A long time ago on a planet not so far away . . . very small extraterrestrials once lived.

Well, probably.

Call them Martian microbes, and credit NASA's long-lived Mars rovers for the discovery.

NASA has been exploring the surface and atmosphere of Mars for a decade now, and its rovers have found evidence that the planet was likely inhabited - at least by microscopic life - around the same time as life began on Earth.

Moreover, a life-supporting atmosphere probably persisted for hundreds of million of years, NASA scientists say.

But in the three billion years or so since that epoch, the Red Planet has become quite inhospitable, with a thin atmosphere more than 95 percent carbon dioxide and a surface temperature of minus 82 degree Fahrenheit.

It's the most important discovery made by the golf-cart-sized rover named Opportunity in its 10 years on the planet. Powered by solar panels, and directed by its Earth-based controllers, it found a type of clay associated with a benign watery environment. Scientists inferred that the water on Mars nearly 4 billion years ago was of the type, with low acidity, that supports life.

Opportunity also kicked up a curious rock that had scientists scratching their heads. Its appearance was unexpected because it showed up in an area where previous photographs showed nothing.

And it prompted "Star Trek" actor William Shatner to submit a question to the NASA press conference: "Are you going to cover the alien rock throwers?" A straight-faced Steven W. Squyres, the principal investigator for Opportunity, responded that he doubted their existence.

The rock "looks like a jelly doughnut," Dr. Squyres continued: "White around the outside, red in the middle. We've looked at it with our microscope. It's clearly a rock," but with a strange composition. "Mars keeps throwing new stuff at us," he quipped.

NASA is scrubbing its budget for spacecraft that have outlived their original missions. But it would be a pity to end the inexpensive Opportunity mission before the doughty little rover finally wears out.

Knowing more about Mars' ancient life-sustaining atmosphere, and why it collapsed, could even help Earthlings learn about how to improve our own planetary chances a few million years hence.