Chapter 11. The Evolution of Terrestrial Atmospheres
11.1 Introduction
Author: Janelle M. Bailey, University of Nevada, Las Vegas
Editor: Beth Hufnagel, Anne Arundel Community College
The goals of this module: At the end, you should be able to:
- Identify similarities and differences between the atmospheres of Earth, Venus, and Mars.
- Explain how these differences probably came about.
In this module you will explore:
- Topographical maps showing the tectonic surface features of Earth, Venus and Mars.
- Models of how the interior of a planet affects these surface features.
Why you are doing it: Comparison of other planets' volcanic features with those on Earth will help us understand not only how their surfaces differ, but also how the interiors of other planets are different from Earth's interior.
11.2 Background
The atmospheres of Venus, Earth, and Mars began in much the same way. As material in the early Solar System coalesced, the hot interiors of these young planets created volcanoes that continually changed the planets' surfaces. Today's volcanoes - and we have no reason to think that early volcanoes were any different - emit water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), and nitrogen (N2); scientists expect that these gases, therefore, should have been important parts of the atmospheres of the three young planets.
Today, the atmospheres of Venus, Earth, and Mars are very different from one another. Venus's thick, acidic atmosphere creates surface temperatures exceeding 450°C - it is far too hot for liquid water on the surface. Mars's atmosphere is thin and wispy, containing mostly CO2 at a temperature near or below the freezing point of water - it is much too cold for liquid water on the surface. Earth's atmosphere contains significant amounts of N2 and oxygen (O2) and keeps our planet's surface to a comfortable range of temperatures that allows liquid water to exist - indeed, most of the surface of Earth is covered with water. Somewhere on Earth, the temperature is just right for humans and all other known life.
So how did these atmospheres, which started out so much alike, end up so different from one another? The story is one that depends on more than just the atmosphere itself.
11.3 Reviewing the Greenhouse Effect
As you may know from paying attention to the news, the greenhouse effect is a process where sunlight is trapped in a planet's atmosphere by particular gases. This can cause an increase in the temperature of the planet. Use the animation below to review the greenhouse effect.
On Earth, the greenhouse effect is responsible for maintaining moderate temperatures at which water can remain liquid - this in turn allows life to flourish. However, it is a delicate equilibrium - too many greenhouse gases can cause the planet to heat up dramatically, as is the case on Venus.
Question 11.1
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On Earth, the crust is broken into about ten major plates that move around on the mantle underneath; in other words, Earth has plate tectonics. This allows the gases emitted by volcanoes to be "recycled" through several processes, maintaining a moderate atmosphere, denser than Mars's but less dense than Venus's.
The moderate temperatures created by Earth's greenhouse effect allowed water to remain liquid on the planet's surface. Oceans formed and rain fell, washing the carbon dioxide (CO2) and sulfur dioxide (SO2) from the atmosphere to the oceans.
Question 11.2
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Life is needed to explain where Earth's current oxygen and nitrogen atmosphere came from. The fossil and genetic evidence tells us that single-celled life dominated the Earth for billions of years. Once living organisms developed on Earth, some of them immediately began to convert CO2, H2O, and sunlight into stored chemical energy (sugars), and then released oxygen (O2) as a waste product. When O2 became abundant enough, other life arose (e.g., humans) that used the chemical energy stored in the O2 and released CO2 as a waste product.
The N2 in the Earth's atmosphere has its own cycle. Certain types of bacteria break apart nitrates into N2 and O2, and release those gases. Plants and other types of bacteria convert them back. (Humans help this process by adding nitrates to crops and their lawns as fertilizer, but this is not a significant piece of the process.) Lightning also has enough energy to force atmospheric nitrogen and oxygen to combine into nitrogen oxides, which dissolve in rainwater, fall into the oceans, and form nitrates.
Over billions of years, these processes have transformed our atmospheric composition to today's levels, with 78% of the molecules being N2 and nearly 21% O2.
11.5 The Evolution of Venus's Atmosphere
Open animation if not still open.
When Venus was young, it probably had liquid water on its surface for a while. However, the Sun's energy output increased over time, raising the temperature of the surface. Eventually, liquid water could not remain on the surface and instead evaporated into the atmosphere. Water vapor is, as we saw earlier, a greenhouse gas, so this served to increase the greenhouse effect. This eventually reached levels that are considered a runaway greenhouse effect. The hotter it got, the more water vapor evaporated, making it even hotter.
Question Sequence
Question 11.3
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.6 The Evolution of Venus's Atmosphere - (continued)
Although Venus's atmosphere began about the same way as Earth's, it is now very different. Carbon dioxide (CO2) does not wash out with rain, so it remains in the atmosphere. Today, 96.5% of the molecules in Venus's atmosphere are carbon dioxide.
Question Sequence
Question 11.5
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In addition to the CO2, sulfur dioxide (SO2) emitted from volcanoes is present in the atmosphere of Venus. This combines with water vapor (H2O) to make sulfuric acid (H2SO4), which remains in the atmosphere as corrosive clouds.
The atmosphere of Venus, containing all these gas molecules, is very dense. At the surface, the atmospheric pressure is 90 times the average air pressure at sea level on Earth! (To reach 90 times the pressure on Earth, we would have to travel about one kilometer below the surface of the oceans.) The atmosphere retains heat for a very long time, and the slow rotation of Venus results in little wind on the surface. As a result, the temperature barely changes between daytime and nighttime, and from the poles to the equator.
11.7 The Evolution of Mars's Atmosphere
Open animation if not still open.
Even though Mars's atmosphere is now thin, scientists think that Mars had a thicker atmosphere early in its existence. Mars's greater distance from the Sun means that the intensity of sunlight at the planet is lower than that at Earth, resulting in cooler temperatures. With a thicker atmosphere, water vapor could easily fall as rain or snow. Today, observations by the Mars Exploration Rovers and orbital craft such as Mars Odyssey show features that are consistent with water erosion, indicating that the liquid water probably flowed on the surface in the past. Although there is no liquid water present today, water is trapped as ice in the polar caps and permafrost below the surface in several areas.
Click here to begin the animation. Read the statement on the animation, then click "Next". Continue working through the steps of the animation for Mars and read the text to answer the questions below.
Question Sequence
Question 11.6
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Four billion years ago, Mars's atmosphere was probably thicker than it is today (although still thinner than the atmospheres of Venus or Earth). The chemical composition is very similar to Venus's atmosphere - 95.3% carbon dioxide (CO2) and 2.7% nitrogen (N2).
The surface pressure on Mars is only 0.006 that of the surface pressure on Earth. The temperature is much colder than either Venus or Earth - daytime highs on Mars are only about 20°C (68°F) and nighttime lows may reach -140°C (-220°F). At these temperatures and pressures, liquid water cannot exist on the surface of Mars.
What about those big polar caps on Mars that look like ice and snow? They are mostly made of frozen CO2! Like Earth, Mars experiences seasons because of its tilt. In the winter, temperatures get so low that the CO2 cannot remain a gas in the atmosphere, so it freezes (turns into a solid) and the ice cap grows. Scientists think that some of the residual polar caps may contain frozen water, which is less easily evaporated back into the atmosphere.
11.9 Quick Check Quiz
Indepth Activity: The Evolution of Terrestrial Atmospheres