In this session we will discuss the general principles of planetary atmospheres in general and consider the atmospheres of Earth, Venus and Mars in particular, covering the topics of

• Goldilocks Paradox
• Atmospheric Characteristics
• Greenhouse Effect
• Atmospheric Sources and Losses

Once we have a basic idea of how atmospheres behave, we can consider climate evolution: how the Earth's climate is changing, how Earth and Venus's atmospheres evolved to be so different (despite these two planets being very similar and so close to each other) and how Mars' has climate evolved from what may have been a comfortable environment for life to a cold, harsh, barren world

• Planetary Atmospheric Evolution
• Why are Earth and Venus so Different?
• Earth's Climate Change
• What Happened on Mars?

Goldilocks Paradox

Why are Mars and Venus apparently barren while Earth has abundant life? Mars is too cold, Venus is too hot, and Earth is just right. Why? To answer this question we need to consider the factors that control a planet's surface temperature.

Distance from the Sun
Clearly, the closer a planet is to the Sun, the warmer it will be. This relationship is not as strong as you might expect it to be, however. Figure 10.3 illustrates how the temperature of planet is controlled, on average, by the balance of energy incoming received from the sun and the outgoing thermal radiation from the planet. Mathematically, the relationship can be expressed:

Temperature  1/D

Where D is the distance from the Sun (which we usually measure in Astronomical Units, AU). The symbol means "is proportional to". This relationship means that a planet that the farther a planet is from the Sun, the lower the temperature (D goes up, then T goes down). But the square-root means that the temperature decreases slowly with distance. If you double the distance, the temperature decreases by only 2. A planet that is 4 times closer to the Sun, will only be 4= 2 times warmer.

Albedo
The color of a planet, particularly how light or dark is also an important controlling factor.

(2) (a) Have you ever gotten into a car that has been sitting out in the sun and sat down on black seats? Did the seat feel hot or cold? (b) Would it have been the same if the seats were white? (c) Is it coincidence that people wear white clothing in summer, particularly in hot countries?

The issue is really not "color" - that is, red, green, yellow, etc.; so much as reflectivity. We have a special word for the reflectivity of planetary bodies - albedo. The albedo of an object is a number between 0 and 1 and it is just the fraction of light that is reflected. See Table 10.2 for the terrestrial planets.

We can now combine the effects of distance from the Sun and albedo on the temperature of a planet. We are interested in the equilibrium temperature which is the temperature of an object that is just sitting in the Sun, absorbing sunlight. The object will be loosing energy by thermal radiation - this was discussed in Session 13 on Light - remember the Stefan-Boltzmann law? (Go back and remind yourself if necessary). When the amount of energy emitted by the planet balances the amount of energy received from the Sun, the planet is in equilibrium. Mathematically, we can write:

Teq = (1 - A)1/4 x 1/D  x 280 Kelvin

where T_eq is the equilibrium temperature, A is albedo and D is the distance from the Sun (in units of AU). (Dont' worry! You will not be expected to derive or remember this equation). Just think about what it means, for now. Pages 278-9 of the text book gives the full story in full glory.

I have calculated the equilibrium temperature for the terrestrial planets so we can compare this with the observed temperatures at night and during the day.

(6) (a) Fill in the difference between night and day temperatures for each planet. (Subtract the night temperature from the day temperature). (b) Mercury heats up during its long `day' and cools down at night. How many Earth-days between sunrise and sunset on Mercury? (c) Venus has a high albedo (0.72) - why? (d) For which planet(s), if any, does the equilibrium temperature match the observed temperature?

Clearly, the temperatures of the planets are being affected by more factors than just distance from the Sun and albedo. In particularly, our calculation of equilibrium temperature completely ignored the planets' atmospheres.

Atmospheric Characteristics

Moon and Mercury
Look at the color of the sky as seen on the Moon (check your book). The Moon has no atmosphere, nor does Mercury. Gravity is the simplest explanation why Mercury and Moon do not have atmospheres while Earth, Venus and Mars do.

(7) (a) Using the same technique as in session 8 on Gravity to fill in the values of the acceleration due to gravity on the surfaces of Mercury and Moon compared with the Earth: (b) WHY is gravity an important factor in determining how much of an atmosphere a planet has?

The differences in gravity explains why Mercury and Moon have essentially no atmosphere compared with Earth and Venus, but does not explain why Venus has a much denser atmosphere than Earth, nor why Mars has much more atmosphere than Mercury. For further explanation we will have to look for other factors.

