By Gina Keating

PASADENA, Calif. (Reuters) - NASA scientists on Thursday scrambled to find out what had gone wrong with the Mars rover Spirit after 24 hours of garbled communications and intermittent silence apparently caused by an unknown malfunction.

The scientists reported that the Spirit had radioed a signal indicating it was receiving Earth transmissions even though it had not resumed sending data back to Earth.

The grim news dampened the atmosphere at the Jet Propulsion Laboratory in California, where mission controllers have delighted up until now at the virtually flawless landing on Mars and three weeks of spectacular photographs of the barren Martian surface.

But despite the somber mood, scientists vowed to keep trying to find and correct the problem.

"Gosh, don't give up on us yet," Mars program manager Firouz Naderi told reporters. "This is a team that has been through this before."

Also, scientists said they did not think the problems with Spirit would affect the planned landing on Saturday night of another rover, Opportunity, on the other side of Mars.

Spirit project manager Pete Theisinger told a news briefing that there was a "very serious anomaly" in communications with the six-wheeled craft, which landed on Mars on Jan. 3 on a planned three-month mission to explore the geologic history of the planet.

The craft has been unable to return any science or telemetry data since early Wednesday but has sent "carrier" signals during twice-daily passes by two satellites orbiting Mars. Attempts to communicate with the rover's three onboard antenna also failed on Wednesday.

Two satellite passes on Wednesday yielded no signal from Spirit, and on a third pass by Mars Global Surveyor at 8:30 p.m. PST (11:30 p.m. EST), NASA received a transmission from Spirit but no data. The rover also cut off its expected 13-minute transmission to the satellite after just two minutes, Mars program manager Firouz Naderi said.

Naderi said scientists and engineers were puzzled by contradictory "symptoms" coming from Spirit -- one indicating that it had not switched to "fault" mode and the other showing that it has.

He added, "It is precisely like trying to diagnose a patient but with some symptoms that don't correlate. In this case it's worse because we have two symptoms that are contradictory."

On Thursday morning, the Spirit team established contact with the rover's low-gain onboard antenna by switching to a radio frequency used when the rover goes into "fault" mode, Naderi said.

Spirit is able to decide independently to shut down if it experiences conditions, such as high temperatures or a power failure, that could damage it, Naderi said.

The rover is programmed to send a signals indicating that it had switched to fault mode and the type of problem was having, if necessary.

"This morning when we tried to communicate with the low gain (antenna) it worked," he said. "We communicated with it on a channel it would have been listening on if it thought it was in fault mode."

Since Spirit did not specify what type of fault had occurred, scientists and engineers planned to piece together information received in upcoming passes by the Mars Global Surveyor and Odyssey satellites.

Naderi said mission scientists had worked throughout the night on scenarios ranging from a major power failure to a software or memory corruption.

Spirit last week began its first tentative journeys sampling the surface soil of the Gusev Crater -- a barren, wind-swept basin that scientists believe may have been the site of an ancient lake bed once fed by a Martian river.

The two robotic rovers are the most advanced missions to date in man's 40-year quest to discover the geologic history of Mars and whether it was ever sufficiently warm or wet enough to sustain a recognizable form of life. (Additional reporting by Jill Serjeant)