Title: Atmospheric Composition and Temperature Regulation

Overview & Outcomes:

This lesson helps learners:

• determine that the higher the amount (mass) of carbon dioxide in the atmosphere, the higher the mean temperature and the narrower or lower the temperature range of a planet or moon.
• associate this phenomena with the "greenhouse effect."
• recognize that the greenhouse effect is a necessary condition for life on earth.

The concept map shows model relationships among concepts this lesson seeks to develop. Concepts introduced in this lesson are bolded on that concept map and concepts from other lessons are in plain text (not bolded).

Background Notes for the Teacher:

Percentage composition vs. Mass Activity-As part of this lesson, students will be examining temperature and atmospheric composition data about Mars, Venus, Earth and the moon. Unfortunately, when students look at the data and see that the atmosphere of both Mars and Venus contains over 95% CO2, they may draw the wrong conclusion that these two planets have similar atmospheres. As will be discussed in more detail later, Mars has very little atmosphere. Thus its "high" concentration of CO2 is misleading. For this reason, this lesson begins with an activity designed to help students experience the relationship between percentage composition and mass. Students will be first be asked to determine the percentage composition of 2 mixtures of colored beans or beads. Both will have similar percentage compositions, but contain different quantities (masses) of material. These mixtures will be representative of the relative concentrations of CO2, N2 and O2 in the atmospheres of Venus and Mars, although in the initial activity these planets will not be identified, only referred to as Planets A & B. Later in the lesson, students will reexamine their calculations and match the percentage concentration of the atmospheres of Planets A & B to Venus and Mars.

Information about Mars, Venus, the moon and the Earth-One way to investigate conditions on this planet (as well as to determine whether certain human activities can alter Earth's ability to sustain life), is to examine other planets with some characteristics similar to Earth, yet which are hostile to the life forms found on our world. For example, scientists think that about one and a half billion years ago, Mars and Earth were more similar than different. Although Mars still shares more of Earth's surface characteristics than any other planet in the solar system, somewhere in history the two planets took different evolutionary paths, leading to considerable differences in their present surface and atmospheric characteristics. Venus is the planet so like Earth that astronomers call it Earth's sister. The thick, carbon dioxide-rich atmosphere of Venus provides the perfect comparison to illustrate the effect that a higher carbon dioxide concentration can potentially have on temperature. The moon, which essentially shares the same "space" as the earth and therefore receives similar energy from the sun, also provides a comparison that illustrates how essential an atmosphere is. The information that the students will use to make their comparison is provided in short reading passages. A summary of that information is provided in the following

*dry air
** However, Mars has very little atmosphere, and the actual amount of CO2 is very small. Most of its CO2 is locked in crustal materials and ice.
*** Venus' atmospheric pressure is 90 times greater than Earth (equal to our at 3000 feet below the Atlantic Ocean!). Therefore, most of its CO2 is in the air.

The chart also gives the albedo of each planet. It's obvious from the data that albedo isn't enough to account for the temperature differences. For example, Venus has the highest albedo (least incoming solar radiation absorbed), yet it has the highest temperature. So, there must be a more important factor regulating temperature. Earth and Mars have similar albedos, but different temperature ranges, and the Moon has the lowest albedo (which means it absorbs most of the incoming solar radiation), but its temperature fluctuates from -173 to 130oC.

A common misconception is that the distance of a planet from the sun is the main variable controlling a planet's temperature. More important to planetary habitability is the planet's atmospheric composition (and all the process that control atmospheric composition). For example, it we put Earth in the orbit of Mars or Venus, changing its distance to the Sun, but holding constant its atmospheric composition and albedo, we find that although the new climate (temperature) is less pleasant, it is nothing like the present conditions on Mars or Venus.

It is important to note that while the atmosphere of Mars and Venus have a similar concentration of CO2 by volume (approximately 95%), they do not contain similar masses (i.e, actual amounts) of CO2. In other words, Mars has very little CO2 in its atmosphere whereas Venus has a tremendous amount. Also important is the fact that the temperature at the uppermost layer of Venus' clouds averages about 13_C or 55_F, quite different from the 475_C temperature on the surface. Two essential ideas are revealed by comparing the earth to these other planetary bodies:

1. The moon, in spite of the fact that it receives similar energy from the sun, has wide temperature extremes due to the fact that it has no atmosphere.

2. Venus has very high surface temperatures due to its high CO2 concentration and its surface temperature does not vary to any significant degree.

The bottom line is that the present climate or temperature regime and the ability of the Earth to support life is not an accident strongly dependent on the Earth being just the right distance from the Sun. Other variables, namely atmospheric composition and related processes, are paramount.

There are other "greenhouse gases" besides CO2 which were mentioned in Investigations Lesson 6 and will be described in more detail in Lesson 8. This lesson affords students the opportunity to examine actual data and draw their own conclusions about the role of the atmosphere in temperature regulation.

Additional Teacher References. Earth and space science texts and encyclopedias are useful. Much of the information presented here is from "Planet Earth-Can other planets tell us where we are going?" by A. Cherif and G. Adams (The American Biology Teacher, vol. 56 (1), January, 1994).

