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Studying the atmospheres of Biosphere 2, Venus, and Mars helps us to appreciate Earth's amazing atmosphere. A balance of gases in the atmosphere enables for respiration and photosynthesis and helps maintain a habitable temperature. The mass of the Earth's atmosphere and updrafts causing circulation buffer the planet against wild temperature swings between day and night.
Stories about colonization of other planets are constantly in the news. Venus, the moon, and Mars are not habitable, but visionaries, scientists, and engineers are trying to design structures that would enable humans to live and produce food on the moon and Mars. Several years ago, a wealthy businessman named Edward P. Bass entered into a relationship with a group that was interested in colonizing other planets. Biosphere 2 (Figure 5‑18) is an artificial atmosphere/ecosystem that simulates an enclosed environment on another planet. The cost of the project was $450 million, and it provided a habitat for eight people. There were unforeseen problems, such as reduced oxygen, that degraded the lives of the Biospherians. Comparing the Biosphere 2 atmosphere to Earth's atmosphere helps one appreciate the special qualities of our atmosphere that make Earth a pleasant place to live. The University of Arizona now runs Biosphere 2 as a research laboratory.
Figure 5-18. The artificial ocean inside the University of Arizona’s Biosphere 2, Oracle, Arizona.
In the following video, Jane Poynter describes her experience as a Biospherian who lived inside Biosphere 2. You might also be interested in this video, https://youtu.be/DHS2KdAZLIc, which describes the Tucson company that she and her fellow Biospherian and husband started. The video describes how Paragon Space Development is sending a manned balloon into the stratosphere. Why does Jane get to work on all of the interesting projects?
One of the challenges for Biosphere 2 engineers was maintaining the desired temperature and pressure inside the structure. Because the atmosphere is a gas, it expands when it gets hot, which presents a problem for a closed container such as Biosphere 2: thousands of plates of glass would blow out if the inside pressure increased significantly above atmospheric pressure. The other major challenge turned out to be maintaining the oxygen concentration in the Biosphere 2 atmosphere within the range that is optimal for humans.
Biosphere 2 engineers needed to design the Biosphere 2 structure with two gas laws in mind. The first law was discovered by Robert Boyle (1627–1691), who was already mentioned in chapter 2. Boyle used the scientific method and conducted careful experiments with an air pump and an animal lung. He found that pressure and volume of a gas are inversely related. As the volume of the animal lung increased, the pressure of the gas decreased. In fact, Boyle found that the product of pressure and volume of any gas remains constant as long as temperature does not change. This relationship is called Boyle’s law:
PV = constant
where
P = pressure, kPa,
V = volume, m3.
Figure 5‑19. Molecules moving in a gas.
Gases consist of molecules that continually bounce off each other and the walls of containers (Figure 5‑19). Pressure on the walls of containers is the cumulative force of all the molecules bouncing off the walls. If the volume of a closed container decreases such as the compression stroke of an automobile piston, then each molecule has less space. Then the molecules bounce off each other and the walls of the container more frequently and the pressure increases.
Heat in a gas is kinetic energy (energy from movement). If temperature goes up, then the molecules in a gas move faster. When molecules move faster, they impact the walls of containers more frequently and with greater force, and the pressure increases. Gay-Lussac (1778–1850) discovered that there was a direct relationship between the temperature of a gas and the pressure of a gas. He found that pressure increases linearly with temperature if volume is held constant:
P = cT
where
c = constant,
T = temperature, K.
Zero Kelvin is absolute zero temperature (nothing moves, not even atoms). Zero degrees Celsius is the same as 273 K. Biosphere 2 is kept at approximately room temperature, 20 0C or 293 K. Standard pressure in Biosphere 2 is atmospheric pressure (1 atmosphere or 100 kPa). If the volume of the Biosphere remains constant, then an increase in temperature would result in an increase in pressure, according to Gay-Lussac’s Law. This would blow the windows out.
Example. Calculate the pressure increase if the temperature in Biosphere 2 increases from 20 0C to 40 0C because of solar radiation. This would increase Kelvin temperature from 293 K to 313 K. Use Gay Lussac’s Law to find the new pressure at the higher temperature, assuming that the volume of Biosphere 2 remains constant. Use Boyle’s law to calculate the change in volume required to keep Biosphere 2 at a constant pressure.
P = cT à c = P/T = 100 / 293 = 0.34
Solve for the new pressure at 313 K: P = cT = (0.34) (313 K) = 106 kPa
Boyle’s law can be used to calculate the required expansion in volume to avoid a pressure increase. The volume of Biosphere 2 is 5.5 million cubic feet.
P1V1 = P2V2 V2 = (P1V1) / P2
where P1 and V1 represent the increased pressure (106 kPa and 5.5 mcf)
where P2 and V2 represent the desired pressure and expanded volume Expansion V2
V2 = (P1V1) / P2 = (106)(5.5)/(100) = 5.83 mcf
Extra expansion volume = 5.83 – 5.5 = 0.33 mcf = 330,000 cf.
