Dec. 2nd - Atmospheres of Venus & Earth

Question 1

How does Earth’s atmosphere make our lives possible?

Answer 1

• It provides our planet with just enough warmth and pressure to enable water to cycle between all three phases (solid ice, liquid water, and gaseous water vapor)
• It protects us from harmful solar radiation
• It produces the weather patterns that variously bring us days of sunshine, clouds, and rain or snow

Question 2

How did Earth’s atmosphere end up so different?

Outgassing should have released the same gases on Venus, Earth, and Mars. How, then, did Earth’s atmosphere end up so different?

We can break down this general question into four separate questions:

Answer 2

1. Why did Earth retain most of its outgassed water—enough to form vast oceans—while Venus and Mars lost theirs?
2. Why does Earth have so little carbon dioxide (CO2) in its atmosphere compared to Venus, when Earth should have outgassed about as much of it as Venus?
3. Why is Earth’s atmosphere composed primarily of nitrogen (N2) and oxygen (O2), when these gases are only trace constituents in the atmospheres of Venus and Mars?
4. Why does Earth have an ultraviolet-absorbing stratosphere, while Venus and Mars do not?

Question 3

How did Earth’s atmosphere end up so different?

Why did Earth retain most of its outgassed water—enough to form vast oceans—while Venus and Mars lost theirs?

Answer 3

• On Mars, some of the outgassed water was lost after solar ultraviolet light (from a small stratosphere) broke water vapor molecules apart, and the rest froze and may remain in the polar caps or underground.
• On Venus, it was too hot for water vapor to condense, so virtually all the water molecules were ultimately broken apart, allowing the hydrogen atoms to escape to space.
• Earth retained its outgassed water because temperatures were low enough for water vapor to condense into rain and form oceans.

Question 4

How did Earth’s atmosphere end up so different?

Why does Earth have so little carbon dioxide (CO2) in its atmosphere compared to Venus, when Earth should have outgassed about as much of it as Venus?

Answer 4

• Evidence from tiny mineral grains suggests that Earth may have had oceans as early as 4.3–4.4 billion years ago.
• The oceans, in turn, explain the low level of carbon dioxide in our atmosphere. Most of the carbon dioxide outgassed by volcanism on Earth dissolved in the oceans, where chemical reactions turned it into carbonate rocks.

Question 5

How did Earth’s atmosphere end up so different?

Why is Earth’s atmosphere composed primarily of nitrogen (N2) and oxygen (O2), when these gases are only trace constituents in the atmospheres of Venus and Mars?

NITROGEN:

Answer 5

Because most of Earth’s water ended up in the oceans and most of the carbon dioxide ended up in rocks, our atmosphere was left with nitrogen as its dominant ingredient.

Question 6

How did Earth’s atmosphere end up so different?

Why is Earth’s atmosphere composed primarily of nitrogen (N2) and oxygen (O2), when these gases are only trace constituents in the atmospheres of Venus and Mars?

OXYGEN:

Answer 6

Molecular oxygen (O2) is not a product of outgassing or any other geological process. Moreover, oxygen is a highly reactive chemical that is easily removed from the atmosphere. (called oxidation reactions)

Without continual replenishment, these types of chemical reactions would remove all the oxygen in Earth’s atmosphere in just a few million years. We must therefore explain not only how oxygen got into Earth’s atmosphere in the first place, but also how it is replenished as chemical reactions remove it.

Question 7

How did Earth’s atmosphere end up so different?

The answer to the oxygen mystery is…

Answer 7

• LIFE!
• Plants and many microorganisms release oxygen through photosynthesis. Photosynthesis takes in CO2 and, through a complex chain of chemical reactions, releases O2. Because chemical reactions can remove oxygen, it took a long time for oxygen to accumulate in Earth’s atmosphere
• Today, plants and single-celled photosynthetic organisms return oxygen to the atmosphere in approximate balance with the rate at which animals and chemical reactions consume oxygen, keeping the oxygen levels relatively steady.

