The orbiting Mars Global Surveyor has detected powerful magnetic objects buried in the crust of the Red Planet and unexpectedly dramatic features such as a vast Sahara-like expanse of stunningly flat terrain that may have been the floor of an ancient sea, scientists reported yesterday.

At the same time, managers of the Mars Pathfinder robots on the surface said the lander's battery appears to be dead after three months in the Martian deep-freeze, causing a communications disruption that began last Saturday. But they said the craft is otherwise healthy and expressed confidence they will be able to continue limited operations using solar energy alone.

"Rumors of the death of Pathfinder are greatly exaggerated," said project manager Brian Muirhead. "I'm confident we'll get to the bottom of this and learn how to operate without a battery."

Overhead, the recently arrived Mars Surveyor craft is still maneuvering into position for a formal mapping mission that will not begin for six months. But as the spacecraft's handlers send it dipping repeatedly into the Martian atmosphere as low as 69 miles above the surface, using the drag to reshape its orbital path, they have begun to check out their equipment.

"All the instruments are operating magnificently," said lead scientist Arden L. Albee, of the California Institute of Technology. "We're astounded by the things we're getting back in the first weeks," he added, and this is only a taste of the flood of data to come.

A couple of weeks ago, when the Surveyor had been in orbit just a few days, its scientists reported the first conclusive evidence of a global magnetic field around Mars. The field was relatively weak-just one-eight-hundredths the strength of Earth's-but its presence holds important implications for the history of the planet.

Yesterday, Surveyor scientist Jack Connerney of NASA's Goddard Space Flight Center in Greenbelt said the craft's electronic compass has shown that Mars no longer has a global magnetic field like Earth and most of the other planets. "That magnetic field is lost. It's gone but not forgotten."

Instead, scientists were shocked to realize that what they are measuring is a remnant "memory" of the ancient global field, locked inside at least two and possibly numerous subsurface objects emplaced like giant magnets in the crust of Mars and detected on several of Surveyor's 12 orbits so far, he said.

These objects, up to perhaps 100 miles across, are most likely rocky formations of molten rock that somehow flowed up through the crust and solidified. "When they cool, they acquire a memory of the primordial magnetic field" of that epoch, he said. It appears that "the crust of Mars is strewn with multiple magnetic anomalies" and that their magnetic power dwarfs that of any such objects ever measured on Earth or anywhere else.

On modern-day Earth, of course, a compass needle swings fairly reliably toward magnetic north. On Mars, these massive magnets would keep the needle spinning, producing navigational chaos.

"If you're a Boy Scout on Mars with a compass, you're lost," Connerney said.

Deciphering the magnetic field data would yield important insights on the planet's evolution. Magnetic fields are generated-at least on Earth, Jupiter and Saturn-by "dynamos" of electrically conducting molten metal at their core. If Mars once had an active dynamo and an internal heat source, as indicated by the presence of ancient volcanoes, said Mario H. Acuna, also of Goddard, this lends weight to theories that it also once had a thicker atmosphere and liquid water on its surface, conditions where primitive life might have arisen. A magnetic field also shields a planet from the supersonic wind of charged particles flowing from the sun, and from cosmic rays, which impede the development of life.

Daniel Winterhalter, of the Jet Propulsion Laboratory in Pasadena, Calif., also on the magnetic field team, said the new finding seems to introduce "a third type of signature" somewhere between planets with a global magnetic field and those, such as Venus, with none.

Researcher David Smith reported that measurements of the vast Northern Plain of Mars had shown it to be much flatter than expected, over an expanse of more than 1,500 miles, leading to a volcanic peak twice as high as previously thought. It's possible that the plain was once covered by an ocean larger than the Pacific, said Smith, also of Goddard. His laser altimeter instrument has also detected a chasm about 2.5 miles deep, two or three times deeper than the Grand Canyon, and yet it is a modest feature in the dramatic Martian landscape.

The scientists emphasized that these findings are preliminary, with much more work to be done. "Hypotheses and theories are flying fast," said Winterhalter. "We really are excited. Mars is certainly turning out to be more interesting than [earlier spacecraft findings] indicated. We're slowly taking the outer wrappers off."