Materials:

For class:

1. Photographs, posters, diagrams or other references on the solar system, including photos of Venus, Earth, Mars and the Moon.

2. For optional activity: Terrarium (with material for dividing into parts, caulking compound or clay, Saran Wrap and another brand of plastic wrap, dark gravel and stones, high intensity light, thermometers). Posted data sheet.

For each group of students:

1. Materials for percentage composition activity: beans, beads or other small, sortable objects (3 different colors), Petri dishes or plastic bags for holding the beans/beads
2. Readings about Venus, Mars, the moon and Earth

For each student.

1. PLANET ATMOSPHERE Worksheets
2. WHAT'S IT LIKE ON...? or similar Worksheet to compile data about Venus, Mars the moon and Earth

Preparation:

Day of the lesson. Duplicate readings for cooperative groups and worksheets for each student. Assemble beans/beads for percentage composition of atmosphere activity.

Instructional Procedures: (1 Day, 40 minutes)

1. Students will be given beans or beads in quantities representing the relative concentrations of the 3 major gases that make up the atmosphere of Mars and Venus. The planets are not identified, only referred to as Planets A and B. Students will later be asked to identify the hypothetical planets based on the percentage composition of the atmospheres. Specific instructions for this activity follow. A model worksheet is provided at the end of the lesson, entitled WHAT'S IT LIKE ON...?. Set up the Unknown Planet beans/beads as follows:

   Obtain three different colors or types of beans that can be easily distinguished from one another. Establish a key which identifies one type or color as N2, another as O2 and the third as CO2. Then assemble the mixtures as follows:

   Unknown Planet A (representing Venus):		Total beans/beads - 200
   	N2	7
   	O2	0
   	C02	193
   Unknown Planet B (representing Mars):		Total beans/beads - 20
   	N2	1
   	O2	0
   	CO2	19

2. Students should be assigned to 4 home and (eventually) expert groups. Students will begin in their home groups. Each group will be given 2 plastic bags or dishes containing an assortment of beans or beads labeled Planet A and Planet B and a corresponding worksheet (one for each group member). Students should be instructed to empty all the beans/beads on the worksheet and count the number of each color. The total of each should be placed in the chart and the percentage of each calculated. Worksheets should be temporarily set aside.

3. Begin the next part of the lesson by determining what students know about Mars, Venus and the moon. (Review percent as needed.) Using cooperative group strategy Jigsaw I, assign students to expert groups (Mars, Venus, Earth and the moon).

4. Inform students that each of them will become an "expert" on either the moon, the earth, Mars or Venus, after which they will share their knowledge with other members of their home group. They will then be comparing the data they gathered to try to explain why the conditions on each surface are so different, and why only the earth can support life (as we know it).

5. Distribute readings, photos and worksheets. Allow sufficient time for expert groups to meet. Instruct students to return to their home groups, "teach" their group about their planet/moon and fill in all data on their worksheet. Students should be encouraged to discuss conditions on each planet. Next, each group should reexamine their findings and calculations from the first activity and determine the identities of Planets A and B. Finally, each group should answer all remaining worksheet questions.

6. As a class, discuss students' conclusions and summarize them on the board. Be sure to discuss the fact that life as we know it exists only on our planet because life has evolved and adapted to the conditions here.

7. Inform students that the role that the atmosphere plays in temperature regulation is referred to as the "greenhouse effect."

Optional Activity: Inform students that you have set up a terrarium which will represent the moon, the earth and Venus and that they will be collecting temperature data in order to test the conclusions they have just drawn. Temperature data may be collected in a number of ways and even graphed for making comparisons. The activity will be more likely to be successful if done in a cooler area of the room. Be sure to note beginning temperatures and take readings at close enough intervals to get a sense of the rate of heating (and cooling when the light is turned off to represent night). It is expected that the "moon" section will heat and cool at the fastest rate and that "Venus" will attain the highest temperature and exhibit the smallest range of temperature change.

Assessment/Portfolio Items:

Journal Entry.

Is the greenhouse effect necessary for life on earth?

Other.

Terrarium Data

The moon is a big ball of gray rock that has broad, flat plains covered with powdery rock dust. There are rugged mountains and thousands of billions of round pits called craters.

There is no air on the moon to scatter the sunlight, so even in the daytime, the sky is pitch black and filled with stars. (The blue colored sky on the Earth is caused by the atmosphere scattering sunlight.) The sole source of the moon's heat is from the Sun. Since the moon is essentially the same distance from the Sun as the earth is, you might expect it to have similar temperatures. However, this is not the case. During the moon's daylight hours, the rocky surface grows so hot it would burn you if you touched it (up to 130°C or 266°F). During the night, the moon grows colder than the coldest place on earth (down to -173°C or -280°F). The moon has an average albedo value of 7.

With no air, there is no sound on the moon and no wind to stir the rock dust. If it ever did have a surrounding layer of gases, it would have leaked away into space because of the moon's weak gravity. The moon is a still, silent, barren, lifeless place, unlike any part of the earth.

