See more of the story

The Mars rover Curiosity has found something noteworthy in a pinch of Martian sand. But what is it?

The scientists working on the mission are not saying. Outside that team, lots of people are guessing.

The intrigue started last week when John Grotzinger, the mission's project scientist, told National Public Radio: "This data is going to be one for the history books. It's looking really good."

And then he declined to say anything more.

Fossils? Living microbial Martians? Maybe the carbon-based molecules known as organics, which are the building blocks of life?

That so much excitement could be set off by a passing hint reflects the enduring fascination of both scientists and nonscientists with Mars.

"It could be all kinds of things," said Peter Smith, a planetary scientist at the University of Arizona who was the principal investigator for NASA's earlier Phoenix mission to Mars. "If it's historic, I think it's organics. That would be historic in my book."

Grotzinger and other Curiosity scientists will announce their latest findings on Monday in San Francisco at a meeting of the American Geophysical Union.

Do not expect pictures of Martians, though.

Guy Webster, a spokesman for NASA's Jet Propulsion Laboratory in Pasadena, Calif., which operates Curiosity, said the findings would be "interesting" rather than "earthshaking."

Whatever is revealed will be linked to the work of Curiosity's sophisticated chemistry laboratory instrument, Sample Analysis at Mars -- SAM, for short. The rover's robotic arm dropped the first bit of sand and dust into the instrument on Nov. 9, and the scientists have been analyzing and contemplating ever since.

One of the main goals of SAM is to identify organic molecules, but it would be a big surprise for organics to show up in a first look at a sand sample selected more as a test exercise than with the expectation of a breakthrough discovery.

Curiosity will be headed toward layers of clays, which could be rich in organics and are believed to have formed during a warm and wet era early in the planet's history. But Curiosity has months to drive before arriving at those locations.

The Curiosity scientists have learned through experience that it pays to double-check their results before trumpeting them. An initial test of the Martian atmosphere by the same instrument showed the presence of methane, which would have been a major discovery, possibly indicating the presence of methane-generating microbes living on Mars today. But when the scientists ran the experiment again, the signs of methane disappeared, leading them to conclude that the methane found in the first test had come from air that the spacecraft had carried to Mars from its launching spot in Florida.

Webster, who was present during the interview with NPR, said Grotzinger had been talking more generally about the quality of data coming back from Curiosity and was not suggesting that the data contained a breakthrough surprise.

"I don't think he had in mind, 'Here's some particular chemical that's been found,' " Webster said. "That's not my impression of the conversation."

"I do want to temper expectations," Webster said. "But then again, I don't know exactly what they're going to say they've found."