’s “The Big Rain” – the planet-obscuring clouds were believed to be formaldehyde polymer dust and the surface was a dry 100 degrees C. Clever chemistry would convert the clouds into water and eventually the Big Rain would fall.

By 1960 the clouds were believed to be water and the surface was roughly 270 degrees C with about 2 bars pressure – thus Carl Sagan’s famous suggestion: seed the clouds with algae to convert the CO2 into oxygen. But by 1963 Sagan had abandoned the water cloud model and the temperature was estimated to be a scorching 700 K with about 100 bars of nitrogen causing a super-greenhouse effect and “cloudiness” was due to scattering. Larry Niven famously described the surface conditions as “a searing black calm” and proposed Earth only avoided such a fate by the early Moon stripping excess air away.

By 1968 Russian Veneras had shown the surface to be even hotter and the atmosphere to be almost all CO2. The clouds had become a total mystery because the measured water vapour levels were so low. Venus’ surface was depicted as a stormy darkness with red-hot patches of glowing rock. Carl Sagan’s idea of seeding the clouds had become SF mythology and remained unchallenged.

After the probes of 1974/5 – Venera camera landers and Mariner X – the clouds were known to be sulphuric acid and the surface was surprisingly bright. More like an overcast day on Earth than an abyss of Hell, though even hotter at 735 K. By 1976 serious studies for terraforming Mars had also led to a re-examination of Venus and the realisation that it was too dry for algae. To make O2 from CO2 requires H2O – and 90 bars of CO2 needs an ocean of water to turn into algae and oxygen. But all that oxygen was far too much oxygen. James Oberg’s “New Earths” proposed combining the oxygen with hydrogen tanked in from the Outer Planets – Saturn being a favourite. A bit “cart before the horse” because there would be no oxygen made without water…

A number of approaches were proposed through the 1980s, but the scenarios all required millennia. David Brin mentions a 10,000 year terraforming project being undertaken by the Earth Clan, to impress the Galactics that we “wolflings” weren’t too hot-headed and impatient to join Galactic society. Nice fiction, but unlikely for a standard human society to undertake. Paul Birch proposed in 1991 a different approach – why not cool Venus enough to freeze out the atmosphere and then bury it for later export off-world?

That’s the new “orthodoxy” of terraforming – chilling Venus’ atmosphere with a gigantic soletta parked in the Sun-Venus L1 point. A major question, then, is just how long would it take to cool, condense and freeze? Currently Venus’ “photosphere” – the region heat escapes from – is at a temperature of about 231 K, but this is due to the high cloud deck giving the planet a high albedo in visible and IR light. Chilled just a bit and the cloud deck would probably collapse since its main component is sulphuric acid, which boils at over 338 degrees C at 1 atm. With a higher emissivity Venus would lose heat somewhat faster, but just how much heat is there?

Carbon Dioxide (96.5% of the atmosphere), unlike nitrogen (3.5%), has quite a variable specific heat capacity – it stores more heat, the hotter it gets in the temperature range in Venus’ atmosphere. Nitrogen remains pretty stable, being a diatomic molecule, but carbon dioxide is triatomic and thus its ways of storing energy are quite complex. I’ve created a model of this process in an Excel spread-sheet and it has some interesting results, which I’ll elaborate on in a future post. But for now basically the hotter Venus’ photosphere is the quicker it’ll lose heat.

Once the temperature of the lowest layers reaches about 31 C the carbon dioxide will start condensing at the 74 bar pressure level, with interesting results – the phase-change heat liberated will drive convection, perhaps keeping the upper layers at a roughly constant 31 C until all the condensible CO2 below the 74 bar pressure level has rained out. Then it will only continue condensing, as the lower levels cool, down to a partial pressure of about 5 bar. Liquid CO2 can’t exist below that pressure – it’s either ice or gas past that point, and as it cools it will increasingly freeze-out, perhaps coating the underlying seas of cold CO2 in a pressure cap – except, unlike water ice, it’s heavier than its liquid phase. A bit of water might be needed to ice over the cold CO2 seas, which will be percolating into Venus’ regolith, and probably making geysers all over as it cools the underlying rocks. A big fraction might then be trapped in the regolith, but Venus’ sub-surface will probably be too hot for it to remain there indefinitely.

Once the CO2 is frozen out and capped over what remains? The nitrogen won’t freeze or liquefy under such conditions and so will make an atmosphere of about 3 bars, which would be a bit much for a prolonged human presence. The regolith might soak up a bit and some will probably “dissolve” in the CO2. Big thinkers have proposed haulling it off-world for Mars and free-space habitats, providing a long term export product. All the CO2 would be even more valuable as carbon nanotubes might eventually be the macro-engineering material of choice, with a theoretical 2 teraPascal strength. Stephen Baxter has Venus supplying carbon across the Galaxy for the War against the Xeelee in his novel “Exultant.” I would hope for something more peaceful.

My daughter’s first-typing… apryll jane georgia crowl, 5