After a martian dust storm. (Astronomy).

The largest dust storm seen on Mars in more than 2 decades (SN: 7/28/01, p. 53) is now beginning to wane, according to data from the Mars Global Surveyor spacecraft. Warmed by dust, the atmosphere is still about 10 [degrees] C hotter than a year ago, says Surveyor scientist Philip Christensen of Arizona State University in Tempe.

In mid-June, Surveyor researchers reported finding dust clouds in the Hellas basin in the south. They later revealed that similar activity had been going in a northerly region. The clouds somehow merged and by June 26 had engulfed most of the planet.

The storm was notable because, unlike others, it occurred before Mars' closest approach to the sun. Surveyor and the Hubble Space Telescope are providing researchers for the first time with a bonanza of data on atmospheric conditions before and after a major martian storm, James F. Bell of Cornell University noted at an Oct. 11 NASA briefing.

Bell says the atmosphere will take a while to clear because the dust grains are as small as smoke particles. He adds that the poles weren't engulfed by the storm, probably because the vortexlike winds there kept the dust out.

On Oct. 23, Mars Odyssey became the first U.S. probe to reach the Red Planet after two failures. Because it entered orbit at high altitude, it hasn't yet been affected by dust. However, the craft will encounter the material as it dips into the planet's atmosphere in order to slow down and reduce its long, elliptical orbit into a 2-hour, circular path. --R. C.