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Contact: Diane Ainsworth

FOR IMMEDIATE RELEASEMay 20, 1997

HUBBLE SEES CLOUDY SKIES, COLD WEATHER FOR MARS ARRIVAL

      New images from the orbiting Hubble Space Telescope suggest that NASA's two Mars-bound spacecraft -- Mars Pathfinder and Mars Global Surveyor -- will experience considerably different weather conditions than those witnessed by the last U.S. spacecraft to land on Mars 21 years ago.

      Astronomers have been using the Hubble Space Telescope to provide updated planetary weather reports to help plan both Mars missions. Martian atmospheric conditions will affect the Mars Pathfinder spacecraft landing and rover rollout on July 4 and the arrival of the Mars Global Surveyor on September 11.

      Images taken barely three weeks apart, on March 10 and March 30, reveal dramatic changes in some local conditions, and show overall cloudier and colder conditions than Viking encountered more than two decades ago.

      "Because Pathfinder uses the atmosphere to decrease its velocity for landing, and because the lander and rover are solar powered, understanding the state of the atmosphere prior to landing is important," said Dr. Matthew Golombek, Pathfinder project scientist at NASA's Jet Propulsion Laboratory, Pasadena, CA.

      "On July 4, Mars Pathfinder will enter the atmosphere directly from approach and slow itself behind an aeroshell with a parachute, small solid rockets and giant airbags. The lander carries a small rover to explore the surface and investigate the kinds of materials present on the Martian surface," Golombek said. "Hubble images of Mars are helping us to adjust our flight path for landing and effectively plan surface operations."

      "This is not the dusty Mars of the Viking days (mid- 1970s to early-1980s) or the habitable oasis of science fiction stories," said Todd Clancy of the Space Science Institute in Boulder, CO. "We're finding a Mars that's colder, clearer and cloudier. Hubble is rapidly changing our view of Mars' environment. The planet's weather apparently has a flip-side to it."

      Hubble's findings also offer new insights into the differences and similarities of weather on the other terrestrial planets. "The planets are similar in many important ways, so the very major differences between them are interesting from a viewpoint of better understanding meteorology," said team leader Phil James of the University of Toledo. "Hubble is allowing us to look at Mars in ways never before seen."

      In September, NASA's Mars Global Surveyor will skim across the upper Martian atmosphere to slow down by friction and enter orbit around the red planet. Atmospheric density is a key factor in precisely executing this complex and delicate aerobraking maneuver. Hubble is ideal for tracking regional dust storms which could pose a threat to Surveyor by drastically changing the planet's air density. Such storms can cause a tenfold increase in the Martian atmosphere's drag at 96 kilometers (60 miles) above the surface.

      Comparing the appearance of Mars to that of earlier spacecraft observations, Hubble has found some areas of the Martian surface that have been changed dramatically by wind-blown dust. The most prominent example is the classic "dark feature" called Cerberus, which is roughly the size of California (1,250 kilometers by 400 kilometers or 800 miles by 250 miles). This feature has been seen as a low albedo, or dark region by ground- based telescopic observers since early in this century, and was studied in detail by the Mariner 9 and Viking orbiters in the 1970s.

      In Hubble's view, only three dark splotches remain, probably related to dark sand being carried out of craters by the wind. The astronomers think that dust storms in the region have covered the formerly dark surface with bright dust, effectively erasing Cerberus from the map.

      Hubble is ideally suited for long-term study of Mars. When Mars has been closest to Earth, Hubble has resolved surface details as small as 40 kilometers (25 miles) across. This allows astronomers to track subtleties in the shifting cloud patterns and periodic dust storms. This planet-wide, "weather satellite" view is complementary to the close-up views which will be provided by Mars Pathfinder and Mars Global Surveyor.

      The Mars Pathfinder and Mars Global Surveyor missions are managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, D.C. The Hubble Space Telescope is managed by the Space Telescope Science Institute in Baltimore, MD.

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