Pressure
The atmospheric pressure at sea-level on Earth is called 1 bar. Look at the atmospheric pressures at different heights for Venus

and for Mars.

A plot with the 3 atmospheres of Earth, Venus and Mars is shown in Figure 10.9.

(8) (a) How many times the Earth sea-level pressure is it on the surface of Venus? (b).. and on the surface of Mars? (c) How high up above the ground on Venus do you have to be before the atmospheric pressure is the same as sea-level on the Earth?

Composition
The chemical composition of an atmosphere is also very important.

 

(9) (a) What are the main constituents of the atmospheres of: (i) EARTH: (ii) VENUS: (iii) MARS: (b) In designing missions for exploration of the terrestrial planets we have to consider what kinds of space suits you need to be comfortable walking on each of the planets.  Considering the temperature, pressure and composition of each of the atmospheres of Venus, Mars and Mercury, describe the properties of space suits humans would need to explore these planets.

Greenhouse Effect

How does the greenhouse effect work? The principle involves the fact that the sunlight received by a planet is in the visible part of the spectrum while the energy that is radiated (lost) by the planet is in the infrared part of the spectrum.

(10) Remind yourself of the "two hump" spectrum of radiation from a planet from Session 13 on Light. In which 2 regions of the electromagnetic spectrum are the 2 humps?

Here is a sketch of the situation - the greenhouse effect - or check out Figure 10.8 in the textbook.

So, what exactly is a greenhouse gas? Here is a definition:

Some examples of greenhouse gases are: CO₂, H₂O, O₃, CH₄, and chloro-fluorocarbons CFCs. (CFCs are better known for causing the Ozone holes over the Earth's polar regions. They are gases made by humans and used in various manufacturing processes. They are also strong greenhouse gases.) Note that 2 common gases on Earth's atmosphere, N₂ and O₂ are NOT greenhouse gases.

These gases let sunlight in to warm the planet but prevent the planet's heat from escaping--keeping the planet's surface warm - warmer than the equilibrium temperature we calculate only considering the distance to the Sun and albedo.

(11) (a) Not only does VENUS have more atmosphere then EARTH, but it is nearly all CO2--a green house gas--while Earth's atmosphere is mostly N2 with only a small amount of CO2. Hence, Venus is much hotter than Earth. What is Venus' temperature in Fahrenheit? (b) MARS' atmosphere is mostly CO2. Why is Mars not has hot as Venus? (There are 2 reasons.)

Atmospheric Sources and Losses

Where to planetary atmospheres come from? The simple answer is that since gases are lighter than solids or fluids they tend to bubble to the top. This is true of gases coming out of a planet as much as steam coming out of soup or oatmeal.

Here is a cartoon of the atmospheric sources - Figure 10.20 of the text elaborates.

 

(12) (a)What is it about the 4 geological processes that makes it likely that a large planet will have more atmosphere? (b) Return to Session 12 where we discussed the 4 geological processes. From just the occurance of volcanism among the 5 terrestrial planets, which would you expect to have substantial atmospheres? (c) What is it about a planet's gravity that makes it likely that a large planet will have more atmosphere? (d) Which of the 5 terrestrial planets do you expect to have lost any atmosphere it originally may have had? (e) Why is hydrogen particularly easily lost from the terrestrial planets' atmospheres?

Next we will discuss the evolution of planetary atmospheres after they are produced by outgassing.

Summary - Basic Issues of Planetary Atmospheres

Equilibrium Temperature

Energy absorbed from VISIBLE sunlight Decreases away from the Sun Black/white objects absorb/reflect more sunlight

Energy radiated in IR thermal emission Radiates out to space Greenhouse gases (CO2, H2, O3, CH4) absorb IR as it tries to escape

Which solar system objects have atmospheres

• Moon, Mercury - no atmosphere to speak of
• Earth - yes, atmosphere of mostly N₂ and some O₂
• Venus - yes, thick atmosphere of mostly CO₂
• Mars - yes, tenuous atmosphere of mostly CO₂
• Other objects? Titan, Triton, Io
• What do these objects have in common?
  • Volcanoes - source of atmospheric gases
  • Larger objects - with enough gravity to hold gases

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Planetary Atmospheric Evolution

In discussing the properties of the atmospheres of the different terrestrial planets, we mentioned that the source of planetary atmospheres is mainly volcanism where gases from the interior of the planet are brought up to the surface. The main gases produced by volcanoes are H₂O, H₂S, CO₂, N₂, and CO. Here is the cartoon again that shows the atmospheric sources. Figure 10.20 shows evaporation and bombardment as additional sources of atmosphere. These were more important than volcanic sources in the early stages of planetary evolution (and remain important in the outer solar system) but are less important than volcanism for the bulk of the geological history for the terrestrial planets.

