Lesson 5: Science and Resources
7/24/03 2:31 PM
The Science
Overview
By Activity
The Greenhouse Effect
Planetary Cycles
Habitable Worlds
Resources
Overview
This lesson focuses on why the Earth has environmental conditions that support liquid
water and life, while Venus and Mars do not. It ties together what students have
learned about the differences among Mars, Venus, and Earth in the rest of the module,
and answers the question, "Why Earth?" The answer lies in the global carbon cycle.
Venus has no liquid water, thus breaking the carbon cycle and leaving carbon dioxide
trapped in its atmosphere. This resulted in a very hot Venus. Mars likely had liquid
water at some time in its past, but its interior cooled (because of its small size, which
could not generate enough heat through radioactive decay to maintain a molten
interior). This ended plate tectonics, breaking the carbon cycle and leaving carbon
dioxide trapped in its rocks. With little carbon dioxide left in the atmosphere, the
temperature on Mars dropped to below freezing. The unbroken carbon cycle on Earth
helps maintain moderate temperatures. Changes in one part of the carbon cycle cause
changes in other parts, which serve to bring the system back into equilibrium.
The Greenhouse Effect
There are several factors that contribute to the temperature and climate of a planet.
Distance from the Sun is the most obvious factor affecting the temperature of a planet;
however, distance from the Sun alone cannot explain the warmth of Venus and Earth.
Albedo is another factor affecting temperature. Albedo is a measure of how much light
is reflected and how much is absorbed by an object, expressed as a fraction of the
sunlight reflected. A perfectly black body has an albedo of zero (no light reflected),
while a perfectly white body has an albedo of one (all the light is reflected). Features
such as clouds, ice, and snow often have albedos of approximately 0.7 or more, which
means that they reflect 70% or more of the sunlight that hits them. For comparison,
rocks typically have low albedos, from 0.1 to 0.25. A lower albedo increases the
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temperature of a planet because more light is absorbed than reflected. Features such
as clouds, ice, and white sand give a planet high albedo. Features such as oceans and
dark rock give a planet low albedo.
The third factor affecting the temperature of a planet is greenhouse strength. This is a
combination of several factors: the pressure of the atmosphere (related to how much
atmosphere there is; its thickness); the percentage of the total atmosphere that is
composed of greenhouse gases; and the effectiveness of those greenhouse gases.
Credit: SETI Institute
The greenhouse effect is a physical phenomenon that we have all experienced directly.
Think of getting into a car on a hot summer day and finding that the temperature in
your car is several degrees hotter than the temperature outside. The light from the Sun
shines in through your car windows and heats up the interior. The resulting heat travels
upward as infrared radiation and cannot escape through the glass window. This traps
the heat inside the car. The same thing happens to the Earth. Light from the Sun heats
up the surface. The resulting heat travels upward, but is trapped by the Earth's
atmosphere. Some gases, such as CO2 and water vapor, are more effective than other
gases at trapping this heat. Such gases are known as greenhouse gases. The
greenhouse gases absorb the infrared (heat) radiation rising from the Earth's surface.
As a result they warm up and emit their own infrared (heat) radiation in all directions.
Some of this re-emitted infrared radiation is directed back toward the surface and the
lower atmosphere, warming them even more than the sunlight alone did. High in the
atmosphere there is not enough gas to trap the heat, so the greenhouse effect is
concentrated in the lower atmosphere, near the surface.
Without the greenhouse effect, the Earth would be a much colder place. Greenhouse
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gases in the Earth's atmosphere keep the planet 23 °C (73 °F) warmer than it
otherwise would be. This may have been especially helpful early in the Earth's history,
when scientists think the Sun gave off less light and energy than it does now. An
earlier—and perhaps stronger (due to the presence of more carbon dioxide in the
early Earth's atmosphere)—greenhouse effect may have helped make the Earth more
conducive to the development of life.
