Weathering the storm: Mars' climate features lots of cloud and dust. Credit: © NASA

Missions that landed on Mars revealed a still surface, undisturbed by volcanoes, earthquakes or landslides. The red planet's atmosphere is nowhere near so boring, say researchers who have been watching clouds, winds and dust storms racing around the planet for a full martian year1.

Studying Mars' climate in the coming years should yield important information about the transport of water and dust around the planet, and provide new insights into what climatic conditions were like before Mars became the barren globe it is today.

"The surface of Mars may be quiescent; the atmosphere certainly is not," says John Pearl at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who is analysing data from the Mars Global Surveyor spacecraft, which began orbiting Mars in 1997.

Clouds on Mars were first seen in detail in the 1970s by the Viking Orbiter spacecraft, "but no obvious general trends were observed," says Francois Forget, who studies the martian atmosphere at the University of Paris, France.

Mars Global Surveyor is equipped with an instrument that measures how light is reflected from the martian atmosphere, giving real-time data on its changes. "These are the first really quantitative observations of clouds," says Forget.

Valleys filled with low-lying morning fog and high peaks shrouded in mist indicate local-scale variations - or 'microclimates' - in the martian environment. At night, the carbon dioxide and water that make up martian clouds seem to freeze out and fall to the ground. "This tells us that water-ice clouds may play a significant role in the climate," says Forget.

Because Mars doesn't have any oceans - which drive cloud formation on Earth - researchers had expected the martian atmosphere to be relatively easy to understand.

"It's turning out to be very complex," says Pearl. His team mapped a large cloud belt around the middle latitudes of northern Mars; the cloud stayed put for the entire summer season, only to disappear in early autumn. They also watched as a winter dust storm banished clouds from the southern hemisphere.

Large-scale weather patterns like these are the key to understanding long-term patterns in martian climate, says Pearl. Their job now is to start to interpret their observations.

Other researchers will now incorporate Surveyor's weather observations into 'global circulation models' of martian climate. As data accumulates over the next few years, these models should be able to predict martian climate patterns.

By running theses models backwards in time, says Pearl, it could be possible to see what the climate of Mars used to be like. This information is crucial for those studying the planet's recent past, when liquid water is thought to have flowed over its surface.

Real-time climate data from Surveyor may also be crucial to Mars missions. Dust storms, which are turning out to be common on Mars, can interfere with the 'aerobraking' manoeuvres that are used to get spacecraft into orbits around the planet. Knowing the size and position of a dust storm can allow spacecraft trajectories to be adjusted to avoid this problem, says Pearl.

A few months ago, a giant dust storm began to form on Mars; now at its peak, the storm encircles the entire planet. It is the first time that a major dust storm has been followed from the start. Says Pearl: "Our observations would have made 19th-century astronomers drool."

The researchers are confident that the storm will have passed by the time NASA's Mars Odyssey mission arrives at the planet in October.