Procedures

1. Introduction

• Discuss greenhouse gases and the greenhouse effect. Refer to background section and to Tables 1 and 2 found in the Tables Section below.
• Ask students "Why do you think living organisms thrive on Earth but not on Venus or Mars?" 
• Discuss the "Goldilocks Principle" and how the Earth has just the right amount of greenhouse gases to keep Earth just the right temperature for living organisms.
• Tell students (in teams or pairs) that in this activity they will build models of the atmospheres of Earth and the other planets. Emphasize that models are critical tools for planetary scientists trying to understand phenomena too distant to experience directly.

2. Depending on the material available, ask students to represent the atmospheric gases with different colored beans, cotton balls, or jelly beans. We will use jellybeans in this activity.
   Example:

• Nitrogen (N₂) with red jellybeans
• Oxygen (O₂) with green jellybeans
• Argon (Ar) with purple jellybeans
• Carbon dioxide (CO₂) with yellow jellybeans
•              Methane              (CH₄) with white jellybeans

4. Challenge the students to produce a model atmosphere for each planet by placing the appropriate number of each color of jellybeans in each of three small, re-sealable plastic bags. The fraction of each different gas in the atmospheres of Venus, Earth and Mars is provided in Table 1 below. For additional information, refer to the Preparation section. Students will have to translate percentages into numbers of jellybeans, and in many cases, will face the difficulty of cutting the jellybeans into small enough pieces to represent small atmospheric concentrations.
    
5. Have students explain what they found to the class.

Tables needed for activities

Table 1

The table below lists the atmospheric factors responsible for the planetary differences. The relative distance from the Sun influences planetary temperature, but the greenhouse gases and atmospheric density also have a large impact on surface temperature. Venus has an extremely dense atmosphere (with a surface pressure more than 90 times Earth's). Conversely, Mars has an extremely thin atmosphere (with a surface pressure less than 1/100th of Earth's).

	Venus	Earth	Mars
Carbon Dioxide (CO2)	96.5%	0.03%	95%
Nitrogen (N2)	3.5%	78%	2.7%
Oxygen (O2)	Trace	21%	0.13%
Argon (Ar)	0.007%	0.9%	1.6%
Methane (CH4)	0	0.002%	0

Table 2

The chemical compositions of the atmospheres are important, especially in determining suitability for life. The major greenhouse gases (GHG) and their percentages are listed below.

	Venus	Earth	Mars
Surface pressure relative to Earth	92 bars	1 bar	0.006 bar
Major greenhouse gases (GHG)	CO2	H2O and CO2	CO2
Temperature if no GHG	-47° C (-52° F)	-19° C (-2° F)	-63° C (-82° F)
Actual Temperature	464° C (867° F)	15° C (59° F)	-63° C (-82° F)
Temperature change due to GHG	+511° C (919° F)	+34° C (61° F)	+0.2° C (0.4° F)

Note: Earth's atmosphere always has some water vapor in it; on a humid day, water vapor can account for more than 2% of the atmospheric gas in a specific place. However, the average amount of water vapor in Earth's atmosphere is around 1%. To make things simpler, scientists often refer to Earth's dry atmosphere, meaning all the gases except for the variable amount of water vapor. In this "dry atmosphere", nitrogen and oxygen comprise almost 99% of the gases in Earth's atmosphere. The remaining trace gases add up to less than 1% of Earth's dry atmosphere. The totals for each planet in Table 1 do not add up to exactly 100% because not all of the trace gases have been listed.