The Goldilocks Principle: A Model of Atmospheric Gases
Modified with permission from Global Climates - Past, Present, and Future,
S. Henderson, S. Holman, and L. Mortensen (Eds.). EPA Report No. EPA/600/R-93/126,
U.S. Environmental Protection Agency, Office of Research and Development, Washington,
DC. pp. 53 - 61.
This activity introduces students to the atmospheric differences between the
three "sister" planets in a graphic and hands-on way. They should
get an overall appreciation of the important similarities and differences. Students
will use this understanding later as they begin to appreciate the scope and
importance of the greenhouse effect on earth.
On earth, two elements, nitrogen ()
and oxygen (), make up almost
99% of the volume of clean, dry air. Most of the remaining 1% is accounted for
by the inert gaseous element, argon (Ar). Argon and the tiny percentage of remaining
gases are referred to as trace gases. Certain trace atmospheric gases help to
heat up our planet because they appear transparent to incoming visible (shortwave)
light but act as a barrier to outgoing infrared (longwave) radiation. These
special trace gases are often referred to as "greenhouse gases"
because a scientist in the early 19th century suggested that they function much
like the glass plates found on a greenhouse used for growing plants.
The earth's atmosphere is composed of gases (for example,
and ) of just the right types
and in just the right amounts to warm the earth to temperatures suitable for
life. The effect of the atmosphere to trap heat is the true "greenhouse
We can evaluate the effect of greenhouse gases by comparing Earth with its
nearest planetary neighbors, Venus and Mars. These planets either have too much
greenhouse effect or too little to be able to sustain life as we know it. The
differences between the three planets have been termed the "Goldilocks
Principle" (Venus is too hot, Mars is too cold, but Earth is just right).
Mars and Venus have essentially the same types and percentages of gases in
their atmosphere. However, they have very different atmospheric densities.
- Venus has an extremely dense atmosphere, so the concentration of
is responsible for a "runaway"
greenhouse effect and a very high surface temperature.
- Mars has almost no atmosphere; therefore the amount of
is not sufficient to supply a warming effect and the surface temperatures
of Mars are very low.
- Mars is much further away from the Sun than is Venus.
Earth has a very different type of atmosphere. Our atmosphere has much less
than Venus or Mars and our
atmospheric pressure is close to midway between the two (1/90th that of Venus
and 100 times that of Mars).
Many scientists believe that the composition of our atmosphere is due to the
presence of life. Life acts to keep Earth's atmosphere in a dynamic balance.
In other words, if life were to completely disappear, eventually our atmospheric
composition could come to closely resemble Mars or Venus. Only with life continually
producing oxygen through photosynthesis and removing and re-circulating
does Earth's atmosphere remain fairly stable.
This activity introduces students to the atmospheric differences between the
three "sister" planets in a graphic and hands-on way. Students need
not memorize the chemical compositions and pressures of the three atmospheres;
rather, the activity should give them an overall appreciation of the important
similarities and differences. Students will use this understanding later as
they begin to appreciate the scope and importance of the greenhouse effect on
earth and realize that rather than being a bad thing, the greenhouse effect
is critical for the survival of the biosphere.
- Students will understand that our two closest neighbors, Venus and Mars,
have very different atmospheres than Earth does in pressure and chemical composition.
- Students will be able to explain the "Goldilocks Principle" and
understand that Earth's moderate temperature is due primarily to its unique
Alignment to National Standards
National Science Education Standards
- Unifying Concepts and Processes, Grades K to 12, pg.117: "Models are
tentative schemes or structures that correspond to real objects, events, or
classes of events and that have explanatory power."
- Earth and Space Science, Grades 5 to 8, pg.160, Earth in the Solar System,
Item #1: "The earth is the third planet from the sun in a system that
includes the moon, the sun, eight other planets and their moons, and smaller
objects, such as asteroids and comets. The sun, an average star, is the central
and largest body in the solar system."
- Earth and Space Science, Grades 5 to 8, pg.160, Structure of the Earth
System, Item #8: "The atmosphere is a mixture of nitrogen, oxygen, and
trace gases that include water vapor. The atmosphere has different properties
at different elevations."
