Photographs taken 12 days apart recently by one of the two Viking spacecraft that landed on Mars three years ago show what scientists are convinced is a thin covering of frost on the surface.

"It can't be [frozen] carbon dioxide because it would have melted in the noonday sun," Viking Project scientists Conway Snyder said yesterday from the Jet Propulsion Laboratory in Pasadena, Calif., where Viking is directed. "So since we see it 12 days apart in the Martian winter, we believe it has to be water ice."

The source of the pictures is the Viking that landed on Mars Sept. 3, 1976. The second Viking to land on the red planet, Viking 2 lies on the Utopia Plains about 48 degrees north of the Martian equator where it is now winter.

Since there are no signs of water in the Martian soil, scientists are convinced the frost seen on the rocks and soil of the Utopia Plains condensed out of the atmosphere. And since there is not enough water in the atmosphere to freeze out and show up as ice, scientists believe the frost fell out of the atmosphere as a result of the periodic dust storms that sweep over the entire planet.

"There is so little water in the Martian atmosphere that if all of it was frozen out on the surface, it would still be too thin to see," Snyder said. "This means that the water ice came from somewhere else, probably the southern hemisphere where dust storms carried the moisture up north and settled it out where the air is clear."

Snyder speculated that what little water is in the Martian atmosphere clung to dust particles along with the carbon dioxide droplets that make up most of the atmosphere. The carbon dioxide droplets made the dust particles heavy enough to fall to the surface, where the sun in the cleared northern hemisphere evaporated the carbon dioxide ice and left the thin water ice cover seen in the photographs.

The photograph of the Utopia Plains was taken May 18, relayed to Earth June 7 by the Viking 1 in orbit around Mars and just reproduced at the Jet Propulsion Laboratory.

"This may have been the last picture we get back from the Viking 2 lander," Snyder said. "We have no plans for any further photographs from the surface."

The Viking 1 spacecraft, which relayed the surface picture to Earth, will go on taking pictures from orbit for the next two months, when it will have photographed a broad stretch of the Martian equator from the Isidis Basin to the Chryse Basin. CAPTION: Picture, Scientists believe this white film on Utopia Plains rocks and soil is a thin coating of water ice. NASA photo