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Today it's well known that the surface of Venus is incredibly hot, over 800 °F (427 ºC). Depictions of Venus in fiction, however, used to portray it as something close to Earth temperatures, generally as a humid swamp.

When did we first figure out that it was so hot? Was it known to be hot before the Mariner 2 mission in 1962? Was it even suspected to be hot before then?

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up vote 11 down vote accepted

Googling "history of Venus knowledge" yields a link to this:

The discovery of hot millimeter waves radiation omitted [sic] by Venus made from Earth-based radioastronomy observations at the end of the 1950’s was the first evidence that Venus is a hot planet Mayer et al. (1958).

[Update] and this:

The first successful flyby of Venus was performed by the NASA Mariner 2 spacecraft whose radiometer confirmed the radioastronomy observations of an extremely high surface temperature of 460 C, under a cloud-topped carbon dioxide atmosphere (Sonett 1963).

in [pdf] History of Venus Observations - Springer where the reference to Mayer is to this paper:

C.H. Mayer, T. McCullough, R. Sloanker, Observations of venus at 3.15 cm wavelength. Ab. J. 127, 1–10 (1958)

and that to Sonett to this one:

C. Sonett, A summary review of the scientific findings of the Mariner Venus mission. Space Sci. Rev. 2, 751–777 (1963) 127.

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Locking for a few minutes to let everyone calm down. If you have a fundametal issue with editing or posting philosophy that needs to be hashed out, please take it to meta (and don't make it personal!). –  T.E.D. Aug 16 '13 at 13:21
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In your title you address habitability, but your question refers only to temperature, so I'm not sure which you're asking about. This answer focuses on habitability in general (although it includes temperature). That is why I am citing Venera 9, which landed on the surface, and dispelled certain doubts about Mariner's measurements, as well as adding information about factors theretofore not known or measured. (Question was subsequently edited to refer only to heat, so most of the discussion here becomes irrelevant to the question as posed, but does contribute something IMO to the question of 'Life on Venus' and the history of our exploration of that planet.)

I think it's safe to say that the first time we acquired what appears to be conclusive evidence concerning Venus's inhabitability was in 1975 was when the Russians orbited and landed on Venus for the first time with their Venera 9 mission:

Venera 9 - the orbiter was the first spacecraft to orbit Venus, while the lander was the first to return images from the surface of another planet

Here are some of the things we learned from Venera 9:

Supra:

Venera 9 measured clouds that were 30–40 km thick with bases at 30–35 km altitude. It also measured atmospheric chemicals including hydrochloric acid, hydrofluoric acid, bromine, and iodine. Other measurements included surface pressure of about 90 atmospheres (9 MPa), temperature of 485 °C, and surface light levels comparable to those at Earth mid-latitudes on a cloudy summer day. Venera 9 was the first probe to send back black and white television pictures from the Venusian surface showing shadows, no apparent dust in the air, and a variety of 30 to 40 cm rocks which were not eroded.

Let's see:

  • Atmospheric chemicals such as hydrochloric acid, hydrofluoric acid, bromine, and iodine:

    Certainly not life friendly material.

  • Surface pressure of about 90 atmospheres:

    Such pressures would be a virtually insurmountable challenge to life as we know it.

  • Temperature of 485 °C:

    That kind of heat would decompose and vaporize any of the chemical compounds necessary for life .

  • No apparent dust in the air, and a variety of 30 to 40 cm rocks which were not eroded:

    This would indicate no precipitation and an atmosphere that is perhaps dense and stagnant. Life as we know it needs water from rain and something circulating to breath

But, considering what we know today about terrestrial extremophiles, one could argue that perhaps in spite of all these apparent obstacles to life, life could still theoretically exist on Venus. Also, perhaps Venera landed in a very exceptional location, as has been mentioned in the comments.

In addition there are those who believe that in much earlier times, Venus might have been more hospitable to life. See: Studies have suggested that billions of years ago, the Venusian atmosphere was much more like Earth's than it is now, perhaps not unlike the depictions of Venus in fiction that you mentioned, and it is conceivable that life arose on Venus then and proceeded to move above or below the planet's surface as conditions there become inhospitable. See: Although the surface conditions on the planet are no longer hospitable to any Earthlike life that may have formed prior to this event, the possibility that a habitable niche still exists in the lower and middle cloud layers of Venus cannot yet be excluded

Conclusion:

According to our knowledge of conventional earthly life forms, based on data from Venera 9 we first learned that the surface of Venus is indeed uninhabitable in all likelihood.

Nonetheless, even assuming Venera landed in a typical Venusian location, and excluding the possibility of previous surface life that migrated, we still have no conclusive proof that the entirety of Venus is uninhabitable for any and all forms of Life. We don't even know about all the niches where life might exist on Earth - new habitats and novel forms of life are discovered here not infrequently, such as the extremophile forms mentioned above. As our knowledge of earth expands, it's becoming increasingly difficult to say with certainty that any particular celestial body is uninhabitable.

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and theoretically it could be an anomaly on the surface of Venus. E.g. were they to land that probe in one of the open pit sulfur mines in Indonesia, which are located in the calderas of active volcanoes, they'd also get a rather toxic atmosphere, landed in the midst of a sandstorm on a superhot day in the middle of the Sahara would lead to anomalous data too, as would landing it in the Marianas trench. Not saying Venus isn't a hellhole, but from a single data point provided by a probe that melted after a few minutes there's little to tell about the planet at large. –  jwenting Aug 16 '13 at 5:11
    
As @jwenting points out, the probe landed in just one place, and that could have been an anomaly. As such it really only confirmed the earlier findings of Mariner 2, which doesn't seem particularly "refutable", so I don't see why this would be seen as more irrefutable and conclusive than earlier results. –  Lennart Regebro Aug 16 '13 at 7:45
    
@jwenting - Atmospheric pressure is not local; The rest? Maybe-but such places on earth are the exception not the rule-it's not probable. But thanks, I edited my answer, my language was too strong, and I also realized the question is dubious but I pounced on overall habitability, not just temperature, which is why I went with Venera 9.(And I remember the excitement I felt when Venera 9 landed: Landing on Venus - wow!) –  comeAndGo Aug 16 '13 at 10:43
    
@LennartRegebro - in terms of temperature, I agree. But I focused on habitability as mentioned in the title - in that respect a surface landing and the data it produced exceeded what we knew from Mariner and clarified the situation greatly. (Barring the possibility that Venera did indeed land in an extraordinary place on the surface). –  comeAndGo Aug 16 '13 at 10:50
    
@Vector say that to the probe that descended to the bottom of the Marianas trench and got crushed. Or the climber at the top of Everest without an oxygen supply who suffocated for lack of breathable air. –  jwenting Aug 16 '13 at 13:26
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