The Runaway Greenhouse Effect on Venus
Why is Venus so hot? And if, as seems likely, Venus started off like Earth then why is its atmosphere now so different from Earth’s? The answer to the first question is easy Venus is hot because of the greenhouse effect (the trapping of heat by gases in the atmosphere) Venus dense atmosphere is made up almost entirely of a prime greenhouse gas, carbon dioxide. This thick blanket absorbs about 99 percent of all the infrared radiation released from the surface of Venus and is the immediate cause of the planet’s sweltering 457°C surface temperature. The answer to the second question is more complex, however, and requires us to consider in detail how the atmospheres of the terrestrial planets—Mercury, Venus, Earth, and Mars—formed and evolved.
The atmospheres of terrestrial planets did not come into being along with the planets themselves. Instead, they developed over many millions of years from gases released from the planets’ interiors by volcanic activity—a process called outgassing. Volcanic gases are rich in water vapor, carbon dioxide, sulfur dioxide, and compounds containing nitrogen. On Earth, as the surface temperature fell and the water vapor condensed, oceans formed. Most of the carbon dioxide and sulfur dioxide dissolved in the oceans or combined with surface rocks. Solar ultraviolet radiation liberated nitrogen from its chemical bonds with other elements, and a nitrogen-rich atmosphere slowly appeared.
The basic mechanism controlling the level of carbon dioxide in Earth’s atmosphere is the competition between its production by volcanic and human activity and its absorption by the rocks and oceans that make up our planet’s surface. This constant recycling of atmospheric carbon dioxide is known as the carbon cycle. The presence of liquid water accelerates the absorption process—carbon dioxide dissolves in water eventually reacting with the surface material to form carbonate rocks. At the same time, plate tectonics (the movement of the plates that form Earth’s outer shell) steadily releases carbon dioxide back into the air. The level of carbon dioxide in Earth’s atmosphere today is the result of a balance between these opposing forces.
The initial stages of atmospheric development on Venus probably took place in much the same way as just described for our own planet. The real difference between Earth and Venus is that the greenhouse gases in Venus’ atmosphere never left the atmosphere as they did on Earth. Indeed, if all the dissolved or chemically combined carbon dioxide on Earth were released back into our present-day atmosphere, its new composition would be 98 percent carbon dioxide and 2 percent nitrogen, and it would have a pressure about 70 times its current value. In other words, apart from the presence of oxygen (which is a consequence of life on Earth) and water (whose absence on Venus will soon be explained). Earth’s atmosphere would look a lot like that of Venus.
To understand what happened on Venus, imagine taking Earth from its present orbit and placing it in that of Venus. Being closer to the Sun, our planet would warm up. More water would evaporate from the oceans, leading to an increase in atmospheric water vapor, which, like carbon dioxide, is a greenhouse gas. Because the ability of both the oceans and surface rocks to hold carbon dioxide diminishes with increasing temperature, more carbon dioxide would enter the atmosphere. The additional greenhouse heating would warm our planet still further, leading to a further increase in atmospheric greenhouse gases, and so on. This runaway greenhouse effect would eventually lead to the complete evaporation of the oceans, restoring all the original greenhouse gases to the atmosphere. Essentially the same thing must have happened on Venus long ago, leading to the planetary furnace we see today.
The greenhouse effect on Venus was even more extreme in the past, when the atmosphere also contained water vapor. By adding Io the blanketing effect of the carbon dioxide, the water vapor helped push the surface of Venus to temperatures perhaps twice as hot as at present. At those high temperatures, the water vapor was able to rise high into the planet’s upper atmosphere—so high that solar ultraviolet radiation broke it up into its components hydrogen and oxygen. The light hydrogen rapidly escaped, the reactive oxygen quickly combined with other atmospheric gases, and all water on Venus was lost forever.
