# Case Study: Contrasting Earth, Mars and the Moon's Atmospheres

The Ontario Science Center describes an atmosphere as "an envelope of mixed gases that surrounds a celestial body such as a planet, moon, or star. An atmosphere is held to the body by the body's gravity". This is a very good definition, as you can basically think of an atmosphere as all the gases surrounding any planet or star in the universe. All planets in the solar system, along with their moons, have different atmospheres. Some of these atmospheres are so small, meaning the planets have so little gas, that they are considered non-existent. The size and composition of an atmosphere has a large impact on the temperature, density, and atmospheric pressure as we will explore later in this module. Recent studies by NASA have determined the atmospheric composition of Earth, the moon, and Mars.

### Earth

What is the swirly, blue transparent blanket that surrounds the Earth? It’s the Earth's atmosphere. The atmosphere is most commonly known as air. What most people do not realize is that air is actually made up of several elements and compounds beside Nitrogen and Oxygen. Other components found in air are listed below along with their volume percent found in the atmosphere.

Figure 1: Photograph of Earth from a spaceship at an altitude of 181 nautical miles (335 km). Image courtesy of the Image Science & Analysis Laboratory, NASA Johnson Space Center.

#### Components

Here is a table with the volume percent of different gases found in air.  Volume percent means that for 100 L of air, there are 78.084 L Nitrogen, 20.946 L Oxygen, 0.934 L Argon and so on; Volume percent mass is different from the composition by mass or composition by amount of moles. It should also be noted that Nitrogen, Oxygen and Hydrogen atoms are not found alone in the atmosphere; instead they are all molecules of N2, H2 and O2.

 Elements Name Volume Percent (v/v) Nitrogen 78.084 Oxygen 20.946 Argon 0.934 Carbon Dioxide 0.0379 Neon 0.001818 Helium 0.000524 Methane 0.0002 Krypton 0.000114 Hydrogen 0.00005 Dinitrogen Monoxide 0.00005 Xenon 0.000009

From Table 1, it is simple to determine the most abundant element in the Earth’s atmosphere, nitrogen. In the graphs below it shows how the element's volume percent compare to each other.

#### Quick details about the Earth's atmospheric layers

There are five layers in the atmosphere: Troposphere (1-12 km above Earth’s surface), Stratosphere (12-55 km above Earth’s surface), Mesosphere (55-80 km), Thermosphere, and Exosphere.

• Troposphere: This is where the weather is made! The temperature in this area is extremely dependent on the altitude, the higher the altitude the lower the temperature. The minimum temperature in this region is about 220 K (or -53.15°C) and can go up to about 1500K (1226.85°C).
• Stratosphere: Like the troposphere the region’s temperature changes at different altitudes. However, higher up in the stratosphere is warmer and lower is cooler, so the change clearly does not occur due to elevation. The reason the stratosphere changes temperatures is because higher up, monoatomic oxygen atoms exist because the UV light from the sun breaks up O2 and ozone, O3. However, lower down in the stratosphere there isn't as much monoatomic oxygen since the sun is farther away and there aren't as many UV rays. As we know from our bonding unit, molecules try to bond in order to form molecules of lower energy, so it would make sense the oxygen gas and monoatomic oxygen would combine to form Ozone. Since the ozone is lower energy than molecular oxygen or monoatomic oxygen, energy is released from the reaction that causes the stratosphere to be warmer near the top where this reaction occurs more. Some fun facts: right before the space age, the supersonic aircrafts use to puncture the lower levels of the stratosphere in order to break the sound barrier.
• Mesosphere: The mesosphere is the region between fifty-five to eighty km, where the temperature is constantly falling to about 180K (-93.15°C).
• Ionosphere or Thermosphere: Most commonly known as the Thermosphere. In this region the temperature can get to roughly 1500 K. It contains both positive and negative ions neutral atoms/ molecules and free electrons. In order to ionize the atoms or disassociate the molecules it requires absorption of energy. The electromagnetic radiation from the sun is the source of energy. The thermosphere is where the Northern Lights or rather aurora borealis happen.
• Exosphere: This is the outermost layer of earth's atmosphere, and it extends for thousands of km out into space, allowing atoms to reach a velocity at which they can escape the force of gravity.

Below is a diagram of the Earth's Atmosphere:

Figure 2: Diagram of atmospheric temperature versus altitude in the various layers of the atmosphere. Figure courtesy of NOAA.

