The surface of Venus is about 492ºC, under a sky containing 92 atmospheres of mostly carbon dioxide, and it probably has been for many millions of years. This means the entirety of the planet's 50 km thick crust is at least that hot as well; the mantle and core being even hotter from radioisotopic decay.

If we magically removed 91 atmospheres of the Veneran atmosphere, and then made the remaining one identical to Earth's, how long do you estimate it would take for the planet to radiate away its internal heat to a point where an unshielded human could stand on its surface and survive? Hundreds of years? Thousands? Longer?

  • $\begingroup$ Welcome to Worldbuilding! No need for the "Edit:" note. You can just change what you're asking to better reflect what you actually want answered. $\endgroup$ – Jason Clyde Sep 20 '19 at 1:04
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    $\begingroup$ Welcome to Worldbuilding Ed. To my mind, this is actually the wrong way around; cooling the planet will in itself reduce the pressure of the atmosphere. What is needed to cool Venus is to change the composition of the atmosphere, not reduce its volume. I don't have time frames to hand, but if you could somehow get an algae into the upper atmospheric levels of Venus to photosynthesise (I don't know if there's enough water there though) then you would eventually reduce the CO2 and increase non-greenhouse gases in the mix. But such a move, if possible, will take either centuries or even millennia $\endgroup$ – Tim B II Sep 20 '19 at 1:15
  • $\begingroup$ Hi Tim. I was curious about the cooling time because all the terraforming schemes I've heard of generally describe the first part - removing the bulk of the Veneran atmosphere - but ignore how long it would take to cool the surface to habitable temperatures. As for removing CO2 with algae, I've read that the issue is that as the algae grows, it eventually circulates (or rains, if heavy enough) down towards the hot surface layers of the atmosphere and would burn into CO2 again. The wikipedia page on the Terraforming of Venus has a bunch of suggestions on higher-tech solutions. $\endgroup$ – Ed Beaty Jan 4 at 15:11

You can actually observe most of the answer here on Earth. The key point is the heat conductivity of the Venusian rock is not very large. If the dense atmosphere was removed, the surface of Venus would be losing heat by radiating it into space. (It's also losing heat by convection into the atmosphere, but I believe that is a much smaller effect.) It is gaining heat by conduction from the bulk of Venus which is hot rock. As the surface cools, the rate of heat loss into space decreases and the rate of heat flow from the interior increases and the end-point temperature is where those two come into balance.

You can get a useful estimate of the time by considering the cooling of lava from a surface flow on Earth. We know that a flow only a few tens of feet thick will solidify entirely on the surface while the interior remains liquid (this is how lava tubes form), so we know that the lava is thick enough to be a good model for the much deeper hot rock on Venus.

A new lava flow cools to a solid surface within hours and is cool enough to walk on in days (provided no new lava underneath cracks the surface and allows new liquid lava out.) See this web page for some specifics.

This is a lower limit for the time needed on Venus, since atmospheric cooling is much more effective on Earth than it would be on Venus, but it points strongly to the time needed to cool to be safe to stand on being days to months and definitely less than years.


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