Question 8

How did Earth’s atmosphere end up so different?

Why does Earth have an ultraviolet-absorbing stratosphere, while Venus and Mars do not?

Answer 8

Life and oxygen also explain the presence of Earth’s ultraviolet-absorbing stratosphere.

In the upper atmosphere, chemical reactions involving solar ultraviolet light transform some of the O2 into molecules of O3, or ozone. The O3 molecule is more weakly bound than the O2 molecule, which allows it to absorb solar ultraviolet energy even better.

• The absorption of solar energy by ozone heats the upper atmosphere, creating the stratosphere.
• This ozone layer prevents harmful ultraviolet radiation from reaching the surface. Mars and Venus lack photosynthetic life and therefore have too little O2, and consequently too little ozone, to form a stratosphere.

Question 9

Maintaining Balance

Our oceans exist because Earth has a greenhouse effect that is “just right” to keep them from either freezing or boiling away

Why does the amount of carbon dioxide in our atmosphere stay “just right”?

Answer 9

The Carbon Dioxide Cycle: The mechanism by which Earth self-regulates its temperature

1. Atmospheric carbon dioxide dissolves in rainwater, creating a mild acid.
2. The mildly acidic rainfall erodes rocks on Earth’s continents, and rivers carry the broken-down minerals to the oceans.
3. In the oceans, calcium from the broken-down minerals combines with dissolved carbon dioxide and falls to the ocean floor, making carbonate rocks such as limestone.
4. Over millions of years, the conveyor belt of plate tectonics carries the carbonate rocks to subduction zones, where they are carried downward.
5. As they are pushed deeper into the mantle, some of the subducted carbonate rocks melt and release their carbon dioxide, which then outgasses back into the atmosphere through volcanoes.

Question 10

Maintaining Balance

How does the CO2 cycle acts as a long-term thermostat for Earth

Answer 10

The CO2 cycle acts as a long-term thermostat for Earth, because it has a built-in form of self-regulating feedback that returns Earth’s temperature toward “normal” whenever it warms up or cools down

The self-regulating feedback occurs because the overall rate at which carbon dioxide is pulled from the atmosphere is very sensitive to temperature: the higher the temperature, the higher the rate at which carbon dioxide is removed

Question 11

Ice Ages and Other Long-Term Climate Change

While Earth’s climate has remained stable enough for the oceans to stay at least partly liquid throughout history, significant variations have still occurred.

Such variations are possible because…

Answer 11

…the CO2 cycle does not act instantly. When something begins to change the climate, it takes time for the feedback mechanisms of the CO2 cycle to come into play because of their dependence on the gradual actions of plate tectonics and mineral formation in the oceans.

Question 12

Ice Ages and Other Long-Term Climate Change

CO2 Cycle - Ice Ages

Answer 12

occur when the global average temperature drops by a few degrees. The slightly lower temperatures lead to increased snowfall, which may cover continents with ice down to fairly low latitudes.

Over periods of tens or hundreds of millions of years, the Sun’s gradual brightening and the changing arrangement of the continents around the globe have at least in part influenced the climate

the ice ages appear to have been strongly influenced by small changes in Earth’s axis tilt and other characteristics of Earth’s rotation and orbit. (These small changes are the Milankovitch cycles noted earlier, which lead to feedbacks involving the greenhouse effect that can rapidly push the temperature downward into an ice age or back up into a warmer “interglacial” period)

Question 13

Ice Ages and Other Long-Term Climate Change

CO2 Cycle - Ice Ages: Snowball Earth

Answer 13

Because ice can reflect up to about 90% of the sunlight hitting it, this increase in global ice would have set up a self-reinforcing feedback process that would have cooled Earth even further. Geologists suspect that in this way our planet may have entered the periods called snowball Earth

Other models suggest the oceans never froze completely, making Earth more of a “slushball” than a snowball. Either way, it seems that Earth became far colder during these periods than in more recent ice ages.

