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*Venus -- Why is it so HOT?*	Venus is about the same size as Earth and only a bit closer to the Sun…

This page is an interdisciplinary learning
module created by geologists, chemists, physicists, and mathematicians. The module will help students and teachers learn more about why Venus is so HOT by using different tools and methods of these disciplines.

Venus was supposed to have been Earth’s twin sister (brother? cousin?) But its surface temperature is a scorching 450^o 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.

The solar constant is a measure of the power (“energy per unit time”) emitted by the Sun that strikes an area.

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 Bennett, et al., page 301, there is a compact formula that has been used to approximate the surface temperature

(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?

Earth has active volcanoes. These outgas (or emit during eruptions) a number of different compounds, including CO₂,

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

http://www.neiu.edu/~kbartels/VolcanoModule/VolcanoModuleHomePage.htm

We can get some estimates of the amounts and types of gases that are dumped into our atmosphere from this web

site: http://www.volcanoworld.org/vwdocs/vw_hyperexchange/Erupt_Rate.html

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.

CO₂ is present in the atmosphere of Venus at a much higher concentration than that of Earth. What characteristic of CO₂

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.

Links: http://www.jpl.nasa.gov/solar_system/planets/venus_index.html

© 2003 The NASA/UNCF Project at Northeastern Illinois University. Last updated October 24, 2003.
Participating departments: Chemistry Earth Science Mathematics Physics