July 9, 2003: Something is happening on Mars and it's so big you can see it through an ordinary backyard telescope.
On July 1st a bright dust cloud spilled out of Hellas Basin, a giant impact crater on Mars' southern hemisphere. The cloud quickly spread and by the Fourth of July was 1100 miles wide--about one-fourth the diameter of Mars itself.
Above: These pictures of Mars spanning July 2nd through 6th were captured by Donald Parker of Coral Gables, FL, using a 16-inch telescope. The stubby black arrows indicate the growing cloud. More images:, , , , .
"The cloud can be seen now through a telescope as small as 6 inches," says Donald Parker, executive director of the Association of Lunar and Planetary Observers (ALPO). "Its core is quite bright."
Two years ago, a similar cloud from Hellas Basin grew until it circled the entire planet. Features on Mars long familiar to amateur astronomers--the dark volcanic terrain of Syrtis Major, for example--were hidden for months. "The planet looked like an orange billiard ball," recalls Parker.
Will it happen again?
"No one knows," says astronomer James Bell of Cornell University who studied the dust storm of 2001 using the Hubble telescope. "We don't yet understand the mechanism that causes regional clouds to self-assemble into giant dust storms."
"Only 10 global or planet-encircling dust storms have been reported since 1877," notes Parker.
Left: An orange billiard ball: a world-wide dust storm on Mars in 2001 blurred the planet's normally sharp features. [more]
All dust storms on Mars, no matter what size, are powered by sunshine. Solar heating warms the martian atmosphere and causes the air to move, lifting dust off the ground.
Because the martian atmosphere is thin--about 1% as dense as Earth's at sea level--only the smallest dust grains hang in the air. "Airborne dust on Mars is about as fine as cigarette smoke," says Bell. These fine grains reflect 20% to 25% of the sunlight that hits them; that's why the clouds look bright. (For comparison, the reflectivity of typical martian terrain is 10% to 15%.)
Sunlight on Mars is about to become unusually intense. The planet goes around the sun in a 9%-elliptical orbit with one end 40 million km closer to the sun than the other. Mars reaches perihelion--its closest approach to the sun--on August 30th. During the weeks around perihelion, sunlight striking Mars will be 20% more intense than the annual average.
"This means the season for dust storms is just beginning," says Bell.
Above: Mars lies in the constellation Aquarius, which is best seen this month during the hours before local sunrise. Northern-hemisphere sky watchers should look south; southern-hemisphere sky watchers should look northeast to find the bright red planet.
A total of four spacecraft from NASA, the European Space Agency and Japan are en route to Mars now. They include three landers and two orbiters. Will dust storms cause problems for those missions?
Probably not. NASA spacecraft have encountered Mars dust before. The Viking landers of 1976, for instance, weathered two big dust storms without being damaged. As far as researchers were concerned, it was a good opportunity to study such storms from the inside--something Mars colonists may do again one day for themselves. Viking data will give them a head start.
Five years earlier, in 1971, the Mariner 9 spacecraft reached Mars during the biggest dust storm ever recorded. The planet was completely obscured; not even the polar caps were visible. Mission controllers simply waited a few weeks for the storm to subside. Then they carried on with Mariner 9's mission: to photograph the entire surface of the planet. It was a complete success.
Right: John Nemy and Carol Legate took this recent picture of bright Mars and a meteor above their campsite on Blackcomb Mountain, Whistler, British Columbia.
Even a small telescope will reveal the planet's orange disk and its icy south polar cap. And if "seeing is good" you might catch a glimpse of some dust clouds. Swirling, surging, merging with others ... building the next global dust storm? "They're fun to watch," says Parker. Now is a great time to see for yourself.
Learn more about NASA's Mars Exploration Program
Why Hellas Basin? According to Jim Bell, dust storms often begin in Hellas Basin. "It's the biggest hole in the ground in the entire solar system," he explains. Measuring 6 km deep and 2000 km across, Hellas Basin was formed more than 3 billion years ago when a large asteroid hit Mars. During the years since it has accumulated plenty of dust. "Hellas is a dust bowl. Because the basin is so deep, air at the bottom is about 10 degrees or so warmer than air at the top. This gradient drives winds, which can carry dust all the way out of the crater."
Approaching Mars (Science@NASA) -- Earth and Mars are converging for a close encounter in August. The red planet is already an appealing target for sky watchers.
Association of Lunar and Planetary Observers (ALPO) -- The astronomers of ALPO are mostly amateurs, yet many are expert observers. Executive director Donald Parker, for instance, is a retired physician who has been observing Mars since 1956. He has taken over 20,000 photographs and electronic images of Mars and Jupiter, as support for professional astronomers at NASA, JPL, and various observatories. In recognition of his contributions to planetary astronomy, Dr. Parker was honored by the International Astronomical Union in 1994, when an asteroid was given the name "5392 Parker."
Planet Gobbling Dust Storms (Science@NASA) -- An enormous dust storm exploded on Mars in 2001, shrouding the planet in haze and raising the temperature of its upper atmosphere 30 deg. C.