The Biosphere 2 engineers needed to design a novel method to maintain a constant pressure inside even though the temperature might fluctuate. They needed almost 2 million feet of expansion capability to protect the structure in case of very high temperatures. They designed an expandable chamber that allowed for expansion of the gas within the enclosed biosphere.
Figure 5‑20. The Biosphere lung for pressure control. Source of original picture unknown
The Biosphere 2 lung is the large dome in Figure 5‑20. A long tunnel leads from Biosphere 2 to the lung (drawn in red): a massive steel plate (gray) moves up and down within the lung to allow for air expansion. A flexible rubber diaphragm (orange) provides a seal and connects the steel plate to the dome structure that houses the lung. Thus, Biosphere 2 can remain sealed but maintain a constant air pressure with changing temperature
In addition to pressure problems, there is not enough heat buffering capacity in the air within Biosphere 2 to prevent dramatic increases in temperature during the day. Giant air conditioners are needed to keep Biosphere 2 habitable. In contrast, the Earth’s atmosphere stretches for 100 km into interstellar space (Figure 5‑21), and the huge heat buffering capacity of our atmosphere generally makes the heat fluctuation tolerable on Earth.
Figure 5‑21. Temperature vs. elevation in the earth’s atmosphere. Credit: scied.ucar.edu ete.cet.edu. Used here per CC BY-SA 4.0.
The Earth’s atmosphere maintains the average temperature fluctuation on Earth’s surface between day and night within +/- 5 0C. Another key to keeping the Earth’s surface habitable during the day is that there is natural cooling on the ground due to updrafts and air exchange with the bulk atmosphere. Earth’s surface would become extremely hot during the day without updrafts. Even temperatures, gases that support life, the angle of Earth's axis, and the water cycle have made earth extremely habitable for life.
The relationship between atmospheric temperature variation and the mass of an atmosphere is demonstrated by comparing the atmospheres and typical day-night temperature variations of the four terrestrial planets:
Earth: 10 0C to 20 0C (283 K to 293 K) – 1 day rotation (10 degree diurnal range)
Mars: – 89 0C to – 31 0C (184 K to 242 K) – 1 day rotation. Pressure of atmosphere on Mars is 150 times less than Earth (60 degree diurnal range)
Venus: 462 0C to 462 0C (735 K to 735 K) – 243 day rotation. Pressure of atmosphere on Venus is 90 times that of the Earth. High carbon dioxide leads to high Ave. temperature
Mercury: – 173 0C to 427 0C (100 K to 700 K) – 58 day rotation. Almost no atmosphere.
The atmospheric pressure on the surface of Venus is 90 times as high as on Earth, which means that the atmosphere of Venus has approximately 90 times the mass of Earth’s atmosphere. This huge mass can absorb so much heat that there is no temperature difference on Venus between day and night even though a day on Venus is 243 times as long as an Earth day. Plants on earth remove carbon dioxide from the atmosphere and form carbon compounds, which settle on the sea floor and become incorporated into geologic layers. The high concentration of carbon dioxide (96%) on Venus causes a high average temperature.
The surface of Venus is completely dry; however, it appears that Venus once had water. This is because there appears to be granite on Venus. Granite forms when water combines with magma. Unlike Earth, Venus has no magnetic field protecting it from solar winds. Venus lost its water from the atmosphere due to the solar wind that sweeps by Venus. The process of solar wind removing gases is called sputtering. High energy photons in solar wind knock loose electrons and ionize atoms in the atmosphere of Mars and Venus. These ionized atoms then float out into interstellar space. As stated previously, the Earth’s core (Figure 5‑22) establishes plate tectonics and a magnetic field, which Venus and Mars are currently missing.
Figure 5‑22. Cores of inner terrestrial planets. Credit: NASA.
Venus has other negative climate factors for life, such as a slow rotation rate. A day lasts almost a year on Venus. Another factor is that the atmosphere on Venus has extremely thick clouds and the clouds rotate at 300 km per hour. There are constant planetary hurricanes in the polar regions. Unlike Venus, Earth has an intensity of solar radiation and atmospheric density that drives gentle winds and the hydrologic climate cycle. Earth's winds and climate are due to the unevenness of solar radiation. The wind currents on earth also drive ocean currents that move equatorial waters toward the poles and warm the higher latitudes. The winds and currents even out the temperatures around the world.
As with Venus, Mars once had water and seems to have also had an atmosphere and cloudy skies at one time. As with Venus, Mars has no magnetic field and lost its atmosphere through sputtering. Mars might have hosted life before losing its atmosphere.
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The barren surface of Mars. Credit: NASA.
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