The Hubble space telescope is snapping the sharpest pictures of Mars ever taken from the vicinity of Earth in a long-term program to monitor seasonal changes in the atmosphere of the red planet.

The work eventually may allow scientists to understand global weather patterns, a prerequisite for any future manned missions to Mars, according to the Space Telescope Science Institute, which coordinates Hubble science activities.Preliminary observations with the $1.5 billion telescope clearly show features of the Martian atmosphere and surface details as small as 31 miles across. The smallest features detectable by ground-based instruments are some 93 miles wide.

One of the first pictures of Mars released by the Space Telescope Science Institute was taken during midsummer in Mars' southern hemisphere. The picture shows a clear sky over much of the planet, but a thick canopy of clouds obscuring the frigid north pole region.

"As on Earth, the periphery of this polar hood region is the locale of intense storm systems which migrate through northern mid latitudes," the science institute said in a news release. "Details of the cloud structures can be seen in the HST images."

Other details visible in the Dec. 13, 1990, image include Isidis Planitia, a 620-mile-wide impact basin, the heavily cratered Arabia Planitia region, the windswept Syrtis Major area and the bright Hellas Planitia basin, which measures 1,118 miles across and 5 miles deep.

While Mars has been studied in detail by a variety of space probes, including the Viking landers, the planet has an active atmosphere and goes through seasonal changes much like Earth.

"To understand Mars' complex meteorology and climate, the planet must be continually monitored over many of its annual cycles, much as the monitoring of Earth by terrestrial weather satellites has improved our ability to understand and forecast weather on our planet," the Space Telescope Science Institute said.

Studies with the Hubble space telescope will allow scientists to monitor the surface and atmosphere of Mars to reveal the global distribution of water and dust clouds at various locations and times of year.

Such research is impossible using current ground-based telescopes because of the blurring effects of Earth's turbulent atmosphere.

In fact, the sharpest images possible, when Mars and Earth are closest together every 780 days or so, cannot show surface features smaller than about 93 miles across. Most of the time, the resolution is between about 370 and 620 miles.