Benchmarks for Science Literacy, Project 2061, AAAS
- Common Themes, Models, Grades 6 to 8, pg. 269, Item #1: "Models are often
used to think about processes that happen too slowly, too quickly, or on too
small a scale to observe directly, or that are too vast to be changed deliberately,
or that are potentially dangerous."
- The Earth, Grades 6 to 8, pg. 68-9, Item #2: "The earth is mostly rock.
Three-fourths of its surface is covered by a relatively thin layer of water
(some of it frozen), and the entire planet is surrounded by a relatively thin
blanket of air. It is the only body in the solar system that appears able
to support life. The other planets have compositions and conditions very different
from the earth's."
- Grade level: 6 to 9 (Note: Younger grades might enjoy the model construction
but may have difficulty translating the abstract model into an understanding
of the real atmospheres.)
- Teacher's introduction and instructions: 30 minutes
- Student model-building: 30 minutes
- Discussion/assessment: 30 minutes
- Colored cotton balls, jellybeans, or different colored beans (or similar
materials) to represent gases in the atmosphere
- Re-sealable plastic bags
- Discuss the "Goldilocks Principle." Use the information in Tables
1 and 2 to engage the class in a discussion of the greenhouse effect. If available,
you may want to share illustrations or slides of Mars, Venus, and Earth.
- After discussing the atmospheres of Earth and the other planets, ask the
students (in teams or pairs) to 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.
- 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 for examples in this activity.) They might represent:
- Nitrogen () with red
- Oxygen () with green
- Argon (Ar) with purple jellybeans
- Carbon dioxide ()
with yellow jellybeans
- Methane () with white
Representing atmospheric density with jellybeans is impractical
- if Earth's atmosphere has 100 jellybeans, Venus will have 9,000,
and Mars will have slightly more than 1/2 jellybean (0.6). Suggest that
the students use 10 or 100 as the base number for each planet. Let the students
know what the real differences in density are.
- Challenge the students to produce a model atmosphere for each planet by
placing the appropriate number of jellybeans in three small, re-sealable plastic
bags. The necessary information is provided in Table 2. They 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.
- Have students explain what they found to the class.
- To extend this activity, you could take one set of bags and distribute the
contents in areas measured to represent atmospheric pressure of each planet.
For example, the jellybeans representing Earth might be distributed in a meter
square. You might have to go outside to find an area big enough to represent
the thin atmosphere found on Mars. To concentrate the beans representing the
dense atmosphere of Venus, you could use a food processor or mortar and pestle
to concentrate the jellybeans.
- Have the students display their work in the classroom and allow time for
them to observe and discuss each other's work as you circulate.
- Have students respond in writing to the following questions:
- Describe the atmospheric conditions you might encounter as an astronaut
setting foot on Venus and Mars.
- Name at least two ways that the atmospheres of Venus and Mars are similar
to each other, and one way that both differ from Earth's.
- What new information did you learn in this lesson? What did you already
know? What was the hardest thing about the activity?
Modifications for Alternative Learners
- Students who have difficulty with math concepts should be paired with students
strong in math to avoid frustration with the fractions.
- Students with language difficulty should be assisted directly at the beginning
of the model building to make sure they understand the instructions.
The table below lists the atmospheric factors responsible for the planetary
differences. The relative distance from the Sun has some influence on planetary
temperature, but the greenhouse gases and atmospheric density have more of an
impact on temperature. Venus has an extremely dense atmosphere (with a surface
pressure 90 times that relative to Earth's). Conversely, Mars has an extremely
thin atmosphere (with a surface pressure less than 1/100th of that relative
The chemical composition of the atmospheres are important (at
least to the presence of life). The major greenhouse gases (GHG) and their
percentages are listed below.
Note: In the Earth's dry atmosphere, nitrogen and oxygen comprise
almost 99% of the gases. The remaining trace gases (with the exception of
water vapor), add up to less than 1% of gases found in Earth's atmosphere.
The amounts on the table below do not add up to exactly 100% because not all
of the trace gases have been included. The table below includes only the principal
gases found in the Earth's dry air. Water vapor, an important greenhouse gas,
is not addressed in this activity.
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