Gas giants and icy moons
Despite their frigidity, the four planets past Mars (Jupiter, Saturn, Neptune,and Uranus) manage to produce some interesting weather. One of their moons even has an atmosphere surprisingly similar to Earth's. All four planets have been dubbed gas giants, because the bulk of their interiors are made up of gases rather than solids (though Neptune and Saturn also hold large amounts of ice). The makeup of these planets' atmospheres resembles the Sun more than earthly air: hydrogen predominates, followed by helium. Small amounts of methane, ammonia, sulphides and other ingredients help produce their wide variety of clouds and colours as seen from Earth.
Jupiter, the solar system's largest planet by far, features an ever-evolving array of clouds, as well as vivid lightning (observed by NASA) and spectacular aurorae. The clouds, which consist of ammonia and other compounds as well as water, change within an overall pattern long noted by astronomers: alternating bright and dark bands at varying latitudes. The bands reflect a more intricate version of a closer-to-home pattern. On Earth, air tends to rise and form clouds along the Equator and at roughly 60°N and S, while sinking air and dry conditions are common around 30°N and S and at the poles. These bands are a product of incoming sunlight and Earth's rotation. Jupiter spins far more rapidly – almost two and a half times faster than Earth – and it's encircled by more than a dozen alternating light and dark bands. NASA's Cassini spacecraft has spotted tiny, bright storms – too small to be visible from Earth – speckling the darker bands.
Since there's no solid surface to help brake Jupiter's circulation, its winds can scream at more than 600kph/380mph, often spinning around longlasting cyclones and anticyclones. The most famous is the Great Red Spot, which is bigger than Earth itself. It's the Methuselah of weather systems: first observed in the mid-1600s, the spot is still going strong. Though it's often referred to as a storm, the Great Red Spot is actually an anticyclone, which usually brings fair weather on Earth. The spot gets a continual supply of spin from its location, nestled between two of Jupiter's circulation bands and their alternating winds. Spacecraft have tracked other, smaller circulations across Jupiter, but none with the persistence and size of the Great Red Spot.
Saturn may have spectacular rings, but its visible weather features are more muted than Jupiter's. Its latitudinal bands are less vivid, and except near the poles, its winds typically blow from a constant direction (west), albeit at astounding speeds – sometimes topping 1600kph/1000mph. Like Jupiter, Saturn experiences water-based thunderstorms and lightning. Saturn's large-scale storm systems are generally shorter-lived than Jupiter's. A recurring feature called the Great White Spot sets in about every 30 years, roughly the amount of time Saturn takes to complete an orbit – but it only lasts for a few weeks at a time. The next appearance of the Great White Spot is expected in 2020.
Of the four outer planets, Uranus has the most humdrum weather. The atmospheric pots on both Jupiter and Saturn get stirred by the immense heat generated by their respective cores (both emit far more heat than they receive from the Sun). Uranus is much less dense, though, and with relatively little heat coming into its atmosphere from either its core or the Sun, there's isn't much action to observe. Perhaps the most distinctive thing about weather on Uranus is the incredibly slow transitions between light and dark, thanks to the planet's odd rotation. Unlike the other planets, which feature only slight season-producing tilts, Uranus lies nearly on its side, spinning around an axis just eight degrees off its orbital plane. Much like our own North and South Pole, this means that each year has just one sunrise and one sunset, in the case of Uranus, each separated by nearly 42 Earth years (since a Uranian year lasts about 84 of ours).
Neptune, in contrast, is a surprisingly lively place weatherwise. It boasts the strongest winds observed anywhere in the solar system, ripping along at speeds estimated to be as high as 2500kph/1600mph. The winds feed into massive, but surprisingly transient storms. In 1989 NASA's Voyager 2 discovered a cyclone the size of the Indian Ocean, quickly dubbed the Great Dark Spot. Five years later, the Hubble Space Telescope found that the Great Dark Spot was gone, with a similar storm now evident in the opposite hemisphere. Pluto hasn't got much of an atmosphere at all – its air is almost a million times less dense than Earth's. The biggest changes in Plutonian air are driven by the planet's markedly elliptical orbit. During its 248-year-long trip around the Sun, Pluto gets within 4.5 billion km/2.8 million miles of the Sun at its closest – closer even than Neptune – but it's more than 7.3 billion km/4.5 billion miles at its farthest. Some of the planet's icy coating evaporates as it nears the Sun and then freezes again during the more distant periods.
More than 100 moons have been found spinning around the four gas giants. Nearly all of these lack an atmosphere and thus are featureless weatherwise, though many are either composed largely of ice or coated with it (such as Europa, which orbits Jupiter and which may harbour liquid water below its ice-covered surface). Neptune's most famous moon, Triton, manages an ultra-thin atmosphere, fed by volcanoes that emit nitrogen and methane. A few other moons harbour a smattering of trace gases. But the only moon with a bona fide atmosphere is Saturn's Titan, which is actually larger than Mercury. Titan boasts a surface air pressure even higher than Earth's, and like our own atmosphere, that air consists mainly of nitrogen. Smaller amounts of methane and other hydrocarbons react in sunlight to produce an orange, smog-like haze that shrouds the surface in perpetual dusk. Occasionally methane and ethane drop from the Titanian sky in the form of rainstorms. Right now Titan is a dependably cold place – temperatures average around –180°C/–292°F – but some astronomers have speculated that it could serve as a potential platform for life in a few billion years, as the Sun expands and warms its atmosphere.