If you think the weather on Earth is unpredictable, try living on Mars. One week, the sky is pink and cloudless, filled with windblown dust raised from the rusty Martian surface. By Martian standards, it's warm, about minus 40 degrees Fahrenheit. Then, in a matter of days, the dust is swept from the atmosphere, temperatures plummet 40 degrees, and brilliant water ice clouds appear against a dark blue sky.
Dramatic weather changes like these may not seem very different from a batch of severe thunderstorms passing through your home town, but for Mars these changes can sweep over the entire planet every week. It appears that Mars' roller coaster-like weather is more chaotic and unpredictable than scientists first thought. Observations by the Hubble Space Telescope and the National Radio Astronomy Observatory (NRAO) radio telescope at Kitt Peak, Ariz., show that the atmosphere of Mars is more complex and variable than the picture revealed by the Viking and Mariner 9 orbiters. These spacecraft collected information from the planet in the 1970's and painted a fairly one-dimensional picture of Mars' climate. Images snapped by the orbiters revealed huge dust storms spreading throughout the entire atmosphere when Mars was closest to the sun (perihelion). These dusty conditions continued to dominate the planet's climate when it was farthest from the sun (aphelion). (Perihelion and aphelion occur every Mars year, which equals two Earth years. Aphelion occurs in northern summer, perihelion in southern summer.)
But information captured by Hubble and NRAO show that Mars is more often cloudy than dusty, experiencing abrupt planet-wide swings between dusty and hot and cloudy and cold. A state of emergency would be declared on Earth if an ice or dust storm blanketed the entire planet.
These shifts in climate are driven by three important factors: Mars' thin atmosphere, its elliptical orbit around the sun, and strong climatic interactions between dust and water ice clouds in the atmosphere. Mars' atmosphere is so thin that it weighs less than 1 percent of Earth's atmosphere. Because Mars' atmosphere is so paper-thin and there are no oceans to store up heat from the sun, the planet's temperatures respond more quickly and intensely to surface changes and atmospheric heating by the sun. There are also much larger annual changes in sunlight falling on Mars than on Earth, because Mars' distance from the sun varies by 20 percent in its orbit around the sun every two years.
Mars' elliptical orbit leads to planet-wide changes in atmospheric and surface temperatures over the course of a Mars year. During perihelion, when Mars is closest to the sun (summer in the southern hemisphere), the planet receives 40 percent more sunlight than during aphelion, when it is farthest from the sun (summer in the northern hemisphere). This annual variation in sunlight causes 35-degree Fahrenheit increases during southern summer (perihelion), forcing continental-scale dust storms at the planet's surface. The dust is swept aloft to altitudes of tens of miles, where it spreads globally, absorbs light from the sun, and heats the entire atmosphere by another 30 to 50 degrees Fahrenheit. This dusty perihelion climate was observed by Viking and Mariner 9 and by NRAO in 1992, 1994, and 1996.
But what the 1970's orbiters did not identify was the very distinctive Mars aphelion climate, with its planet-wide belts of water ice clouds. These clouds are as striking as the perihelion global dust storms. During the aphelion climate, surface dust raised by low dust storms is confined to low altitudes (about 10 km or 6 miles), and is eventually swept to the ground by water ice clouds. These clouds surround the planet at altitudes of 3 to 10 km (2 to 6 miles). It is the cold atmospheric conditions of Mars during aphelion, when the sun is much weaker, that stimulate the formation of these water ice clouds. The clouds further reduce atmospheric temperatures by forming around the dust. Without sunlight, the dust freezes and falls to the ground. This strong competition between dust heating and cloud cooling drives sweeping annual and short-term regional changes in Mars' climate.