Question 14

Ice Ages and Other Long-Term Climate Change

How did Earth recover from a “snowball” phase?

Answer 14

The drop in surface temperature would not have affected Earth’s interior heat, so volcanic outgassing would have continued to add CO2 to the atmosphere.

Oceans covered by ice would have been unable to absorb this CO2 gas, which therefore would have accumulated in the atmosphere and strengthened the greenhouse effect. Eventually (perhaps after as long as 10 million years), the strengthening greenhouse effect would have warmed Earth enough to start melting the ice.

The feedback processes that started the snowball Earth episode then moved in reverse: from snowball to hothouse

Question 15

Ice Ages and Other Long-Term Climate Change

Earth’s Long-Term Future Climate

Answer 15

The continuing brightening of the Sun will eventually overheat our planet.

According to some climate models, the warming Sun could cause Earth to begin losing its water as soon as a billion or so years from now.

If such models are correct, life on Earth has already completed about 75% of its history on this planet. However, there are enough uncertainties in the models to make it possible that the CO2 cycle will keep the climate steady much longer.

At that point, the effect will be the same as if we moved Earth to Venus’s orbit (see Figure 10.34): a runaway greenhouse effect. Our planet will become a Venus-like hothouse, with temperatures far too high for liquid water to exist and all the CO2 baked out of the rocks released into the atmosphere.

Question 16

What is the evidence for human-caused climate change?

Answer 16

Our planet may regulate its own climate quite effectively over long time scales, but fossil and geological evidence tells us that substantial and rapid changes in global climate can occur on shorter ones.

In some cases, Earth’s climate appears to have warmed several degrees Celsius in just decades.

Evidence also shows that these past climate changes have had dramatic effects on local climates by:
* Raising or lowering sea level as much as tens of meters
* Altering ocean currents that keep coastlines warm
* Transforming rain forests into deserts.

Human activity is rapidly increasing the atmospheric concentration of carbon dioxide and other greenhouse gases: GLOBAL WARMING

Question 17

The Basic Science of, and Evidence for, Global Warming

Global warming is surprisingly simple, and can be laid out with two facts that lead to an inevitable conclusion:

Answer 17

• Fact 1: Carbon dioxide and other greenhouse gases trap heat and therefore make Earth (or any other world) warmer than it would be otherwise.
• Fact 2: Human activity, especially the use of fossil fuels (coal, oil, and gas), has been adding significantly more carbon dioxide and other heat-trapping greenhouse gases to Earth’s atmosphere.

Question 18

The Basic Science of, and Evidence for, Global Warming

How we can be certain that the rising carbon dioxide concentration is due to human activity?

Answer 18

1. The amount of carbon dioxide released into the atmosphere by human activity has risen in lockstep with the carbon dioxide concentration. In contrast, the total amount of carbon dioxide released by natural sources such as volcanoes and the oceans is only about 1% of what would be needed to explain the observed rise in carbon dioxide
2. The burning of fossil fuels consumes oxygen at the same time that it releases carbon dioxide, and measurements confirm the expected small decrease in the oxygen concentration of oxygen in the atmosphere and oceans
3. Measurements show that the increase in the carbon dioxide concentration has been accompanied by a change in the overall carbon isotope ratio (that is, the relative amounts of carbon-12, carbon-13, and carbon-14) of atmospheric carbon dioxide—and this change can be explained only if the added carbon dioxide has the distinct ratio of carbon isotopes that is found in fossil fuels.

Question 19

Predicting Future Warming

We can use the climate models to predict how much can we expect Earth to warm in the future.