P1: Why is Venus so hot? And if, as seems likely, Venus started off like Earth then why is its atmosphere now so different from Earth’s? The answer to the first question is easy Venus is hot because of the greenhouse effect (the trapping of heat by gases in the atmosphere) Venus dense atmosphere is made up almost entirely of a prime greenhouse gas, carbon dioxide. This thick blanket absorbs about 99 percent of all the infrared radiation released from the surface of Venus and is the immediate cause of the planet’s sweltering 457°C surface temperature. The answer to the second question is more complex, however, and requires us to consider in detail how the atmospheres of the terrestrial planets—Mercury, Venus, Earth, and Mars—formed and evolved.
1.The word “immediate” in the passage is closest in meaning to
A.likely
B.specific
C.direct
D.obvious
2.According to paragraph 1. which of the following is true about Venus’ atmosphere?
A.It prevents most of the heat from Venus surface from going into space.
B.It is easily damaged by infrared radiation.
C.It becomes less dense as it absorbs infrared radiation from the surface.
D.It consists of a greater variety of greenhouse gases than are present in the atmospheres of other terrestrial planets.
P2: The atmospheres of terrestrial planets did not come into being along with the planets themselves. Instead, they developed over many millions of years from gases released from the planets’ interiors by volcanic activity—a process called outgassing. Volcanic gases are rich in water vapor, carbon dioxide, sulfur dioxide, and compounds containing nitrogen. On Earth, as the surface temperature fell and the water vapor condensed, oceans formed. Most of the carbon dioxide and sulfur dioxide dissolved in the oceans or combined with surface rocks. Solar ultraviolet radiation liberated nitrogen from its chemical bonds with other elements, and a nitrogen-rich atmosphere slowly appeared.
3.According to paragraph 2, which of the following is true about the development of the atmospheres of terrestrial planets?
A.The atmospheres of terrestrial planets formed later than the terrestrial planets themselves did.
B.The original atmospheres of terrestrial planets contained large amounts of nitrogen until nitrogen formed compounds with other elements.
C.The formation of the atmospheres of terrestrial planets was disrupted by volcanic activity in the planets’ interiors.
D.After the atmospheres of terrestrial planets first formed, ultraviolet radiation from the Sun maintained the chemical bonds within their gases.
4.Paragraph 2 suggests that the cooling of Earth’s surface caused which of the following?
A.An increase in the amount of water vapor in Earth’s atmosphere
B.An increase in the strength of chemical Ponds between nitrogen and other elements
C.A decrease in the amount of sulfur dioxide in Earth’s atmosphere
D.A reduction in the amount of nitrogen released into Earth’s atmosphere
P3: The basic mechanism controlling the level of carbon dioxide in Earth’s atmosphere is the competition between its production by volcanic and human activity and its absorption by the rocks and oceans that make up our planet’s surface. This constant recycling of atmospheric carbon dioxide is known as the carbon cycle. The presence of liquid water accelerates the absorption process—carbon dioxide dissolves in water eventually reacting with the surface material to form carbonate rocks. At the same time, plate tectonics (the movement of the plates that form Earth’s outer shell) steadily releases carbon dioxide back into the air. The level of carbon dioxide in Earth’s atmosphere today is the result of a balance between these opposing forces.
5.According to paragraph 3, which TWO of the following cause a reduction in the level of carbon dioxide in Earth’s atmosphere? To receive credit, you must select TWO answers.
A.The presence of liquid water on Earth’s surface
B.The activity of volcanoes
C.The formation of carbonate rocks
D.The movement of Earth’s plates
P4: The initial stages of atmospheric development on Venus probably took place in much the same way as just described for our own planet. The real difference between Earth and Venus is that the greenhouse gases in Venus’ atmosphere never left the atmosphere as they did on Earth. Indeed, if all the dissolved or chemically combined carbon dioxide on Earth were released back into our present-day atmosphere, its new composition would be 98 percent carbon dioxide and 2 percent nitrogen, and it would have a pressure about 70 times its current value. In other words, apart from the presence of oxygen (which is a consequence of life on Earth) and water (whose absence on Venus will soon be explained). Earth’s atmosphere would look a lot like that of Venus.
