Huge rock formations on Mars that resemble a human face, a pyramid and a fortress could be the work of a lost civilization or "just a trick of nature," say scientists urging a close-up look at the outcroppings.

Four scientists including a former astronaut said Thursday that the mysterious formations should be scrutinized further by spacecraft.Photographs taken in 1976 by the Viking 1 spacecraft as it orbited Mars show a sphinxlike face staring outward into space. The image resembles a death mask, with a long shadow obscuring one side of the face while emphasizing a humanlike brow, nose, eye socket and mouth.

Nearby is an angular mountain, suggesting the sharp lines of a pyramid, and a grouping of rocks

that some view as a fortress near the center of what they suggest could have been an ancient city of a lost civilization.

Or, the scientists said at a news conference Thursday, the images captured by the Viking camera could all be "a trick of light and shadow."

"We have found something that is so interesting that it demands we go back to Mars and get more data," said Richard C. Hoagland, founder of The Mars Project, an organization that is studying the Viking photographs.

Brian T. O'Leary, a former astronaut and an expert on Mars, said there is so much uncertainty about the origin of the rock face that it should be a major target for future spacecraft sent to Mars.

"The feature is sufficiently exciting that we must go back to Mars and photograph it at a higher resolution," said O'Leary.

Gerald Soffel, the mission scientist on the Viking project and currently chief of the space and Earth sciences branch at the Goddard Space Center, said in a telephone interview that the facelike image captured by the camera has been dismissed by most scientists as an illusion caused by bright sunlight and shadow.

He said some of the world's finest geologists have examined the photos and "they said what we were seeing is light and shadow."

But Mark Carlotto, an optical engineering expert, said a new, sophisticated computer study of the photos suggests that the face and other formations are not of natural origin.

In an article published in Applied Optics, Carlotto said a computer enhancement of the Viking photographs shows the rock shapes appear to have been carved by "intelligent design" and not by the random forces of nature.

In one study, Carlotto said, the face was compared statistically with other shapes photographed on Mars, and he concluded that "the face is not natural."

"It's extremely unlikely that it's a trick of light and shadow," said Carlotto.

O'Leary said Soviet scientists showed great interest in the Mars formations when he talked with them in January but was told their spacecraft was not technically designed for such a survey of the Cydonia region of Mars where the structures are located.

On Thursday, a Soviet unmanned probe that will scout out possible manned landing sites on Mars for the next century headed toward the red planet on a seven-month journey that began with a spectacular nighttime launch.

Phobos I, named for the potato-shaped Martian moon to which it will eventually descend, was piggybacked aloft atop a 198-foot Proton booster.