What happens to these gases when they enter the atmosphere?

• Volcanoes produce mostly water, carbon dioxide, nitrogen, carbon monoxide - plus some other gases such as sulfur dioxide and hydrogen sulphide (both very smelly!)
• If the planet is cold enough the water condenses to form an ocean
• Carbon dioxide dissolves into the water - if there is life in the ocean, the carbon dioxide is converted to calcium carbonate - rock - by shellfish and coral, etc.
• Carbon monoxide and water can react in the early planet's atmosphere to make hydrogen and carbon dioxide. The hydrogen, a light gas, can escape more easily than other gases. Thus, more carbon dioxide is left behind.
• If the planet does not have water, then the atmosphere collects carbon dioxide.
• Plants on Earth convert carbon dioxide to O₂ - the gas that humans (and other mammals) breath in the Earth's atmosphere.

Earth and Venus both have volcanoes, both have the same amount of gravity and Venus is really not that much closer to the Sun than the Earth. Yet Venus as a thick, poisonous atmosphere and a surface temperature equivalent to a "high broil" in your oven. The critical difference is the presence of liquid oceans on the Earth:

First, let us look at the evidence for climate change on Earth. There is a great deal of talk in the public media about "global warming" - what is the evidence? Look at these graphs of global climate change.

The top plot is for long term variations - over hundreds of thousands of years. The bottom one is for the past 150 years. They both have temperature as the left axis and carbon dioxide concentration on the right axis. How these data obtained? Well, for the modern data we can just average measurements of temperature taken all over the world. The carbon monoxide measurements are made at a few special atmospheric observatories (such as the one run by the National Center for Atmospheric Research on Mauna Loa, Hawaii). The long-term data come from ice cores from the Greenland icesheets. The ratio of oxygen isotopes is believed to be an accurate indicator of temperature, so by measuring the oxygen isotope and CO₂ concentrations with depth down a core we can get an estimate of the climate at the time that the layer of ice was deposited.

The temperature scales are in degrees Celsius as the change from average values in the 1970s. The CO₂ concentration scale is in parts of CO₂ per million parts of air ("parts" just means volume - so that 300 parts per million of CO₂ means that for every million cubic meters of air, there is 300 cubic meters of CO₂). The important things to notice, however, are the changes in these quantities with time. Look at how the red CO₂ concentration lines match the black temperature lines. This just says that it is warmer when the CO₂ concentration is higher.

(15) (a) How is a warmer temperature consistent with more CO2 in the atmosphere? (b) About 140 thousand years ago there was a large increase in temperature and CO2 concentration. By how many degrees did the temperature increase? How much did the CO2 concentration increase? (c) These long-term variations are believed to be caused by wobbles in the Earth's orbit and the tilt of the Earth's spin axis. The times of colder temperatures are often called "ice ages" since there were extensive glaciers that spread south from artic regions into the mid-latitudes. From the plot you can see that there are roughly 2 timesscales - one between deep, freezing temperatures (or between very hot spells - this is perhaps easier to measure) and the other, much shorter variations. What are the two timescales for these variations? (d) Now, look at the lower plot of modern variations. Note that the temperature scale has now changed to a range of -0.8 C to +0.4 C - about a factor of 10 smaller range that the upper plot. How much is the variation in average, global temperature from year to year? (e) How much is the variation in temperature, smoothing out the year-to-year variations, over the past 150 years? Are YOU convinced that there is global warming? (f) The red plot is really as much a plot of human activity as a plot of CO2 concentration. What are the ways in which human activity contributes to CO2 production? (g) What is the prognosis for the future? The top plot shows that the Earth has suffered much larger ranges in CO2 and temperature in the distant past. What would be the impact on present flora and fauna - including humans - of similar ice ages / global warmings?

Climate Models
So far we have explored the effects of distance of a planet from the Sun, albedo and greenhouse gases on a planet's surface temperature. We are able to do a reasonable job of modeling the temperature of the planets - at least to an accuracy of a few degrees. However, the Earth's environment is very sensitive to small changes in temperature. A few degrees can have drastic effects on vegetation and melting of the ice caps (which would flood low-lying areas of land), for example. This means that the effects of changing the composition of the atmosphere need to be calculated with sophisticated models.

