Mars has a much thinner atmosphere than Earth but it is 95% composed of CO2. One might therefore expect a larger greenhouse effect, since there are 20 times more CO2 molecules in the Martian atmosphere than there are on Earth. To quantify this I have run my greenhouse model (described here) for Mars, using the following parameters:

- lapse rate = 10.5 K/km
- g=3.7 m/s2
- P0 = 600 Pa
- Ts = 210 deg.C.
- Following Nasa we take the top of the troposphere as 7km.

The result is shown in Figure 1.

Fig 1: The up-going radiation flux within the 13-17 micron CO2 band for Martian atmosphere. The zero level is the flux for temperature 210K with zero CO2.

The reduction in outgoing flux at the ground from CO2 is 8 watts/m2. Using the formula DS/DT = 4.sigmaT^3 results in a net CO2 induced warming effect of just 2 degrees.C ! In fact measurements show that the greenhouse effect on Mars is in fact only between 1-5 degrees C. So how come the total warming is so small on Mars ? There are a lot of hand waving arguments about the low density being responsible, but the real difference between Mars and Earth is that the surface temperature is much cooler to start with. Total IR radiation to space goes as T^4 so there is far nearly 4 times less flux to be absorbed by CO2 molecules on Mars than on the Earth. So although there are 20 times more CO2 molecules above each square meter on Mars than on Earth, they absorb less net energy and consequently the greenhouse warming is less.

What about Venus ? I’ll look into that next.

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