Venus, the second planet, is found about 108 million kilometers or 67, 250,000 miles from the sun. It is often called the Earth's sister planet because it so closely resembles our world in size and composition. Venus however is covered in thick clouds that completely hide the surface of the planet. Because these bright clouds act like a mirror and reflect 80% of the sunlight, many people thought that the surface temperatures on Venus would be similar to here on Earth. That is not the case though! The surface is much too hot (475°C) to support any life such as we have here. The temperature does not even vary from day to night, but stays very hot all the time. In fact, Venus is actually hotter than the planet Mercury which is closer to the sun! The bright clouds also contribute to Venus' average albedo of 80.

Studies of Venus have shown that its atmosphere is different from the earth's. First of all, there is more atmosphere than here on Earth, which causes the total surface pressure or atmospheric pressure to be much higher. Venus' atmospheric pressure is 90 times greater than the Earth's which is roughly equal to the pressure you would find if you went down 3000 feet below the Atlantic Ocean (about 90,000 millibars)! Why is this atmospheric pressure so much higher? It is mostly because of the enormous amounts of carbon dioxide in the atmosphere. 96.5% of the atmosphere is made up of CO2. (By contrast, CO2 only makes up about 0.03% of the earth's atmosphere.) The remainder of the atmosphere is mostly nitrogen (3.5%) and a trace of oxygen (less than .01%).

The clouds of Venus are made of droplets of sulfuric acid - a chemical so strong it can dissolve metal. Terrible windstorms rage high in Venus' atmosphere with winds that blow harder than the strongest hurricane that ever blew on Earth. The sky is lit by constant flickers of lightning, flashing as often as twenty times a minute. There is a constant rumble of thunder.

The surface of Venus is one great, bleak desert. The ground is so hot, it would melt lead. There is no water, nor any other liquid, anywhere. Not exactly a great place to visit!

Mars, the red planet, is the fourth planet from the sun. It is 1.5 times farther from the sun than the earth (about 228 million kilometers or 141,700,000 miles) and about half as large as the earth. Mars is similar to Earth in its rotation around the sun and its moons (it has 2).

The atmosphere of Mars is VERY thin. Its atmospheric pressure ranges from 6 to 10 millibars (compared to 1000 millibars on Earth). When this thin atmosphere is analyzed it is found to be mostly carbon dioxide (approximately 95.32%). While this sounds like a lot of CO2, it really is very little due to the fact that Mars has so little atmosphere to begin with. This large number simply means that of the few gas molecules there are, most of them are CO2. The remainder of the atmosphere is nitrogen (2.7%) and oxygen (.13%).

The surface temperature of Mars is also different from Earth. Because Mars is tilted similarly to the earth, it also has seasons. In the winter, it can get as cold as -124°C (-191°F) and in the summer as "warm" as -31°C (or -24°F). It's average albedo is 22.

Mars has thin blue and thick white clouds that move across its sky. Fierce windstorms whirl sand up from the plains and fill the air with dust. This dust gives the Martian sky its strange orange color. There are inactive volcanoes on Mars. One of them is three times higher and many times wider than Mount Everest.

The earth looks like a big, blue marble to astronauts flying in outer space. The blue color is due to the fact that nearly three-fourths of the surface of the earth is covered with water. There is also water in the air called clouds.

All around the surface of the earth is a layer of air which is hundreds of miles high. It is made up mostly of nitrogen gas (78%), oxygen (21%), and argon (1%). Carbon dioxide, which is essential to organisms that carry out photosynthesis, makes up only 0.03% of the atmosphere. The atmospheric pressure varies depending on elevation, but at sea level is it measured at about 1000 millibars.

The earth is the third planet from the sun (at a distance of about 150 million kilometers or 93,000,000 miles). It is between Venus and Mars. The temperature of the earth varies according to the seasons and the latitude (distance from the equator). Generally, temperatures can get as cold as -40°C and as warm as 75°C. The earth has an average albedo of 30.

1. Compare the above data to the Planet A & B data and calculations. Identify Planets A & B.

   Planet A ___________________________ Planet B _____________________________
   Compare the percentage of CO2 in each atmosphere. Are they similar?

   Explain: _________

   _______________________________________________________________________

   Now, compare the relative quantity or mass of CO2 in each atmosphere. Are they similar?

   Explain: _________________________________________________________________

2. Could the earth have more molecules of CO2 in its atmosphere than Venus? ________

   Could the earth have more molecules of CO2 in its atmosphere than Mars? _________

   Explain your answers: _________________________________________________________

   ___________________________________________________________________________

3. Which of the three planets' atmospheres has the greatest amount of CO2?

   ___________________________________________________

4. Which planet has the narrowest atmospheric temperature range? _________________________

5. Is there an association? What is the relationship between the amount of CO2 in a planet's atmosphere and its temperature range?

   ______________________________________________________________________________

   ______________________________________________________________________________

   ______________________________________________________________________________

6. If the moon is about the same distance from the sun, and therefore receives about the same amount of radiant energy, why is the temperature there so different?

   ______________________________________________________________________________

   ______________________________________________________________________________

   ______________________________________________________________________________

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