Re: Venus' Uniform Global temperature could be dropped to habitable if ...?

Gabriel,

The average surface temperature on the planet Venus is
734.85K(=461.85C=863.33F).  For comparison, lead melts at only
600.5K(=327.5C=621.5F).  The high temperature is due not only to the
proximity to the Sun, but also to the runaway greenhouse effect in
Venus' incredibly thick atmosphere.  The atmospheric pressure on
Venus' surface is 92 times that of Earth; this is like being crushed 
under half a mile of water on Earth.
 http://en.wikipedia.org/wiki/Venus
 http://en.wikipedia.org/wiki/Runaway_greenhouse_effect#Positive_feedback_and_runaway_greenhouse_effect

Now we need to decide what "habitable" means in terms of temperature.  
On Earth, the temperature extremes are:

329.67K(=56.67C=134F) in Death Valley, California, July 10, 1913.
(http://hypertextbook.com/facts/2000/MichaelLevin.shtml)

184.72K(=-88.28C=-126.9F) in Vostok, Antarctica, August 25, 1960.
(http://hypertextbook.com/facts/2000/YongLiLiang.shtml)

To see the effect of the Cytherean atmosphere, consider the planet
Mercury, which is closer to the Sun but lacks an atmosphere.  Near its
equator Mercury's surface temperatures vary between 100K=(-173C=-279.4F)
on the night side of the planet and 700K(=427C=800.6F) at the subsolar
point, with a mean of 340K=(67C=152.6F).  Notice that even the hottest
spot on Mercury is not as hot as the average temperature on Venus.  At
Mercury's north pole, the average temperature is 200K=(-73C=-99.4F)
which is warmer than the coldest temperature recorded on Earth.  Is
Mercury therefore habitable?
 http://en.wikipedia.org/wiki/Mercury_%28planet%29

"In the absence of an atmosphere the Earth would average about 30
Celsius degrees (about 50 Fahrenheit degrees) lower than it does at
present."
(http://www.ems.psu.edu/~fraser/Bad/BadGreenhouse.html)

One answer to your question is that if Venus were not shrouded in its
thick atmosphere, the AVERAGE temperature would be habitable at its
present distance from the Sun.  Of course, the temperature would vary
quite a but from equator to pole, and from day to night just as it
does on Mercury.  The time from one sunrise to the next on Venus is
116.75 Earth days, which gives the day side a long time to heat up and
the night side a long time to cool down.

Another answer is that if Venus' atmosphere is maintained, then the
planet would need to orbit the Sun between Earth and Mars (between 1.1
and 1.4 astronomical units) in order for the surface temperature to be
habitable.  Doubling a planet's distance from the Sun reduces the
energy received from Sun by a factor of four -- it's an inverse square
law.  Of course, Venus would still be unhabitable for other reasons:
the crushing atmospheric pressure, the corrosive sulfuric acid in the
atmosphere, the lack of a strong magnetic field to shield the planet
from cosmic radiation, etc.


--Dr. Randall J. Scalise    http://www.phys.psu.edu/~scalise/