Planetary Atmospheres

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Planetary Atmospheres

The term planetary atmosphere refers to the envelope of gases that surrounds any of the planets or dwarf planets within or outside the solar system. A complete understanding of the properties of a planet’s atmosphere involves a number of different areas including atmospheric temperatures, chemical composition of the atmosphere, atmospheric structure, and circulation patterns within the atmosphere.

The study of planetary atmospheres is traditionally sub-divided into two large categories, separating the planets nearest the sun (the terrestrial planets) from the planets outside Earth’s orbit (the giant planets). Included in the first group are Mercury, Venus, Earth, Mars. The second group includes Jupiter, Saturn, Uranus, and Neptune. On the basis of distance from the sun the former ninth planet, Pluto, might be included in this second group but it is not a giant planet and little is now known about the planet and its atmosphere.

Since, 2006, Pluto has been demoted to a dwarf planet, although its study within planetary atmospheres is still valid. In addition, a third category of planetary atmospheres—extrasolar planets or exoplanets—has been added as of the early 1990s. During this period of time, many tiny bodies beyond the orbit of Neptune (what are called trans-Neptunian objects) were discovered. These icy bodies were similar in composition and size to Pluto. In addition, hundreds of exoplanets (planets orbiting stars other than the sun) were found to exist. These discoveries added a wide variety of sizes and characteristics when describing planets.

Until recently scientific knowledge of planetary atmospheres consisted almost entirely of telescopic observations and intelligent guesses based on what scientists already know about Earth’s atmosphere. This situation began to change in the early 1960s when Soviet and American space scientists launched space probes designed to study the inner planets first and later the outer planets. The most successful of the early flights were the NASA’s Mariner 2, which flew past Venus in December 1962; its Mariner 4, which flew past Mars in July 1965; and the Soviet Union’s Venera 3 space probe, which landed on Venus on March 1, 1966.

Studies of the outer planets have been conducted under the auspices of the United States Pioneer and Voyager programs. On December 3, 1972, Pioneer 10 flew past Jupiter exactly nine months after its launch. Flybys of Jupiter and Saturn were accomplished with the Voyager I space probe on March 5, 1979 and November 13, 1980, while Uranus and Neptune were first visited by the Voyager 2 spacecraft on January 24, 1986 and August 25, 1989, respectively.

The 1990s saw advancement in the type of probes launched to explore planetary atmospheres. After a six-year journey, the Galileo Probe entered Jupiter’s atmosphere on December 7, 1995. During its parachute descent, it studied the atmosphere of Jupiter with seven different scientific experiments, with the results radioed back to the Earth. Galileo may have entered Jupiter’s atmosphere at a somewhat special point, but the results indicated that the upper atmosphere of Jupiter was much hotter and more dense than expected—about 305°F (152°C), with an atmospheric pressure of about 24 bars. Galileo also found that winds below Jupiter’s clouds were about 435 mph (700 km/h), and that the atmosphere was surprisingly dry, containing very little water vapor.

As of October 2006, four spacecraft—Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter —are orbiting Mars for the purpose of investigating atmospheric conditions on Mars, along with other aspects of Mars. NASA’s Mars Global Surveyor, launched in 1996, is in its extended mission phase, which is expected to last two more years. NASA’s Mars Odyssey, launched in 2001, has had its mission extended to at least 2008. The European Space Agency’s (ESA’s) Mars Express, launched in 2003, had its lander, Beagle 2, destroyed. However, its orbiter, Mars Express Orbiter, is currently working fine in its orbit about the planet. NASA’s Mars Reconnaissance Orbiter, launched in 2004, began its primary mission in November 2006 to monitor weather and surface conditions on Mars.

The Cassini mission, launched in September 1997, arrived at Saturn in 2004 after a 2.175 billion mi (3.5 billion km) journey. One of the largest, heaviest, and most complex interplanetary spacecraft ever built, Cassini, for the next four years, will observe Saturn’s atmosphere, magnetic field, rings, and moons, along with many other bodies within the Saturnian system. It will also deploy a probe, called the Huygens probe, to Saturn’s largest moon Titan. Titan is unique in the solar system, having a dense atmosphere consisting of nitrogen, and other chemicals in smaller proportions. The atmospheric pressure at Titan’s surface is about twice that of Earth’s.

One interesting proposal for future exploration of planetary or lunar atmospheres are aerobots. Aerobots would be unmanned scientific exploration vehicles designed to float like balloons for up to several months in the atmospheres of planets, conducting scientific experiments and radioing results back to Earth. Aerobots are being studied by NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.