#### Effect of elevation on atmosphere

We can see the effect of the size of the atmosphere as we consider a change in elevation on earth. Since gravity pulls the atmosphere down, more air molecules are present near the surface than in say, the mountains. Therefore, as you increase elevation, the density of air becomes less due to the smaller amount of molecules, and therefore mass in the air. Likewise, Atmospheric pressure is basically the weight of the air on the surface of the earth, so the higher the elevation, the lower the air pressure since less amount of the atmosphere presses down. Finally, it is a fairly common known fact that as you increase elevation, the temperature decreases. But why is this? Wouldn't someone with little scientific knowledge think that the higher you go, the warmer it would be since the sun would be closer to the earth?  However, it is important to understand that the sun itself does not warm the earth; instead, the sun strikes the surface of the earth and becomes absorbed in the air. Knowing this, it now makes sense that the higher you go, the cooler it gets because there are less molecules to absorb the heat from the sun to heat the earth.

#### Water Vapor in the Atmosphere

Water vapor occurs when water evaporates off the surface of the earth, and goes into the atmosphere. Water vapor is less dense than air, meaning it rises through the atmosphere. Eventually, the water rises to a point where it becomes cooled and condenses and reaches a point of equilibrium with the surrounding air. Sound familiar?  Yes, a mass of condensed water vapor in the air is called a cloud!  As the water  in the clouds become heavier and heavier, droplets begin to form and eventually become more dense than air, which causes the phenomenon of rain. In the atmosphere there is a maximum 4% of water vapor, meaning that no more than 4% of the air can be made of water vapor.  Only in very tropical and warm climates will the water vapor in the air reach 4%, and after this point the water condenses. This cycle of evaporation, condensation and precipitation is the water cycle, and is vital to life on earth as the it allows us to reuse water over and over as it evaporates and then eventually falls back down. Using the ideal gas equation, it is quite simple to determine the amount of water vapor (H2O) or the partial pressure of the water vapor. Note: these are proportional to each other since in the equation PV=nRT, "n" and "P" are on separate sides.

 Example

The partial pressure of 0.1 moles water vapor is 17.5 mmHg in a enclosed container of finite volume and constant temperature. The more vapor is added, to make the total moles 0.5.  What is the final partial pressure?

Solution: Use general gas law to make the formula P1/n1=P2/n2. Make P1=17.5 mmHg, make n1=0.1, make n2=0.5 and make P2=x.

so 17.5/.1=x/.5   => x= 87.5 mmHg

Water vapor in the air is most commonly known as humidity.  However, weather reporter  usually use the term Relative humidity or Dew point to give a quantified assessment about the humidity on a certain day. Relative humidity is given by the following formula.

Relative humidity=  partial pressure  /  vapor pressure of water  *100%

This formula is basically dividing the partial pressure of water vapor, by the maximum amount of water vapor that can be in the air at any given temperature before the vapor must condense.

Since we now see how humidity and Vapor pressure are related, it makes sense how Dew Point, a temperature, could let us now how humid it is.  Dew Point is the temperature needed for the vapor  in the air to condense. If you notice how in the morning the grass is sometimes wet, this is because the temperature reached the Dew Point, meaning the vapor condensed into water.

#### Effects of Carbon Dioxide in the Atmosphere

The atmosphere acts as a protective shield around the earth. It protects against heat radiation that is produced from the sun. If the earth didn’t have an atmosphere it would not be able to retain thermal energy as like we discussed before, the atmosphere absorbs the heat from the sun to heat the earth, rather than the sun directly warming the earth. This would cause major freezing on Earth's surface. The amount of CO2 and water in the atmosphere majorly controls the effects of temperature on the earth’s surface.

### Moon

The moon is about 240,000 miles away from Earth and orbits every 27.3 days. It rotates a complete revolution every 655 hours due to tidal forces. The moon is commonly mistaken for not having any atmosphere because of the fact that it takes so long for footprints and other such markings to go away (meaning no wind would erase the prints). However, the moon does have an atmosphere, but it is so small that is called an exosphere. Just to give an idea of how small the exosphere is let’s compare it to a portion of the earth’s atmosphere. According to NASA on their information on the moon they stated that a cubic centimeter of Earth’s atmosphere is one billion billion molecules, but a cubic centimeter of the moon’s exosphere has only 100 molecules. That’s a more extreme difference than comparing a ten pound to a one ounce candy chocolate bar. The amount of molecules is negligible, but through various methods of  (UV) Spectrophotometry and inference, scientists have detected the presence of Na, K, polonium-210, radon-222, and several other elements and binary compounds.

