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NASA Science News for July 9, 2003

Mars Dust

Using only backyard telescopes, amateur astronomers are enjoying
great views of dust clouds on Mars 

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. 

"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." 

Parker has been tracking the cloud through his own 16-inch
telescope. "A red filter helps," he notes. "Even a piece of red or
orange gelatin held between the eye and ocular will improve the
visibility of the dust." 

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."

Mars Global Surveyor and Mars Odyssey, two NASA spacecraft circling
Mars, have seen many "regional storms" like the cloud near Hellas
Basin now. They persist for a few days or weeks, then dissipate.
Rarely do they become a planet-wide event. 

"Only 10 global or planet-encircling dust storms have been reported
since 1877," notes Parker. 

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.

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. 

As 2003 unfolds, Earth and Mars are drawing together for their
closest approach in some 60,000 years on August 27th. Already in July
Mars is a pleasing sight. Step outside before dawn anytime this
month. Mars will be there in the southern sky, a remarkably bright
red star. (If you live in the southern hemisphere, look northeast
instead.) 

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. 

Credits & Contacts
Author: Dr. Tony Phillips 
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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