Origin and evolution

When the terrestrial planets formed about 4.6 billion years ago, they did so within the solar nebula (a giant disk of gas and dust). The solar nebula’s rocky solids, ice, and nebulan gas aggregated into larger solid bodies over time, eventually becoming the four terrestrial planets. They grew by the accretion (formation by sweeping up smaller bodies) of planetesimals (smaller, pre-planet bodies); their atmospheres formed by heating, outgassing (releasing), and reprocessing volatiles (volatiles are substances that readily vaporize at relatively low temperature). The terrestrial planets probably obtained equal amounts of volatiles, water, carbon, and nitrogen from planetesimals located in the solar system or the asteroid belt. The cratering process and a high ultraviolet flux from the early sun probably drove large amounts of light atmospheric gases into space. Once formed, the atmospheres have changed in oxidation, total mass, and gaseous amount, as the sun and its intensity has changed.

The giant planets’ atmospheres may have similar starting points to the terrestrials’s, but they did not evolve in the same manner over time, nor is much known about this transformation. Jupiter and Saturn grew with the addition of icy solids and the collapse of nebular gas around them. Uranus and Neptune grew too late to capture nebular gas so the icy dominates. Because these planets have no solid surfaces and strong gravitational fields, their atmosphere only resembles the terrestrial planets by having a complex atmospheric chemistry.

For all planets, the escape of some gases and the retention of others due to temperature and surface gravity played an important role in how their atmosphere’s evolved. Distance from the sun affected what could be retained. The transient heat and pressure generated during planetesimals’s impacts drove chemical reactions between the volatile elements and the rock-forming minerals that determined the chemical composition of the gases released. Released gases did not always remain—some were lost to space because of the initial impact and the sun’s ultraviolet radiation.

General principles

The structure and properties of a planet’s atmosphere depend on a number of factors. One is proximity to its parent star (in the case of the solar system, the sun). With respect to the solar system, those planets closest to the sun are less likely to contain lighter gases that are driven off by the sun’s radiant energy. Mercury illustrates this principle. It is so close to the sun that it has essentially no atmosphere. Its atmospheric pressure is only 10^-12 millibars, one-quadrillionth that of Earth’s atmospheric pressure. The major gases found in this planet’s very thin atmosphere are helium and sodium, both of which are probably remnants of the sun’s solar wind rather than intrinsic parts of the planet’s own structure. Some astronomers believe that contributions come from gases seeping out from the planet’s interior.

Another property determining the nature of a planet’s atmosphere is cloud cover or other comparable features. Cloud cover has a variety of contradictory effects on a planet’s atmosphere. As sunlight reaches the planet, clouds will reflect some portion of that sunlight back into space. The amount that is reflected depends partly on the composition of clouds, with whiter, brighter clouds reflecting more light than darker clouds. Gases in the planet’s atmosphere absorb some of the light that does pass through clouds, and the rest reaches the planet’s surface. The distribution of solar radiation that is absorbed and reflected will depend on the gases present in the atmosphere. For example, ozone absorbs radiation in the ultraviolet region of the electromagnetic spectrum, protecting life on Earth from this harmful radiation.

Of the solar radiation that reaches a planet’s surface, some will be absorbed, causing the surface to heat up. In response, the surface emits infrared radiation that consists of wavelengths significantly longer than that of the incoming radiation. Depending on the composition of the atmosphere, this infrared radiation may be absorbed, trapping heat energy in the atmosphere. Carbon dioxide in a planet’s atmosphere will absorb radiation emitted from a planet’s surface, although the gas is transparent to the original incoming solar radiation. This process is known as the greenhouse effect and is responsible for the warmer atmospheres on some planets than would be predicted based on their proximity to the sun.

A planet’s rotational patterns also influence its atmospheric properties. One can describe the way gases would flow in an idealized planet atmosphere. Since the equator of any planet is heated more strongly than the poles, gases near the equator would tend to rise upward, drift toward the poles, be cooled, return to the surface of the planet, and then flow back toward the equator along the planet’s surface. This flow of atmospheric gases, driven by temperature differences, is called convection. The simplified flow pattern described is named the Hadley cell. In a planet like Venus, where rotation occurs very slowly, a single planet-wide Hadley cell may very well exist. In planets that rotate more rapidly, such as Earth, single Hadley cells cannot exist because the movement of gases is broken up into smaller cells and because Earth’s oceans and continents create a complex pattern of temperature variations over the planet’s surface.

The terrestrial planets

The primary gases present in the atmospheres of Venus, Earth, and Mars are nitrogen, carbon dioxide, oxygen, water, and argon. For Venus and Mars, carbon dioxide is by far the most important of these, making up 96% and 95% of the two planets’ atmospheres, respectively. The reason that Earth’s carbon dioxide content (about 335 parts per million, or 0.0335%) is so different is that the compound is tied up in rocky materials such as limestone, chalk, and calcite, having been dissolved in seawater and deposited in carbonate rocks such as these. Nitrogen is the most abundant gas in Earth’s atmosphere (77%), although it is also a major component of the Venusian (3.5%) and the Martian (2.7%) atmospheres.

