From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of Queensland, 2004.
Activity # 8
ALIEN WORLDS*
Lesson at a Glance: Students compare data about Venus, Earth, and Mars and try to determine reasons for differences in global average temperatures.
Suggested Prerequisites: None
Focus Question: What are the principal factors that determine global temperature on Earth?
Background:
The radiant energy from the sun received by a planet in our solar system is called insolation (for incoming solar radiation). The amount of this energy that reaches the surface of a planet depends primarily on its:
- distance from the sun (if the Earth were 5% closer to sun, the oceans would boil off and form a dense atmosphere; if it were 1% farther away, the oceans would permanently freeze)
- reflectivity or planetary albedo (due in large part to cloud cover)
If we compare Venus, Earth, and Mars (see Planetary Comparisons Data table), we can see that there is a great difference between the expected average temperature of the planet (based on the amount of insolation reaching the surface) and the actual average temperature. This difference is due to a combination of the composition of the atmosphere and the total atmospheric pressure.
Solar energy enters the atmosphere principally in the form of shortwave radiation, much of it as visible light. Of the energy that is not reflected or scattered back into space, some is absorbed by the atmosphere, and some of it passes through and is absorbed by the planetary surface. There the energy is transformed into a longwave radiation and emitted to the atmosphere as heat. Molecules of certain atmospheric gases such as carbon dioxide, methane, and water vapour absorb infrared radiation that is radiated from the surface of the planet, keeping it warmer than it would otherwise be (the so-called greenhouse effect).
Venus, Earth, and Mars all formed in a similar manner by the accretion of solid material from condensation within the solar nebula. All probably had similar primary atmospheres, but these initial atmospheres were lost and replaced by secondary atmospheres generated by volatile compounds that were released to the atmosphere from the planetary interiors through volcanic degassing.
Because it is closer to the sun, Venus receives almost twice as much solar radiation as the Earth. As the planet formed, water vapour was unable to condense and remained in the upper atmosphere. High energy particles split the water molecules into hydrogen and oxygen. The lighter hydrogen atoms were lost to space in the solar winds and the oxygen combined with rocks at the planet’s surface. Sulphur gases generated by volcanism react with remaining moisture to form sulphuric acid clouds. These clouds reflect about 80% of the incoming radiation back to space. The surface of Venus receives only a little more than half the amount of radiation that the Earth’s surface does. In spite of this, the surface temperature on Venus is high because of an enhanced greenhouse effect due to high levels of carbon dioxide and high atmospheric pressure. The planet has no water, so carbon dioxide is not removed from the atmosphere through the hydrological cycle or deposited in the oceans as carbonate rock. Carbon dioxide accumulated (and is still accumulating) in the atmosphere through volcanic outgassing. The total atmospheric pressure (the weight of the atmosphere at sea level) on Venus is about 92 bars.
Mars is a smaller planet, having about half the diameter of Earth. Because of its lower mass, it cooled more rapidly and had a shorter period of volcanic outgassing to release carbon dioxide. Much of its early atmosphere was probably lost to space because there was too little gravity to hold it. Mars now has a very thin atmosphere with a total atmospheric pressure of only 0.007 – 0.010 bars. Therefore, although the atmosphere is composed of more than 95% carbon dioxide, the total atmospheric pressure and the amount of carbon dioxide (i.e., the partial pressure) are too low to induce a substantial greenhouse effect.
Earth has an atmosphere composed mostly of nitrogen (78%) and oxygen (21%) with a total atmospheric pressure of 1.014 bars. Water vapour is variable, and makes up from 0% to 4% of the atmosphere. Carbon dioxide averages about 0.035%. Earth’s early atmosphere was like those of early Venus and Mars, composed largely of carbon dioxide, water vapour, and nitrogen with a strong greenhouse effect. Because the Earth was further from the sun, the atmosphere cooled enough for water vapour to condense and fall as rain (for millions of years). The water filled basins to form the early oceans, where carbon dioxide was drawn from the atmosphere and reacted with dissolved salts to form carbonate rock. The appearance of carbonate-depositing cyanobacteria (stromatolites) about 3.5 billion years ago also helped to draw carbon dioxide from the atmosphere. The loss of water vapour and carbon dioxide left an atmosphere largely composed of nitrogen. The evolution of photosynthesizing bacteria and algae added oxygen to the atmosphere, which slowly rose to the current value of 21% by about 500 million years ago. If all of the carbonate rock in the crust of the Earth today was heated to release the carbon dioxide, the Earth would have an atmosphere similar to that of Venus.
