Talk about a long trip: An unmanned rover has been driving on Mars for more than seven years, though not constantly. Now the solar-powered exploratory vehicle with six wheels has reached a promising destination -- a sunken crater called Endeavour. And the first rock its long robotic arm picked up for remote-control analysis has opened a new chapter in the study of Mars.

NASA mission scientists are excited because the rock has sizable quantities of two elements that suggest geology formed by heat and water -- based on how things work here. Earlier evidence and now the presence of zinc and bromine, shown in data transmitted home, suggest a warmer, wetter period in Mars' past that may have offered friendlier conditions for some form of life. "The excitement level within the engineering and science teams is way up," says project scientist Ray Arvidson of Washington University in St Louis.

The dune buggy-size rover, named Opportunity, could spend years examining the new terrain. It's one of two that arrived in January 2004, landing on different sides of the planet with the goal of exploring the surface for signs of past water. Each already found evidence of water that suggests life might have been possible on ancient Mars, but that water was highly acidic. The other rover went silent after a sand trap last year. NASA plans to launch an SUV-size mobile lab in November that will reach Mars next summer. Its name, aptly, is Curiosity.

Astronomer says: "This rock doesn't look like anything else we've seen before" on Mars. -- Steven Squyres, Cornell University professor in Ithaca, N.Y.

NASA official says: "We have a very senior rover that's showing her age. . . . But in general, she's in good health." -- John Callas, project manager

Science writer says: "Nobody is predicting that [the rover] will find signs of life, ancient or modern. . . . The search is simply for evidence that Mars could at least have been hospitable to life, and the clues will be more geological than biological." -- Michael Lemonick, Time magazine