Planetary Atmospheres

The term planetary atmosphere refers to the envelope of gases that surrounds any of the planets in our 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 often 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, and, sometimes, the Moon. The second group includes Jupiter, Saturn, Uranus, and Neptune. On the basis of distance from the sun the 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.

Until recently our 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 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 700 km/hr (435 mi/hr), and that the atmosphere was surprisingly dry, containing very little water vapor.

On December 11, 1998, NASA launched a space probe to explore Mars, called the Mars Climate Orbiter. It is expected to reach Mars in September, 1999, when it will study Martian weather and climate. The Orbiter will generate weather maps and profile the thin but dusty Martian atmosphere over a full Martian year (687 days).

The Cassini mission, launched in September 1997, will arrive at Saturn in 2004. One of the largest, heaviest, and most complex interplanetary spacecraft ever built, Cassini will 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 the Jet Propulsion Lab in Pasadena, California.