Suppose I want to cool down the surface of Venus with a planet sized sunshade; with our sister planet plunged into eternal night, how long would it take for heat to radiate away and the atmosphere to precipitate fully to dry ice?
One problem is that your shade will heat up, then start to radiate itself toward the planet. I’m not sure how cool you can practically get the surface with a simple shield.
In the sf novel The Merchant’s War*, Frederick Pohl imagined using Ranque-Hilsch Tubes** to speed the cooldown of Venus. This would invariably lead to loss of atmosphere (which would, in essence, be carrying away the excess heat), but that might be a good thing, considering the makeup of the atmosphere there.
- The sequel to the classic The Space Merchants, whicvh he co-authored with C.M. Kornbluth.
** AKA Vortex Tubes – the closest thing in Real Life to Maxwell’s Demon. Vortex tube - Wikipedia
Apparently a rule here is that thought experiments are not allowed.
We have a magic shield. Or the sun goes out. Or the shield has heat exchangers pointed at deep space. Or it had a massive, refillable supply of cryogenic stuff.
If the shield reflected 90% of the light hitting it, wouldn’t it’s equilibrium temperature be much lower than a black body?
It doesn’t matter. For the purpose of addressing the OP, none of it maters.
This thesis on “The Stability of Climate on Venus” will give the fundamentals you would need to model the Venusian atmosphere. Unfortunately, even this won’t answer the question simply as once the atmosphere becomes cool enough that sulphuric acid concentrations start to drop the entire cloud distribution and emissiveness in the infrared spectrum (which is where almost all of the radiative energy loss will occur) changes dramatically, and of course once water vapor and gaseous carbon dioxide starts to condense the greenhouse effects that keep the atmosphere of Venus so intolerably warm and dense will be diminshed. To answer this question to any degree of fidelity would require at least a model including the chemical kinetics of the atmosphere at different temperatures and a metric shitton of assumptions about the thermal load of the planet’s surface and what that does to the convection patterns which will transfer heat from the lower levels up to the upper cloud deck where it can be radiated to space.
My totally wild ass guess is that it would be at least a few months before measurable changes start to occur and years or decades before the atmosphere condenses enough to change optical and radiative properties of the atmosphere.
As for the Sun shield, making it workable is a relatively straightforward if logistically challenging problem of putting the shield in the Sun-Venus L1 libration point with a keel to keep the input face oriented toward the Sun, of sufficient diameter to assure that Venus is in perpetual eclipse, and sporting a fan of radiators on the back side pointing at an outward angle that shadows Venus; think of a conical frustum with the base oriented toward the Sun and the conical surface facing outward such that its aspect has a zero view angle of the planet. The resulting envrionment may not be quite the 2.7 K ambient background of interstellar space, but it should be close enough not to matter.
Stranger
I think a bigger problem is that the actual surface of the planet is also hot. You have to cool down not only the atmosphere, but also an unknown number of kilometres of the crust before you reach some new equilibrium.
With our theoretical sunshade cutting Venus’ solar thermal intake to zero. I wonder how efficient heat shedding is due to atmospheric convection working hand-in-hand with the condensation and evaporation/sublimation of acid rain and dry ice, respectively.
(Note: condensation may not…at first…even reach the surface. Sometimes this phenomenon happens on Earth deserts, as rain that evaporates mid-air before hitting the sand)
Heat flow between the upper atmosphere and surface probably is pretty efficient. The bottleneck, though, is going to be radiation from the top of the atmosphere out to space. Clouds, even sulfuric clouds, have a very high albedo.
Agreed, and Venus has a lot of heat locked into both its dense atmosphere and the thick lithosphere which will maintain a high enough temperature, not to mention heat released from the highly active volanism, to keep carbon dioxide and sulphuric acid vapor thick enough to prevent more than a small radiation flux from the surface or lower atmosphere out to space. That means at heat is primarily conveyed to the opaque upper cloud deck above the troposphere where it can then be radiated to space. The amount of energy to lift the heat-laden atmosphere this high puts a very restrictive limit how just how fast heat can be transfered out to space from the ground and low atmosphere. Actually, the cloud deck rises even higher on the night side, which places further limits on thermal throughput.
Stranger
FWIW…the hard numbers.
http://www.tak2000.com/data/planets/venus.htm
Here is a JPL presentation which explains the greenhouse effect on Venus, albeit in the context of comparison with Earth.
Stranger
Earth has a significant amount of internal heating from radioactive decay. This has come up in conversations about the sun disappearing, earth getting flung out into space by a rogue planet, etc. Apparently our planet wouldn’t be a complete iceball. However, we’re talking pW/kg of isotopes that are hard to quantify in our own planet, let alone another. Plus there’s primordial heat from the planet’s accretion.
If we want to assume Venus is like Earth, that’s 10[sup]31[/sup] J, according to a cite I cannot vouch for in the least. Geothermal gradient - Wikipedia
And 47 TW flow from inside to the surface.Earth's internal heat budget - Wikipedia
Granted, that’s small compared to other inflows and outflows.
Venus’ energy budget, less all sun input, could probably be estimated now. However, it’s going to change as the atmosphere chunks out and the emissions spectrum opens up.