The isotopes radon-222 and polonium-222 are produced by radioactive decay beneath the Moon’s surface. Small amounts of metals are introduced to the Moon’s exosphere upon the collision of micrometeorites, which melt and vaporize on impact. These elements are of insufficient concentration to compare to Earth’s atmosphere in a meaningful way, but through UV Spectophotometry they have been helpful in determining the composition of the Moon’s regolith- the loose heterogeneous layer of sediment and debris deposit. This is because the resonance lines of the elements likely to be found in the exosphere fall in the Ultra Violet wavelength (Morgan, TH). The highest concentration of metal atoms will be found at an altitude between 1000-2000 km, during a meteor-shower (Berezhony, AA).

The range of the temperature on the moon is very extreme due to the lack of an atmosphere. It ranges -153°C (in dark) to +107°C(in light). There can’t be any life on the moon due to the high radiation environment that is caused because of the lack of an atmosphere as there are no molecules to block the heat or radiation from the sun (this also accounts for the very high temperatures). Conversely on earth, the atmosphere traps the heat from the sun to keep a livable temperature even after the sun has set for the day. However, the moon has such a thin atmosphere that almost no heat becomes absorbed, which accounts for the extremely cold temperatures of the moon when it is dark.

### Mars

Mars like the moon has a very thin atmosphere, and consists of carbon dioxide, nitrogen, argon, oxygen, and water. The pressure on Mars is almost exactly like that of Earth. However, the scale-heights (the heights where the pressures are 1/e times the pressures at the surfaces) of Mars and Earth are 11km and 7km, respectively, which accounts for their different pressures. Mars has an atmospheric pressure of less than 1% than the pressure of Earth's pressure, measuring in at 7 millibars. However,like the earth,  Mars pressures vary depending on the altitude. The pressure on the planet ranges from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. Although Mars has a very thin atmosphere, it can create very strong windstorms and dust storms that can last for months; creating a greenhouse effect like that of Earth. The average temperature of Mars is about -63 Celsius; this makes sense because like the moon, Mars has a very thin atmosphere that cannot store heat like the earth.

 Element Mass Percent Carbon dioxide 95.3 nitrogen 2.7 Argon 1.6 Oxygen 0.15 Water 0.03

### Summary of Earth, Moon and Mars differences

Size of Atmosphere: Earth has a much larger atmosphere than either Mars or the moon. For example, the moon has approximately 100 molecules per cubic centimeter, while the air on Earth has about $$3 \times 10^{19}$$ molecules per cubic centimeter. Mars also has a much smaller atmosphere than earth, but larger than the moon.

Pressure- Of Earth, Moon and Mars, the Earth has the greatest amount of Pressure, followed by Mars and the Moon. Since Earth has the largest atmosphere, followed by Mars and the Moon this pressure trend makes sense because the larger the atmosphere, the more weight that presses down on the surface of the planet or moon.

Temperature: Compared to the Earth, the moon has very extreme temperatures, on average -173 degrees Celsius when dark and 107 degrees Celsius when light.  Also compared to Earth, Mars is much colder, with an average temperature of -63 degrees Celsius.

### Problems

1. Does the Moon have an atmosphere?
2. What are the five spheres of the Earth's atmosphere?
3. What planet is the closest to the Earth’s atmosphere?
4. The relative humidity is 30.% and the vapor pressure of water is 40.mm Hg. What is the partial pressure of water vapor?
5. The partial pressure of a unknown compound vapor in the atmosphere is 65 mmHg, and the total pressure is 220 mmHg. Also the vapor pressure of water is 317 mmHg. What is the relative humidity?
6. The landscape on Mars varies in altitude, from the peak of Olympus Mon’s to the bottom of Hellas Planitia, a massive impact crater. Where would you expect to find the LOWEST and HIGHEST atmospheric pressures, respectively?
7. Why does the Moon experience such dramatic temperature changes?
8. Reseachers use UV Spectophotometry to examine the Moon's exosphere in order to determine:

A- the composition of the Moon's regolith B- the speed at which the Moon rotates C- The age of the Moon

1. Mars has a relatively thin atmosphere, (5.25 Torr) at mean-ground-level, compared to Earth. What is the atmospheric pressure on Mars in atm?
2. If the temperature in the Hellas Planitia, Mars, is 0 °C at 1155 Pa, what is the temperature at the peak of Olympus Mon's where the pressure is 30 Pa?

### References

1. DeCoste,Zumdahl : Introductory Chemistry Sixth Ed. Houghton Mifflin Company. 2008
2. Petrucci, Hardwood, Herring and Madura General Chemistry Principles and Modern Applications. 9th Edition. Prentice Hall, New Jersey.
3. Berezhony, AA (Berezhony, AA), Sternberg Aston Inst., Univ. Skij, Pr 13, Moscow, 119992, Russia
4. Morgan, TH (Morgan, TH), SW Res Inst., Instrumentat and Space and Res. Div, 6220 Culebra Rd, San Antonio, Texas