Answer 19

• Some scientists worry about climate “tipping points” that may not yet be properly accounted for in models.
• There’s the possibility we could find ways in the future to remove some of the carbon dioxide that we have added to the atmosphere, helping to restore the climate to a more natural state.
• In summary, the basic science of global warming makes it clear that we should expect human activity to be causing global warming, and the evidence makes clear that the warming is occurring as expected. Moreover, unless we make rapid and dramatic changes to slow or stop the warming, by the end of this century you—and your children and grandchildren—will be living in the warmest climate that any generation of Homo sapiens has ever experienced.

Question 20

Consequences of Global Warming

A temperature increase of a few degrees might not sound so bad, but small changes in average temperature can lead to much more dramatic changes in climate patterns

Answer 20

Almost all regions of the world are now significantly warmer than they were a few decades ago, but some regions warmed much more than others. This is what we mean by “climate change”—the idea that regional effects can be very different from the global average. Models suggest that regional climate changes will be further amplified as the global average temperature continues to increase.

Question 21

Consequences of Global Warming

We can separate the many consequences of global warming into five major categories.

Answer 21

1. Local and regional changes in weather patterns and precipitation
2. Storms and extreme weather
3. Melting of sea ice
4. Sea level rise
5. Ocean acidification

Question 22

Five major categories of Global Warming: Local and regional changes in weather patterns and precipitation.

Answer 22

Changes in regional average temperatures are only the beginning of the local and regional climate changes that are occurring.

For example, data show that nights have generally warmed more than days, and winters more than summers. Moreover, there has been a tendency toward more extreme heat, particularly in summer

Question 23

Five major categories of Global Warming: Storms and extreme weather

Answer 23

A second major category of expected consequences is an increase in the frequency and/or severity of extreme weather events such as hurricanes, thunderstorms, floods, droughts, heat waves, and even winter storms.

The reason is easy to understand: Global warming means more heat and energy being trapped in the lower atmosphere and oceans, and heat and energy are the drivers of weather events.

Question 24

Five major categories of Global Warming: Sea level rise

Answer 24

Global warming is also causing a rise in sea level, for two reasons:

• First, water expands very slightly as it warms. While this thermal expansion is much too small to notice in a cup of water, it has a measurable effect on the oceans.
• Second, the melting of glacial ice, particularly in Greenland and Antarctica, creates runoff that adds water to the oceans.

Together, these effects have already caused global sea level to rise by more than 20 centimeters (8 inches) in the past century or so, and thermal expansion alone could easily raise sea level another 20 centimeters by the year 2100

Question 25

Five major categories of Global Warming: Ocean Acidification

Answer 25

The oceans are suffering due to rising water temperatures caused by global warming, but they are also suffering in another way.

Some of the carbon dioxide released into the atmosphere by human activity (roughly 30% of it) ends up dissolving in the oceans, where it undergoes chemical reactions that make the oceans more acidic.

Together with the warming temperatures, this “ocean acidification” has been tied to the demise of many coral reefs around the world and to less productive fisheries.

Question 26

Five major categories of Global Warming: Melting of sea ice

Answer 26

Data clearly show that ice coverage over the Artic Ocean has been declining dramatically for the past few decades.

The ice now covers less area in the summers and is thinner in the winters. If the current trend continues, summers may see an ice-free Arctic Ocean by about mid-century.

Question 27

Overall notes about global warming as a human activity:

Answer 27

Climate change can occur naturally, but we are currently conducting a dramatic experiment on our planet for which we do not fully know what the outcome will be.

Question 28

Global Warming Solutions—Building a Post-Global Warming World: we can slow or stop the warming by reducing or eliminating these emissions

Answer 28

Through “futuristic” technologies, which could provide clean and abundant energy for the future, including new ways of doing nuclear fission that recycle past nuclear waste, or even solar energy from space, in which huge solar collectors are unfurled high above Earth (where it’s never dark and never cloudy) and energy is beamed down to collecting stations on the ground. (such as ITER in France)

However, this would NOT reverse the damage that has been done!