6.Why does the author discuss what Earth’s atmosphere would be like “if all the dissolved or chemically combined carbon dioxide on Earth were released back into our present-day atmosphere”?
A.To challenge the idea that the atmospheres of Earth and Venus underwent similar initial stages of development
B.To argue that Earth’s atmosphere was highly affected by the presence of life on Earth
C.To identify developments in Earth’s atmosphere that made the appearance of oxygen and water possible
D.To explain why Earth never became as hot as Venus despite similarities in the planets initial development
P5: To understand what happened on Venus, imagine taking Earth from its present orbit and placing it in that of Venus. Being closer to the Sun, our planet would warm up. More water would evaporate from the oceans, leading to an increase in atmospheric water vapor, which, like carbon dioxide, is a greenhouse gas. Because the ability of both the oceans and surface rocks to hold carbon dioxide diminishes with increasing temperature, more carbon dioxide would enter the atmosphere. The additional greenhouse heating would warm our planet still further, leading to a further increase in atmospheric greenhouse gases, and so on. This runaway greenhouse effect would eventually lead to the complete evaporation of the oceans, restoring all the original greenhouse gases to the atmosphere. Essentially the same thing must have happened on Venus long ago, leading to the planetary furnace we see today.
7.Why does the author ask the reader to “imagine taking Earth from its present orbit and placing it in that of Venus”?
A.To help the reader understand the sequence of events that likely led to Venus’ intense heat
B.To convince the reader that Earth’s orbit has remained the same since Earth’s origins
C.To correct a misunderstanding that the reader may have had about Venus orbit
D.To raise a question in the reader’s mind about the future of Earth’s atmosphere
8.All of the following are mentioned in paragraph 5 as possible causes of greenhouse heating EXCEPT
A.an increase in the amount of water vapor in the atmosphere
B.a decrease in the ability of the oceans to hold carbon dioxide
C.a decrease in the amount of carton dioxide in surface rocks
D.an increase in the total amount of greenhouse gases in the oceans
P3: The basic mechanism controlling the level of carbon dioxide in Earth’s atmosphere is the competition between its production by volcanic and human activity and its absorption by the rocks and oceans that make up our planet’s surface. ■This constant recycling of atmospheric carbon dioxide is known as the carbon cycle.■ The presence of liquid water accelerates the absorption process—carbon dioxide dissolves in water eventually reacting with the surface material to form carbonate rocks. ■At the same time, plate tectonics (the movement of the plates that form Earth’s outer shell) steadily releases carbon dioxide back into the air.■ The level of carbon dioxide in Earth’s atmosphere today is the result of a balance between these opposing forces.
9.Look at the four squares [■] that indicate where the following sentence could be added to the passage.
It is made up of two separate stages.
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
10.
A.Both Venus’ and Earth’s atmospheres were originally made up of gases released from their interiors by the activity of volcanoes but they later underwent different developments.
B.There is debate about why Earth and Venus developed so differently, given the similarities between their origins and their similar geological and chemical compositions.
C.Venus’ high surface temperature led to greenhouse gases increasing in its atmosphere, which led to further temperature rises, eventually causing all of Venus’ liquid water to disappear.
D.Conditions on Earth allowed its atmosphere to become rich in nitrogen, oxygen, and controlled levels of greenhouse gases, while Venus’ atmosphere still consists largely of carbon dioxide.
E.The carbon cycle works differently on Venus than it does on Earth because there are fewer rocks on Venus to absorb carbon dioxide, leading to very high surface temperatures.
F.The runaway greenhouse effect on Venus is more dramatic at present than it was in the past because of an ongoing increase in carbon dioxide levels in Venus’ atmosphere.
