An enormous dust storm is taking place on Mars, shrouding the planet in haze and raising the temperature of its atmosphere an amazing 54 degrees Fahrenheit. It?s the largest such storm in 25 years and still growing. The storm is so big that amateur astronomers using 8 to 10 inch telescopes can see it from Earth.

NASA?s Mars Global Surveyor is in orbit around Mars right now and has been measuring the temperature and dust content of the Martian atmosphere on a daily basis. ?This storm began as a small dust cloud inside the Hellas Basin, a 9-km deep impact crater in Mars?s southern hemisphere,? according to Phil Christensen, of Arizona State University. At first the cloud did little — alternately growing and retreating as days passed, but never getting very large until June 27th. ?That’s when the storm exploded,? he says. ?It crossed some critical threshold and really began to grow? This is a big deal.?

By early July the dust cloud had spilled out of the Basin and wrapped itself around the entire planet. It?s the equivalent of a dust storm on Earth that covers every continent at once. No one knows exactly how Martian dust storms grow to such huge proportions. ?One theory holds that airborne dust particles absorb sunlight and warm the Martian atmosphere in their vicinity. Warm pockets of air rush toward colder regions and generate winds. Strong winds lift more dust off the ground, which further heats the atmosphere,? says Christensen.

Dust storms on Earth are smaller than Martian storms for two reasons. First, Earth isn?t a global desert like Mars. Most of our planet offers little fuel for self-sustaining dust storms. For example, winds in the Gobi desert frequently stir up dust clouds that migrate over the vast Pacific Ocean. With no source of dust to feed them from the ocean below, such storms quickly die.

Second, dust clouds don?t raise the temperature of air on Earth the way they do on Mars. ?The temperature of Earth?s atmosphere is controlled by the latent heat of water vapor,? explains Christensen. ?Airborne dust can?t compete.? On Mars, however, sunlight-absorbing dust can substantially heat the dry, thin atmosphere — raising winds and, of course, more dust. ?The global air temperature on Mars is now about [86 degrees Fahrenheit] higher than it was before this dust storm began,? says Christensen.

The ultimate energy source for Martian dust storms is sunlight. For that reason the dusty season on Mars begins each year near perihelion, the planet?s closest approach to the Sun. Mars will be at perihelion on October 12, 2001. ?The biggest dust storms don?t usually begin until one or two months after perihelion,? says Christensen. ?This one coming so early in the season makes me think Mars is heading for a spell of big dust storms.?

Not only is Mars near perihelion, it?s also near Earth. Just now the two planets are closer than they have been in about a dozen years, which makes Mars unusually big and bright in the midnight sky. Before the ongoing dust storm began, you could see the dark markings and polar caps on Mars with a small telescope. ?But now those details are fading away as dust fills the air, says Christensen. ?Martian dust is made of the same rust-colored stuff that covers the ground, so the planet is looking pretty bland.?

Martian dust may be a problem for future astronauts. Fast-blowing (60 to 100 mph) dust particles can gum the joints of spacesuits and infiltrate cracks in doors and windows. And once they begin, Marian dust storms don?t end quickly. ?Big events tend to last for weeks or months,? says Christensen. ?In fact, we?re not certain what makes them stop.?

When Mariner 9 arrived at Mars in 1971, it was the first spacecraft to orbit the planet. Scientists were anxious to study the new pictures it was expected to send back, since much of Mars had never been seen in any detail. But when the first images arrived at mission control they revealed a world-wide haze. The surface of the entire planet was hidden by the biggest dust storm anyone had ever seen. Only Olympus Mons, a giant volcano 15 miles high, could be seen above the clouds. After a month the dust settled and Mariner 9 mapped the Red Planet with great success.

Like its predecessors, the new dust storm is unpredictable — no one knows when it will end. Indeed, it could be just the first of a series. ?Energy in the atmosphere from one storm makes it easier for the next dust storm to begin,? Christensen explains.

?Atmospheric scientists have been waiting for a beautiful storm like this. The data we?re collecting are marvelous, and I suspect there will be a rush of papers in the months ahead answering some of the questions we have about these events.?

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