The presence of oxygen in Earth’s atmosphere is a consequence of the presence of living organisms on the planet. The widespread incorporation of carbon dioxide into rocky materials can also be explained on the same basis. Water is present in all three planets’ atmospheres but in different ways. On Venus, a trace amount of the compound occurs in the atmosphere in combination with oxides of sulfur in the form of sulfuric acid (most of the water that Venus once had has long since disappeared). On Earth, most water has condensed to the liquid form and can be found in the massive oceans that cover the planet’s surface. On Mars, the relatively small amounts of water available on the planet have been frozen out of the atmosphere and have condensed in polar ice caps, although substantial quantities may also lie beneath the planet’s surface, in the form of permafrost.

On the basis of solar proximity alone one would expect the temperatures of the four terrestrial plants to decrease as a function of their distance from the sun. That pattern tends to be roughly true for Mercury, Earth, and Mars, whose average surface temperatures range from 333°F (167°C) to 59°F (15°C) to -67°F (-55°C), respectively. But the surface temperature on Venus—855°F (457°C)—reflects the powerful influence of the planet’s very thick atmosphere of carbon dioxide, sulfur dioxide, and sulfuric acid, all strong greenhouse gases.

Atmospheric circulation patterns

The gases that make up a planet’s atmosphere are constantly in motion—convection and rotation are key to understanding circulation. The patterns characteristic of any given planetary atmosphere depend on a number of factors, such as the way the planet is heated by the sun, the rate at which it rotates, and the presence or absence of surface features. As indicated above, solar heating is responsible for at least one general circulation pattern, known as a Hadley cell, and observed on all terrestrial planets except Mercury. In the case of Venus and Mars, one cell is observed for the whole atmosphere, while the Earth’s atmosphere appears to consist of three such cells but with a vast complexity introduced by temperature contrasts between oceans and continents.

The presence of extensive mountain ranges and broad expanses of water in the oceans on Earth are responsible for an atmospheric phenomenon known as stationary eddies. In most cases, these eddies involve the vertical transport of gases through the atmosphere, as when air is warmed over land adjacent to water and then pushed upward into the atmosphere. Eddies of this kind have also been observed in the Venusian and Martian atmospheres. The dynamics by which such eddies are formed are different from those on Earth, since neither planet has oceans comparable to Earth.

One interesting example of a circulation pattern is the famous Red Spot on Jupiter. It is a giant storm in Jupiter’s atmosphere, similar to a hurricane, 25,000 mi (40,000 km) across. It has been continuously observed for more than 300 years, and while the spot itself has never disappeared, the circulation patterns within the spot are continuously changing.

The giant planets

Two critical ways in which the giant planets differ from the terrestrial planets are their distance from the sun and their size. For example, Jupiter, the giant planet closest to Earth has an average mean distance of 483 million mi (778 million km) from the sun, more than five times that of Earth. Its mass is 1.9 × 10²⁷ kg, about 300 times greater than that of Earth. These two factors mean that the chemical composition of the giant planet atmospheres is very different from that of the terrestrial planets. Lighter gases such as hydrogen and helium that were probably present at the formation of all planets have not had an opportunity to escape from the giant planets as they have from the terrestrial planets. Light gases never condensed in the inner solar nebula and so were absent from the terrestrial planets to begin with.

An indication of this fact is that these two gases make up almost 100% of the atmospheres of Jupiter, Saturn, Uranus, and Neptune. Other gases, such as water vapor, ammonia, methane, and hydrogen sulfide, also occur in their atmospheres but in very small concentrations. The atmosphere of Jupiter contains about 0.2% methane, 0.03% ammonia, and 0.0001% water vapor.

One of the intriguing features of the giant planets’ atmospheres is the existence of extensive cloud systems. These cloud systems appear to be carried along by rapidly moving winds that have velocities reaching a maximum of 1,640 ft (500 m) per second on Saturn to a maximum of about 300 ft (100 m) per second on Jupiter. The most rapid winds are found above the equators of the planets, with wind speeds dropping off to near zero near the poles.

The cloud systems tend to be confined to narrow latitudinal bands above the planets’ surfaces. Their composition appears to be a function of height within the atmosphere. On Jupiter and Saturn the lowest clouds seem to be composed of water vapor, while those at the next higher level of an ammonia/hydrogen sulfide compound, and those at the highest level, of ammonia.

The Hubble Space Telescope found that Jupiter’s second moon, Europa (which is about the size of Earth’s moon), has a very thin atmosphere that consists of molecular oxygen. While its surface pressure is only one-hundred billionth that of Earth’s. Unlike Earth, though, Europa’s oxygen atmosphere is produced purely by non-biological processes. Though Europa’s surface is icy, its surface temperature is 230°F (-145°C), too cold to support life.