Scientists at the National Center for Atmospheric Research (up on the mesa above Boulder) have developed models of the atmosphere that include many more effects - multiple layers, clouds, geographical variations across the globe, coupling to the oceans, etc.; in their Global Climate Models (GCMs). For an example, go to the Genesis site. Clicking on a picture will give you a full-page version.

(16) (a) What are the NCAR scientists predicting for the increase in the amount of carbon dioxide over the next (i) 20 years? (iI) 300 years? (b) What are the predicted changes in temperature (i) in your current location? (ii) maximum on the globe? (iii) minimum on the globe? (Note: Only values for the continents are valid - ignore the ocean and Greenland icecaps.) (c) Described the changes in vegetation that are predicted by the NCAR model.

If you click on the word CHANGE (highlighted in blue at the top of the page) you will go to a site that has little movies showing the deforestation. If you click on "QT" you can download QuickTime movies.

This issue of whether there are going to be major changes to the habitability of Earth due to human activity is a major political issue and one that affects everyone on the planet - though the people who are causing the changes (e.g. by generating CO₂) are not necessarily those who will be affected the most. Here is an article from an international paper The Guardian Weekly, November 8, 1998, that discusses results of climate models presented at a meeting for talks on controlling CO₂ production.

(17) What are the predictions discussed in the article in terms of (a) flooding (b) temperature (c) food production?

Theories of Planetary Climate Change

These models are just for the Earth. The political issues of global warming are very immediate for us on Earth. In a more philosophical way, we can look at the Earth as the only (seriously) habitable planet in the solar system and wonder what the implications may be. Here are 2 ideas about planetary climates:

What Happened on Mars?

While Venus is much more similar to Earth, in size and location, Mars is more likely to have had life. Yet, presently there is no evidence of life existing on Mars right now. An important factor is Mars' tenuous atmosphere. As we discovered when discussing erosion in session 12, there is plenty of evidence of there having been liquid water on the surface of Mars in the past. What happened to Mars' atmosphere? What happened to the water?

• Mars is a little farther from the Sun, making it a little colder
• Mars is smaller:
  • interior cools off faster, has less volcanism to generate an atmosphere
  • has less gravity to hold in an atmosphere
• As a result:
  • Water is frozen
  • CO₂ may also be frozen
  • Little greenhouse effect -> runaway "icebox effect"

(19) (a) Why does Mars have so much less atmosphere than Earth? (2 reasons) (b) What factors cause Mars to have fewer volcanoes? (c) What happened to the water on Mars?

Seasons
Remind yourself about what causes the seasons on the Earth. Then look at Figure 10.11 and think about the orbit of Mars.

(20) (a) Look up the tilt of the spin axes for Earth, Venus and Mars. (b)Explain why Earth and Mars have seasons, but Venus does not. (c) How do seasons on Mars differ from seasons on Earth? (d) Why does atmospheric pressure on Mars change during the seasons, while atmospheric pressure on Earth remains steady year-round?

Summary - Climate Change

On the topic of the evolution of planetary atmospheres, we are left with lots of questions, rather than facts, to summarize:

• Could the change in the Earth's atmosphere - such as due to human-generated CO₂ - lead to a change in Earth's climate - such as send us to a runaway greenhouse effect, like Venus?
• Direct measurements of CO₂ and the global, average temperature show increases since the industrial revolution - suggesting they are due to human activity. What will happen as human activity is increasing exponentially?
• Measurements of CO₂ and temperature from Greenland ice cores going back hundreds and thousands of years, suggest that there have been much stronger variations in CO₂ and temperature in the past - due to changes in the Earth's orbit. Does this geological evidence of climate change in the past raise concerns or reassure us that the Earth's climate is stable? (That is change - but not become "runaway")
• There are several theories about climate evolution. Right now we do not know enough about our atmosphere (and its relationship to the ocean) or about the atmospheres of other planets to say which theory is correct.
• Climate change - theory 1: Subtle changes in temperature can send a planet towards either runaway greenhouse effect or towards a runaway icebox. This limits the "habitable zone" in a solar system to a very narrow region. This would mean that the earth's atmosphere is precariously balanced - it could tip catastrophically in either direction.
• Climate change - theory 2: The biosphere is self-stabilizing (Gaia hypothesis) - it reacts to changes to maintain habitable environment for life. This means there is a large range in conditions that could be habitable and there could be life all over the solar system. This also means that we do not need to worry about pollution...or does it?

Model answers to the comprehension questions.