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

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

The intrigue started last week when John P. Grotzinger, the Mars 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.

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