Planetary Atmospheres

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 778 million km (483 million mi) from the Sun, more than five times that of Earth. Its mass is 1.9 x 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.

We know very little about the atmosphere of the most distant 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.

In 1995 the Hubble Space Telescope found that Jupiter's second moon, Europa (which is about the size of our 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.