Martian dust storms are very much like the severe ones on Earth--"only more so," Jet Propulsion Laboratory planetary scientist says.
The towering storms which obscured Mars' southern hemisphere in 1971 appear to have been triggered by the same mechanism which kicks up giant dust clouds on Earth in winter and spring--polar air sweeping down onto warmer mountain slope, basin or plain.
Peter M. Woiceshyn of JPL reported this finding after twoyear comparative study of Martian and Earth dust storm data. He said Martian storms, particularly in the Hellas area, are "quite similar" to some in the arid regions of Russia, Persia, the high plains of the United States, and the Arizona and Sahara deserts.
The JPL investigator used Lowell Observatory data on July l9, 1971, Martian dust storm to determine that wall of dust over 30 miles high (50 kilometers) swept down the west slopes of Hellas at speeds greater than 300 miles per hour. Mariner 9 radio occultation data provided by Dr. Arvidas J. Kliore, also of JPL, verified that such high-velocity winds would be required to raise surface dust in Mars' low atmospheric pressure.
(Air density on Mars is only l/l00th that on Earth. Mariner 9 is the unmanned spacecraft laboratory which JPL sent to orbit Mars in 1970-71 for the National Aeronautics and Space Administration.)
When Mariner 9 arrived in November, 1971, second dust storm had been in progress several weeks. Dust cloud tops were estimated by Mariner 9 experimenters at heights of 50 to 70 kilometers (30 to 40 miles) above the surface.
In their joint written report, Woiceshyn and Kliore said the two 1971 storms (and another in 1956) began in the same location on Hellas slopes and apparently were triggered by cold jet stream from the Martian north pole, funneling down long valley across the planet's equator.
Hellas extends from about 65 degrees to 30 degrees south latitude on Mars. Its long sloping topography is strong factor in producing giant dusters--the bottom of the Hellas basin lying 8 km (5 miles) lower than the highest rim of the surrounding mountains.
"The gravity flow produced from cold air streaming over the top of mountain ridge is like combination of waterfall and tidal wave," Woiceshyn told members of the Division of planetary Sciences of the American Astronomical Society in Austin, Tex., March 30. He will make further report at the annual spring meeting of the American Geophysical Union April 11 in Washington, D.C.
Such frigid air cascade over mountain barrier onto slope and plain is known to meteorologists as bora, or norther. The best Earth examples, Woiceshyn points out, are found in Russia, where polar winds sweep the steppes; in the mountain-ringed valleys of Persia, and to some extent on the U.S. plains just east of the Rockies.
Typical dust storms on these plains have been reported to reach heights of more than 20,000 feet, with winds near the surface ranging from 60 to l00 miles per hour. recent (March 19) dust storm obscured the Colorado-Kansas border region, whipped by 80 to l00 mph winds.
Data from Project Dustorm (cq), organized by the Aerosol Project Group and headed by Dr. Ed Danielsen of the National Center for Atmospheric Research, Boulder, Colo., is now being analyzed to determine the soil erosion damage caused by severe dusters in this country and around the world.
The most intense and dangerous storms on Earth have occurred during prolonged periods of drought, such as the early 1930s in the U.S. dustbowl area. In Russia winds of prolonged 1928 storm raised more than 15 million tons of black earth dust from an area of 250 million acres, according to Woiceshyn's research.
Similar erosion is caused by the heavy winds on Mars, too, Mariner 9 revealed. And there were indications other factors may be at work on the Red Planet.
The yellow cast of the Mars' dust clouds gave them the appear ance of so-called desert dusters, known as haboobs (Arabic for wild winds). Haboobs are the dust-laden gusts which occasionally cool Sahara and Arizona desert regions during the summer. But there is no firm proof yet that the right conditions exist to produce that type of storm on Mars.
However, more conclusive data on Martian storms could be provided in the coming year by the two 1976 Viking soacecraft and their landers. Viking arrives at Mars in mid-June and drops its lander on or about July 4. Viking II reaches Mars in mid-August, with lander descendinq about Sept. 4.
The Woiceshyn-Kliore study was sponsored by NASA's Office of Space Science. Caltech operates JPL for NASA.
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