The sun has become about 25-30% brighter since the Earth first formed. The loss of greenhouse gases from the atmosphere, in part controlled by the biosphere, has fortunately kept pace with this increase to maintain Earth’s surface temperatures within the range suitable for life.
Subjects: Earth Science, Geography
Key Syllabus Concepts:
Earth Science – the solar system & its formation; major earth systems (atmosphere)
Geography – living in physical systems
Assessment: Students can effectively interpret a data sheet and offer reasonable explanations for possible causes of temperature differences among the planets.
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From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of Queensland, 2004.
Key Vocabulary: albedo
atmospheric pressure
insolation
radiation
reflectance
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From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of Queensland, 2004.
Time: 45 minutes to one hour
Materials Needed:
(per team or small group of students)
Part A
Þ 3 thermometers
Þ ruler
Þ modelling clay
Þ heat lamp
Þ instruction/data sheet
Part B
Þ 2 thermometers
Þ 2 Styrofoam drink cups with lids
Þ black construction paper
Þ sticky tape
Þ sand or other weights (enough to stabilise the cups)
Þ heat lamp
Þ instruction/data sheet
Activity:
1. Show the students pictures of Venus, Earth, and Mars and ask them which planet they think would be the hottest? the coldest? Why? What factors do they think might influence the temperature? Record their ideas on the board.
2. Divide the students into teams or small groups and give each group a set of materials and an instruction sheet. Give them time to conduct the experiments and record their results. Pool the results from each group and discuss with the whole class.
3. Give each student a copy of the “Planetary Comparisons” data table and ask them to answer the following questions:
a) What is the difference between the expected and the actual average temperatures for each of the planets?
b) Which planet has the greatest difference between the expected and actual temperatures?
c) Which has the least difference?
d) What factors might be responsible for the differences? (This would include both the composition of the atmosphere and the total atmospheric pressure. This is illustrated by the fact that both Venus and Mars have mostly carbon dioxide atmospheres, but only Venus has a large difference between expected and actual temperatures)
Discussion Questions:
1. What do you think would be the difference between the expected and actual temperatures on Mercury? (none; both the expected and the observed mean temperatures are 167°C) Why? (Mercury has no atmosphere)
2. What are some possible consequences of changing the composition of our atmospheric?
Adaptations/Extensions:
Explore the relationship of increased atmospheric carbon dioxide and temperature by doing Activity # 7 – Balancing the Budget.
Activity adapted in part from “Experiments to Study our Atmospheric Environment” by Steve Businger, 1996.
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From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of Queensland, 2004.
Venus
Distance from Sun
107,000,000 km
Planetary Albedo
75%
(Photo credit NASA)
Earth
Distance from Sun
149,000,000 km
Planetary Albedo
30%
(Photo credit NASA)
Mars
Distance from Sun
223,000,000 km
Planetary Albedo
15%
(Photo credit NASA)
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From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of Queensland, 2004.
Planetary Comparisons Investigations
Part A: Distance
1. Below is a table showing the relative distance from the sun to Venus, Earth, and Mars. Distances are given in astronomical units (AU). One astronomical unit is the mean distance from the Sun to the Earth.
2. Put a strip of masking tape on the table with one end lined up with your heat lamp. Mark the positions of the planets using a scale of 1 AU = 10 cm.
3. Use the modelling clay to stand a thermometer at the location of each of the planets. Offset the thermometers slightly so that they are not shading each other.
4. Record the starting temperature of each of the thermometers. Turn the lamp on and record the temperatures every 3 minutes for the next 15 minutes.
