h into snow density, lead by David E. Smith of NASA's Goddard Space Flight Center, confirm that much of the Martian snow is in fact composed of carbon dioxide.The study involved more than 400 million elevation measurements spanning more than one Martian year, from February 1999 through May of 2001. The orbiting spacecraft bounced a beam of laser light to the surface and back, recording the round-trip time to determine elevations within 4 inches (10 centimeters). To determine snow density, this data was compared with measurements of tiny variations in the gravity field caused when there is more or less snow at given locations.Smith and his colleagues also measured for the first time how the elevation of Mars' surface changes during the seasons, as ice builds up in winter and returns to the atmosphere in summer. As expected, each polar ice cap is highest in the dead of its winter, when it is in total darkness. Also no surprise was the finding that the biggest changes in snow depth -- more than 6 feet (2 meters) occurred close to the poles.But the overall bulk of snow accumulation seems to take place at in thinner but vast sheets at lower latitudes, the study found.Strange snowsAs with Earth, the weather on Mars can be strange.Smith's study also found odd off-season snowfalls on Mars. Because carbon dioxide does not like to be a liquid, it freezes directly out of the atmosphere into surface dry ice. It's possible shadowed areas could accumulate this "snow" regardless of the season, said Maria T. Zuber, an MIT geophysicist who also worked on the study.In one case, patches of snow disappeared during autumn in the northern hemisphere -- a time when cooler temperatures should have generated accumulations. A huge dust storm that raged in recent months and for a time covered the entire planet may have been responsible, temporarily raising global temperatures.But Zuber said the deviations are not yet understood....