An increase in greenhouse gases, such as an increase in CO2 due to car exhaust
emissions, could increase the temperature of a planet. Many scientists argue that
human emissions of greenhouse gases, especially carbon dioxide from the burning of
fossil fuels, have caused an already measurable increase in the surface temperature of
the Earth. While an increase has been observed, it is not entirely clear if this increase is
solely the result of human activities, or part of the natural cycle of variations in
temperature. Earth's climate is extremely complicated and complex, and untangling
what causes climate changes is difficult. Nevertheless, most scientists agree that,
whatever the cause, global warming is taking place.
This activity refers to temperatures in degrees Kelvin. In the Kelvin temperature scale
(named after the British physicist Lord Kelvin, William Thompson, who studied heat
and temperature), zero degrees corresponds to absolute zero, the temperature at
which all molecular motion stops. Absolute zero is the coldest temperature possible. In
the Kelvin temperature scale, water freezes at 273 K and boils at 373 K. Conversions
between the Kelvin, Celsius, and Fahrenheit temperature scales are as follows:
K = °C + 273
°C = 0.555 x (°F – 32)
°F = (1.8 x °C) + 32
Planetary Cycles
The global carbon cycle has played an important role in determining the climatic fates
of Mars, Earth, and Venus. The carbon cycle refers to the movement of carbon, in
one form or another, from the biosphere to the atmosphere, oceans, geosphere, and
back. Within this cycle there are reservoirs, where carbon is stored, and processes by
which the carbon moves from one reservoir to another. The following image shows the
major reservoirs and processes.
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Credit: SETI Institute
The Earth is a closed system, which means that there is a fixed total amount of carbon
in the world (the combined total amount of carbon in all the reservoirs). However, the
amount of carbon in any given reservoir can change over time. Here's how the carbon
cycle on Earth works. Carbon dioxide in the atmosphere is dissolved into the oceans
and into rainwater. The rainwater (which acts like a weak acid called carbonic acid)
weathers and erodes rocks, forming bicarbonate ions in the groundwater. Plankton
and other sea organisms use the bicarbonate ions to build calcium carbonate shells. As
living things die and are buried on the ocean floor, the carbonate
becomes part of sedimentary rocks. Those rocks are carried to the edge of the ocean
plates by seafloor spreading. There they are subducted and melted. The melting
releases carbon back into the atmosphere as CO2 through volcanic eruptions. The
table below lists some of the major reservoirs of carbon and how much carbon is
locked up in each.
Estimated Major Stores of Carbon on the Earth
Sink
Atmosphere
Soil Organic Matter
Ocean
Marine Sediments and Sedimentary
Rocks
Terrestrial Plants
Fossil Fuel Deposits
Amount of Carbon
(in billions of metric tons)
578 (as of 1700) – 766
(as of 1999)
1,500 – 1,600
38,000 – 40,000
66,000,000 – 100,000,000
540 – 610
4,000
Credit: Okanagan University College
"Fundamentals of Physical Geography" online textbook
Clearly, the primary reservoir for carbon on the Earth is in marine sediments and
sedimentary rocks. Note that the Earth's carbon cycle is a relatively fast moving
recycling process. Each atom of carbon has been recycled many times. The following
diagram gives a detailed view of the various reservoirs and processes involved in the
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carbon cycle.
Credit: NASA
This cycle of carbon through the system allows the Earth to obtain a climatic
equilibrium. When one part of the cycle changes, other parts also change to bring the
system back into equilibrium. For example, an increase in temperature results in an
increase in evaporation. This increases rainfall and the removal of CO2 from the
atmosphere via erosion and weathering. The rate of CO2 being returned to the
atmosphere is not changing, so the amount of CO2 in the atmosphere decreases, and
the temperature drops again. This is an example of a negative feedback loop that will
help maintain moderate temperatures on Earth, even as the energy output from the Sun
increases over billions of years as it ages.