Scientists know very little about the atmosphere of the distant dwarf planet, Pluto. On June 9, 1988, a group of astronomers watched as Pluto occulted a star of the 12th magnitude. What they observed was that the star’s light did not reappear suddenly after occultation but was restored gradually over a period of a few minutes. From this observation, astronomers concluded that Pluto must have some kind of atmosphere that would smudge out the star light that had been occulted. They have hypothesized that the major constituent of Pluto’s atmosphere is probably methane, which exists in a solid state for much of the Pluto’s very cold year. Depending upon the exact temperature, a certain amount of methane should form a tenuous atmosphere around Pluto. As the temperature changes, the atmosphere’s pressure on Pluto’s surface could vary up to 500 times as the methane evaporates and redeposits on the surface. Alternatively, based on the 1988 observations, a haze of photochemical smog might be suspended above the planet’s surface. Others, like William Hubbard, theorize that it may contain carbon monoxide or nitrogen.

NASA’s New Horizon spacecraft will cross the orbits of all of the planets past Earth on its way to fly past Pluto and Charon in 2015. It will be the first spacecraft to visit the Pluto-Charon system. The spacecraft is expected to fly within 6,200 mi (10,000 km) of Pluto with a relative velocity of 8.6 mi/sec (13.78 km/sec) at closest approach. When it flies by Charon it will come as close as 16,800 mi (27,000 km). The seven science instruments onboard the spacecraft will help scientists understand the global atmospheres of Pluto and Charon, along with the chemical compositions of their atmospheres. The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) will measure the composition and density of plasma (ions) escaping the atmosphere of Pluto. The Solar Wind Around Pluto (SWAP) instrument will measure Pluto’s atmospheric escape rate and observe its interaction with the solar wind. The Radio Science

KEY TERMS

Atmosphere— The envelope of gases that surrounds a planet.

Giant planets— Relatively large planets more distant from the Sun than the terrestrial planets. The giant planets are Jupiter, Saturn, Uranus, and Neptune.

Greenhouse effect— The phenomenon that occurs when gases in a planet’s atmosphere capture radiant energy radiated from a planet’s surface thereby raising the temperature of the atmosphere and the planet it surrounds.

Hadley cell— A circulation of atmospheric gases that occurs when gases above a planet’s equator are warmed and rise to higher levels of the atmosphere, transported outward toward the planet’s poles, cooled and return to the planet’s surface at the poles, and then transported back to the equator along the planet’s surface.

Stationary eddy current— A movement of atmospheric gases caused by pronounced topographic features, such as mountain ranges and the proximity of large land masses to large water masses.

Terrestrial planets— Planets with Earth-like characteristics relatively close to the Sun. The terrestrial planets are Mercury, Venus, Earth, and Mars.

Experiment (REX) will measure the composition of Pluto’s atmosphere, along with its temperature, with the use of radio waves. An ultraviolet imaging spectrometer (nicknamed ALICE) will analyze the composition and structure of Pluto’s atmosphere and will examine the atmospheres around Charon and various Kuiper Belt Objects (KBOs).

Resources

BOOKS

Bauer, Siegfried J. Planetary Aeronomy: Atmosphere Environments in Planetary Systems. Berlin, Germany: Springer, 2004.

Irwin, Patrick. Giant Planets of Our Solar System: Atmospheres, Composition, and Structure. Berlin, Germany, and New York: Springer, 2003.

Mendillo, Michael, Andrew Nagy, and J.H. Waite, eds. Atmospheres in the Solar System: Comparative Aeronomy. Washington, DC: American Geophysical Union, 2002.

Sobel, Dava. The Planets. New York: Viking, 2005.

PERIODICALS

Clark, B.C. “Planetary Interchange of Bioactive Material: Probability Factors and Implications.” Origins of Life and Evolution of the Biosphereno. 31 (2001): 185-197.

OTHER

Jet Propulsion Laboratory, National Aeronautics and Space Administration. “Planet Quest: The Search for Another Earth.” <http://planetquest.jpl.nasa.gov/gallery/gallery_index.cfm> (accessed October 14, 2006).

TeachNet-lab.org. “A Tour of the Planets.” <http://www.teachnet-lab.org/miami/2001/salidoi2/a_tour_of_the_planets.htm> (accessed October 14, 2006).

SpaceKids, National Aeronautics and Space Administration. “Tour the Solar System and Beyond.” <http://space-kids.hq.nasa.gov/osskids/animate/mac.html> (accessed October 14, 2006).

Jet Propulsion Laboratory, National Aeronautics and Space Administration. “Welcome to the Planets.” <http://pds.jpl.nasa.gov/planets/> (accessed October 21, 2006).

David E. Newton

The Gale Encyclopedia of Science