Overview (The Solar System)
Atmospheres are an attribute shared by seven of the eight planets in the solar system. (Since 2006 Pluto has been classified by the International Astronomical Union not as a planet but as a dwarf planet.) Crater-covered Mercury is the only planet that does not have a gaseous envelope. Even Titan, a large Satellite of Saturn, possesses a significant atmosphere. Nevertheless, these atmospheres differ considerably in chemical composition, structure, and density, and comparative planetologists’ study of these differences is key to an understanding of Earth’s atmosphere and its future.
Planetary scientists believe the planets acquired their atmospheres in one of two ways. In some cases, the atmosphere formed with the planet around the Sun out of the solar Nebula around 4.5 billion years ago. In such cases, a planet is said to have a primordial, or captured, atmosphere. In other examples, an atmosphere appears to have been created from gaseous material released from within the planet after it formed. These are called secondary, or outgassed, atmospheres.
Atmospheres of the Jovian planets (Jupiter, Saturn, Uranus, and Neptune) seem to be of the primordial type. Like the Sun itself, they are composed largely of hydrogen and helium. On the other hand, three of the terrestrial planets (Venus, Earth, and Mars) may have secondary rather than primary atmospheres. Characteristics and compositions of these atmospheres vary...
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Methods of Study (The Solar System)
Understanding planetary atmospheres beyond Earth has been facilitated greatly by data returned by such spacecraft as Mariner 10, a host of Venus and Mars probes, Pioneer 10 and 11, the two Voyagers, and the Galileo and Cassini orbiters. Prior to the epic journeys of these robotic laboratories, knowledge of the gaseous envelopes around the planets was severely limited by the difficulties of extracting meaningful data from faint images seen in telescopes.
Techniques for determining the chemical constituents of an atmosphere from afar are not essentially different whether the measurements are made through a telescope on Earth or from aboard a spacecraft coming within tens of thousands of kilometers of the planet. In both cases, the viewing telescope must be equipped with a spectroscope, an instrument that breaks the reflected light of the planet up into its component colors. It thereby reveals the “fingerprints” of chemicals present in the source of the light. A spacecraft in close proximity to a planet, however, has more reflected light with which to work and its target appears big enough that the instrument can be very discriminating about what it samples.
Coupled with the accuracy and specificity of the chemical assays provided by planetary space probes are the results of radio science experiments. These can probe the temperature and density of the deeper atmosphere. A wide variety of other instruments can image...
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Context (The Solar System)
Accompanying the great increase in data about the nature of the solar system has been a quantum improvement in the understanding of how each of the planets formed. This knowledge rests heavily on information concerning the planetary atmospheres themselves, particularly as the planets with primordial atmospheres retain vestiges of the chemical makeup of the Solar nebula from which the Sun and all its planets were born.
Some fundamental questions regarding the atmospheres of the other planets remain unanswered. This is particularly true of the Jovian planets, since by 2008 only one entry probe, detached from the Galileo orbiter, had entered Jupiter’s atmosphere. The depth to which the visible patterns extend is not known in general, nor is it clear whether and to what depth the atmospheres are layered below the cloud tops. Whether heat from the Sun or heat generated in the interior of the planet has the primary role in driving the circulation of the atmosphere has also not yet been determined. The next generation of planetary probes will seek to answer these questions.
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Further Reading (The Solar System)
Beatty, J. Kelly. “Getting to Know Neptune.” Sky and Telescope 79 (February, 1990): 146-155. A synopsis of the findings from Voyager 2’s encounter with Neptune in August, 1989. Several paragraphs discuss the atmosphere itself, and color diagrams enrich the text. The journal is intended for amateur astronomers and others with some general science background, but Beatty’s expertise in science writing bridges the gap to the general reader.
Burgess, Eric. Uranus and Neptune: The Distant Giants. New York: Columbia University Press, 1988. Written after Voyager 2’s Uranus encounter but before the spacecraft reached Neptune, this book’s value is in its comprehensive discussion. Burgess presents both the findings of the spacecraft’s instrumentation and the interpretation of these findings by scientists. Advanced reading.
De Pater, Imke, and Jack J. Lissauer. Planetary Sciences. New York: Cambridge University Press, 2001. A challenging and thorough text for students of planetary geology, and an excellent reference for the most serious reader with a strong science background. Covers extrasolar planets and provides an in-depth explanation of the solar system’s formation and evolution.
Esposito, Larry W., Ellen R. Stofan, and Thomas E. Cravens, eds. Exploring Venus as a Terrestrial Planet. New York: American Geophysical Union, 2007. This is a collection of articles covering...
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