The breakdown of the carbon cycle on Venus and Mars is responsible for the
different temperatures on those planets, and provides the answer to what scientists
refer to as the "Goldilocks problem:" Why is Venus too hot, Mars too cold, and Earth
just right for liquid water and life? There is no liquid water on Venus into which carbon
dioxide can dissolve. As a result, carbon dioxide pumped into the atmosphere by
volcanic activity remains there, trapping heat, and raising the temperature of the
planet's surface to an extremely hot 464 °C (867 °F). The presence of so much
carbon dioxide in its atmosphere can also explain why Venus' atmosphere is so thick,
with nearly 100 times the pressure on its surface as that on Earth.
Mars, on the other hand, likely did have liquid water at one time. The break in the
carbon cycle for Mars occurred when the interior of the planet cooled. The planet is
too small to generate enough heat from radioactive decay to maintain a molten interior.
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Without a molten interior, plate tectonics could no longer take place. As a result,
Martian rocks were no longer subducted and melted, which meant that carbon dioxide
was no longer released through volcanic activity. However, carbon dioxide continued
to be extracted from the atmosphere by liquid water, resulting in a thinning of the
atmosphere, and a general planetary cooling as the amount of greenhouse gases in its
atmosphere steadily decreased. Today the average surface temperature on Mars is
below freezing, about -65 °C (-85 °F).
The fully functioning carbon cycle on the Earth moderates the surface temperature, as
described above, maintaining perfect temperatures for liquid water and life as we
know it.
Habitable Worlds
The definition of a habitable planet has expanded considerably with the discovery of
life in extreme environments such as hydrothermal ocean vents, Antarctic ice, and
boiling hot springs. The one requirement for life seems to be the stable presence of
liquid water. In our solar system, the habitable zone is the area around the Sun where
it is possible for liquid water to exist in oceans for the billions of years necessary for
life to develop and survive. This area extends from just inside the Earth's orbit (0.8
AU, where one AU, or Astronomical Unit, is the distance from the Earth to the Sun)
to just outside Mars's orbit (1.5 AU). Falling within that zone does not guarantee the
presence of liquid water, however. (Mars does not currently have liquid water
because it did not maintain enough of an atmosphere to allow it to stay warm.)
A habitable zone can also occur near sources of heat other than the Sun. The moons
of Jupiter, for example, have internal heating due to the tidal forces exerted by the
giant planet. Europa appears to have liquid water beneath the deep layer of surface
ice, evidenced by large fractures in the ice. If such an ocean does exist, Europa could
potentially be host to organisms similar to those found at hydrothermal ocean vents on
Earth.
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Resources
See
Web Help (available if you are connected to the internet) for updated
resources on the topics covered in this lesson. This will bring up a separate page of
updated web links with sites containing additional background science and teacher's
guides on global climate, the greenhouse effect, the carbon cycle, the water cycle, and
life in extreme conditions.
Print Recommendations
Hocking, Colin, et al. Global Warming and the Greenhouse Effect . Berkeley, CA:
LHS Publications, 1990. This book of classroom activities was produced by the Great
Explorations in Math and Science program at the Lawrence Hall of Science,
University of California, Berkeley.
Pollack, James B. "Atmospheres of the Terrestrial Planets." The New Solar System.
Eds. J. Kelly Beatty, and Andrew Chaikin. Cambridge, MA: Sky Publishing
Corporation & Cambridge University Press, 1990.
Kasting, James, et al. "How the Climates Evolved on
the Terrestrial Planets." Scientific American, Feb. 1988.
Soffen, Gerald A. "Life in the New Solar System?" The New Solar System. Eds. J.
Kelly Beatty, and Andrew Chaikin. Cambridge, MA: Sky Publishing Corporation &
Cambridge University Press, 1990.
Jakosky, Bruce M. "Searching for Life in Our Solar System." Scientific American,
March 1998. This article explores where in the Solar System life, including
extremophiles, could exist.
Madigan, Michael T. and Barry L. Marrs. "Extremophiles." Scientific American,
April 1997.
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