Question 29

To stop or reverse that future damage, we will need technology that can actually remove carbon dioxide from the atmosphere, so that the climate might be restored to a more natural balance

Passive vs. Active

Answer 29

Some of this removal can be done “passively” by such things as planting more trees and farming practices than can store carbon in the soil.

But we’ll likely also need “active” removal strategies, such as machines that directly capture carbon dioxide from the air.

Question 30

Putting these idea together, we can envision a two-step pathway to a much more promising future.

Answer 30

The first step is to stop making the situation worse by implementing technologies that can end our greenhouse gas emissions.

The second step is to begin removing carbon dioxide from the atmosphere, until we restore the climate to a more natural balance.

Question 31

The Atmospheric History of Venus

What is Venus like today?

Answer 31

• If you stood on the surface of Venus, you’d feel a searing heat
• Venus’s atmosphere consists almost entirely of carbon dioxide (CO2). It has virtually no molecular oxygen (O2), so you could not breathe the air even if you cooled it to a comfortable temperature.
• Moving through the thick air near Venus’s surface would feel somewhat like a cross between swimming and flying: Because the air density is about 10% that of water, you would be able to propel yourself by flapping your arms, though you might have a hard time getting off the ground.
• Because the thick atmosphere scatters nearly all the blue light away, the dimly lit sky appears reddish-orange in color.

Question 32

The Atmospheric History of Venus

What is Venus like today?
WEATHER:

Answer 32

Venus’s slow rotation (243 Earth days) means a very weak Coriolis effect.

• As a result, Venus has little wind on its surface and never has hurricane-like storms.
• No rain falls, because droplets that form and fall from the cool upper atmosphere evaporate long before they reach the ground.
• The weak Coriolis effect also means that Venus’s atmosphere has just two large circulation cells, much like what Earth would have if rotation didn’t split its cells
• The thick atmosphere makes the circulation so efficient at transporting heat from the equator to the poles that the surface temperature is virtually the same everywhere: The poles are no cooler than the equator, and night is just as searingly hot as day. Moreover, Venus has no seasons because it has virtually no axis tilt, so temperatures are the same year-round.

Question 33

The Atmospheric History of Venus

What is Venus like today?
WEATHER AT HIGH ALTITUDES:

Answer 33

Strong convection drives hot air upward; high in the troposphere, where the temperature is 400°C cooler than on the surface (see Figure 10.10), sulfuric acid (H2SO4) condenses into droplets that create Venus’s bright, reflective clouds.
* The droplets sometimes fall through the upper troposphere as sulfuric acid rain, but they evaporate at least 30 kilometers above the surface.
* In addition, high-altitude winds circle the planet in just 4 days—much faster than the planet rotates. No one knows why these fast winds blow, but they are responsible for the dynamic cloud patterns

Question 34

How did Venus get so hot?

Answer 34

The simple answer is that Venus has a huge amount of carbon dioxide in its atmosphere—nearly 200,000 times as much as in Earth’s atmosphere.

However, a deeper question still remains. Given their similar sizes and compositions, we expect Venus and Earth to have had similar levels of volcanic outgassing, and the released gas ought to have had about the same composition on both worlds.

Question 35

Why, then, is Venus’s atmosphere so different from Earth’s?

The Fate of Outgassed Water and Carbon Dioxide

Answer 35

We expect that huge amounts of water vapor and carbon dioxide should have been outgassed into the atmospheres of both Venus and Earth. Venus’s atmosphere does indeed have an enormous amount of carbon dioxide, but it has virtually no water.

Earth’s atmosphere has very little of either gas. We conclude that Venus must have somehow lost its outgassed water, while Earth lost both water vapor and carbon dioxide. But where did these gases go?

Question 36

The Fate of Outgassed Water and Carbon Dioxide: Where did the outgassing go?

