Planetary Atmospheres

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 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, 40,000 km (25,000 mi) 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.