Planet / Distance from Sun (AU) / Scale Distance(10cm = 1AU) / Temperature (°C)
Start / 3 min. / 6 min. / 9 min. / 12 min. / 15 min. / Total Temp. Change
Venus / 0.72
Earth / 1.00
Mars / 1.52
5. Graph your results on a separate piece of paper and answer the following questions:
a) Which “planet” had the greatest overall temperature change?
b) Was the change in temperature linear (a straight line) on your graph? Why or why not?
Planetary Comparisons Investigations
Part B: Reflectivity
1. Wrap the black paper around one of the Styrofoam cups and attach with tape. Leave the other cup white.
2. Put enough sand or other weights in the two cups so that they will not tip over when you insert a thermometer. Put on the lids and insert thermometers through the centre of the lids. Both thermometers should the same distance in, about halfway to the bottom of the cup.
3. Place the two cups an equal distance from the heat lamp. Record the starting temperatures.
4. Predict what will happen when you heat both cups for 10 minutes. Give a reason for your prediction.
I predict that:
because:
5. Turn on the lamp and record the temperatures every 2 minutes for the next 10 minutes.
Temperature (°C)Start / 2 min. / 4 min. / 6 min. / 8 min. / 10 min.
White
Black
6. Answer the following questions:
a) Was your initial prediction correct? Give a possible reason for the results you obtained.
b) Would the results be different if you had cups of other colours? Why or why not?
c) Would you get different results if you had water in the bottom of the cups? Why or why not?
How does this experiment relate to planetary temperatures?
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From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of Queensland, 2004.
Planetary Comparisons Data SheetPLANET / Distance from Sun
(x 100,000 km) / Total Sunlight
(w/m2) / Planetary Albedo
(%) / Sunlight reaching surface
(w/m2) / Expected Temperature (based on sunlight reaching surface)
(°C) / Surface Atmospheric Pressure
(mb) / Major Atmospheric Composition / Actual Temperature
(°C)
Venus / 107 / 2613.9 / 75 / 652 / -40 / 9200 / 96.5% CO2
3.5% N2 / 464
Earth / 149 / 1367.6 / 30 / 956 / -18 / 1014 / 78.1% N2
20.9% O2
~1% H2O / 15
Mars / 223 / 589.2 / 15 / 500 / -56 / 6.36 / 95.3% CO2
2.7% N2
1.6% Ar
0.1% O2 / -63
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From Focus on Corals: Global Climate & Reef Health, Bright Minds™, University of Queensland, 2004.
Activity # 10
BALANCING THE BUDGET
Lesson at a Glance: Students look at the effect of greenhouse gases on atmospheric temperatures by heating and cooling containers with different concentrations of carbon dioxide and measuring the temperature change.
Suggested Prerequisites: Activity # 5 – Alien Worlds, Activity # 6 – Detecting CO2
Focus Question: How could a change in atmospheric composition affect global temperature?
Background:
Maintaining a stable temperature and climate on Earth depends on sustaining a global balance of incoming and outgoing radiation averaged over time. Only a tiny fraction of the total energy radiated by the sun falls on the Earth. Averaged over the entire outer boundary of the atmosphere this equals about 342 watts per square metre. Most of this energy is in the form of electromagnetic radiation in short wavelengths that range from visible light to ultraviolet to x-rays. It is the flow of this energy through Earth systems that makes life on Earth possible.
Earth’s Energy Budget
About 6% of the radiation that reaches the upper atmosphere is reflected back into space by atmospheric gases and dust. Approximately another 20% is reflected back into space by clouds, and around 3% is absorbed by the water droplets in the clouds. An average of 16% is absorbed by the molecules of gases of the atmosphere, especially water vapour (H2O), carbon dioxide (CO2), methane (CH4), ozone (O3), and chlorofluorocarbons (CFCs). These are known as greenhouse gases because they are able to absorb certain wavelengths of electromagnetic energy and then release the energy at infrared wavelengths (i.e., as heat).