Answer 36

We can easily account for the missing gases on Earth. The huge amounts of water vapor released into our atmosphere condensed into rain, forming our oceans. In other words, the water is still here, but mostly in liquid rather than gaseous form. The huge amount of carbon dioxide released into our atmosphere is also still here, but in solid form: Carbon dioxide dissolves in water, where it can undergo chemical reactions to make carbonate rocks such as limestone.

Earth has about 200,000 times as much carbon dioxide locked up in rocks as in its atmosphere—which means that Earth does indeed have almost as much total carbon dioxide as Venus. Of course, the fact that Earth’s carbon dioxide is mostly in rocks rather than in the atmosphere makes all the difference in the world: If this carbon dioxide were in our atmosphere, our planet would be nearly as hot as Venus and certainly uninhabitable.

Question 37

The Fate of Outgassed Water and Carbon Dioxide: Where did the water go?

Answer 37

Venus today is incredibly dry. It is far too hot to have any liquid water or ice on its surface, or even be chemically bound in surface rock

Hypothesis for the disappearance of Venus’s water (similar to Mars):
* Ultraviolet light from the Sun broke apart water molecules in Venus’s atmosphere.
* The hydrogen atoms then escaped to space (through thermal escape), ensuring that the water molecules could never re-form.
* The oxygen from the water molecules was lost to a combination of chemical reactions with surface rocks and stripping by the solar wind; Venus’s lack of a magnetic field leaves its atmosphere vulnerable to the solar wind.
* Acting over billions of years, the breakdown of water molecules and the escape of hydrogen can easily explain the loss of an ocean’s worth of water from Venus

Question 38

Why didn’t Venus, like Earth, end up with oceans to trap its carbon dioxide in carbonate rocks and prevent its water from being lost to space?

Feedback processes

Answer 38

• To understand why Venus does not have oceans, we need to consider the role of feedback processes—processes in which a change in one property amplifies or counteracts the behavior of the rest of the system.
• You are probably familiar with feedback processes in daily life. For example, if someone brings a microphone too close to a loudspeaker, it picks up and amplifies small sounds from the speaker.
• These amplified sounds are again picked up by the microphone and further amplified, causing a loud screech.
• This sound feedback is an example of self-reinforcing (or positive) feedback, because it automatically amplifies itself.
• The screech usually leads to a form of self-regulating (or negative) feedback: The embarrassed person holding the microphone moves away from the loudspeaker, thereby stopping the sound feedback.

Question 39

Why didn’t Venus, like Earth, end up with oceans to trap its carbon dioxide in carbonate rocks and prevent its water from being lost to space?

The Runaway Greenhouse Effect

Answer 39

With the idea of feedback in mind, let’s consider what would happen if we could magically move Earth to the orbit of Venus

The greater intensity of sunlight would almost immediately raise Earth’s global average temperature by about 30°C, from its current 15°C to about 45°C (113° F). Although this is still well below the boiling point of water, the higher temperature would lead to increased evaporation of water from the oceans. The higher temperature would also allow the atmosphere to hold more water vapor before the vapor condensed to make rain.

Question 40

OVERALL: Why is Venus so much hotter than Earth?

Answer 40

Even though Venus is only about closer to the Sun than Earth is, this difference was critical.
* On Earth, it was cool enough for water to rain down to make oceans. The oceans dissolved carbon dioxide and chemical reactions locked it away in carbonate rocks, leaving our atmosphere with only enough greenhouse gases to make our planet pleasantly warm.
* On Venus, the greater intensity of sunlight made it just warm enough that oceans either never formed or soon evaporated, leaving Venus with a thick atmosphere full of greenhouse gases.

Question 41

Early Venus

Answer 41

It’s possible that Venus might have been more moderate in its early history.

Recall that the Sun has gradually brightened with age; some 4 billion years ago, the intensity of sunlight shining on the young Venus was not much greater than it is on Earth today.

Rain might have fallen, and oceans could have formed. It’s even conceivable that life could have arisen on the young Venus.As the Sun gradually brightened, however, any liquid water or life on Venus’s surface was doomed