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I'm looking at designing a neo-Old Solar System version of Venus which is habitable. The following conditions hold:

  1. The cloud cover still exists, hiding Venusian civilization from the outside world and vice versa, though not necessarily with the same composition as in reality (see the next point) - though the albedo provided by the cloud cover is the more-or-less the same as in reality.
  2. Venus is habitable: beneath the cloud cover, Venus has an Earth-like atmosphere, a temperature in a hospitable range for humans (no runaway greenhouse effect), and for whatever reason - perhaps the cloud cover is composed of different chemical than in real life - acid rain is not a significant problem, except possibly as a rare natural disaster. (This is a huge break from reality, I know, but crucial to what I'm doing.
  3. The physical composition of Venus is broadly similar to Earth, and it has a nearly 24-hour day, in line with older calculations.

Given these conditions, which is actually more likely: an average temperature higher than that of Earth, or an average temperature lower than that of Earth?

The former is in line with common sense and older SF stories. My argument for the latter is that the cloud cover (point #1) gives the planet an albedo of 0.75, more than twice that of Earth, which when combined with a lack of runaway greenhouse effect (point #2) and a physical composition broadly similar to Earth (point #3), suggests to me that a habitable Venus should actually be colder than Earth, possibly in a permanent ice age.

The sort of setting I'm developing obviously doesn't demand strict scientific accuracy, but I'm wondering if there's any glaring flaws in my argument other than the false premeses of points 2-3.

Update: Just to make it clear, by "permanent ice age" I don't mean pole-to-equator glaciation, just something like what Earth went through not that long ago.

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    $\begingroup$ The greenhouse effect is exactly what causes temperature to be high so if you magic that away, you can magic the planet to any temperature you want. $\endgroup$
    – A. C. A. C.
    Jul 20, 2017 at 21:43
  • $\begingroup$ I guess what I'm asking is whether an ice age Venus, if explained this way, would likely really annoy a scientifically-literate reader who otherwise accepts points 2 and 3. For example, as a programmer, seeing an HTML document presented as the boot code for an operating system would annoy me even if I'm already swallowing FTL drives and psionics. $\endgroup$
    – James Jensen
    Jul 20, 2017 at 21:47
  • $\begingroup$ @notstoreboughtdirt Well, the basic assumption of a neo-Old Solar System universe is that the assumptions made by early 1900's SF such as Edgar Rice Burrough's Barsoom and Amtor series -- that other planets are habitable and usually inhabited -- are basically correct. Advances in scientific knowledge since then, especially info garnered by probes, can be ignored if they get in the way. Accepting that a fictional Venus can have an Earth-like atmosphere and biosphere -- including Earth-like pressure -- under its clouds is simply one of the preconditions of enjoying the genre. $\endgroup$
    – James Jensen
    Jul 21, 2017 at 2:09
  • $\begingroup$ @ A. C. A. C.: No, you can't get any temperature you want. Remove all the CO2, or reduce it to an (earlier) Earthlike 250 ppm or so (which you'd need for life), and you still have a certain minimum temperature. But I strongly doubt it'd be an Ice Age Venus. I think the only way to get a decent answer to the question would be to run a climate model. $\endgroup$
    – jamesqf
    Jul 21, 2017 at 5:05
  • $\begingroup$ @jamesqf Hmm. Judging from the last glacial period, to get the sort of results I'm looking for, Venus would need to be 5.6 degrees C (about 10 F) cooler than Earth is now. Given that Earth managed those temperatures despite receiving modestly more solar energy once you take albedo into account, that should be within the bounds of plausibility but as you say, it would really take a climate model to be sure. $\endgroup$
    – James Jensen
    Jul 21, 2017 at 10:06

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The answer is in the question, really. Assuming that the physical composition of Venus is Earthlike, it's unlikely that you'll find a radically different kind of life (different chemical building blocks or whatnot).

With that in mind, unless the cloud cover is a recent phenomenon, the temperatures would also have to be Earthlike. Ice Ages do not generally lend themselves to the continued survival of a population, particularly if they're global in scope. You could potentially fiddle with the composition of the clouds to allow them to be effectively transparent to non-visible spectrum (for us) light, still allowing a great deal of solar radiation to strike and heat the planet, provide power for photosynthesis, etc.

Unfortunately, you might run into a problem there; if life on Venus adapted to better utilize the wavelengths passing through their cloud cover, it would be much less of a visual obstacle to them than it would be to us. The impact of that would probably depend on how important the hiding of the outside world is to the Venusians.

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  • $\begingroup$ You make some good points I'll have to think about. I'm definitely OK with Venusians being a similar kind of life -- some of the sources I'm drawing information from were written before DNA's role in inheritance was known. $\endgroup$
    – James Jensen
    Jul 21, 2017 at 2:16
  • $\begingroup$ @James Jensen: For life, you could always invoke panspermia. Life originated on Earth (or perhaps on early Mars), microbes were blasted into space by asteroid impacts, so both planets share basic DNA &c. $\endgroup$
    – jamesqf
    Jul 21, 2017 at 5:08
  • $\begingroup$ Also, I edited the question to clarify that I don't mean pole-to-equator ice, just something like what Earth went through in the last glacial period. Does that change the issue you expressed in the middle paragraph? $\endgroup$
    – James Jensen
    Jul 21, 2017 at 9:57

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