Craft tracks giant dust storm on Mars.

It's the beginning of dust season on Mars, and the first dust storm is a whopper. Engulfing the entire planet, the storm is the largest observed on the Red Planet in the past 25 years and already has raised the temperature of the frigid Martian atmosphere by 30 [degrees] C.

Daily observations by a temperature and dust detector aboard the orbiting Mars Global Surveyor spacecraft revealed the storm on June 15. At that time, the storm amounted to a small cloud confined to Hellas Basin, a 9-kilometer-deep crater in the southern hemisphere.

Alternately shrinking and expanding for several days, the storm exploded on June 27, says Philip R. Christensen of Arizona State University in Tempe, the investigator in charge of the craft's thermal-emission spectrometer. By July 4, the cloak of dust had wrapped around the globe. Says Christensen: "Atmospheric scientists have been waiting for a giant storm on Mars with Surveyor in orbit, and this is it."

Although dust storms are common on the cold desert covering Mars, most remain small or at their largest cover a portion of the planet comparable to the dust bowl of the U.S. heartland in the 1930s. A Martian storm of the current magnitude is "a relatively rare event," notes James F. Bell of Cornell University.

Dust season, which can last 6 months, typically occurs about every 2 years, soon after Mars makes its closest approach to the sun. A storm begins brewing when dust particles suspended in the atmosphere absorb sunlight. Warmed gases stream toward colder regions, and this wind lifts more dust off the ground, further heating the atmosphere. The dust can blow at more than 100 km per hour.

Previous observations had only revealed large Martian dust storms 1 or 2 months after the planet's closest approach to the sun. Yet this time, the global storm erupted just 2 days after passing that milestone.

Bell cautions that it's hard to determine how unusual that timing is or if, in fact, this storm ranks as the biggest in 25 years, since visits by spacecraft to the Red Planet have been rare. Christensen argues, however, that if a Martian storm as big as the present one had erupted any time since the mid-1970s, astronomers on Earth would have spotted it.

The intensity of sunlight, the amount of dust in the Martian atmosphere, and the planet's desert conditions all conspire to keep Mars poised at a delicate balance, says Christensen. Just a slight shift in climatic conditions may cause some dust storms to rage out of control.

With Surveyor closely monitoring the Martian climate, scientists hope to determine which factors transformed this storm from a small cloud hovering inside a crater to a planetwide event. If the handful of other large storms observed on Mars are any indication, this one could last another month and be followed by another giant storm, Christensen adds.

The storm shouldn't affect the expected October arrival of another orbiting craft, Mars Odyssey. In contrast, the current storm would have presented a problem for a Martian lander, Bell notes. Since such a craft would rely on solar power, a major storm "would seriously diminish the amount of power available for surface operations like driving and making science measurements," he says.

Observing the dusty doings from Earth, amateur and professional astronomers are benefiting by a lucky coincidence: Mars and Earth lie closer to each other than they have in about 12 years. Earthlings will have an even better view in 2003, during the next Martian dust season, Bell says. At that time, the Red Planet will come within 56 million km of Earth--its nearest pass until 2287--and will appear about 20 percent bigger than it does now.