ASTR 1100

Group activity: Planetary atmospheres and the greenhouse effect

Introduction:

Two of the factors that affect a planet’s surface temperature are its distance from the Sun and the greenhouse effect.

[Another important factor is a planet’s reflectivity (or “albedo”) – this is a measure of how much  incoming sunlight is reflected back into space and it is largely determined by the planet’s colour and composition. A lighter colour (e.g. from snow or ice or clouds) will lead to more incoming sunlight being reflected back into space and therefore less warming. Due to its bright clouds, Venus has a much larger reflectivity than Earth, Mars, and Mercury. Despite this, it still has a much higher surface temperature than those planets due to an extreme greenhouse effect and  its relative closeness to the Sun. For the purposes of this activity, we will focus on the greenhouse effect and the distance from the Sun as the main determinants of a planet’s temperature.]

Planetary Data:

The following table shows some important properties of the atmospheres of the terrestrial planets. An important contributor to the greenhouse effect on a planet is the total amount of CO2 in its atmosphere.

The table shows both the total mass of each planet’s atmosphere and the percentage of the atmosphere that is made up of CO2 .

[Note: the mass of Mercury’s atmosphere is so small that, for the purposes of this activity, we can say that it effectively has no atmosphere. That is what is meant by the word “negligible” .]

Comparing the total amounts of CO2  in the terrestrial planets’ atmospheres:

To make the calculations a bit easier, in the table the atmospheric masses have also been “normalized” to the Earth’s atmospheric mass. This just means that we have set the mass of the Earth’s atmosphere equal to 1, and that the masses of the other atmospheres are written relative to that. For example, Venus’ atmosphere is 81 times more massive than Earth’s atmosphere.

To compare the total amount of CO2  in a planet’s atmosphere to the total amount of CO2  in Earth’s atmosphere, we could do the following calculation:

(Percentage of the planet’s atmosphere that is CO2) x (Mass of the planet’s atmosphere relative to Earth) Percentage of Earth’s atmosphere that is CO2

For example, using the data in the table, the calculation for comparing Venus with Earth is as follows:

96 x 81 0.04

= 194,400

This result tells us that there is almost two hundred thousand times more CO2  in Venus’ atmosphere than in Earth’s atmosphere!

Group questions

1.   Rank the four terrestrial planets in terms of the total amount of CO2  in their atmospheres. Show your calculations.

2.   What two factors do you think make Venus’ surface temperature so much higher than Earth’s?

3.   Why is Mercury’s night-time surface temperature so vastly different from its day-time surface temperature?

4.   Hopefully your calculation in question 1 showed that Mars has about 10 times more CO2  in its atmosphere than Earth does. Despite this, Mars’ average surface temperature is significantly lower than Earth’s. Is there a non-atmospheric factor that helps explain why Mars is cooler than the Earth?

5.   The greenhouse effect can have both beneficial and harmful impacts on the habitability of a  planet. Discuss the implications of both the beneficial and harmful effects of the greenhouse effect for life on Earth. Write down the main points of your discussion.