Venus -- Why is it so HOT?
Venus is about the same size as
This page is an interdisciplinary
Venus was supposed to have been Earth’s twin sister (brother? cousin?) But its surface temperature is a scorching 450o C!
Why? What went wrong?
a. Well, we know it’s closer to the Sun than Earth is;
b. Its reflectivity is greater than Earth’s;
c. It seems to have active volcanoes on its surface;
d. It’s cloud cover is predominantly carbon dioxide.
The purpose of this site is to see what we can come up with and whether we can get some idea as to the
source of Venus’ problem.
On our Earth, this has been determined to be approximately 1400 watts/m2. Now, Earth is 1 A.U. from the Sun;
Venus is about 0.7 A.U. (that is, about 30% closer). Can we use this information to estimate the value of the
Sun’s radiative power falling on Venus per square meter?
There is a nice web site that describes a bit more how you (from your own back yard!) can determine your own
value for the solar constant of our Sun: http://www.amtsgym-sdbg.dk/as/solarconstant/
-- try it and see how close you can get (some students report getting within a factor of “2” of the experts’ value!)
How can we use this to learn the “solar constant” for Venus? Well, we know that as one moves away from a light
source, the intensity we see (its “apparent brightness”) decreases. The “rule” that describes this is:
apparent brightness = Luminosity (total power output) / 4pd2
where “d” is the distance from the observer to the source, and “Luminosity” is the “watt” rating of the source.
Since “apparent brightness” has units of watts/m2, it seems reasonable to identify it with the “solar constant”.
We will do this!
Now: If our distance to the Sun is “d”, and Venus’ distance is 0.7d, can we take a simple ratio to determine the
solar constant on the surface of Venus?
(in Kelvins) of a planet that lies a distance d from the Sun and that reflects a fraction of solar energy e:
Temp [Kelvins] = 280 * ( (1 - e) / d2 )1/4
(Be careful; here d must again be given in terms of the number of A.U. it lies from the Sun, and not in terms of kilometers,
as one might expect.) For Venus, use a value of 0.72 for e and 0.7 A.U. for d. You might be surprised at the result!
Repeat this calculation for our own Earth (use a value for e of 0.4 and 1 A.U. for d). Based on these, where would you
expect to be able to go cross-country skiing -- on Venus or on Earth?
sulfur dioxide, and water vapor. Yet the amount of CO2 in Earth’s atmosphere is quite low (a fraction of a percent),
whereas the amount of CO2 in Venus’ atmosphere is close to 98.5%. Where does Earth’s “excess” CO2 go?
Why couldn’t Venus do the same? Does Venus have “too many” volcanoes (many more than Earth)?
A great site for the uninitiated in volcano lore can be found at
We can get some estimates of the amounts and types of gases that are dumped into our atmosphere from this web
If we assume that Venus’ volcanic activity is similar to that of Earth, then we might also feel justified in applying these
data to Venus as well.
is responsible for the greenhouse effect? (Hint: See Bennett, pages 299-300.) How would the differences in the
amounts of CO2 account for the temperature differences of Earth and Venus?
A web site that provides an overview of the greenhouse effect is http://www.pbs.org/wgbh/nova/ice/greenhouse.html
More information can be found at a site maintained by the University of Oregon: http://zebu.uoregon.edu/1998/es202/l13.html
This site may help you to learn some techniques that scientists use in estimating the effect that different concentrations of
greenhouse gases may have on our atmosphere.